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Sample records for point-defect migration energy

  1. Atomistic studies of point defect migration rates in the iron-chromium system

    NASA Astrophysics Data System (ADS)

    Hetherly, Jeffrey

    Generation and migration of helium and other point defects under irradiation causes ferritic steels based on the Fe-Cr system to age and fail. This is motivation to study point defect migration and the He equation of state using atomistic simulations due to the steels' use in future reactors. A new potential for the Fe-Cr-He system developed by collaborators at the Lawrence Livermore National Laboratory was validated using published experimental data. The results for the He equation of state agree well with experimental data. The activation energies for the migration of He- and Fe-interstitials in varying compositions of Fe-Cr lattices agree well with prior work. This research did not find a strong correlation between lattice ordering and interstitial migration energy.

  2. Ion beam injected point defects in crystalline silicon: Migration, interaction, and trapping phenomena

    SciTech Connect

    Priolo, F.; Libertino, S. |; Privitera, V.; Coffa, S.

    1997-11-01

    The recent work on the room temperature migration and trapping phenomena of ion beam generated point defects in crystalline Si is reviewed. It is shown that a small fraction ({approximately}10{sup {minus}6}) of the defects generated at the surface by a shallow implant is injected into the bulk. These defects undergo a long range trap-limited diffusion and interact with both impurities, dopants and preexisting defects along their path. In particular, these interactions result in dopant deactivation and/or partial annihilation of pre-existing vacancy-type defect markers. It is found that in highly pure, epitaxial Si layers, these effects extend to several microns from the surface, demonstrating a long range migration of point defects at room temperature. By a detailed analysis of the experimental evidences the authors have identified the Si self-interstitials as the major responsible for the observed phenomena. This allowed them to give a lower limit of 6 {times} 10{sup {minus}11} cm{sup 2}/s for the room temperature diffusion coefficient of the Si self-interstitials. Room temperature trap-limited migration of vacancies is also detected as a broadening in the divacancy profile of as implanted samples. In this case the room temperature diffusion coefficient of vacancies has been found to be {ge}3 {times} 10{sup {minus}12} cm{sup 2}/s. These data are presented and their implications discussed.

  3. Energy barriers for point-defect reactions in 3C-SiC

    NASA Astrophysics Data System (ADS)

    Zheng, Ming-Jie; Swaminathan, Narasimhan; Morgan, Dane; Szlufarska, Izabela

    2013-08-01

    Energy barriers of the key annealing reactions of neutral and charged point defects in SiC are calculated with ab initio density functional theory methods. In order to effectively search for the lowest energy migration paths the preliminary path is first established based on ab initio molecular dynamics (AIMD) simulations. The energy barrier of each hop is then calculated via climbing image nudged elastic band methods for paths guided by the AIMD simulations. The final paths and barriers are determined by comparing different pathways. The annealing reactions have important implications in understanding the amorphization, recovery, and other aspects of the radiation response of SiC. The results are compared with the literature data and experimental results on SiC recovery and amorphization. We propose that the C interstitial and Si antisite annealing reaction may provide a critical barrier that governs both the recovery stage III and amorphization processes.

  4. Long range lateral migration of intrinsic point defects in n-type 4H-SiC

    SciTech Connect

    Loevlie, L. S.; Vines, L.; Svensson, B. G.

    2012-05-15

    The lateral distributions of intrinsic point defects in n-type (0001) 4H-SiC have been investigated following room temperature irradiation with a focused beam of 10 keV protons. Laterally resolved deep level transient spectroscopy measurements reveal that the well-known and prominent Z{sub 1/2} and S{sub 1/2} centers display lateral diffusion lenghts on the order of 1 mm with negligible (if any) motion parallel to the direction of the c-axis. The migration occurs only in the presence of excess charge carriers generated during the proton irradiation, and no further motion takes place even under subsequent optical excitation of high intensity. Assuming one-dimensional geometry, an effective defect diffusivity in excess of 10{sup -6} cm{sup 2}/s is deduced by numerical modelling of the experimental data, corresponding to an energy barrier for migration of {approx}0.2 eV. Possible mechanisms for the rapid migration, invoking charge carrier recombination as a necessary condition, are discussed, and especially, an association with the glide of partial dislocations along the (0001) basal plane is scrutinized in some detail.

  5. Migration of point defects and a defect pair in zinc oxide using the dimer method

    SciTech Connect

    Chen, Dong; Gao, Fei; Dong, Mingdong; Liu, Bo

    2012-09-24

    The migration mechanism and the minimum energy path of vacancies, interstitials and an interstitial-vacancy pair in zinc oxide have been studied by the dimer method. The in-plane and out-of-plane migrations of zinc and oxygen vacancies are found to be anisotropic. The kick-out mechanism is energetically preferred to zinc and oxygen interstitials that can easily migrate through the ZnO crystal lattice. In addition, the migration process of an interstitial-vacancy pair as a complex of an octahedral oxygen interstitial and a zinc vacancy is dominated by an oxygen interstitial/zinc vacancy successive migration. The energy barriers indicate that the existence of oxygen interstitial in the defect pair can promote the mobility of zinc vacancy, whereas the migration of oxygen interstitial is slowed down due to the presence of zinc vacancy. In the end, we show a possible migration path of the interstitial-vacancy pair that can be dissociated through a set of displacement movements.

  6. Migration mechanisms and diffusion barriers of carbon and native point defects in GaN

    NASA Astrophysics Data System (ADS)

    Kyrtsos, Alexandros; Matsubara, Masahiko; Bellotti, Enrico

    2016-06-01

    Carbon related defects are readily incorporated in GaN due to its abundance during growth both with MBE and MOCVD techniques. Employing first-principles calculations, we compute the migration barriers of carbon interstitials and we discuss possible relevant mechanisms of diffusion in the wurtzite GaN crystal. In addition, we calculate the migration barriers for the diffusion of the native defects of the crystal, i.e., gallium and nitrogen interstitials and vacancies. The minimum energy path and the migration barriers of these defects are obtained using the nudged elastic band method with the climbing image modification. In addition, the dimer method is used to independently determine the results. The results yield a quantitative description of carbon diffusion in GaN allowing for the determination of the most preferable migration paths.

  7. Electron and chemical reservoir corrections for point-defect formation energies

    NASA Astrophysics Data System (ADS)

    Freysoldt, Christoph; Lange, Björn; Neugebauer, Jörg; Yan, Qimin; Lyons, John L.; Janotti, Anderson; Van de Walle, Chris G.

    2016-04-01

    Point-defect formation energies calculated within the framework of density functional theory often depend on the choice of the exchange and correlation (xc) functional. We show that variations between the local density approximation (LDA), generalized gradient approximation (GGA), and hybrid functionals mainly arise from differences in the position of the bulk valence-band maximum, as well as in the reference energies for the chemical potential obtained with distinct xc functionals. We demonstrate for point defects relevant for p -type GaN that these differences can be accounted for by corrections, reducing the maximum disagreement between the different functionals from more than 2 eV to below 0.2 eV. Our correction scheme should be useful for performing high-throughput calculations in cases where full hybrid functional calculations are prohibitively expensive.

  8. Formation energies of intrinsic point defects in monoclinic VO2 studied by first-principles calculations

    NASA Astrophysics Data System (ADS)

    Cui, Yuanyuan; Liu, Bin; Chen, Lanli; Luo, Hongjie; Gao, Yanfeng

    2016-10-01

    VO2 is an attractive candidate for intelligent windows and thermal sensors. There are challenges for developing VO2-based devices, since the properties of monoclinic VO2 are very sensitive to its intrinsic point defects. In this work, the formation energies of the intrinsic point defects in monoclinic VO2 were studied through the first-principles calculations. Vacancies, interstitials, as well as antisites at various charge states were taken into consideration, and the finite-size supercell correction scheme was adopted as the charge correction scheme. Our calculation results show that the oxygen interstitial and oxygen vacancy are the most abundant intrinsic defects in the oxygen rich and oxygen deficient condition, respectively, indicating a consistency with the experimental results. The calculation results suggest that the oxygen interstitial or oxygen vacancy is correlated with the charge localization, which can introduce holes or electrons as free carriers and subsequently narrow the band gap of monoclinic VO2. These calculations and interpretations concerning the intrinsic point defects would be helpful for developing VO2-based devices through defect modifications.

  9. Structure and energy of point defects in TiC: An ab initio study

    NASA Astrophysics Data System (ADS)

    Sun, Weiwei; Ehteshami, Hossein; Korzhavyi, Pavel A.

    2015-04-01

    We employ first-principles calculations to study the atomic and electronic structure of various point defects such as vacancies, interstitials, and antisites in the stoichiometric as well as slightly off-stoichiometric Ti1 -cCc (including both C-poor and C-rich compositions, 0.49 ≤c ≤0.51 ). The atomic structure analysis has revealed that both interstitial and antisite defects can exist in split conformations involving dumbbells. To characterize the electronic structure changes caused by a defect, we introduce differential density of states (dDOS) defined as a local perturbation of the density of states (DOS) on the defect site and its surrounding relative to the perfect TiC. This definition allows us to identify the DOS peaks characteristic of the studied defects in several conformations. So far, characteristic defect states have been discussed only in connection with carbon vacancies. Here, in particular, we have identified dDOS peaks of carbon interstitials and dumbbells, which can be used for experimental detection of such defects in TiC. The formation energies of point defects in TiC are derived in the framework of a grand-canonical formalism. Among the considered defects, carbon vacancies and interstitials are shown to have, respectively, the lowest and the second-lowest formation energies. Their formation energetics are consistent with the thermodynamic data on the phase stability of nonstoichiometric TiC. A cluster type of point defect is found to be next in energy, a titanium [100] dumbbell terminated by two carbon vacancies.

  10. A study on density functional theory of the effect of pressure on the formation and migration enthalpies of intrinsic point defects in growing single crystal Si

    NASA Astrophysics Data System (ADS)

    Sueoka, Koji; Kamiyama, Eiji; Kariyazaki, Hiroaki

    2012-05-01

    In 1982, Voronkov presented a model describing point defect behavior during the growth of single crystal Si from a melt and derived an expression to predict if the crystal was vacancy- or self-interstitial-rich. Recently, Vanhellemont claimed that one should take into account the impact of compressive stress introduced by the thermal gradient at the melt/solid interface by considering the hydrostatic pressure dependence of the formation enthalpy of the intrinsic point defects. To evaluate the impact of thermal stress more correctly, the pressure dependence of both the formation enthalpy (Hf) and the migration enthalpy (Hm) of the intrinsic point defects should be taken into account. Furthermore, growing single crystal Si is not under hydrostatic pressure but almost free of external pressure (generally in Ar gas under reduced pressure). In the present paper, the dependence of Hf and Hm on the pressure P, or in other words, the pressure dependence of the formation energy (Ef) and the relaxation volume (vf), is quantified by density functional theory calculations. Although a large number of ab initio calculations of the properties of intrinsic point defects have been published during the last years, calculations for Si crystals under pressure are rather scarce. For vacancies V, the reported pressure dependences of HfV are inconsistent. In the present study, by using 216-atom supercells with a sufficient cut-off energy and mesh of k-points, the neutral I and V are found to have nearly constant formation energies EfI and EfV for pressures up to 1 GPa. For the relaxation volume, vfI is almost constant while vfV decreases linearly with increasing pressure P. In case of the hydrostatic pressure Ph, the calculated formation enthalpy HfI and migration enthalpy HmI at the [110] dumbbell site are given by HfI = 3.425 - 0.057 × Ph (eV) and HmI = 0.981 - 0.039 × Ph (eV), respectively, with Ph given in GPa. The calculated HfV and HmV dependencies on Ph given by HfV = 3.543 - 0

  11. Point defect survival and clustering fractions obtained from molecular dynamics simulations of high energy cascades

    SciTech Connect

    Stoller, R.E.

    1995-12-31

    Evolution of high-energy displacement cascades in iron has been investigated for times up to 200 ps using molecular dynamics simulation. The simulations were carried out using the MOLDY code and a modified version of the many-body interatomic potential developed by Finnis and Sinclair. Previously reported results have been supplemented by a series of 10 keV simulations at 900 K and 20 keV simulations at 100K. Results indicate that the fraction of the Frenkel pairs escaping in-cascade recombination is somewhat higher and the fraction of the surviving point defects that cluster is lower in iron than in materials such as copper. In particular, vacancy clustering appears to be inhibited in iron. Many of the larger interstitial clusters were observed to exhibit a complex, three-dimensional morphology. Apparent mobility of the <111> crowdion and clusters of <111> crowdions was very high.

  12. Role of defect coordination environment on point defects formation energies in Ni-Al intermetallic alloys

    NASA Astrophysics Data System (ADS)

    Tennessen, Emrys; Rondinelli, James

    We present a relationship among the point defect formation energies and the bond strengths, lengths, and local coordination environment for Ni-Al intermetallic alloys based on density functional calculations, including Ni3Al, Ni5Al3, NiAl,Ni3Al4, Ni2Al3 and NiAl3. We find the energetic stability of vacancy and anti-site defects for the entire family can be attributed primarily to changes in interactions among first nearest neighbors, owing to spatially localized charge density reconstructions in the vicinity of the defect site. We also compare our interpretation of the local coordination environment with a DFT-based cluster expansion and discuss the performance of each approach in predicting defect stability in the Ni-Al system.

  13. Evolution of point defect clusters in pure iron under low-energy He+ irradiation

    NASA Astrophysics Data System (ADS)

    Arakawa, K.; Imamura, R.; Ohota, K.; Ono, K.

    2001-05-01

    The formation process of point defect clusters in high-purity (99.999%) iron as a typical bcc metal under the irradiation with low-energy (5 keV)He+ is studied by in situ transmission electron microscopy (TEM). Using conventional TEM techniques, clusters induced by the irradiation are determined to be interstitial-type dislocation loops (I loops) at temperatures ranging from 85 to 770 K and cavities from 300 to 770 K. Most of the I loops are determined to lie on {100} planes, and their Burgers vector are determined to be a[100]. The temporal variation in the volume density of I loops is measured by stereomicroscopy at several temperatures. The volume density of I loops is about two orders of magnitude higher in comparison with the case of high-energy electron irradiation at the same dpa rate; hence, it is deduced that helium atoms have an effect enhancing the nucleation of I loops. The depth distribution of the loop densities becomes broad above around 235 K at which vacancies become thermally mobile, and the dependence of loop formation on He+ beam flux is weak. From these results, it is suggested that helium-vacancy complexes act as trapping sites of self-interstitial atoms and I-loop nucleation is enhanced.

  14. Formation energy and lattice relaxation for point defects in Li and Al

    SciTech Connect

    Benedek, R.; Yang, L.H. ); Woodward, C. ); Min, B.I. )

    1992-02-01

    Calculations were performed for both a vacancy and an Al solute atom in bcc Li and for a vacancy in fcc Al. The purpose of this work was (i) to test optimization algorithms that allow a unified determination of ground-state electronic structure and lattice relaxation, and (ii) to compare calculated properties with experiment and with previous pair-potential simulations. 16- and 54-site supercells were employed in the Li-host calculations and a 32-site supercell was employed for the Al vacancy. The self-consistent Kohn-Sham orbitals, expanded in a plane-wave basis, were obtained using the modified steepest-descents algorithm of Williams and Soler and the band-by-band iteration method of Teter, Payne, and Allan. Electron-ion interactions were represented by generalized norm-conserving psuedopotentials cast in separable form. The relative performance of the two optimization algorithms is discussed. The equilibrium lattice relaxation was calculated by the Newton-Raphson method, with the Hessian matrix determined from numerical derivatives of the Hellman-Feynman forces. Calculated vacancy-formation energies are in excellent agreement with experiment.

  15. A fourfold coordinated point defect in silicon.

    PubMed

    Goedecker, Stefan; Deutsch, Thierry; Billard, Luc

    2002-06-10

    Vacancies, interstitials, and Frenkel pairs are considered to be the basic point defects in silicon. We challenge this point of view by presenting density functional calculations that show that there is a stable point defect in silicon that has fourfold coordination and is lower in energy than the traditional defects.

  16. Molecular dynamics simulations of point defect production in cementite and Cr23C6 inclusions in α-iron: Effects of recoil energy and temperature

    NASA Astrophysics Data System (ADS)

    Henriksson, K. O. E.

    2016-06-01

    The number of point defects formed in spherical cementite and Cr23C6 inclusions embedded into ferrite (α-iron) has been studied and compared against cascades in pure versions of these materials (only ferrite, Fe3C, or Cr23C6 in a cell). Recoil energies between 100 eV and 3 keV and temperatures between 400 K and 1000 K were used. The overall tendency is that the number of point defects — such as antisites, vacancy and interstitials — increases with recoil energy and temperature. The radial distributions of defects indicate that the interface between inclusions and the host tend to amplify and restrict the defect formation to the inclusions themselves, when compared to cascades in pure ferrite and pure carbide cells.

  17. Point defects and the binding energies of boron near defect sites in Ni{sub 3}Al: A first-principles investigation

    SciTech Connect

    Fu, C.L.; Painter, G.S.

    1997-02-01

    First-principles local-density-functional calculations have been used to investigate the equilibrium point defect structure and boron-defect interactions in Ni{sub 3}Al. The dominant point defect types in off-stoichiometric Ni{sub 3}Al are substitutional antisite defects on both sublattices. The boron binding energy is dependent on lattice coordination; it is strongest near vacancy sites with a nearest-neighbor nickel coordination number of about four (instead of six as in the defect-free interstitial site) and with no aluminum atom nearest-neighbors. This suggests that boron tends to segregate to open defect sites and to enhance cohesion through the formation of localized Ni-B covalent bonds. Comparison of the binding energies of boron and carbon in Ni{sub 3}Al shows that boron has a stronger tendency to segregate to open sites than carbon.

  18. Point defect creation by low fluence swift heavy ion irradiation-induced low energy electrons in YBa2Cu3O7-y

    NASA Astrophysics Data System (ADS)

    Biswal, R.; John, J.; Behera, D.; Mallick, P.; Kumar, Sandeep; Kanjilal, D.; Mohanty, T.; Raychaudhuri, P.; Mishra, N. C.

    2008-08-01

    The effect of 200 MeV Ag ion irradiation on the superconducting and normal state properties of the high-Tc superconductor YBa2Cu3O7-y (YBCO) is studied by in situ temperature-dependent resistance measurements. We show that irradiating YBCO thin films (~150 nm) at low temperature result in a softly defected region of about 85 nm radius due to swift heavy ion-induced secondary electrons around the highly amorphized latent tracks of ~5 nm radius. This leads to a decrease of Tc at fluences three orders of magnitude less than the threshold fluence, where overlapping of tracks block the supercurrent path. Due to their low energy (4.1 keV for 200 MeV Ag ions), the secondary electrons can induce point defects by inelastic processes rather than by direct elastic collision.

  19. Displacement Threshold Energy and Recovery in an Al-Ti Nanolayered System with Intrinsic Point Defect Partitioning

    SciTech Connect

    Gerboth, Matthew D.; Setyawan, Wahyu; Henager, Charles H.

    2014-01-07

    A method is established and validated using molecular dynamics (MD) to determine the displacement threshold energies as Ed in nanolayered, multilayered systems of dissimilar metals. The method is applied to specifically oriented nanolayered films of Al-Ti where the crystal structure and interface orientations are varied in atomic models and Ed is calculated. Methods for defect detection are developed and discussed based on prior research in the literature and based on specific crystallographic directions available in the nanolayered systems. These are compared and contrasted to similar calculations in corresponding bulk materials, including fcc Al, fcc Ti, hcp Al, and hcp Ti. In all cases, the calculated Ed in the multilayers are intermediate to the corresponding bulk values but exhibit some important directionality. In the nanolayer, defect detection demonstrated systematic differences in the behavior of Ed in each layer. Importantly, collision cascade damage exhibits significant defect partitioning within the Al and Ti layers that is hypothesized to be an intrinsic property of dissimilar nanolayered systems. This type of partitioning could be partly responsible for observed asymmetric radiation damage responses in many multilayered systems. In addition, a pseudo-random direction was introduced to approximate the average Ed without performing numerous simulations with random directions.

  20. Point Defects Creation by Swift Heavy Ion Irradiation Induced Low Energy Electrons in YBa2Cu3O7-y through Dissociative Recombination

    NASA Astrophysics Data System (ADS)

    Biswal, R.; John, J.; Behera, D.; Kanjilal, D.; Avasthi, D. K.; Mishra, N. C.

    2008-10-01

    Our in-situ temperature dependent resistance studies in a set of YBa2Cu3O7-y (YBCO) thin films irradiated with 200 MeV Ag ions at 79 K show that in addition to amorphized latent tracks, a large concentration of point defects are created by the secondary electrons emitted radially from the ion path. Detailed calculation of the energetics indicates that in the YBCO matrix, these secondary electrons cannot create defect by direct elastic knock-on process. We propose an inelastic interaction of the secondary electrons with the YBCO matrix, which results into defect creation by a process similar to dissociative recombination. Our study shows that accumulation of point defects during irradiation is accompanied by self-organization of point defect into clustering and phase segregation.

  1. Influence of point defects on grain boundary motion.

    SciTech Connect

    Foiles, Stephen Martin

    2010-09-01

    This work addresses the influence of point defects, in particular vacancies, on the motion of grain boundaries. If there is a non-equilibrium concentration of point defects in the vicinity of an interface, such as due to displacement cascades in a radiation environment, motion of the interface to sweep up the defects will lower the energy and provide a driving force for interface motion. Molecular dynamics simulations are employed to examine the process for the case of excess vacancy concentrations in the vicinity of two grain boundaries. It is observed that the efficacy of the presence of the point defects in inducing boundary motion depends on the balance of the mobility of the defects with the mobility of the interfaces. In addition, the extent to which grain boundaries are ideal sinks for vacancies is evaluated by considering the energy of boundaries before and after vacancy absorption.

  2. Electroneutral intrinsic point defects in cadmium chalcogenides

    SciTech Connect

    Kharif, Ya.L.; Kudryashov, N.I.; Strunilina, T.A.

    1987-12-01

    Low-mobility electrically neutral intrinsic point defects were observed in cadmium chalcogenides. It was shown that the concentration of these defects is proportional to the cadmium vapor pressure to the 1/3 power at a constant temperature, and a mechanism for the formation of these defects were proposed.

  3. Point Defects in SiC

    NASA Astrophysics Data System (ADS)

    Zvanut, Mary Ellen

    2004-03-01

    Production of high frequency, high power electronic devices using wide bandgap semiconductors has spurred renewed interest in point defects in SiC. Recent electron paramagnetic resonance (EPR) spectroscopy studies focus on centers in as-grown high purity semi-insulating substrates because intrinsic defects are thought to compensate unavoidable shallow centers, thus creating the high resistivity required. The EPR studies address the chemical/structural composition of the defects, the defect level (energy with respect to a band edge with which the defect can accept or release an electron) and thermal stability. Thus far, the positively charged carbon vacancy, the Si vacancy, a carbon-vacancy/carbon antisite pair, and several as yet-unidentified centers have been observed in as-grown electronic-grade 4H-SiC [1-3]. The talk will review the types of defects recently identified in SiC and discuss their possible relationship to compensation. The photo-induced EPR experiments used to determine defect levels will be discussed, with a particular focus on the carbon vacancy. The use of high frequency EPR to resolve the many different types of centers in SiC will also be covered. Finally, the presentation will review the thermal stability of the intrinsic defects detected in as-grown 4H SiC. 1. M. E. Zvanut and V. V. Konovalov, Appl. Phys. Lett. 80, 410 (2002). 2. N.T. Son, Z. Zolnai, and E. Janzen, Phys. Rev. B64, 2452xx (2003). 3. W.E. Carlos, E.R. Glaser, and B.V. Shanabrook, in Proceedings of the 22nd conference on Defects in Semiconductors, Aarhus, Denmark, July 2003.

  4. Correlated recombination and annealing of point defects in dilute and concentrated Fe-Cr alloys

    NASA Astrophysics Data System (ADS)

    Terentyev, D.; Castin, N.; Ortiz, C. J.

    2012-11-01

    In this work, we present a comprehensive combined modelling approach to study the annealing of lattice defects in dilute and concentrated metallic alloys. The developed approach consists in the combination of molecular dynamics, atomistic kinetic Monte Carlo (AKMC) and mean field rate theory methods, linked at appropriate time and space scales. For the first time, the AKMC tool has been designed to model the evolution of point defects (both vacancies and self-interstitial atoms) in random concentrated alloys, taking into account the influence of lattice distortion on the local migration energy barrier due to the mutual interaction of point defects and solutes. Good accuracy and outstanding speed of calculations has been achieved by introducing the artificial neural network regression as an engine of the AKMC applied to calculate migration barriers for mobile defects. The developed method was applied to study correlated recombination in bcc Fe and random Fe-Cr alloys, aiming at the reproduction of a set of experimental studies after electron irradiation. The obtained results agree well with the available experimental data, implying that the developed modelling procedure correctly captures the undergoing physical process.

  5. Charged point defects in semiconductors and the supercell approximation

    NASA Astrophysics Data System (ADS)

    Lento, J.; Mozos, J.-L.; Nieminen, R. M.

    2002-03-01

    The effects of the supercell approximation in first-principles calculations for isolated, charged point defects in semiconductors and insulators are studied. The convergence of the Coulomb energy with respect to the supercell size is investigated. Quantitative numerical results for the standard uniform compensating charge and the newly proposed localized compensating charge scheme are presented for a prototypical defect, the doubly positive silicon self-interstitial.

  6. Atomistic simulation of point defects and dislocations in bbc transition metals from first principles

    SciTech Connect

    Xu, W; Moriarty, J.A.

    1996-01-19

    Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we have been studying point defects and dislocations in bcc transition metals, with molybdenum (Mo) as a prototype. For point defects in Mo, the calculated vacancy formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations in Mo have also been investigated. The <110> split dumb-bell is found to have the lowest formation energy, as is experimentally observed, but the corresponding migration energy is calculated to be 3--15 times higher than previous theoretical estimates. The atomic structure and energetics of <111> screw dislocations in Mo are now being investigated. We have found that the ``easy`` core configuration has a lower formation energy than the ``hard`` one, consistent with previous theoretical studies. The former has a distinctive 3-fold symmetry with a spread out of the dislocation core along the <112> directions, an effect which is driven by the strong angular forces present in these metals.

  7. Quantum Monte Carlo calculations for point defects in semiconductors

    NASA Astrophysics Data System (ADS)

    Hennig, Richard

    2010-03-01

    Point defects in silicon have been studied extensively for many years. Nevertheless the mechanism for self diffusion in Si is still debated. Direct experimental measurements of the selfdiffusion in silicon are complicated by the lack of suitable isotopes. Formation energies are either obtained from theory or indirectly through the analysis of dopant and metal diffusion experiments. Density functional calculations predict formation energies ranging from 3 to 5 eV depending on the approximations used for the exchange-correlation functional [1]. Analysis of dopant and metal diffusion experiments result in similar broad range of diffusion activation energies of 4.95 [2], 4.68 [3], 2.4 eV [4]. Assuming a migration energy barrier of 0.1-0.3 eV [5], the resulting experimental interstitial formation energies range from 2.1 - 4.9 eV. To answer the question of the formation energy of Si interstitials we resort to a many-body description of the wave functions using quantum Monte Carlo (QMC) techniques. Previous QMC calculations resulted in formation energies for the interstitials of around 5 eV [1,6]. We present a careful analysis of all the controlled and uncontrolled approximations that affect the defect formation energies in variational and diffusion Monte Carlo calculations. We find that more accurate trial wave functions for QMC using improved Jastrow expansions and most importantly a backflow transformation for the electron coordinates significantly improve the wave functions. Using zero-variance extrapolation, we predict interstitial formation energies in good agreement with hybrid DFT functionals [1] and recent GW calculations [7]. [4pt] [1] E. R. Batista, J. Heyd, R. G. Hennig, B. P. Uberuaga, R. L. Martin, G. E. Scuseria, C. J. Umrigar, and J. W. Wilkins. Phys. Rev. B 74, 121102(R) (2006).[0pt] [2] H. Bracht, E. E. Haller, and R. Clark-Phelps, Phys. Rev. Lett. 81, 393 (1998). [0pt] [3] A. Ural, P. B. Griffin, and J. D. Plummer, Phys. Rev. Lett. 83, 3454 (1999). [0pt

  8. Impact of point defects on III-nitride tunnel devices

    NASA Astrophysics Data System (ADS)

    Wickramaratne, Darshana; Lyons, John; van de Walle, Chris G.

    Heterostructures using GaN and InGaN are being pursued in designs of tunnel field-effect-transistors (TFETs) to enable low-power switching devices. Point defects and impurities in these heterostructures can adversely affect the performance of these devices through Shockley-Read-Hall (SRH) and Trap-Assisted-Tunneling (TAT) processes. Using first-principles calculations based on a hybrid functional, we calculate the thermodynamic and charge-state switching levels as well as nonradiative recombination rates of point defects and impurities in GaN and InGaN. Gallium vacancies and their complexes, in particular, are found to be potentially detrimental centers. We then investigate how these defects can contribute to SRH and TAT processes in a nitride TFET device. This work was supported by the Center for Low Energy Systems Technology (LEAST), one of the six SRC STARnet Centers, sponsored by MARCO and DARPA.

  9. Point Defect Characterization in CdZnTe

    SciTech Connect

    Gul,R.; Li, Z.; Bolotnikov, A.; Keeter, K.; Rodriguez, R.; James, R.

    2009-03-24

    Measurements of the defect levels and performance testing of CdZnTe detectors were performed by means of Current Deep Level Transient Spectroscopy (I-DLTS), Transient Charge Technique (TCT), Current versus Voltage measurements (I-V), and gamma-ray spectroscopy. CdZnTe crystals were acquired from different commercial vendors and characterized for their point defects. I-DLTS studies included measurements of defect parameters such as energy levels in the band gap, carrier capture cross sections, and defect densities. The induced current due to laser-generated carriers was measured using TCT. The data were used to determine the transport properties of the detectors under study. A good correlation was found between the point defects in the detectors and their performance.

  10. Modeling of point defects and rare gas incorporation in uranium mono-carbide

    NASA Astrophysics Data System (ADS)

    Chartier, A.; Van Brutzel, L.

    2007-02-01

    An embedded atom method (EAM) potential has been established for uranium mono-carbide. This EAM potential was fitted on structural properties of metallic uranium and uranium mono-carbide. The formation energies of point defects, as well as activation energies for self migration, have been evaluated in order to cross-check the suitability of the potential. Assuming that the carbon vacancies are the main defects in uranium mono-carbide compounds, the migration paths and energies are consistent with experimental data selected by Catlow[C.R.A. Catlow, J. Nucl. Mater. 60 (1976) 151]. The insertion and migration energies for He, Kr and Xe have also been evaluated with available inter-atomic potentials [H.H. Andersen, P. Sigmund, Nucl. Instr. and Meth. B 38 (1965) 238]. Results show that the most stable defect configuration for rare gases is within uranium vacancies. The migration energy of an interstitial Xe is 0.5 eV, in agreement with the experimental value of 0.5 eV [Hj. Matzke, Science of advanced LMFBR fuels, Solid State Physics, Chemistry and Technology of Carbides, Nitrides and Carbonitrides of Uranium and Plutonium, North-Holland, 1986].

  11. Energetics and electronic structure of UAl4 with point defects

    NASA Astrophysics Data System (ADS)

    Kniznik, L.; Alonso, P. R.; Gargano, P. H.; Rubiolo, G. H.

    2015-11-01

    UAl4 ideal and defect structures were studied within the framework of the Density Functional Theory. The structural and magnetic ordering of UAl4 in paramagnetic, ferromagnetic, and antiferromagnetic states have been investigated, within the collinear and non-collinear spin approximation, using the GGA model, as embedded in the program package VASP. An antiferromagnetic (0 1 bar 1) layer structure with spins aligned to the [010] direction was found to be energetically preferred. The analysis of density of states and bonding charge density point out that the bonding mechanism consists primarily of band mixing between the U 5f and Al 3p states. Supercells were built from UAl4 unit cells with the established magnetic structure. For those supercells we calculated the energy of formation of vacancies and antisite defects taking into account the existence of three distinct aluminum sites. Point defect formation energies, local lattice relaxations, as well as the defect induced magnetic ordering and electronic density redistribution, are discussed. It is shown that antiferromagnetism is locally broken. Al antisites and U antisites in Al 4e Wyckoff positions are the constitutional point defects in Al-rich and U-rich oI20 UAl4, respectively. In this way we have presented here the first set of data which makes it possible to discuss and quantify the point defects concentrations in the experimental composition range for existence of this uranium aluminide.

  12. Point Defects Quenched in Nickel Aluminide and Related Intermetallic Compounds

    NASA Astrophysics Data System (ADS)

    Fan, Jiawen

    Point defects in the highly ordered B2 compounds NiAl, CoAl and FeAl were studied using the perturbed gammagamma angular correlations (PAC) technique. Quadrupole interactions detected at dilute ^{111}In probes on Al sites in NiAl and CoAl were identified with complexes containing one or two vacancies in the first atomic shell. Measurements on rapidly quenched NiAl and CoAl exhibited increases in site fractions of vacancy-probe complexes caused by formation of thermal defects. Site fractions were analyzed using the law of mass action to obtain absolute vacancy concentrations. PAC is shown to be a powerful new technique for the quantitative study of equilibrium defects in solids. For NiAl, the vacancy concentration quenched-in from a given temperature was found to be independent of composition over the range 50.4 -53.5 at.% Ni, identifying the Schottky defect (vacancy pair) as the dominant equilibrium defect, and ruling out the so-called triple defect. Formation energies and entropies of Schottky pairs were determined to be 2.66(8) and 3.48(12) eV, and 12(1) and 17(2) k_{rm B}, respectively, for NiAl and CoAl. The entropies suggest huge vacancy concentrations, 13%, at the melting temperatures of NiAl and CoAl. Migration energies of Ni and Co vacancies were found to be 1.8(2) and 2.5(2) eV, respectively. FeAl exhibited complex behavior. A low-temperature regime was detected in NiAl and CoAl within which vacancies are mobile but do not anneal out, so that the vacancy concentration remains constant. In NiAl, this "bottleneck" regime extends from 350 to 700 ^circC. Vacancies were found to be bound to the In probes with an energy very close to 0.20 eV in NiAl and CoAl. An explanation of the bottleneck is proposed in terms of saturation of all lattice sinks. This annealing bottleneck should exist in a wide range of intermetallic compounds when there is a sufficiently high vacancy concentration.

  13. Native point defects in GaSb

    SciTech Connect

    Kujala, J.; Segercrantz, N.; Tuomisto, F.; Slotte, J.

    2014-10-14

    We have applied positron annihilation spectroscopy to study native point defects in Te-doped n-type and nominally undoped p-type GaSb single crystals. The results show that the dominant vacancy defect trapping positrons in bulk GaSb is the gallium monovacancy. The temperature dependence of the average positron lifetime in both p- and n-type GaSb indicates that negative ion type defects with no associated open volume compete with the Ga vacancies. Based on comparison with theoretical predictions, these negative ions are identified as Ga antisites. The concentrations of these negatively charged defects exceed the Ga vacancy concentrations nearly by an order of magnitude. We conclude that the Ga antisite is the native defect responsible for p-type conductivity in GaSb single crystals.

  14. Photosensitive Point Defects in Optical Glasses: Science and Applications

    SciTech Connect

    Potter, B.G. Jr.; Simmons-Potter, K.

    1999-07-28

    The understanding and manipulation of the point defect structure in oxide glasses have been critical to the enhanced performance and reliability of optical-fiber-based, photosensitive photonic devices that currently found widespread application in telecommunications and remote sensing technologies. We provide a brief review of past research investigating photosensitive mechanisms in germanosilicate glasses, the primary material system used in telecommunications fibers. This discussion motivates an overview of ongoing work within our laboratories to migrate photosensitive glass technologies to a planar format for integrated photonic applications. Using reactive-atmosphere, RF-magnetron sputtering, we have demonstrated control of glass defect structure during synthesis, thereby controlling both the material photosensitivity (i. e. dispersion and magnitude of the refractive index change) and its environmental stability.

  15. Formation and stability of point defects in monolayer rhenium disulfide

    NASA Astrophysics Data System (ADS)

    Horzum, S.; ćakır, D.; Suh, J.; Tongay, S.; Huang, Y.-S.; Ho, C.-H.; Wu, J.; Sahin, H.; Peeters, F. M.

    2014-04-01

    Recently, rhenium disulfide (ReS2) monolayers were experimentally extracted by conventional mechanical exfoliation technique from as-grown ReS2 crystals. Unlike the well-known members of transition metal dichalcogenides (TMDs), ReS2 crystallizes in a stable distorted-1T structure and lacks an indirect to direct gap crossover. Here we present an experimental and theoretical study of the formation, energetics, and stability of the most prominent lattice defects in monolayer ReS2. Experimentally, irradiation with 3-MeV He+2 ions was used to break the strong covalent bonds in ReS2 flakes. Photoluminescence measurements showed that the luminescence from monolayers is mostly unchanged after highly energetic α particle irradiation. In order to understand the energetics of possible vacancies in ReS2 we performed systematic first-principles calculations. Our calculations revealed that the formation of a single sulfur vacancy has the lowest formation energy in both Re and S rich conditions and a random distribution of such defects are energetically more preferable. Sulfur point defects do not result in any spin polarization whereas the creation of Re-containing point defects induce magnetization with a net magnetic moment of 1-3μB. Experimentally observed easy formation of sulfur vacancies is in good agreement with first-principles calculations.

  16. Point defects in thorium nitride: A first-principles study

    NASA Astrophysics Data System (ADS)

    Pérez Daroca, D.; Llois, A. M.; Mosca, H. O.

    2016-11-01

    Thorium and its compounds (carbides and nitrides) are being investigated as possible materials to be used as nuclear fuels for Generation-IV reactors. As a first step in the research of these materials under irradiation, we study the formation energies and stability of point defects in thorium nitride by means of first-principles calculations within the framework of density functional theory. We focus on vacancies, interstitials, Frenkel pairs and Schottky defects. We found that N and Th vacancies have almost the same formation energy and that the most energetically favorable defects of all studied in this work are N interstitials. These kind of results for ThN, to the best authors' knowledge, have not been obtained previously, neither experimentally, nor theoretically.

  17. Theoretical investigation of thermodynamic stability and mobility of the intrinsic point defects in Ti3AC2 (A = Si, Al).

    PubMed

    Wang, Jiemin; Liu, Bin; Wang, Jingyang; Zhou, Yanchun

    2015-04-14

    Nano-laminated Ti3AC2 (A = Si, Al) are highlighted as nuclear materials for a generation IV (GIV) reactor because they show high tolerance to radiation damage and remain crystalline under irradiation of high fluence heavy ions. In this paper, the energetics of formation and migration of intrinsic point defects are predicted by density functional theory calculations. We find that the space near the A atomic plane acts as a point defect sink and can accommodate lattice disorder. The migration energy barriers of Si/Al vacancy and TiSi anti-site defects along the atomic plane A are in the range of 0.3 to 0.9 eV, indicating their high mobility and the fast recovery of Si/Al Frenkel defects and Ti-A antisite pairs after irradiation. This layered structure induced large disorder accommodation and fast defect recovery must play an important role in the micro-structural response of Ti3AC2 to irradiation.

  18. Autonomous basin climbing method with sampling of multiple transition pathways: application to anisotropic diffusion of point defects in hcp Zr

    NASA Astrophysics Data System (ADS)

    Fan, Yue; Yip, Sidney; Yildiz, Bilge

    2014-09-01

    This paper presents an extension of the autonomous basin climbing (ABC) method, an atomistic activation-relaxation technique for sampling transition-state pathways. The extended algorithm (ABC-E) allows the sampling of multiple transition pathways from a given minimum, with the additional feature of identifying the pathways in the order of increasing activation barriers, thereby prioritizing them according to their importance in the kinetics. Combined with on-the-fly kinetic Monte Carlo calculations, the method is applied to simulate the anisotropic diffusion of point defects in hcp Zr. Multiple migration mechanisms are identified for both the interstitials and vacancies, and benchmarked against results from other methods in the literature. The self-interstitial atom (SIA) diffusion kinetics shows a maximum anisotropy at intermediate temperatures (400~700 K), a non-monotonic behavior that we explain to originate from the stabilities and migration mechanisms associated with different SIA sites. The accuracy of the ABC-E calculations is validated, in part, by the existing results in the literature for point defect diffusion in hcp Zr, and by benchmarking against analytical results on a hypothetical rough-energy landscape. Lastly, sampling prioritization and computational efficiency are demonstrated through a direct comparison between the ABC-E and the activation relaxation technique.

  19. First-principles study of point defects in thorium carbide

    NASA Astrophysics Data System (ADS)

    Pérez Daroca, D.; Jaroszewicz, S.; Llois, A. M.; Mosca, H. O.

    2014-11-01

    Thorium-based materials are currently being investigated in relation with their potential utilization in Generation-IV reactors as nuclear fuels. One of the most important issues to be studied is their behavior under irradiation. A first approach to this goal is the study of point defects. By means of first-principles calculations within the framework of density functional theory, we study the stability and formation energies of vacancies, interstitials and Frenkel pairs in thorium carbide. We find that C isolated vacancies are the most likely defects, while C interstitials are energetically favored as compared to Th ones. These kind of results for ThC, to the best authors' knowledge, have not been obtained previously, neither experimentally, nor theoretically. For this reason, we compare with results on other compounds with the same NaCl-type structure.

  20. A first principles investigation of point defects in monolayer, few-layer, and bulk WS2

    NASA Astrophysics Data System (ADS)

    Li, Wun-Fan; Fang, Changming; Dijkstra, Marjolein; van Huis, Marijn A.; Soft Condensed Matter Team

    We present the results of a systematic study of physics of point defects in 2D WS2 materials conducted by means of density functional theory. First, we investigate the physics of point defects in monolayer (ML) WS2. Second, we examine the impact of point defects on the physical properties of multi-layer defective WS2 as a function of slab thickness. The studied point defects are: monovacancies, interstitials and anti-sites, and the considered physical properties include local geometry, defect formation energy, electronic structure and magnetism. Van der Waals interaction, spin-polarization and spin-orbit coupling effects are also incorporated in the calculations to ensure accurate results. In a ML WS2, we predict that IS is the most favorable defect inside WS2 having a low formation energy of 1.21 eV. WS and WS2 anti-sites result in a total magnetic moment of 2 μB. By studying ML, few-layer (up to 4 layers), and bulk WS2 slabs we find that, all point defects cause only localized perturbation, thus have little influence on the thickness-dependent evolution of the physical properties. The depth-dependence of the defect formation energy is also found: VS prefers to stay on the surface, while VW prefers the slab center. This work is supported by the Dutch Science Foundation NWO, VIDI Grant (Grant Nr. 723.012.006).

  1. Half-Integer Point Defects in the Q-Tensor Theory of Nematic Liquid Crystals

    NASA Astrophysics Data System (ADS)

    Di Fratta, G.; Robbins, J. M.; Slastikov, V.; Zarnescu, A.

    2016-02-01

    We investigate prototypical profiles of point defects in two-dimensional liquid crystals within the framework of Landau-de Gennes theory. Using boundary conditions characteristic of defects of index k/2, we find a critical point of the Landau-de Gennes energy that is characterised by a system of ordinary differential equations. In the deep nematic regime, b^2 small, we prove that this critical point is the unique global minimiser of the Landau-de Gennes energy. For the case b^2=0, we investigate in greater detail the regime of vanishing elastic constant L → 0, where we obtain three explicit point defect profiles, including the global minimiser.

  2. Influence of point defects on grain boundary diffusion in oxides

    SciTech Connect

    Stubican, V.S.

    1991-03-15

    The influence of point defects on grain boundary diffusion of Co ions in NiO was studied using polycrystalline films and bicrystals. Grain boundary diffusion was studied at 750 C at oxygen partial pressure. Two diffusion regions were found. At low oxygen pressures, extrinsic diffusion was observed. Above oxygen pressure of 10{sup {minus}7}, influence of intrinsic point defects was detected. It was determined that grain boundary diffusion was > 3 orders of magnitude faster than volume diffusion. However, it seems that grain boundary diffusion is influenced by the point defects in a similar way as the volume diffusion. 4 figs.

  3. First-principles study of point defects in an fcc Fe-10Ni-20Cr model alloy

    NASA Astrophysics Data System (ADS)

    Piochaud, J. B.; Klaver, T. P. C.; Adjanor, G.; Olsson, P.; Domain, C.; Becquart, C. S.

    2014-01-01

    The influence of the local environment on vacancy and self-interstitial formation energies has been investigated in a face-centered-cubic (fcc) Fe-10Ni-20Cr model alloy by analyzing an extensive set of first-principle calculations based on density functional theory. Chemical disorder has been considered by designing special quasirandom structures and four different collinear magnetic structures have been investigated in order to determine a relevant reference state to perform point defect calculations at 0 K. Two different convergence methods have also been used to characterize the importance of the method on the results. Although our fcc Fe-10Ni-20Cr would be better represented in terms of applications by the paramagnetic state, we found that the antiferromagnetic single-layer magnetic structure was the most stable at 0 K and we chose it as a reference state to determine the point defect properties. Point defects have been introduced in this reference state, i.e., vacancies and Fe-Fe, Fe-Ni, Fe-Cr, Cr-Cr, Ni-Ni, and Ni-Cr dumbbell interstitials oriented either parallel or perpendicular to the single layer antiferromagnetic planes. Each point defect studied was introduced at different lattice sites to consider a sufficient variety of local environments and analyze its influence on the formation energy values. We have estimated the point defect formation energies with linear regressions using variables which describe the local environment surrounding the point defects. The number and the position of Ni and Cr first nearest neighbors to the point defects were found to drive the evolution of the formation energies. In particular, Ni is found to decrease and Cr to increase the vacancy formation energy of the model alloy, while the opposite trends are found for the dumbbell interstitials. This study suggested that, to a first approximation, the first nearest atoms to point defects can provide reliable estimates of point defect formation energies.

  4. Phase-operation for conduction electron by atomic-scale scattering via single point-defect

    SciTech Connect

    Nagaoka, Katsumi Yaginuma, Shin; Nakayama, Tomonobu

    2014-03-17

    In order to propose a phase-operation technique for conduction electrons in solid, we have investigated, using scanning tunneling microscopy, an atomic-scale electron-scattering phenomenon on a 2D subband state formed in Si. Particularly, we have noticed a single surface point-defect around which a standing-wave pattern created, and a dispersion of scattering phase-shifts by the defect-potential against electron-energy has been measured. The behavior is well-explained with appropriate scattering parameters: the potential height and radius. This result experimentally proves that the atomic-scale potential scattering via the point defect enables phase-operation for conduction electrons.

  5. Structures, mobilities, electronic and magnetic properties of point defects in silicene.

    PubMed

    Gao, Junfeng; Zhang, Junfeng; Liu, Hongsheng; Zhang, Qinfang; Zhao, Jijun

    2013-10-21

    In the fabrication and processing of silicene monolayers, structural defects are almost inevitable. Using ab initio calculations, we systemically investigated the structures, formation energies, migration behaviors and electronic/magnetic properties of typical point defects in silicene, including the Stone-Wales (SW) defect, single and double vacancies (SVs and DVs), and adatoms. We found that SW can be effectively recovered by thermal annealing. SVs have much higher mobility than DVs and two SVs are very likely to coalesce into one DV to lower the energy. Existence of SW and DVs may induce small gaps in silicene, while the SV defect may transform semimetallic silicene into metallic. Adatoms are unexpectedly stable and can affect the electronic properties of silicene dramatically. Especially, Si adatoms as self-dopants in silicene sheets can induce long-range spin polarization as well as a remarkable band gap, thus achieving an all-silicon magnetic semiconductor. The present theoretical results provide valuable insights into identification of these defects in experiments and understanding their effects on the physical properties of silicene.

  6. Interplay between intrinsic point defects and low-angle grain boundary in bcc tungsten: effects of local stress field

    NASA Astrophysics Data System (ADS)

    Niu, Liang-Liang; Zhang, Ying; Shu, Xiaolin; Jin, Shuo; Zhou, Hong-Bo; Gao, Fei; Lu, Guang-Hong

    2015-06-01

    We have used molecular statics in conjunction with an embedded atom method to explore the interplay between native point defects (vacancies and self-interstitials (SIAs)) and a low-angle grain boundary (GB) in bcc tungsten. The low-angle GB has biased absorption of SIAs over vacancies. We emphasize the significance of phenomena such as vacancy delocalization and SIA instant absorption around the GB dislocation cores in stabilizing the defect structures. Interstitial loading into the GB can dramatically enhance the interaction strength between the point defects and the GB due to SIA clustering (SIA cloud formation) or SIA vacancy recombination. We propose that the ‘maximum atom displacement’ can complement the ‘vacancy formation energy’ in evaluating unstable vacancy sites. Calculations of point defect migration barriers in the vicinity of GB dislocation cores show that vacancies and SIAs preferentially migrate along the pathways in the planes immediately above and below the core, respectively.

  7. Role of silicon surface in the removal of point defects in ultra-shallow junctions

    SciTech Connect

    Sultan, A.; Banerjee, S.; List, S.; Rodder, M.

    1996-12-31

    The role of the Si surface in the annihilation of point defects has been studied for ultra-shallow p{sup +}/n junctions. The dopant and defect distributions for low energy implants lie within a few hundred Angstroms of the surface. The proximity of the Si surface has been shown to help in the efficient removal of point defects for the shallower junctions. A 5 keV, 1{times}10{sup 15} cm{sup -2} BF{sub 2} implant and a 30 keV, 3.3{times}10{sup 14} cm{sup -2} BF{sub 2} implant were estimated to create comparable damage at different depths. After identical anneals, the higher energy implant sample showed end-of-range dislocation loops in cross-sectional transmission electron microscopy analysis, while the low energy sample, for which the point defect distribution was closer to the surface, was defect-free. This is attributed to the role of the Si surface as an efficient sink for the removal of point defects.

  8. Diffusion of point defects in crystalline silicon using the kinetic activation-relaxation technique method

    SciTech Connect

    Trochet, Mickaël; Béland, Laurent Karim; Joly, Jean -François; Brommer, Peter; Mousseau, Normand

    2015-06-16

    We study point-defect diffusion in crystalline silicon using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo method with on-the-fly catalog building capabilities based on the activation-relaxation technique (ART nouveau), coupled to the standard Stillinger-Weber potential. We focus more particularly on the evolution of crystalline cells with one to four vacancies and one to four interstitials in order to provide a detailed picture of both the atomistic diffusion mechanisms and overall kinetics. We show formation energies, activation barriers for the ground state of all eight systems, and migration barriers for those systems that diffuse. Additionally, we characterize diffusion paths and special configurations such as dumbbell complex, di-interstitial (IV-pair+2I) superdiffuser, tetrahedral vacancy complex, and more. In conclusion, this study points to an unsuspected dynamical richness even for this apparently simple system that can only be uncovered by exhaustive and systematic approaches such as the kinetic activation-relaxation technique.

  9. Diffusion of point defects in crystalline silicon using the kinetic activation-relaxation technique method

    NASA Astrophysics Data System (ADS)

    Trochet, Mickaël; Béland, Laurent Karim; Joly, Jean-François; Brommer, Peter; Mousseau, Normand

    2015-06-01

    We study point-defect diffusion in crystalline silicon using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo method with on-the-fly catalog building capabilities based on the activation-relaxation technique (ART nouveau), coupled to the standard Stillinger-Weber potential. We focus more particularly on the evolution of crystalline cells with one to four vacancies and one to four interstitials in order to provide a detailed picture of both the atomistic diffusion mechanisms and overall kinetics. We show formation energies, activation barriers for the ground state of all eight systems, and migration barriers for those systems that diffuse. Additionally, we characterize diffusion paths and special configurations such as dumbbell complex, di-interstitial (IV-pair+2I) superdiffuser, tetrahedral vacancy complex, and more. This study points to an unsuspected dynamical richness even for this apparently simple system that can only be uncovered by exhaustive and systematic approaches such as the kinetic activation-relaxation technique.

  10. Diffusion of point defects in crystalline silicon using the kinetic activation-relaxation technique method

    DOE PAGESBeta

    Trochet, Mickaël; Béland, Laurent Karim; Joly, Jean -François; Brommer, Peter; Mousseau, Normand

    2015-06-16

    We study point-defect diffusion in crystalline silicon using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo method with on-the-fly catalog building capabilities based on the activation-relaxation technique (ART nouveau), coupled to the standard Stillinger-Weber potential. We focus more particularly on the evolution of crystalline cells with one to four vacancies and one to four interstitials in order to provide a detailed picture of both the atomistic diffusion mechanisms and overall kinetics. We show formation energies, activation barriers for the ground state of all eight systems, and migration barriers for those systems that diffuse. Additionally, we characterize diffusion pathsmore » and special configurations such as dumbbell complex, di-interstitial (IV-pair+2I) superdiffuser, tetrahedral vacancy complex, and more. In conclusion, this study points to an unsuspected dynamical richness even for this apparently simple system that can only be uncovered by exhaustive and systematic approaches such as the kinetic activation-relaxation technique.« less

  11. Imaging atomic-level random walk of a point defect in graphene

    NASA Astrophysics Data System (ADS)

    Kotakoski, Jani; Mangler, Clemens; Meyer, Jannik C.

    2014-05-01

    Deviations from the perfect atomic arrangements in crystals play an important role in affecting their properties. Similarly, diffusion of such deviations is behind many microstructural changes in solids. However, observation of point defect diffusion is hindered both by the difficulties related to direct imaging of non-periodic structures and by the timescales involved in the diffusion process. Here, instead of imaging thermal diffusion, we stimulate and follow the migration of a divacancy through graphene lattice using a scanning transmission electron microscope operated at 60 kV. The beam-activated process happens on a timescale that allows us to capture a significant part of the structural transformations and trajectory of the defect. The low voltage combined with ultra-high vacuum conditions ensure that the defect remains stable over long image sequences, which allows us for the first time to directly follow the diffusion of a point defect in a crystalline material.

  12. Point defect weakened thermal contraction in monolayer graphene

    SciTech Connect

    Zha, Xian-Hu; Zhang, Rui-Qin; Lin, Zijing

    2014-08-14

    We investigate the thermal expansion behaviors of monolayer graphene and three configurations of graphene with point defects, namely the replacement of one carbon atom with a boron or nitrogen atom, or of two neighboring carbon atoms by boron-nitrogen atoms, based on calculations using first-principles density functional theory. It is found that the thermal contraction of monolayer graphene is significantly decreased by point defects. Moreover, the corresponding temperature for negative linear thermal expansion coefficient with the maximum absolute value is reduced. The cause is determined to be point defects that enhance the mechanical strength of graphene and then reduce the amplitude and phonon frequency of the out-of-plane acoustic vibration mode. Such defect weakening of graphene thermal contraction will be useful in nanotechnology to diminish the mismatching or strain between the graphene and its substrate.

  13. Study of Near-Neighbor Structure of Point Defects in α-FE by Displacement Cascade

    NASA Astrophysics Data System (ADS)

    Wang, Wei-Lu; Liu, Wei; Wu, Xuebang; Fang, Q. F.; Liu, C. S.; Huang, Qun-Ying; Wu, Y. C.

    2012-07-01

    Molecular dynamics simulations have been performed to study the primary damage formation in α-Fe by collision cascades in the recoil energy range 0.5-20 keV. Two near-neighbor analysis methods including the near-neighbor defect density (NPDD) analysis and cluster analysis were introduced to characterize the spatial aggregation of point defects and the morphologies of clusters, respectively. It is found that the NPDD of self-interstitial atom (SIA) and the number of Frenkel pairs show a similar variation trend, while the NPDD of vacancy exhibits a peak at shorter time than that of SIA. Furthermore, we find that the clusters of point defects exist mostly in the form of chainlike structure in the course and the end of cascades, but the proportion of chainlike clusters decreases with increasing the number of point defects included in one cluster. Therefore, the present methods are found to be effective to characterize the aggregation and the near-neighbor structure of point defects by displacement cascades at any time.

  14. Time scale for point-defect equilibration in nanostructures

    SciTech Connect

    Millett, Paul C.; Wolf, Dieter; Desai, Tapan; Yamakov, Vesselin

    2008-10-20

    Molecular dynamics simulations of high-temperature annealing are performed on nanostructured materials enabling direct observation of vacancy emission from planar defects (i.e., grain boundaries and free surfaces) to populate the initially vacancy-free grain interiors on a subnanosecond time scale. We demonstrate a universal time-length scale correlation that governs these re-equilibration processes, suggesting that nanostructures are particularly stable against perturbations in their point-defect concentrations, caused for example by particle irradiation or temperature fluctuations.

  15. ANALYSIS OF THE ANISOTROPY OF POINT DEFECT DIFFUSION IN HCP Zr

    SciTech Connect

    Samolyuk, German D; Barashev, Alexander V; Golubov, Stanislav I; Osetskiy, Yury N; Stoller, Roger E

    2014-01-01

    A combination of density functional theory (DFT), kinetic Monte Carlo and mean-field rate theory is applied to analyze point defect migration and its effect on the observed growth of hcp Zr under 1 MeV electron irradiation. DFT is used to study stability of various configurations of vacancies and self-interstitial atoms (SIAs) and migration barriers. The data are used in kinetic Monte Carlo modelling of defect diffusion at different temperatures. It is found that both defects exhibit anisotropic diffusion, predominantly parallel to the basal planes. The ratio of diffusion coefficients parallel and perpendicular to the basal planes is found to be higher for vacancies as compared to SIAs at temperatures below ~600K. This raises doubts that the observed radiation growth in Zr irradiated with 1 MeV electrons, namely positive strains in prismatic and negative strains in basal directions, and void alignment along basal planes, can be accounted for by the anisotropy of point defect diffusion, which predicts opposite strain signs. It is speculated that formation of small SIA clusters with higher diffusion anisotropy may be responsible for the experimental observations.

  16. Reversal of helicoidal twist handedness near point defects of confined chiral liquid crystals.

    PubMed

    Ackerman, Paul J; Smalyukh, Ivan I

    2016-05-01

    Handedness of the director twist in cholesteric liquid crystals is commonly assumed to be the same throughout the medium, determined solely by the chirality of constituent molecules or chiral additives, albeit distortions of the ground-state helicoidal configuration often arise due to the effects of confinement and external fields. We directly probe the twist directionality of liquid crystal director structures through experimental three-dimensional imaging and numerical minimization of the elastic free energy and show that spatially localized regions of handedness opposite to that of the chiral liquid crystal ground state can arise in the proximity of twisted-soliton-bound topological point defects. In chiral nematic liquid crystal confined to a film that has a thickness less than the cholesteric pitch and perpendicular surface boundary conditions, twisted solitonic structures embedded in a uniform unwound far-field background with chirality-matched handedness locally relieve confinement-imposed frustration and tend to be accompanied by point defects and smaller geometry-required, energetically costly regions of opposite twist handedness. We also describe a spatially localized structure, dubbed a "twistion," in which a twisted solitonic three-dimensional director configuration is accompanied by four point defects. We discuss how our findings may impinge on the stability of localized particlelike director field configurations in chiral and nonchiral liquid crystals.

  17. Modelling the capillary locking of point defects in nematic liquid crystals

    NASA Astrophysics Data System (ADS)

    Peroli, G. Guidone; Virga, E. G.

    1997-06-01

    DEDICATED TO JERRY ERIKSEN: It has long been known that the equilibrium configuration with minimum energy of a nematic liquid crystal within a cylinder subject to homeotropic conditions on the lateral boundary is escaped along the axis of the cylinder, and there is no singularity of the orientation field. So problems of explaining the presence of point defects in capillary tubes and of exploring their stability arise. There is enough evidence to believe that the menisci play a central role in preventing the orientation field around a point defect from unwinding towards the escaped configuration. We propose a variational model which describes how a point defect interacts with a meniscus. This interaction fades away at a finite distance. When active, it is two-sided, being repulsive at first and then attractive when the defect comes closer to the centre of curvature of the meniscus. Thus, when a defect is enclosed between two menisci they can become antagonists, so that there is a metastable equilibrium position where the defect could be locked in.

  18. Reversal of helicoidal twist handedness near point defects of confined chiral liquid crystals.

    PubMed

    Ackerman, Paul J; Smalyukh, Ivan I

    2016-05-01

    Handedness of the director twist in cholesteric liquid crystals is commonly assumed to be the same throughout the medium, determined solely by the chirality of constituent molecules or chiral additives, albeit distortions of the ground-state helicoidal configuration often arise due to the effects of confinement and external fields. We directly probe the twist directionality of liquid crystal director structures through experimental three-dimensional imaging and numerical minimization of the elastic free energy and show that spatially localized regions of handedness opposite to that of the chiral liquid crystal ground state can arise in the proximity of twisted-soliton-bound topological point defects. In chiral nematic liquid crystal confined to a film that has a thickness less than the cholesteric pitch and perpendicular surface boundary conditions, twisted solitonic structures embedded in a uniform unwound far-field background with chirality-matched handedness locally relieve confinement-imposed frustration and tend to be accompanied by point defects and smaller geometry-required, energetically costly regions of opposite twist handedness. We also describe a spatially localized structure, dubbed a "twistion," in which a twisted solitonic three-dimensional director configuration is accompanied by four point defects. We discuss how our findings may impinge on the stability of localized particlelike director field configurations in chiral and nonchiral liquid crystals. PMID:27300955

  19. Reversal of helicoidal twist handedness near point defects of confined chiral liquid crystals

    NASA Astrophysics Data System (ADS)

    Ackerman, Paul J.; Smalyukh, Ivan I.

    2016-05-01

    Handedness of the director twist in cholesteric liquid crystals is commonly assumed to be the same throughout the medium, determined solely by the chirality of constituent molecules or chiral additives, albeit distortions of the ground-state helicoidal configuration often arise due to the effects of confinement and external fields. We directly probe the twist directionality of liquid crystal director structures through experimental three-dimensional imaging and numerical minimization of the elastic free energy and show that spatially localized regions of handedness opposite to that of the chiral liquid crystal ground state can arise in the proximity of twisted-soliton-bound topological point defects. In chiral nematic liquid crystal confined to a film that has a thickness less than the cholesteric pitch and perpendicular surface boundary conditions, twisted solitonic structures embedded in a uniform unwound far-field background with chirality-matched handedness locally relieve confinement-imposed frustration and tend to be accompanied by point defects and smaller geometry-required, energetically costly regions of opposite twist handedness. We also describe a spatially localized structure, dubbed a "twistion," in which a twisted solitonic three-dimensional director configuration is accompanied by four point defects. We discuss how our findings may impinge on the stability of localized particlelike director field configurations in chiral and nonchiral liquid crystals.

  20. Point defect balance in epitaxial GaSb

    SciTech Connect

    Segercrantz, N. Slotte, J.; Makkonen, I.; Kujala, J.; Tuomisto, F.; Song, Y.; Wang, S.

    2014-08-25

    Positron annihilation spectroscopy in both conventional and coincidence Doppler broadening mode is used for studying the effect of growth conditions on the point defect balance in GaSb:Bi epitaxial layers grown by molecular beam epitaxy. Positron annihilation characteristics in GaSb are also calculated using density functional theory and compared to experimental results. We conclude that while the main positron trapping defect in bulk samples is the Ga antisite, the Ga vacancy is the most prominent trap in the samples grown by molecular beam epitaxy. The results suggest that the p–type conductivity is caused by different defects in GaSb grown with different methods.

  1. DFT calculations of point defects on UN(001) surface

    NASA Astrophysics Data System (ADS)

    Bocharov, D.; Gryaznov, D.; Zhukovskii, Yu. F.; Kotomin, E. A.

    2011-02-01

    The density functional theory is used in a study of point defects on both UN(001) surface and sub-surface layers. We compare the results for slabs of different thicknesses (both perfect and containing nitrogen or uranium vacancies) with a full geometry, electronic and spin density optimization. The electronic charge density re-distribution, density of states, magnetic moments of U atoms and local atomic displacements around defects are carefully analyzed. It is predicted that the vacancies are formed easier on the surface, whereas the property of sub-surface layer does not differ significantly from the central one in the slab.

  2. Stacking faults and lamellar twins with intrinsic point defects in poly-crystalline CdTe analyzed by density functional theory

    NASA Astrophysics Data System (ADS)

    Buurma, Christopher; Chan, Maria; Pauluaskas, Tadas; Klie, Robert; Sivananthan, Sivalingam; DOE Bridge Project Collaboration

    2014-03-01

    Polycrystalline CdTe is a prominent photovoltaic material with proven industry success. To develop the next generation of thin film CdTe solar cells, higher open-circuit voltages and longer minority carrier lifetimes must be achieved. Playing a major role in doping, defect migration, recombination, and current transport are grain boundaries and other extended defects within grains of poly-crystalline CdTe. Commonly observed with STEM in CdTe are twins and stacking faults that extend throughout the entire grain. These twins can appear as lamellar repeating twins, or as single column stacking faults occurring in repetition near that of a Wurtzite structure. In this talk, we will use first principles density functional theory to investigate the thermodynamics and electronic structures such structures observed in STEM. The interaction energetics between adjacent twins and sets of twins are investigated. We will also investigate the likelihood of formation of neutral and charged native point defects in and near these extended defect structures. Binding energies of multiple point defects near such structures are also revealed. Implications towards PV efficiencies are discussed.

  3. Visualizing the influence of point defects on the electronic band structure of graphene.

    PubMed

    Farjam, M

    2014-04-16

    The supercell approach enables us to treat the electronic structure of defective crystals, but the calculated energy bands are too complicated to understand or compare with angle-resolved photoemission spectra because of inevitable zone folding. We discuss how to visualize supercell band structures more effectively by incorporating unfolded spectral weights and orbital decompositions into them. We then apply these ideas to gain a better understanding of the band structure of graphene containing various types of point defects, including nitrogen impurity, hydrogen adsorbate, vacancy defects and the Stone-Wales defect.

  4. Surface-assisted defect engineering of point defects in ZnO

    NASA Astrophysics Data System (ADS)

    Gorai, Prashun; Ertekin, Elif; Seebauer, Edmund G.

    2016-06-01

    Semiconductor surfaces facilitate the injection of highly mobile point defects into the underlying bulk, thereby offering a special means to manipulate bulk defect concentrations. The present work combines diffusion experiments and first-principles calculations for polar ZnO (0001) surface to demonstrate such manipulation. The rate behavior of oxygen interstitial injection varies dramatically between the Zn- and O-terminated ZnO surfaces. A specific injection pathway for the Zn-terminated surface is identified, and activation barrier determined from the first-principles calculations agrees closely with the experimental activation energy of 1.7 eV.

  5. Analysis of an optimization-based atomistic-to-continuum coupling method for point defects

    SciTech Connect

    Olson, Derek; Shapeev, Alexander V.; Bochev, Pavel B.; Luskin, Mitchell

    2015-11-16

    Here, we formulate and analyze an optimization-based Atomistic-to-Continuum (AtC) coupling method for problems with point defects. Application of a potential-based atomistic model near the defect core enables accurate simulation of the defect. Away from the core, where site energies become nearly independent of the lattice position, the method switches to a more efficient continuum model. The two models are merged by minimizing the mismatch of their states on an overlap region, subject to the atomistic and continuum force balance equations acting independently in their domains. We prove that the optimization problem is well-posed and establish error estimates.

  6. Impact of point defects on the electronic and transport properties of silicene nanoribbons

    NASA Astrophysics Data System (ADS)

    Iordanidou, K.; Houssa, M.; van den Broek, B.; Pourtois, G.; Afanas'ev, V. V.; Stesmans, A.

    2016-01-01

    We study the impact of various point defects on the structural, electronic and ballistic transport properties of armchair silicene nanoribbons, using the density functional theory and the non equilibrium Green’s function method. The effect of a Stone-Wales defect, an interior/edge vacancy and an edge dangling bond is examined. Our results show that structural imperfections can alter the electronic structure (energy band structure and density of states) of the nanoribbons and can either increase or decrease the ballistic current. The dependence of the transport properties on the position of the defects (sublattice A or B) and on their distance from the contact is also investigated.

  7. Native point defects in CaCu3Ti4O12

    NASA Astrophysics Data System (ADS)

    Delugas, P.; Alippi, P.; Raineri, V.

    2010-02-01

    We report first principles computations on native point defects in CCTO. Vacancies present a general high formation energy, their concentration never exceed 1016 cm-3. Oxygen vacancies present stable positive charge states and are thus able to act as donor. Copper vacancies present instead stable negative charge states and are thus potential native acceptors for the material. As to anti-sites, the CuCa defect results to be the energetically favorite in most of the possible conditions, and may reach concentrations as high as 1019 cm-3.

  8. Point Defects in Two-Dimensional Layered Semiconductors: Physics and Its Applications

    NASA Astrophysics Data System (ADS)

    Suh, Joonki

    Recent advances in material science and semiconductor processing have been achieved largely based on in-depth understanding, efficient management and advanced application of point defects in host semiconductors, thus finding the relevant techniques such as doping and defect engineering as a traditional scientific and technological solution. Meanwhile, two- dimensional (2D) layered semiconductors currently draw tremendous attentions due to industrial needs and their rich physics at the nanoscale; as we approach the end of critical device dimensions in silicon-based technology, ultra-thin semiconductors have the potential as next- generation channel materials, and new physics also emerges at such reduced dimensions where confinement of electrons, phonons, and other quasi-particles is significant. It is therefore rewarding and interesting to understand and redefine the impact of lattice defects by investigating their interactions with energy/charge carriers of the host matter. Potentially, the established understanding will provide unprecedented opportunities for realizing new functionalities and enhancing the performance of energy harvesting and optoelectronic devices. In this thesis, multiple novel 2D layered semiconductors, such as bismuth and transition- metal chalcogenides, are explored. Following an introduction of conventional effects induced by point defects in semiconductors, the related physics of electronically active amphoteric defects is revisited in greater details. This can elucidate the complication of a two-dimensional electron gas coexisting with the topological states on the surface of bismuth chalcogenides, recently suggested as topological insulators. Therefore, native point defects are still one of the keys to understand and exploit topological insulators. In addition to from a fundamental science point of view, the effects of point defects on the integrated thermal-electrical transport, as well as the entropy-transporting process in

  9. Point Defects in Two-Dimensional Layered Semiconductors: Physics and Its Applications

    NASA Astrophysics Data System (ADS)

    Suh, Joonki

    Recent advances in material science and semiconductor processing have been achieved largely based on in-depth understanding, efficient management and advanced application of point defects in host semiconductors, thus finding the relevant techniques such as doping and defect engineering as a traditional scientific and technological solution. Meanwhile, two- dimensional (2D) layered semiconductors currently draw tremendous attentions due to industrial needs and their rich physics at the nanoscale; as we approach the end of critical device dimensions in silicon-based technology, ultra-thin semiconductors have the potential as next- generation channel materials, and new physics also emerges at such reduced dimensions where confinement of electrons, phonons, and other quasi-particles is significant. It is therefore rewarding and interesting to understand and redefine the impact of lattice defects by investigating their interactions with energy/charge carriers of the host matter. Potentially, the established understanding will provide unprecedented opportunities for realizing new functionalities and enhancing the performance of energy harvesting and optoelectronic devices. In this thesis, multiple novel 2D layered semiconductors, such as bismuth and transition- metal chalcogenides, are explored. Following an introduction of conventional effects induced by point defects in semiconductors, the related physics of electronically active amphoteric defects is revisited in greater details. This can elucidate the complication of a two-dimensional electron gas coexisting with the topological states on the surface of bismuth chalcogenides, recently suggested as topological insulators. Therefore, native point defects are still one of the keys to understand and exploit topological insulators. In addition to from a fundamental science point of view, the effects of point defects on the integrated thermal-electrical transport, as well as the entropy-transporting process in

  10. Point Defects in Binary Laves-Phase Alloys

    SciTech Connect

    Liaw, P.K.; Liu, C.T.; Pike, L.M.; Zhu, J.H.

    1998-11-30

    Point defect mechanisms in the binary C15 NbCr{sub 2} and NbCo{sub 2}, and C14 NbFe{sub 2} systems on both sides of stoichiometry was studied and clarified by both bulk density and X-ray lattice parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000 C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with the model predictions. However, thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in NbCr{sub 2} laves phase alloys after quenching from 1400 C. These could be completely eliminated by annealing at 1000 C. Anti-site hardening was found on both sides of stoichiometry for all three Laves phase systems studied. Furthermore, the thermal vacancies in NbCr{sub 2} alloys after quenching from 1400 C were found to soften the Laves phase. The anti-site hardening of the Laves phases is similar to that of the B2 compounds, while the thermal vacancy softening is unique to the Laves phase. Both the anti-site defects and thermal vacancies do not significantly affect the fracture toughness of the Laves phases.

  11. Point Defects in Binary Laves-Phase Alloys

    SciTech Connect

    Liaw, P.K.; Liu, C.T.; Pike, L.M.; Zhu, J.H.

    1999-01-11

    Point defects in the binary C15 NbCrQ and NbCoz, and C 14 NbFe2 systems on both sides of stoichiometry were studied by both bulk density and X-ray Iattiee parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000"C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with the model predictions. Thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in NbCr2 Laves phase alloys after quenching from 1400"C. However, there are essentially no thermal vacancies in NbFe2 alloys after quenching from 1300oC. Anti-site hardening was found on both sides of stoichiometry for all the tie Laves phase systems studied, while the thermal vacancies in NbCr2 alloys quenched from 1400'C were found to soften the Laves phase. The anti-site hardening of the Laves phases is similar to that of the B2 compounds and the thermal vacancy softening is unique to the Laves phase. Neither the anti-site defects nor the thermal vacancies affect the fracture toughness of the Laves phases significantly.

  12. Experimental Determination of Metal Fuel Point Defect Parameters

    SciTech Connect

    Fluss, M J; McCall, S

    2008-06-03

    Nuclear metallic fuels are one of many options for advanced nuclear fuel cycles because they provide dimensional stability, mechanical integrity, thermal efficiency, and irradiation resistance while the associated pyro-processing is technically relevant to concerns about proliferation and diversion of special nuclear materials. In this presentation we will discuss recent success that we have had in studying isochronal annealing of damage cascades in Pu and Pu(Ga) arising from the self-decay of Pu as well as the annealing characteristics of noninteracting point defect populations produced by ion accelerator irradiation. Comparisons of the annealing properties of these two populations of defects arising from very different source terms are enlightening and point to complex defect and mass transport properties in the plutonium specimens which we are only now starting to understand as a result of many follow-on studies. More importantly however, the success of these measurements points the way to obtaining important mass transport parameters for comparison with theoretical predictions or to use directly in existing and future materials modeling of radiation effects in nuclear metallic fuels. The way forward on such measurements and the requisite theory and modeling will be discussed.

  13. Point defect sinks in self-ion-irradiated nickel: A self-diffusion investigation

    SciTech Connect

    Mueller, A.; Naundorf, V.; Macht, M.

    1988-10-01

    The diffusion coefficient of /sup 63/Ni in pure nickel after irradiation with 300 keV Ni ions has been measured directly using a secondary ion mass spectrometer. The calculated displacement rate for this irradiation ranged from 1.2 x 10/sup -5/ to 3.1 x 10/sup -2/ dpa/s, the dose from 0.2 to 102 dpa, and the temperatures from 293 to 950 K. Between 293 and 650 K the irradiation-induced diffusion coefficient is temperature independent for a displacement rate of 1.2 x 10/sup -2/ dpa/s. In this temperature range mass transport by atomic mixing prevails over diffusion via point defects (radiation-enhanced diffusion). Normalized to the displacement rate K' the diffusion coefficient describing atomic mixing attains a value of D/sub mix/ /K' = 1.3 x 10/sup -18/ m/sup 2/ /dpa. The temperature and displacement rate dependence of the radiation-enhanced diffusion coefficient is discussed in the framework of a rate equation model. The analysis yields a production rate of freely migrating defects of about 1.5% of the calculated displacement rate. The effective concentration of point defect sinks was derived for nickel under heavy ion irradiation. This effective sink concentration C/sub s/ showed no dependence on displacement rate but a considerable temperature dependence, e.g., C/sub s/ = 2 x 10/sup -6/ at 850 K and C/sub s/ = 2.5 x 10/sup -7/ at 950 K. The conditions for a stationary effective sink concentration in self-ion irradiated nickel are discussed.

  14. Positron spectroscopy of point defects in the skyrmion-lattice compound MnSi

    NASA Astrophysics Data System (ADS)

    Reiner, Markus; Bauer, Andreas; Leitner, Michael; Gigl, Thomas; Anwand, Wolfgang; Butterling, Maik; Wagner, Andreas; Kudejova, Petra; Pfleiderer, Christian; Hugenschmidt, Christoph

    2016-07-01

    Outstanding crystalline perfection is a key requirement for the formation of new forms of electronic order in a vast number of widely different materials. Whereas excellent sample quality represents a standard claim in the literature, there are, quite generally, no reliable microscopic probes to establish the nature and concentration of lattice defects such as voids, dislocations and different species of point defects on the level relevant to the length and energy scales inherent to these new forms of order. Here we report an experimental study of the archetypical skyrmion-lattice compound MnSi, where we relate the characteristic types of point defects and their concentration to the magnetic properties by combining different types of positron spectroscopy with ab-initio calculations and bulk measurements. We find that Mn antisite disorder broadens the magnetic phase transitions and lowers their critical temperatures, whereas the skyrmion lattice phase forms for all samples studied underlining the robustness of this topologically non-trivial state. Taken together, this demonstrates the unprecedented sensitivity of positron spectroscopy in studies of new forms of electronic order.

  15. Small polarons and point defects in LaFeO3

    NASA Astrophysics Data System (ADS)

    Zhu, Zhen; Peelaers, Hartwin; van de Walle, Chris G.

    The proton-conductive perovskite-type LaFeO3 is a promising negative-electrode material for Ni/metal-hydride (Ni-MH) batteries. It has a discharge capacity up to 530 mAhg-1 at 333 K, which is significantly higher than commercialized AB5-type alloys. To elucidate the underlying mechanism of this performance, we have investigated the structural and electronic properties of bulk LaFeO3, as well as the effect of point defects, using hybrid density functional methods. LaFeO3 is antiferromagnetic in the ground state with a band gap of 3.54 eV. Small hole and electron polarons can form through self- or point-defect-assisted trapping. We find that La vacancies and Sr substitutional on La sites are shallow acceptors with the induced holes trapped as small polarons, while O and Fe vacancies are deep defect centers. Hydrogen interstitials behave like shallow donors, with the donor electrons localized on nearby iron sites as electron polarons. With a large trapping energy, these polarons can act as electron or hole traps and affect the electrical performance of LaFeO3 as the negative electrode for Ni-MH batteries. We acknowledge DOE for financial support.

  16. Positron spectroscopy of point defects in the skyrmion-lattice compound MnSi

    PubMed Central

    Reiner, Markus; Bauer, Andreas; Leitner, Michael; Gigl, Thomas; Anwand, Wolfgang; Butterling, Maik; Wagner, Andreas; Kudejova, Petra; Pfleiderer, Christian; Hugenschmidt, Christoph

    2016-01-01

    Outstanding crystalline perfection is a key requirement for the formation of new forms of electronic order in a vast number of widely different materials. Whereas excellent sample quality represents a standard claim in the literature, there are, quite generally, no reliable microscopic probes to establish the nature and concentration of lattice defects such as voids, dislocations and different species of point defects on the level relevant to the length and energy scales inherent to these new forms of order. Here we report an experimental study of the archetypical skyrmion-lattice compound MnSi, where we relate the characteristic types of point defects and their concentration to the magnetic properties by combining different types of positron spectroscopy with ab-initio calculations and bulk measurements. We find that Mn antisite disorder broadens the magnetic phase transitions and lowers their critical temperatures, whereas the skyrmion lattice phase forms for all samples studied underlining the robustness of this topologically non-trivial state. Taken together, this demonstrates the unprecedented sensitivity of positron spectroscopy in studies of new forms of electronic order. PMID:27388948

  17. Estimation of the temperature dependent interaction between uncharged point defects in Si

    SciTech Connect

    Kamiyama, Eiji; Vanhellemont, Jan; Sueoka, Koji

    2015-01-15

    A method is described to estimate the temperature dependent interaction between two uncharged point defects in Si based on DFT calculations. As an illustration, the formation of the uncharged di-vacancy V{sub 2} is discussed, based on the temperature dependent attractive field between both vacancies. For that purpose, all irreducible configurations of two uncharged vacancies are determined, each with their weight given by the number of equivalent configurations. Using a standard 216-atoms supercell, nineteen irreducible configurations of two vacancies are obtained. The binding energies of all these configurations are calculated. Each vacancy is surrounded by several attractive sites for another vacancy. The obtained temperature dependent of total volume of these attractive sites has a radius that is closely related with the capture radius for the formation of a di-vacancy that is used in continuum theory. The presented methodology can in principle also be applied to estimate the capture radius for pair formation of any type of point defects.

  18. Point Defect Cluster Formation in Iron Displacement Cascades Up to 50 keV

    SciTech Connect

    Stoller, R.E.

    1998-11-30

    The results of molecular dynamics displacement cascade simulations in iron at energies up to 50 keV and temperatures of 100, 600, and 900K are summarized, with a focus on the characterization of interstitial and vacancy clusters that are formed directly within the cascade. The fraction of the surviving point defects contained in clusters, and the size distributions of these in-cascade clusters have been determined. Although the formation of true vacancy clusters appears to be inhibited in iron, a significant degree of vacancy site correlation was observed. These well correlated arrangements of vacancies can be considered nascent clusters, and they have been observed to coalesce during longer term Monte Carlo simulations which permit short range vacancy diffusion. Extensive interstitial clustering was observed. The temperature and cascade energy dependence of the cluster size distributions are discussed in terms of their relevance to microstructural evolution and mechanical property changes in irradiated iron-based alloys.

  19. Effect of point defects on the properties of silicene-like BSi3 sheets from first-principles

    NASA Astrophysics Data System (ADS)

    Jalili, Seifollah; Akhavan, Mojdeh; Schofield, Jeremy

    2016-08-01

    Density functional theory calculations are used to investigate the impact of point defects on BSi3 nanosheets. It is shown that the defects have low formation energies and are likely to form in BSi3 sheets. The divacancies with missing Si an B atoms are more probable than those which have two missing Si atoms and the diantisite (BSi)anti is also more probable than (BB)anti. It is found that the structural reconstructions caused by bond rotation in divacancies continuously lower the formation energies. Interestingly, unlike graphene or many other sheets, the presence of point defects does not change the planar structure and metallic behavior of BSi3 and all systems are nonmagnetic. The planar and metallic BSi3 sheets can therefore be used in devices, without any concern about the presence of defects.

  20. Second workshop role of point defects/defect complexes in silicon device fabrication

    SciTech Connect

    Not Available

    1992-01-01

    Abstracts are presented of 24 papers, arranged under the following session/panel headings: defects and impurities in commercial photovoltaic Si substrates, point defects and point defect processes, impurity gettering for Si solar cells, gettering in Si solar cells, and passivation of impurities and defects.

  1. Tungsten Cluster Migration on Nanoparticles: Minimum Energy Pathway and Migration Mechanism

    SciTech Connect

    Chen, Dong; Hu, Wangyu; Gao, Fei; Deng, Huiqiu; Sun, Lixian

    2011-03-02

    Transition state searches have been employed to investigate the migration mechanisms of W clusters on W nanoparticles, and to determine the corresponding migration energies for the possible migration paths of these clusters. The tungsten clusters containing up to four adatoms are found to prefer 2D-compact structures with relatively low binding energies. The effect of interface and vertex regions on the migration behavior of the clusters is significantly strong, as compared to that of nanoparticle size. The migration mechanisms are quite different when the clusters are located at the center of the nanoparticle and near the interface or vertex areas. Near the interfaces and vertex areas, the substrate atoms tend to participate in the migration processes of the clusters, and can join the adatoms to form a larger cluster or lead to the dissociation of a cluster via the exchange mechanism, which results in the adatom crossing the facets. The lowest energy paths are used to be determined the energy barriers for W cluster migrations (from 1- to 4-atoms) on the facets, edges and vertex regions. The calculated energy barriers for the trimers suggest that the concerted migration is more probable than the successive jumping of a single adatom in the clusters. In addition, it of interest to note that the dimer shearing is a dominant migration mechanism for the tetramer, but needs to overcome a relatively higher migration energy than other clusters.

  2. Intrinsic point defects and their interaction with impurities in mono-crystalline zinc oxide

    NASA Astrophysics Data System (ADS)

    Svensson, Bengt G.

    2015-03-01

    Zinc oxide (ZnO) is a direct and wide band-gap semiconductor with several attractive features, like an exciton binding energy of ~ 60 meV, for light emitting devices, photovoltaics and spintronics. In the past decade, ZnO has received tremendous attention by the semiconductor physics community and many challenging issues have been addressed, especially the ``native'' n-type conductivity, the role of intrinsic point defects, and the realization of reproducible p-type doping. The latter is, indeed, decisive for a true breakthrough of ZnO-based optoelectronics. In this contribution, recent progress in our understanding of the interaction between intrinsic point defects and impurities in ZnO will be discussed. Aluminum (Al) is often introduced intentionally to accomplish high n-type conductivity since Al on Zn-site (AlZn) acts as a shallow donor. However, AlZn was recently found to react strongly with Zn vacancies (VZn) and the resulting complex (AlZn-VZn) is energetically favorable. The AlZn-VZn complex is a deep acceptor limiting the n-type doping efficiency and this finding is expected to hold in general for complexes between VZn and group-III elements. Further, implantation of self-ions (Zn and O) has been demonstrated to affect radically the balance of intrinsic point defects where an excess of Zn interstitials gives rise to a dramatic depletion of residual Li impurities on Zn-site (LiZn) whilst the opposite holds for an excess of O interstitials. In fact, this behavior appears to be of general validity and Li depletion occurs for a wide variety of implanted elements incorporated into the Zn sub-lattice while Li pile-up occurs for elements residing on O-site. Finally, the most prominent deep-level defect in ZnO, labelled E3, will be shown to involve hydrogen. E3 exists in most ZnO materials, irrespective of the growth method used, and evidence for a center formed by reaction between interstitial hydrogen and primary defects on the Zn sub-lattice will be given.

  3. The role of substrate point defects in adhesion of metal films

    SciTech Connect

    Stolyarova, S.

    1996-12-31

    As known, nucleation and epitaxial growth of metal films are affected by point defects of substrate surface, F-centers in particular, but their effect on adhesion of thin films has not yet been thoroughly studied. Despite the fact that the point defects are usually taken into account when the adhesion activation by various irradiation treatments is discussed, their role has not been properly revealed. This is due to the difficulties of the accurate control of the type and the density of the point defects in subsurface region as well as to the fact that the radiation treatment of surfaces can produce some changes in the chemical composition and stoichiometry of the surface, in addition to the creation of the point defects. For the purpose of the study of the effect of point defects on the adhesion of thin films, the author approached the problem in a principally different way: the author created point defects in the bulk of the crystals, controlled the type and the bulk density of the defects and then cleaved the crystals in vacuum - in a stream of metal vapors. The fresh, free from contaminants contact of metal film with the crystal surface enriched with point defects was created in this way.

  4. Point defects in CdTexSe1-x crystals grown from a Te-rich solution for applications in detecting radiation

    SciTech Connect

    Gul, R.; Roy, U. N.; Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A.; Lee, W.; Yang, G.; Burger, A.; James, R. B.; Cui, Y.

    2015-04-15

    We investigated cadmium telluride selenide (CdTeSe) crystals, newly grown by the Traveling Heater Method (THM), for the presence and abundance of point defects. Deep Level Transient spectroscopy (I-DLTS) was used to determine the energies of the traps, their capture cross sections, and densities. The bias across the detectors was varied from (1–30) V. Four types of point defects were identified, ranging from 10 meV to 0.35 eV. Two dominant traps at energies of 0.18 eV and 0.14 eV were studied in depth. Cd vacancies are found at lower concentrations than other point defects present in the material.

  5. Point defects in CdTexSe1-x crystals grown from a Te-rich solution for applications in detecting radiation

    DOE PAGESBeta

    Gul, R.; Roy, U. N.; Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A.; Lee, W.; Yang, G.; Burger, A.; James, R. B.; et al

    2015-04-15

    We investigated cadmium telluride selenide (CdTeSe) crystals, newly grown by the Traveling Heater Method (THM), for the presence and abundance of point defects. Deep Level Transient spectroscopy (I-DLTS) was used to determine the energies of the traps, their capture cross sections, and densities. The bias across the detectors was varied from (1–30) V. Four types of point defects were identified, ranging from 10 meV to 0.35 eV. Two dominant traps at energies of 0.18 eV and 0.14 eV were studied in depth. Cd vacancies are found at lower concentrations than other point defects present in the material.

  6. Visualizing Point Defects in Transition-Metal Dichalcogenides Using Optical Microscopy.

    PubMed

    Jeong, Hye Yun; Lee, Si Young; Ly, Thuc Hue; Han, Gang Hee; Kim, Hyun; Nam, Honggi; Jiong, Zhao; Shin, Bong Gyu; Yun, Seok Joon; Kim, Jaesu; Kim, Un Jeong; Hwang, Sungwoo; Lee, Young Hee

    2016-01-26

    While transmission electron microscopy and scanning tunneling microscopy reveal atomic structures of point defect and grain boundary in monolayer transition-metal dichalcogenides (TMDs), information on point defect distribution in macroscale is still not available. Herein, we visualize the point defect distribution of monolayer TMDs using dark-field optical microscopy. This was realized by anchoring silver nanoparticles on defect sites of MoS2 under light illumination. The optical images clearly revealed that the point defect distribution varies with light power and exposure time. The number of silver nanoparticles increased initially and reached a plateau in response to light power or exposure time. The size of silver nanoparticles was a few hundred nanometers in the plateau region as observed using optical microscopy. The measured defect density in macroscale was ∼2 × 10(10) cm(-2), slightly lower than the observed value (4 × 10(11) cm(-2)) from scanning tunneling microscopy. PMID:26645092

  7. GGA + U study of native point defects in ZnRh2O4

    NASA Astrophysics Data System (ADS)

    Volnianska, O.; Boguslawski, P.

    2014-11-01

    ZnRh2O4 spinel is the one of the most promising transparent conducting oxides for applications in optoelectronic technology. Energy levels and formation energies of native point defects, i.e. vacancies (V), interstitials (I), and cation antisites in ZnRh2O4 were analysed. Generalized gradient approximation (GGA) was supplemented by the +U on-site corrections imposed on d(Rh) and p(O) states. The inclusion of the pronounced distortions of the anion sublattice was necessary to obtain the correct band gap. U was treated as a free parameter, which allowed for the systematic study of the U-induced changes of the defect states. A diagram of the thermodynamic phase stability of ZnRh2O4 was obtained. ZnRh is the dominant acceptor that can be responsible for the observed p-type conductivity in ZnRh2O4. The low formation energy of ZnRh can make the intentional n-doping difficult. In the O-rich conditions the second important acceptor is VZn. The two dominant donors that can compensate ZnRh in Zn-rich and O-rich conditions are VO and RhZn, respectively. Growth conditions leading to the lowest concentrations of native defects were identified. Due to the pronounced occupancy dependence of the +U term, VO and RhZn are ‘negative-Ueff’ centres.

  8. Point-defect-mediated dehydrogenation of AlH3

    NASA Astrophysics Data System (ADS)

    Ismer, Lars; Janotti, Anderson; Van de Walle, Chris G.

    2010-11-01

    Based on hybrid density functional calculations, we propose a microscopic mechanism for the dehydrogenation of AlH3. Our results indicate that mass transport mediated by positively charged hydrogen vacancies (VH+) is likely the rate-limiting mechanism. The calculated activation energy of 1.72 eV is in good agreement with experimental values. The high formation energy and hence low concentration of VH+ explains why AlH3 does not decompose at room temperature, although it is thermodynamically unstable. Issues of maintaining charge neutrality are addressed.

  9. Point-Defect Haloing in Curved Nematic Films

    NASA Astrophysics Data System (ADS)

    Isaku Hasegawa,; Hiroyuki Shima,

    2010-07-01

    We investigate a correlation between the point disclination energies and the surface curvature modulation of nematic liquid crystal membranes with a Gaussian bump geometry. The correlation causes point disclinations to feel an attractive force that confines them to an annulus region, resulting in a halo distribution around the top of the bump. The halo formation is a direct consequence of the nonzero Gaussian curvature of the bump that affects preferable configurations of liquid crystal molecules around the disclination core.

  10. First-principles study of point defects in chalcopyrite ZnSnP2

    NASA Astrophysics Data System (ADS)

    Kumagai, Yu; Choi, Minseok; Nose, Yoshitaro; Oba, Fumiyasu

    2014-09-01

    Chalcopyrite ZnSnP2 is an alternative photoabsorber material for solar cells because of its controllable band gap, high absorption coefficient, and earth abundant constituents. In this study we systematically investigate its native point defects including vacancies, interstitials, and antisites using first-principles calculations with the Heyd-Scuseria-Ernzerhof hybrid functional. We evaluate the defect formation energies and defect single-particle levels at the dilute limit using finite-size image-charge corrections and compare them with those reported for CuInSe2 and CuGaSe2. The most likely donors and acceptors are cation antisites, Sn-on-Zn and Zn-on-Sn, respectively. Because of their significantly low formation energies, they lead to Fermi level pinning in the band gap under any growth condition, and constrain the carrier concentration. The Sn-on-Zn antisite in the neutral charge state becomes an intrinsic DX center, a complex of the Sn interstitial and Zn vacancy, and shows a deep donor level as reported for CuGaSe2.

  11. The effect of native point defect thermodynamics on off-stoichiometry in beta-Mg17Al12

    SciTech Connect

    Wolverton, Christopher

    2012-01-01

    The mechanical strength of Mg-Al-Zn alloys can be affected by a fine spatial dispersion of {beta}-Mg{sub 17}Al{sub 12} precipitates in the Mg matrix. In an effort to understand the phase stability and the unusual asymmetric off-stoichiometry observed in {beta}-Mg{sub 17}Al{sub 12}, we have performed a series of first-principles density functional theory (DFT) calculations of bulk and defect properties of Mg{sub 17}Al{sub 12}. Specifically, we consider native point defects (i.e. vacancies and anti-sites) in all four sublattices of Mg{sub 17}Al{sub 12}, i.e. 2a, 8c, 24g (Mg) and 24g (Al). The T = 0 K static energies of defect Mg{sub 17}Al{sub 12} supercells indicate that anti-site defects are energetically favored over vacancies, and the lowest anti-site defect formation energies are in 24g sites for both Al{sub Mg} and Mg{sub Al}. These Al-rich and Mg-rich anti-site defect formation energies are similar in magnitude, and thus do not explain the asymmetric off-stoichiometry of Mg{sub 17}Al{sub 12}. We also investigate the effect of atomic vibrations via DFT phonon calculations on native point defect free energies of Mg{sub 17}Al{sub 12} and combine these entropic contributions with the point defect formation energies to evaluate the thermodynamics of off-stoichiometry in this phase. We find that the formation of the Al{sub Mg} anti-site is not strongly stabilized by vibrational entropy. Thus, we conclude that the observed asymmetry in the off-stoichiometry of the {beta}-Mg{sub 17}Al{sub 12} phase in the Mg-Al phase diagram is not explained by simple native point defect thermodynamics, and must involve a more complicated defect formation mechanism, such as multi-defect clustering.

  12. Atomic Displacements Due to Point Defects in Ni Dilute Alloys

    NASA Astrophysics Data System (ADS)

    Sharma, Hitesh; Prakash, S.

    The Embedded atom method has been used to investigate the strain field due to substitutional transition metal impurities in Ni. The calculations are carried out in the discrete lattice model of the metal using Kanzaki lattice static method. The results for atomic displacements due to 3d, 4d and 5d impurities (Cu, Pd, Pt and Au) in Ni are given up to 20 NN's of impurity and are compared with the earlier calculations and with the available experimental data. The maximum displacements of 3.6% of 1NN distance are found for NiAu, while the minimum displacements of 0.78% of 1NN distance are found for NiCu alloy respectively. The relaxation energy for Cu are found less than those for Pd, Au and Pt impurities in the Ni host.

  13. Deuterium-Hydrogen Interdiffusion in Olivine: Implications for Point Defects and Electrical Conductivity

    NASA Astrophysics Data System (ADS)

    Tyburczy, J. A.; Du Frane, W. L.

    2011-12-01

    Knowledge about hydrogen diffusivity in mantle minerals is critical for determining mantle hydrogen distribution, and additionally for understanding point defects. Chemical diffusion of hydrogen in olivine depends on self diffusion and concentration of hydrogen and other point defects, such as small polarons and metal vacancies. In this study we measured hydrogen self diffusion in olivine, and we compare these values to those previously reported for chemical 1-H redox exchange DExch. Deuterium 2-H was interdiffused into hydrogen 1-H saturated single crystals of San Carlos olivine between 750-900 °C at 2 GPa. We measured and fit the resulting 2-H and 1-H profiles to obtain the interdiffusion coefficient DH,[100] = 10^(-5.04+/-1.43)*e(-137+/-31kJ/mol)/(RT) m^2/s. This is ~1 log unit lower than DExch,[100], with similar activation energy Ea,[100]. DH is anisotropic with DH,[001] = 10^(-12.0+/-0.2) m^2/s at 900 °C and 2 GPa; only upper bound estimates for DH,[010] could be determined from our experiments. We use DH,[100] and DExch,[100] to calculate the small polaron diffusion coefficient associated with redox exchange Dh,[100] = 10^-3.90*e(-142kJ/mol)/(RT) m^2/s. Dh,[100] is combined with reported values for DExch,[010] and DExch,[010] to calculate values for DH,[010] and DH,[001] that are consistent with our upper bound estimates. These DH values, both measured and derived, are used with the Nernst-Einstein relation to calculate the electrical conductivity σ by hydrogen in olivine σH. We calculate σH = 10^1.63*e(-126kJ/mol)/(RT) S/m, which is similar in magnitude to the lower range of reported σ measurements. This similarly suggests that hydrogen alone cannot account for high σ anomalies observed at asthenospheric depths (~10^-2 to ~10^-1 S/m). The Ea for hydrogen mobility/diffusion we calculate are higher (~40% for 100 ppmw H2O) than those derived from previous σ measurements. This work supported by NSF EAR 0739050 to J. A. Tyburczy and R. Hervig. The SIMS

  14. Electronic structures and optical properties of rutile TiO2 with different point defects from DFT + U calculations

    NASA Astrophysics Data System (ADS)

    Zhu, H. X.; Zhou, P. X.; Li, X.; Liu, J.-M.

    2014-07-01

    The electronic states and formation energies of four types of lattice point defects in rutile TiO2 are studied using the first-principles calculations. The existence of oxygen vacancy leads to a deep donor defect level in the forbidden band, while the Ti interstitial forms two local states. It is predicted that oxygen vacancy prefers to combine with Ti-interstitial to form VO-Tii dimer by a partial 3d electron transfer from the Tii to its neighboring VO. The charge distribution between a Ti interstitial and its neighboring Ti ions partially shields the Coulomb interactions. Lastly, optical properties of these defective lattices are discussed.

  15. Magnetic properties of point defects in proton irradiated diamond

    NASA Astrophysics Data System (ADS)

    Makgato, T. N.; Sideras-Haddad, E.; Ramos, M. A.; García-Hernández, M.; Climent-Font, A.; Zucchiatti, A.; Muñoz-Martin, A.; Shrivastava, S.; Erasmus, R.

    2016-09-01

    We investigate the magnetic properties of ultra-pure type-IIa diamond following irradiation with proton beams of ≈1-2 MeV energy. SQUID magnetometry indicate the formation of Curie type paramagnetism according to the Curie law. Raman and Photoluminescence spectroscopy measurements show that the primary structural features created by proton irradiation are the centers: GR1, ND1, TR12 and 3H. The Stopping and Range of Ions in Matter (SRIM) Monte Carlo simulations together with SQUID observations show a strong correlation between vacancy production, proton fluence and the paramagnetic factor. At an average surface vacancy spacing of ≈1-1.6 nm and bulk (peak) vacancy spacing of ≈0.3-0.5 nm Curie paramagnetism is induced by formation of ND1 centres with an effective magnetic moment μeff~(0.1-0.2)μB. No evidence of long range magnetic ordering is observed in the temperature range 4.2-300 K.

  16. Ab initio thermodynamic properties of point defects and O-vacancy diffusion in Mg spinels

    NASA Astrophysics Data System (ADS)

    Łodziana, Zbigniew; Piechota, Jacek

    2006-11-01

    We report ab initio plane wave density functional theory studies of thermodynamic properties of isolated cation substitutions and oxygen vacancies in magnesium spinel, MgAl2O4 . The formation enthalpy of Ca, Cu, and Zn substitutions of Mg cation indicate that transition metal dopants are energetically stable in the bulk of MgAl2O4 at low oxygen chemical potential. The electronic and thermodynamic properties of isolated defects in ternary spinel show close similarities with those in binary oxides; MgO and α-Al2O3 . The formation enthalpy of the oxygen vacancies are also similar in pure magnesium spinel and in binary oxides, but presence of impurity cations in MgAl2O4 significantly lowers formation enthalpy of the oxygen vacancy in their vicinity. Calculated energy barriers for oxygen vacancy hopping are lower in the vicinity of impurity atoms in the spinel structure. Our calculations indicate that the charge state of doped cation is modified by the accompanying oxygen vacancy and the vacancy diffusion is more facile around impurity. The present studies suggest that point defects play an important role in diffusion of oxygen vacancies in MgAl2O4 .

  17. Ab initio study of point defects near stacking faults in 3C-SiC

    DOE PAGESBeta

    Xi, Jianqi; Liu, Bin; Zhang, Yanwen; Weber, William J.

    2016-07-02

    Interactions between point defects and stacking faults in 3C-SiC are studied using an ab initio method based on density functional theory. The results show that the discontinuity of the stacking sequence considerably affects the configurations and behavior of intrinsic defects, especially in the case of silicon interstitials. The existence of an intrinsic stacking fault (missing a C-Si bilayer) shortens the distance between the tetrahedral-center site and its second-nearest-neighboring silicon layer, making the tetrahedral silicon interstitial unstable. Instead of a tetrahedral configuration with four C neighbors, a pyramid-like interstitial structure with a defect state within the band gap becomes a stablemore » configuration. In addition, orientation rotation occurs in the split interstitials that has diverse effects on the energy landscape of silicon and carbon split interstitials in the stacking fault region. Moreover, our analyses of ionic relaxation and electronic structure of vacancies show that the built-in strain field, owing to the existence of the stacking fault, makes the local environment around vacancies more complex than that in the bulk.« less

  18. Impact of Mg Content on (Mg,Zn)O Native Point Defects

    NASA Astrophysics Data System (ADS)

    Ball, Molly; Restrepo, Oscar; Brillson, Leonard; Windl, Wolfgang; Department of Material Science; Engineering Collaboration; Department of Physics Collaboration

    2015-03-01

    The two most thermodynamically stable defects in ZnO are oxygen vacancies (VO) and zinc vacancies (VZn) . These native point defects are electrically charged and can contribute to free carrier densities. Experiment shows that Mg addition to ZnO significantly changes native defect densities. To better understand this dramatic decrease in VZn and VO-related defects with increasing Mg content up to x =0.44 and the subsequent increase, we performed density functional theory (DFT) calculations using PAW potentials within PBE using VASP. The results showed to be very sensitive to DFT method used and chemical-potential calculation. For the latter, the literature shows that one can assume that the oxygen chemical potential equals that of the atoms in the oxygen molecules at a given surrounding partial oxygen pressure. However, one can also postulate that the total defect concentrations conserve the stoichiometry, or limiting potentials from elemental equilibrium phases can be used. The experimentally observed dependence helped identify the correct way to reproduce the experimental dependence of formation energy on Mg concentration, which will be discussed in detail in this presentation.

  19. Investigation of oxygen point defects in cubic ZrO2 by density functional theory

    SciTech Connect

    Liu, Bin; Xiao, Haiyan; Zhang, Yanwen; Aidhy, Dilpuneet S; Weber, William J

    2014-01-01

    The energetics of formation and migration of the oxygen vacancy and interstitial in cubic ZrO2 are investigated by density functional theory calculations. In an O-rich environment, the negatively charged oxygen interstitial is the most dominant defect whereas, the positively charged oxygen vacancy is the most dominant defect under O-poor conditions. Oxygen interstitial migration occurs by the interstitialcy and the direct interstitial mechanisms, with calculated migration energy barriers of 2.94 eV and 2.15 eV, respectively. For the oxygen vacancy, diffusion is preferred along the <100> direction, and the calculated energy barriers are 0.26 eV for , 0.27 eV for and 0.54 eV for . These results indicate that oxygen diffusivity is higher through the vacancy-migration mechanism.

  20. First principles calculation of point defects and mobility degradation in bulk AlSb for radiation detection application

    SciTech Connect

    Lordi, V; Aberg, D; Erhart, P; Wu, K J

    2007-07-30

    The development of high resolution, room temperature semiconductor radiation detectors requires the introduction of materials with increased carrier mobility-lifetime ({mu}{tau}) product, while having a band gap in the 1.4-2.2 eV range. AlSb is a promising material for this application. However, systematic improvements in the material quality are necessary to achieve an adequate {mu}{tau} product. We are using a combination of simulation and experiment to develop a fundamental understanding of the factors which affect detector material quality. First principles calculations are used to study the microscopic mechanisms of mobility degradation from point defects and to calculate the intrinsic limit of mobility from phonon scattering. We use density functional theory (DFT) to calculate the formation energies of native and impurity point defects, to determine their equilibrium concentrations as a function of temperature and charge state. Perturbation theory via the Born approximation is coupled with Boltzmann transport theory to calculate the contribution toward mobility degradation of each type of point defect, using DFT-computed carrier scattering rates. A comparison is made to measured carrier concentrations and mobilities from AlSb crystals grown in our lab. We find our predictions in good quantitative agreement with experiment, allowing optimized annealing conditions to be deduced. A major result is the determination of oxygen impurity as a severe mobility killer, despite the ability of oxygen to compensation dope AlSb and reduce the net carrier concentration. In this case, increased resistivity is not a good indicator of improved material performance, due to the concomitant sharp reduction in {mu}{tau}.

  1. Lithium-ion drifting: Application to the study of point defects in floating-zone silicon

    NASA Technical Reports Server (NTRS)

    Walton, J. T.; Wong, Y. K.; Zulehner, W.

    1997-01-01

    The use of lithium-ion (Li(+)) drifting to study the properties of point defects in p-type Floating-Zone (FZ) silicon crystals is reported. The Li(+) drift technique is used to detect the presence of vacancy-related defects (D defects) in certain p-type FZ silicon crystals. SUPREM-IV modeling suggests that the silicon point defect diffusivities are considerably higher than those commonly accepted, but are in reasonable agreement with values recently proposed. These results demonstrate the utility of Li(+) drifting in the study of silicon point defect properties in p-type FZ crystals. Finally, a straightforward measurement of the Li(+) compensation depth is shown to yield estimates of the vacancy-related defect concentration in p-type FZ crystals.

  2. Effect of Intrinsic Point Defect on the Magnetic Properties of ZnO Nanowire

    PubMed Central

    Yun, Jiangni; Zhang, Zhiyong; Yin, Tieen

    2013-01-01

    The effect of intrinsic point defect on the magnetic properties of ZnO nanowire is investigated by the first-principles calculation based on the density functional theory (DFT). The calculated results reveal that the pure ZnO nanowire without intrinsic point defect is nonmagnetic and ZnO nanowire with VO, Zni, Oi, OZn, or ZnO point defect also is nonmagnetic. However, a strong spin splitting phenomenon is observed in ZnO nanowire with VZn defect sitting on the surface site. The Mulliken population analysis reveals that the oxygen atoms which are close to the VZn defect do major contribution to the magnetic moment. Partial density states calculation further suggests that the appearance of the half-metallic ferromagnetism in ZnO nanorod with VZn originates from the hybridization of the O2p states with Zn 3d states. PMID:24396300

  3. Point defect modeling in materials: Coupling ab initio and elasticity approaches

    NASA Astrophysics Data System (ADS)

    Varvenne, Céline; Bruneval, Fabien; Marinica, Mihai-Cosmin; Clouet, Emmanuel

    2013-10-01

    Modeling point defects at an atomic scale requires careful treatment of the long-range atomic relaxations. This elastic field can strongly affect point defect properties calculated in atomistic simulations because of the finite size of the system under study. This is an important restriction for ab initio methods which are limited to a few hundred atoms. We propose an original approach coupling ab initio calculations and linear elasticity theory to obtain the properties of an isolated point defect for reduced supercell sizes. The reliability and benefit of our approach are demonstrated for three problematic cases: the self-interstitial in zirconium, clusters of self-interstitials in iron, and the neutral vacancy in silicon.

  4. Point defect dynamics in MOSFETs -- From atomic-scale physics to engineering models

    NASA Astrophysics Data System (ADS)

    Pantelides, Sokrates T.

    2003-03-01

    Dopant impurities are the key ingredient that makes semiconductors so useful in microelectronics. Other point defects, e.g., vacancies and self-interstitial, play major roles in mediating diffusion, which can be both good and bad. Hydrogen is another element that is very useful in passivating point defects at the Si-SiO2 interface of metal-oxide-semiconductor field-effect transistors (MOSFETs), but it also lurks dormant in all kinds of places. Oxygen vacancies also abound dormant and benign in the SiO2 gate layer, but both hydrogen and oxygen vacancies reveal sinister personas when radiation strikes (as in space electronics). This talk will give a brief account of how, over the last few decades, atomic-scale quantum mechanical calculations have impacted the process of constructing engineering models that are used in technology development and then focus on recent results on point defect dynamics in MOSFETs, some of which have been fed into engineering models.

  5. Effect of intrinsic point defect on the magnetic properties of ZnO nanowire.

    PubMed

    Yun, Jiangni; Zhang, Zhiyong; Yin, Tieen

    2013-01-01

    The effect of intrinsic point defect on the magnetic properties of ZnO nanowire is investigated by the first-principles calculation based on the density functional theory (DFT). The calculated results reveal that the pure ZnO nanowire without intrinsic point defect is nonmagnetic and ZnO nanowire with V(O), Zn(i), O(i), O(Zn), or Zn(O) point defect also is nonmagnetic. However, a strong spin splitting phenomenon is observed in ZnO nanowire with V(Zn) defect sitting on the surface site. The Mulliken population analysis reveals that the oxygen atoms which are close to the V(Zn) defect do major contribution to the magnetic moment. Partial density states calculation further suggests that the appearance of the half-metallic ferromagnetism in ZnO nanorod with V(Zn) originates from the hybridization of the O2p states with Zn 3d states. PMID:24396300

  6. Self-compensation due to point defects in Mg-doped GaN

    NASA Astrophysics Data System (ADS)

    Miceli, Giacomo; Pasquarello, Alfredo

    2016-04-01

    Using hybrid density functional theory, we address point defects susceptible to cause charge compensation upon Mg doping of GaN. We determine the free energy of formation of the nitrogen vacancy and of several Mg-related defects. The entropic contribution as a function of temperature is determined within the quasiharmonic approximation. We find that the Mg interstitial shows a noticeably lower free energy of formation than the Mg substitutional to Ga in p -type conditions. Therefore, the Mg impurity is amphoteric behaving like an acceptor when substitutional to Ga and like a double donor when accommodated in an interstitial position. The hybrid-functional results are then linked to experimental observations by solving the charge neutrality equations for semiconductor dominated by impurities. We show that a thermodynamic equilibrium model is unable to account for the experimental hole concentration as a function of Mg doping density, due to nitrogen vacancies and Mg interstitials acting as compensating donors. To explain the experimental result, which includes a dropoff of the hole concentration at high Mg densities, we thus resort to nonequilibrium models. We show that either nitrogen vacancies or Mg interstitials could be at the origin of the self-compensation mechanism. However, only the model based on interstitial Mg donors provides a natural mechanism to account for the sudden appearance of self-compensation. Indeed, the amphoteric nature of the Mg impurity leads to Fermi-level pinning and accounts for the observed dropoff of the hole concentration of GaN samples at high Mg doping. Our work suggests that current limitations in p -type doping of GaN could be overcome by extrinsically controlling the Fermi energy during growth.

  7. The role of point defects and defect complexes in silicon device processing. Summary report and papers

    SciTech Connect

    Sopori, B.; Tan, T.Y.

    1994-08-01

    This report is the summary of the third workshop on the role of point defects and defect complexes in silicon device processing. The workshop was organized: (1) to discuss recent progress in the material quality produced by photovoltaic Si manufacturers, (2) to foster the understanding of point defect issues in Si device processing, (3) to review the effects of inhomogeneities on large- area solar cell performance, (4) to discuss how to improve Si solar cell processing, and (5) to develop a new understanding of gettering, defect passivation, and defect annihilation. Separate abstract were prepared for the individual papers, for the database.

  8. Role of point defects in the photosensitivity of hydrogen-loaded phosphosilicate glass

    SciTech Connect

    Larionov, Yu V

    2010-08-03

    It is shown that point defect modifications in hydrogen-loaded phosphosilicate glass (PSG) do not play a central role in determining its photosensitivity. Photochemical reactions that involve a two-step point defect modification and pre-exposure effect are incapable of accounting for photoinduced refractive index changes. It seems likely that a key role in UV-induced refractive index modifications is played by structural changes in the PSG network. Experimental data are presented that demonstrate intricate network rearrangement dynamics during UV exposure of PSG. (fiber optics)

  9. Density functional theory calculations of point defects and hydrogen isotopes in Li4SiO4

    NASA Astrophysics Data System (ADS)

    Xiang, Xiaogang; Zhu, Wenjun; Lu, Tiecheng; Gao, Tao; Shi, Yanli; Yang, Mao; Gong, Yichao; Yu, Xiaohe; Feng, Lan; Wei, Yongkai; Lu, Zhipeng

    2015-10-01

    The Li4SiO4 is a promising breeder material for future fusion reactors. Radiation induced vacancies and hydrogen isotope related impurities are the major types of point defects in this breeder material. In present study, various kinds of vacancies and hydrogen isotopes related point defects in Li4SiO4 are investigated through density functional theory (DFT) calculations. The band gap of Li4SiO4 is determined by UV-Vis diffuse reflectance spectroscopy experiments. Formation energies of all possible charge states of Li, Si and O vacancies are calculated using DFT methods. Formation energies of possible charge states of hydrogen isotopes substitution for Li and O are also calculated. We found that Li-vacancies will dominate among all vacancies in neutral charge state under radiation conditions and the O, Li, and Si vacancies (VO,VLi,VSi) are stable in charge states +2, -1, -4 for most of the range of Fermi level, respectively. The interstitial hydrogen isotopes (Hi) and substitutional HLi are stable in the charge states +1, 0 for most of the range of Fermi level, respectively. Moreover, substitutional HO are stable in +1 charge states. We also investigated the process of tritium recovery by discussing the interaction between interstitial H and Li-vacancy, O-vacancy, and found that HO + and HLi 0 are the most common H related defects during radiation process.

  10. Point defects stabilise cubic Mo-N and Ta-N

    NASA Astrophysics Data System (ADS)

    Koutná, Nikola; Holec, David; Svoboda, Ondřej; Klimashin, Fedor F.; Mayrhofer, Paul H.

    2016-09-01

    We employ ab initio calculations to investigate energetics of point defects in metastable rocksalt cubic Ta-N and Mo-N. Our results reveal a strong tendency to off-stoichiometry, i.e. defected structures are surprisingly predicted to be more stable than perfect ones with 1:1 metal-to-nitrogen stoichiometry. Despite the similarity of Ta-N and Mo-N systems in exhibiting this unusual behaviour, we also point out their crucial differences. While Ta-N significantly favours metal vacancies, Mo-N exhibits similar energies of formation regardless of the vacancy type (V Mo, V N) as long as their concentration is below ≈ 15~\\text{at}. % . The overall lowest energies of formation were obtained for \\text{T}{{\\text{a}}0.78}\\text{N} and \\text{M}{{\\text{o}}0.91}\\text{N} , which are hence predicted to be the most stable compositions. To account for various experimental conditions during synthesis, we further evaluated the phase stability as a function of chemical potential of individual species. The proposed phase diagrams reveal four stable compositions, \\text{M}{{\\text{o}}0.84}\\text{N} , \\text{M}{{\\text{o}}0.91}\\text{N} , \\text{Mo}{{\\text{N}}0.69} and \\text{Mo}{{\\text{N}}0.44} , in the case of Mo-N and nine stable compositions in the case of Ta-N indicating the important role of metal under-stoichiometry, since \\text{T}{{\\text{a}}0.75}\\text{N} and \\text{T}{{\\text{a}}0.78}\\text{N} significantly dominate the diagram. This is particularly important for understanding and designing experiments using non-equilibrium deposition techniques. Finally, we discuss the role of defect ordering and estimate a cubic lattice parameter as a function of defect contents and put them in the context of existing literature theoretical and experimental data.

  11. Concentration of point defects in 4H-SiC characterized by a magnetic measurement

    NASA Astrophysics Data System (ADS)

    Peng, B.; Jia, R. X.; Wang, Y. T.; Dong, L. P.; Hu, J. C.; Zhang, Y. M.

    2016-09-01

    A magnetic method is presented to characterize the concentration of point defects in silicon carbide. In this method, the concentration of common charged point defects, which is related to the density of paramagnetic centers, is determined by fitting the paramagnetic component of the specimen to the Brillouin function. Several parameters in the Brillouin function can be measured such as: the g-factor can be obtained from electron spin resonance spectroscopy, and the magnetic moment of paramagnetic centers can be obtained from positron lifetime spectroscopy combined with a first-principles calculation. To evaluate the characterization method, silicon carbide specimens with different concentrations of point defects are prepared with aluminum ion implantation. The fitting results of the densities of paramagnetic centers for the implanted doses of 1 × 1014 cm-2, 1 × 1015 cm-2 and 1 × 1016 cm-2 are 6.52 × 1014/g, 1.14 × 1015/g and 9.45 × 1014/g, respectively. The same trends are also observed for the S-parameters in the Doppler broadening spectra. It is shown that this method is an accurate and convenient way to obtain the concentration of point defects in 4H-SiC.

  12. Effect of extended strain fields on point defect phonon scattering in thermoelectric materials.

    PubMed

    Ortiz, Brenden R; Peng, Haowei; Lopez, Armando; Parilla, Philip A; Lany, Stephan; Toberer, Eric S

    2015-07-15

    The design of thermoelectric materials often involves the integration of point defects (alloying) as a route to reduce the lattice thermal conductivity. Classically, the point defect scattering strength follows from simple considerations such as mass contrast and the presence of induced strain fields (e.g. radius contrast, coordination changes). While the mass contrast can be easily calculated, the associated strain fields induced by defect chemistry are not readily predicted and are poorly understood. In this work, we use classical and first principles calculations to provide insight into the strain field component of phonon scattering from isoelectronic point defects. Our results also integrate experimental measurements on bulk samples of SnSe and associated alloys with S, Te, Ge, Sr and Ba. These efforts highlight that the strength and extent of the resulting strain field depends strongly on defect chemistry. Strain fields can have a profound impact on the local structure. For example, in alloys containing Ba, the strain fields have significant spatial extent (1 nm in diameter) and produce large shifts in the atomic equilibrium positions (up to 0.5 Å). Such chemical complexity suggests that computational assessment of point defects for thermal conductivity depression should be hindered. However, in this work, we present and verify several computational descriptors that correlate well with the experimentally measured strain fields. Furthermore, these descriptors are conceptually transparent and computationally inexpensive, allowing computation to provide a pivotal role in the screening of effective alloys. The further development of point defect engineering could complement or replace nanostructuring when optimizing the thermal conductivity, offering the benefits of thermodynamic stability, and providing more clearly defined defect chemistry. PMID:26145414

  13. Migration Mechanisms of Oxygen Interstitial Clusters in UO2

    SciTech Connect

    Xian-Ming Bai; Anter El-Azab; Jianguo Yu; Todd R. Allen

    2013-01-01

    Understanding the migration kinetics of radiation-induced point defects and defect clusters is a key to predicting the microstructural evolution and mass transport in nuclear fuels. Although the diffusion kinetics of point defects in UO2 is well explored both experimentally and theoretically, the kinetics of defect clusters is not well understood. In this work the migration mechanisms of oxygen interstitial clusters of size one to five atoms (1Oi – 5Oi) in UO2 are investigated by temperature-accelerated dynamics simulations without any a priori assumptions of migration mechanisms. It is found that the migration paths of oxygen interstitial clusters are complex and non-intuitive and that multiple migration paths and barriers exist for some clusters. It is also found that the cluster migration barrier does not increase with increasing cluster size and its magnitude has the following order: 2Oi < 3Oi < 1Oi < 5Oi < 4Oi. Possible finite-size effects are checked with three different sized systems. The results show good agreement with other available experimental and theoretical data. In particular, the relatively large migration barriers of cuboctahedral clusters (4Oi and 5Oi) are in good agreement with the experimentally measured oxygen diffusion activation energy in U4O9, which is thought to contain many such clusters. The cluster migration sequence may explain the interesting relationship between the oxygen diffusivity and stoichiometry in UO2+x.

  14. Energy barriers and cell migration in confluent tissues

    NASA Astrophysics Data System (ADS)

    Bi, Dapeng; Lopez, J. H.; Schwarz, J. M.; Manning, M. Lisa

    2014-03-01

    Biological processes such as embryogensis, tumorigenesis and wound healing require cells to move within a tissue. While the migration of single cells has been extensively studied, it has remained unclear how single cell properties control migration through a confluent tissue. We develop numerical and theoretical models to calculate energy barriers to cell rearrangements, which govern cell motility. In contrast to sheared foams where energy barriers are power-law distributed, energy barriers in tissues are exponentially distributed and depend systematically on the cell's number of neighbors. Using simple extensions of `trap' and `Soft Glassy Rheology' models, we demonstrate that these energy barrier distributions give rise to glassy behavior and use the models to make testable predictions for two-time correlation functions and caging times. We incorporate these ideas into a continuum model that combines glassy rheology with active polarization to better understand collective migration in epithelial sheets.

  15. Excitation energy migration in uniaxially oriented polymer films

    NASA Astrophysics Data System (ADS)

    Bojarski, P.; Kamińska, A.; Kułak, L.; Sadownik, M.

    2003-07-01

    Multistep nonradiative excitation energy transport is for the first time studied in uniaxially stretched polymer films. Concentration depolarization studies performed for 3,3 '-diethylthiacarbocyanine iodide (DTCI) for stretched (system I) and unstretched (system II) polyvinyl alcohol films (PVA) yield extremely different results. As expected, disordered system II exhibits strong concentration depolarization due to energy migration between identical fluorophores. However, for stretched films, concentration depolarization effect is much weaker and it occurs at higher dye concentrations compared to system II. This fact is mostly due to strongly modified angular distribution of dipole moments of molecules taking part in the excitation energy migration.

  16. Effect of wire configuration and point defects on the conductance of gold nano-conductors

    NASA Astrophysics Data System (ADS)

    Barzilai, S.; Tavazza, F.; Levine, L. E.

    2014-04-01

    Gold nanowire (NW) chains are considered a good candidate for nano-electronics devices because they exhibit remarkable structural and electrical properties. One promising nano-conductor candidate is called ‘Hexa1’. It has a two-dimensional, one atom thick structure and was found to spontaneously form during simulations of gold NWs elongations. It is stable and a good conductor when adsorbed on a suitable substrate. In this study, we deepened the investigation of such a NW structure, to explore the effect of the NWs length, point defects and NW junctions on its conductance. We found that the conductance is not affected by the NW length, and that conveniently placed point defects can be used to create resistors. We also found that direction changes in circuits produce conductance bottle necks, therefore decreasing the conductance. However, this decrease can be easily overcome by adding a few atoms to the NW junction.

  17. Quantifying point defects in Cu2ZnSn(S,Se)4 thin films using resonant x-ray diffraction

    NASA Astrophysics Data System (ADS)

    Stone, Kevin H.; Christensen, Steven T.; Harvey, Steven P.; Teeter, Glenn; Repins, Ingrid L.; Toney, Michael F.

    2016-10-01

    Cu2ZnSn(S,Se)4 is an interesting, earth abundant photovoltaic material, but has suffered from low open circuit voltage. To better understand the film structure, we have measured resonant x-ray diffraction across the Cu and Zn K-edges for the device quality thin films of Cu2ZnSnS4 (8.6% efficiency) and Cu2ZnSn(S,Se)4 (3.5% efficiency). This approach allows for the confirmation of the underlying kesterite structure and quantification of the concentration of point defects and vacancies on the Cu, Zn, and Sn sublattices. Rietveld refinement of powder diffraction data collected at multiple energies is used to determine that there exists a high level of CuZn and ZnCu defects on the 2c and 2d Wyckoff positions. We observe a significantly lower concentration of ZnSn defects and Cu or Zn vacancies.

  18. Point defect generation, nano-void formation and growth. II. Criterion for ductile failure

    NASA Astrophysics Data System (ADS)

    Saimoto, S.; Diak, B. J.; Lloyd, D. J.

    2012-05-01

    Using the derived relation for point defect generation according to a new constitutive relation, the notion of nano-void formation at grown-in nano-particles is examined and its consequences deduced as the nano-voids grow in size with continued deformation. Assuming that void growth is due only to point defect accumulation, the analysis of fracture strains in tension of natural-aged AA6111 suggests that coalescence by micro-plastic activity occurs when the void diameter becomes about one third of the evolving inter-void spacing. Hence, the derived limit strain to incipient void-coalescence is inversely proportional to the square root of point defect generation as determined from the stress-strain data. Using this criterion, failure prediction maps can be constructed for strain modes of plane-strain and balanced bi-axial to result in the outer bounds of the forming limit diagram. Trial examinations with AA5754 and AA3003 show great promise.

  19. Emerging Diluted Ferromagnetism in High‐T c Superconductors Driven by Point Defect Clusters

    PubMed Central

    Guzman, Roger.; Mishra, Rohan; Bartolomé, Elena; Salafranca, Juan; Magén, Cesar; Varela, Maria; Coll, Mariona; Palau, Anna; Valvidares, S. Manuel; Gargiani, Pierluigi; Pellegrin, Eric; Herrero‐Martin, Javier.; Pennycook, Stephen J.; Pantelides, Sokrates T.; Puig, Teresa; Obradors, Xavier

    2016-01-01

    Defects in ceramic materials are generally seen as detrimental to their functionality and applicability. Yet, in some complex oxides, defects present an opportunity to enhance some of their properties or even lead to the discovery of exciting physics, particularly in the presence of strong correlations. A paradigmatic case is the high‐temperature superconductor YBa2Cu3O7‐δ (Y123), in which nanoscale defects play an important role as they can immobilize quantized magnetic flux vortices. Here previously unforeseen point defects buried in Y123 thin films that lead to the formation of ferromagnetic clusters embedded within the superconductor are unveiled. Aberration‐corrected scanning transmission microscopy has been used for exploring, on a single unit‐cell level, the structure and chemistry resulting from these complex point defects, along with density functional theory calculations, for providing new insights about their nature including an unexpected defect‐driven ferromagnetism, and X‐ray magnetic circular dichroism for bearing evidence of Cu magnetic moments that align ferromagnetically even below the superconducting critical temperature to form a dilute system of magnetic clusters associated with the point defects. PMID:27812469

  20. New Insights into Intrinsic Point Defects in V2VI3 Thermoelectric Materials

    PubMed Central

    Hu, Lipeng; Zhao, Xinbing

    2016-01-01

    Defects and defect engineering are at the core of many regimes of material research, including the field of thermoelectric study. The 60‐year history of V2VI3 thermoelectric materials is a prime example of how a class of semiconductor material, considered mature several times, can be rejuvenated by better understanding and manipulation of defects. This review aims to provide a systematic account of the underexplored intrinsic point defects in V2VI3 compounds, with regard to (i) their formation and control, and (ii) their interplay with other types of defects towards higher thermoelectric performance. We herein present a convincing case that intrinsic point defects can be actively controlled by extrinsic doping and also via compositional, mechanical, and thermal control at various stages of material synthesis. An up‐to‐date understanding of intrinsic point defects in V2VI3 compounds is summarized in a (χ, r)‐model and applied to elucidating the donor‐like effect. These new insights not only enable more innovative defect engineering in other thermoelectric materials but also, in a broad context, contribute to rational defect design in advanced functional materials at large.

  1. First principles calculations of point defect diffusion in CdS buffer layers: Implications for Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4-based thin-film photovoltaics

    NASA Astrophysics Data System (ADS)

    Varley, J. B.; Lordi, V.; He, X.; Rockett, A.

    2016-01-01

    We investigate point defects in CdS buffer layers that may arise from intermixing with Cu(In,Ga)Se2 (CIGSe) or Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers in thin-film photovoltaics (PV). Using hybrid functional calculations, we characterize the migration barriers of Cu, In, Ga, Se, Sn, Zn, Na, and K impurities and assess the activation energies necessary for their diffusion into the bulk of the buffer. We find that Cu, In, and Ga are the most mobile defects in CIGS-derived impurities, with diffusion expected to proceed into the buffer via interstitial-hopping and cadmium vacancy-assisted mechanisms at temperatures ˜400 °C. Cu is predicted to strongly favor migration paths within the basal plane of the wurtzite CdS lattice, which may facilitate defect clustering and ultimately the formation of Cu-rich interfacial phases as observed by energy dispersive x-ray spectroscopic elemental maps in real PV devices. Se, Zn, and Sn defects are found to exhibit much larger activation energies and are not expected to diffuse within the CdS bulk at temperatures compatible with typical PV processing temperatures. Lastly, we find that Na interstitials are expected to exhibit slightly lower activation energies than K interstitials despite having a larger migration barrier. Still, we find both alkali species are expected to diffuse via an interstitially mediated mechanism at slightly higher temperatures than enable In, Ga, and Cu diffusion in the bulk. Our results indicate that processing temperatures in excess of ˜400 °C will lead to more interfacial intermixing with CdS buffer layers in CIGSe devices, and less so for CZTSSe absorbers where only Cu is expected to significantly diffuse into the buffer.

  2. Density functional theory study on the impact of heavy doping on Si intrinsic point defect properties and implications for single crystal growth from a melt

    NASA Astrophysics Data System (ADS)

    Sueoka, Koji; Kamiyama, Eiji; Vanhellemont, Jan

    2013-10-01

    Density functional theory (DFT) calculations are performed to obtain the formation energies of the vacancy V and the self-interstitial I at all sites within a sphere around the dopant atom with 6 Å radius for V and 5 Å radius for I in Si crystals. Substitutional p-type (B and Ga), neutral (C, Ge, and Sn), and n-type (P, As, Sb, and Bi) dopants were considered. The results show that the formation energies of V and I around dopant atoms change depending on the types and sizes of the dopants, i.e., depending on the electrical state and the local strain around the dopants. The dependence of the total thermal equilibrium concentrations of point defects (sum of free V or I and V or I around the dopants) at melting temperature on the type and concentration of each dopant is obtained. Further DFT calculations reveal that most of the total incorporated point defects from the melt contribute to pair recombination. An appropriate model of point defect behavior in heavily doped single crystal Si growing from a melt is proposed on the basis of DFT calculations. (1) The incorporated total V and I concentrations at melting point depend on the types and concentrations of dopants. (2) Most of the total V and I concentrations during Si crystal growth contribute to the pair recombination at temperatures much higher than those to form grown-in defects. The Voronkov model successfully explains all reported experimental results on intrinsic point defect behavior dependence on dopant type and concentration for heavily doped Si while taking the present model into consideration.

  3. Tuning upconversion through energy migration in core-shell nanoparticles.

    PubMed

    Wang, Feng; Deng, Renren; Wang, Juan; Wang, Qingxiao; Han, Yu; Zhu, Haomiao; Chen, Xueyuan; Liu, Xiaogang

    2011-10-23

    Photon upconversion is promising for applications such as biological imaging, data storage or solar cells. Here, we have investigated upconversion processes in a broad range of gadolinium-based nanoparticles of varying composition. We show that by rational design of a core-shell structure with a set of lanthanide ions incorporated into separated layers at precisely defined concentrations, efficient upconversion emission can be realized through gadolinium sublattice-mediated energy migration for a wide range of lanthanide activators without long-lived intermediary energy states. Furthermore, the use of the core-shell structure allows the elimination of deleterious cross-relaxation. This effect enables fine-tuning of upconversion emission through trapping of the migrating energy by the activators. Indeed, the findings described here suggest a general approach to constructing a new class of luminescent materials with tunable upconversion emissions by controlled manipulation of energy transfer within a nanoscopic region.

  4. Research Update: Point defects in CdTe{sub x}Se{sub 1−x} crystals grown from a Te-rich solution for applications in detecting radiation

    SciTech Connect

    Gul, R.; Roy, U. N.; Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A.; Yang, G.; James, R. B.; Lee, W.; Cui, Y.; Burger, A.

    2015-04-01

    We investigated cadmium telluride selenide (CdTeSe) crystals, newly grown by the Traveling Heater Method (THM), for the presence and abundance of point defects. Current Deep Level Transient spectroscopy (I-DLTS) was used to determine the energies of the traps, their capture cross sections, and densities. The bias across the detectors was varied from 1 to 30 V. Four types of point defects were identified, ranging from 10 meV to 0.35 eV. Two dominant traps at energies of 0.18 eV and 0.14 eV were studied in depth. Cd vacancies are found at lower concentrations than other point defects present in the material.

  5. Influence of point defects on grain boundary diffusion in oxides. [Annual report, July 1, 1990--March 1, 1992

    SciTech Connect

    Stubican, V.S.

    1991-03-15

    The influence of point defects on grain boundary diffusion of Co ions in NiO was studied using polycrystalline films and bicrystals. Grain boundary diffusion was studied at 750 C at oxygen partial pressure. Two diffusion regions were found. At low oxygen pressures, extrinsic diffusion was observed. Above oxygen pressure of 10{sup {minus}7}, influence of intrinsic point defects was detected. It was determined that grain boundary diffusion was > 3 orders of magnitude faster than volume diffusion. However, it seems that grain boundary diffusion is influenced by the point defects in a similar way as the volume diffusion. 4 figs.

  6. Self-regulation mechanism for charged point defects in hybrid halide perovskites

    DOE PAGESBeta

    Walsh, Aron; Scanlon, David O.; Chen, Shiyou; Gong, X. G.; Wei, Su -Huai

    2014-12-11

    Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) exhibit unusually low free-carrier concentrations despite being processed at low-temperatures from solution. We demonstrate, through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self-regulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature. Furthermore, this behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.

  7. Structural transformations of stacking fault tetrahedra upon the absorption of point defects

    NASA Astrophysics Data System (ADS)

    Poletaev, G. M.; Starostenkov, M. D.

    2009-01-01

    Mechanisms of the structural modification of stacking fault tetrahedra (SFTs) upon the absorption of point defects have been studied by the method of molecular dynamics. The sequential absorption of vacancies by a perfect SFT is accompanied by the following transformations: (i) the formation of a step on one of the SFT faces, (ii) a change in the step sign upon reaching the middle of the face, (iii) the formation of an SFT with truncated vertex, and (iv) the formation of the perfect SFT. Upon the absorption of interstitial atoms, the stages of SFT transformation follow the reverse order.

  8. Mode confinement in photonic quasicrystal point-defect cavities for particle accelerators

    NASA Astrophysics Data System (ADS)

    Di Gennaro, E.; Savo, S.; Andreone, A.; Galdi, V.; Castaldi, G.; Pierro, V.; Masullo, M. Rosaria

    2008-10-01

    In this letter, we present a study of the confinement properties of point-defect resonators in finite-size photonic-bandgap structures composed of aperiodic arrangements of dielectric rods, with special emphasis on their use for the design of cavities for particle accelerators. Specifically, for representative geometries, we study the properties of the fundamental mode (as a function of the filling fraction, structure size, and losses) via two-dimensional and three-dimensional full-wave numerical simulations, as well as microwave measurements at room temperature. Results indicate that for reduced-size structures, aperiodic geometries exhibit superior confinement properties by comparison with periodic ones.

  9. Periodic surface structure bifurcation induced by ultrafast laser generated point defect diffusion in GaAs

    NASA Astrophysics Data System (ADS)

    Abere, Michael J.; Torralva, Ben; Yalisove, Steven M.

    2016-04-01

    The formation of high spatial frequency laser induced periodic surface structures (HSFL) with period <0.3 λ in GaAs after irradiation with femtosecond laser pulses in air is studied. We have identified a point defect generation mechanism that operates in a specific range of fluences in semiconductors between the band-gap closure and ultrafast-melt thresholds that produces vacancy/interstitial pairs. Stress relaxation, via diffusing defects, forms the 350-400 nm tall and ˜90 nm wide structures through a bifurcation process of lower spatial frequency surface structures. The resulting HSFL are predominately epitaxial single crystals and retain the original GaAs stoichiometry.

  10. Kinetic model for electric-field induced point defect redistribution near semiconductor surfaces

    SciTech Connect

    Gorai, Prashun; Seebauer, Edmund G.

    2014-07-14

    The spatial distribution of point defects near semiconductor surfaces affects the efficiency of devices. Near-surface band bending generates electric fields that influence the spatial redistribution of charged mobile defects that exchange infrequently with the lattice, as recently demonstrated for pile-up of isotopic oxygen near rutile TiO{sub 2} (110). The present work derives a mathematical model to describe such redistribution and establishes its temporal dependence on defect injection rate and band bending. The model shows that band bending of only a few meV induces significant redistribution, and that the direction of the electric field governs formation of either a valley or a pile-up.

  11. First-principles study of point defects at a semicoherent interface

    SciTech Connect

    Metsanurk, E.; Tamm, A.; Caro, A.; Aabloo, A.; Klintenberg, M.

    2014-12-19

    Most of the atomistic modeling of semicoherent metal-metal interfaces has so far been based on the use of semiempirical interatomic potentials. Here, we show that key conclusions drawn from previous studies are in contradiction with more precise ab-initio calculations. In particular we find that single point defects do not delocalize, but remain compact near the interfacial plane in Cu-Nb multilayers. Lastly, we give a simple qualitative explanation for this difference on the basis of the well known limited transferability of empirical potentials.

  12. The impact of mobile point defect clusters in a kinetic model of pressure vessel embrittlement

    SciTech Connect

    Stoller, R.E.

    1998-05-01

    The results of recent molecular dynamics simulations of displacement cascades in iron indicate that small interstitial clusters may have a very low activation energy for migration, and that their migration is 1-dimensional, rather than 3-dimensional. The mobility of these clusters can have a significant impact on the predictions of radiation damage models, particularly at the relatively low temperatures typical of commercial, light water reactor pressure vessels (RPV) and other out-of-core components. A previously-developed kinetic model used to investigate RPV embrittlement has been modified to permit an evaluation of the mobile interstitial clusters. Sink strengths appropriate to both 1- and 3-dimensional motion of the clusters were evaluated. High cluster mobility leads to a reduction in the amount of predicted embrittlement due to interstitial clusters since they are lost to sinks rather than building up in the microstructure. The sensitivity of the predictions to displacement rate also increases. The magnitude of this effect is somewhat reduced if the migration is 1-dimensional since the corresponding sink strengths are lower than those for 3-dimensional diffusion. The cluster mobility can also affect the evolution of copper-rich precipitates in the model since the radiation-enhanced diffusion coefficient increases due to the lower interstitial cluster sink strength. The overall impact of the modifications to the model is discussed in terms of the major irradiation variables and material parameter uncertainties.

  13. Review of Calculations on Point Defect Properties in Delta-Pu

    SciTech Connect

    Allen, P. G.; Wolfer, W. G.

    2015-09-08

    The results of theoretical predictions of properties for vacancies and self-interstitial atoms (SIA) in δ-Pu are presented and reviewed. Three methods have been used for these predictions, namely the modified embedded atom method (MEAM), density functional theory (DFT) with and without spin polarization, and continuum mechanics (CM) models adapted to plutonium. The properties considered are formation and migration energies, and relaxation volumes of vacancies and SIA. Predicted values vary considerably. Nevertheless, all three methods predict that the activation energy for self-diffusion by vacancies is of similar magnitude as the SIA formation energy. Furthermore, the absolute magnitudes of relaxation volumes for vacancies and SIA are also similar, indicating that there exist no large bias for radiation-induced void swelling.

  14. Optical properties of phosphorus-related point defects in silica fiber preforms

    SciTech Connect

    Origlio, Giusy; Messina, Fabrizio; Cannas, Marco; Boscaino, Roberto; Girard, Sylvain; Boukenter, Aziz; Ouerdane, Youcef

    2009-11-15

    We report an experimental study on phosphorus-related point defects in amorphous silica, based on photoluminescence, absorption, and electron spin resonance measurements carried out on P-doped SiO{sub 2} fiber preforms. By photoluminescence measurements excited by laser or synchrotron light we detect an emission band peaked at 3.0 eV with a lifetime in the range of ms. The excitation spectrum of the 3.0 eV emission consists of two transitions peaked at 4.8 and 6.4 eV, the former giving rise also to a measurable absorption band. We attribute this optical activity to a P-related point defect embedded in SiO{sub 2}, based on the spatial correlation between the emission intensity and the P doping level. A detailed spectroscopical investigation allows us to propose a scheme of the electronic levels of this P-related defect, in which the 4.8 and 6.4 eV excitation channels arise from transitions from the ground to two-excited singlet states, while the long-lived 3.0 eV emission is associated to a spin-forbidden transition from an excited triplet to the ground state. Finally, electron spin resonance measurements on X-irradiated samples lead us to propose a tentative microscopic model of the defect as a diamagnetic four-coordinated P impurity substitutional to a Si atom.

  15. Calculation of the High-Temperature Point Defects Structure in Te-Rich CdTe

    NASA Astrophysics Data System (ADS)

    Dai, Shujun; Wang, Tao; Liu, Huimin; He, Yihui; Jie, Wanqi

    2016-06-01

    A thermodynamic equilibrium model for CdTe annealed under Te vapor is established, in which possible point defects and a defect reaction existing in undoped and In-doped Te-rich CdTe crystals are taken into consideration. Independent point defects, such as VCd, Cdi, and Tei, as well as defect complexes, namely TeCd-VCd (B complex), {{Te}}_{{Cd}}^{2 + } - {{V}}_{{Cd}}^{2 - } (D complex), {{In}}_{{Cd}}^{ + } - {{V}}_{{Cd}}^{ - } (A-center) and Tei-VCd (TeCd), are discussed based on the defect chemistry theory. More specially, the mass action law and quasi-chemical equations are used to calculate defects concentration and Fermi level in undoped and doped CdTe crystals with different indium concentrations. It is found that the Fermi level is controlled by a {{V}}_{{Cd}}^{2 - } , TeCd, and B/D-complex in undoped crystal. The concentration of VCd drops down in an obvious manner and that of TeCd rises for doped crystal with increasing [In].

  16. In-situ electron paramagnetic resonance studies of paramagnetic point defects in superconducting microwave resonators

    NASA Astrophysics Data System (ADS)

    Zhang, Shengke; Kopas, Cameron; Wagner, Brian; Queen, Daniel; Newman, N.

    2016-09-01

    The physical nature and concentration of paramagnetic point defects in the dielectrics of superconducting planar microwave resonators have been determined using in-situ electron paramagnetic resonance spectroscopy. To perform this work, the quality factor of parallel plate and stripline resonators was measured as a function of the magnitude of a magnetic-field applied parallel to the electrode surfaces. YBa2Cu3O7-δ thin film electrodes proved to be a preferred choice over Nb and MgB2 because they are readily available and have a small surface resistance (Rs) up to high temperatures (˜77 K) and magnetic fields (i.e., <1 T). Stripline resonators with a widely used high performance microwave dielectric, Co2+-doped Ba(Zn1/3Nb2/3)O3, are shown to have losses dominated by d-electron spin-excitations in exchange-coupled Co2+ point-defect clusters, even in the absence of an applied magnetic field. A significant enhanced microwave loss in stripline and parallel plate resonators is found to correlate with the presence of paramagnetic Mn2+ dopants in Ba(Zn1/3Ta2/3)O3 ceramics and dangling bond states in amorphous Si thin films, although the identification of the dominant loss mechanism(s) in these dielectrics requires further investigation.

  17. Calculation of the High-Temperature Point Defects Structure in Te-Rich CdTe

    NASA Astrophysics Data System (ADS)

    Dai, Shujun; Wang, Tao; Liu, Huimin; He, Yihui; Jie, Wanqi

    2016-10-01

    A thermodynamic equilibrium model for CdTe annealed under Te vapor is established, in which possible point defects and a defect reaction existing in undoped and In-doped Te-rich CdTe crystals are taken into consideration. Independent point defects, such as VCd, Cdi, and Tei, as well as defect complexes, namely TeCd-VCd (B complex), {Te}_{{Cd}}^{2 + } - {V}_{{Cd}}^{2 - } (D complex), {In}_{{Cd}}^{ + } - {V}_{{Cd}}^{ - } (A-center) and Tei-VCd (TeCd), are discussed based on the defect chemistry theory. More specially, the mass action law and quasi-chemical equations are used to calculate defects concentration and Fermi level in undoped and doped CdTe crystals with different indium concentrations. It is found that the Fermi level is controlled by a {V}_{{Cd}}^{2 - } , TeCd, and B/D-complex in undoped crystal. The concentration of VCd drops down in an obvious manner and that of TeCd rises for doped crystal with increasing [In].

  18. Point defects in YBa2Cu3O7-x studied using positron annihilation

    NASA Astrophysics Data System (ADS)

    Chudy, Michal; Eisterer, M.; Weber, H. W.; Veterníková, J.; Sojak, S.; Slugeň, V.

    2012-07-01

    Fast neutron irradiation is a powerful technique for introducing additional pinning centers into high temperature superconductors. The spherical defects with sizes of a few nanometers are considered to be effective pinning centers, enhancing Jc. Their morphology is well-known and has already been investigated by several authors in great detail. However, only very little is known about the nature and density of smaller and point defects, which are invisible in transmission electron microscopy. Positron annihilation lifetime spectroscopy was applied to investigate the nature and the concentration of small point-like defects. In this work, the influence of small point defects, such as vacancies and vacancy clusters, on the superconducting properties of YBa2Cu3O7-x bulks was studied; these were introduced by irradiation in the TRIGA Mark II reactor in Vienna. Jc and Tc measurements were performed prior to and after each irradiation step. The samples were irradiated up to a fast neutron ( > 0.1 MeV) fluence of 6 × 1021 m-2. The two kinds of defects—the large collision cascades and the small point-like defects—contribute to the decrease of Tc as well as to the Jc enhancement in astonishingly similar ways.

  19. Confining energy migration in upconversion nanoparticles towards deep ultraviolet lasing

    NASA Astrophysics Data System (ADS)

    Chen, Xian; Jin, Limin; Kong, Wei; Sun, Tianying; Zhang, Wenfei; Liu, Xinhong; Fan, Jun; Yu, Siu Fung; Wang, Feng

    2016-01-01

    Manipulating particle size is a powerful means of creating unprecedented optical properties in metals and semiconductors. Here we report an insulator system composed of NaYbF4:Tm in which size effect can be harnessed to enhance multiphoton upconversion. Our mechanistic investigations suggest that the phenomenon stems from spatial confinement of energy migration in nanosized structures. We show that confining energy migration constitutes a general and versatile strategy to manipulating multiphoton upconversion, demonstrating an efficient five-photon upconversion emission of Tm3+ in a stoichiometric Yb lattice without suffering from concentration quenching. The high emission intensity is unambiguously substantiated by realizing room-temperature lasing emission at around 311 nm after 980-nm pumping, recording an optical gain two orders of magnitude larger than that of a conventional Yb/Tm-based system operating at 650 nm. Our findings thus highlight the viability of realizing diode-pumped lasing in deep ultraviolet regime for various practical applications.

  20. Investigation of the effect of shock-wave propagation in metals on point defect production using Thomas-Fermi-Dirac microscopical model

    NASA Astrophysics Data System (ADS)

    Ryazanov, A. I.; Pavlov, S. A.; Kiritani, M.

    2002-01-01

    The process of high-speed deformation in metals is considered as a propagation of shock-wave with some velocity. The description of this phenomenon is based on microscopic Thomas-Fermi-Dirac and so called "jelly" model of metal with degenerated electron gas and cold ionic subsystem, This model allows to investigate the stationary highspeed deformation structure in metals on microscopic level as a propagation of shock-wave in cold plasma. It is shown that during the shock wave propagation in metals in the shock wave front an effective electrical field is formed which tends to come back the displaced ions to initial equilibrium positions in crystal lattice. This electrostatic energy plays the role of potential energy of ions. The effective electrical field profile and kinetic energy profile for moved ions formed during the short time of shock wave propagation have been obtained. The developed theoretical model allows to suggest a physical mechanism for the explanation of formation of displaced atoms from nodal points during high speed deformation. When the kinetic energy of ions in shock wave front overcomes the threshold displacement energy (the sum of potential energy of ions in electrical field and energy for Frenkel pair formation) the point defects are produced. The obtained results are used to analyze the point defect formation in shock wave front moved with the different velocities.

  1. Hybrid Hamiltonian and Green's Function Approach for Studying Native Point Defect Levels in Semiconductor Compounds and Superlattices

    NASA Astrophysics Data System (ADS)

    Krishnamurthy, Srini; Van Orden, Derek; Yu, Zhi-Gang

    2016-09-01

    We have developed a hybrid method that can be applied to study isolated defects in semiconductor compounds and superlattices. The method is a combination of (1) a long-range tight-binding (TB) Hamiltonian, (2) a first-principles Hamiltonian, and (3) a Green's function (GF) formalism. The calculation of the GF requires accurate energy band structure, wave functions, and defect potentials. The TB Hamiltonian with sp 3 orbitals basis ensures accurate band gaps and band masses while providing the functional form for the wave functions. We calculated the band gaps of InAs/GaSb and InAs/InAsSb strained-layer superlattices and found them to agree well with measurements. The change in potentials caused by native point defects (NPDs) was obtained from a first-principles method using Spanish Initiative for Electronic Simulations with Thousands of Atoms, which also uses sp 3 basis. We describe the method of calculating NPD energy levels in compounds and superlattices, obtain some defect levels in GaAs, InAs, InSb, and GaSb compounds, and provide details of the NPD-level calculations.

  2. Polarization reduction in half-metallic Heusler alloys: the effect of point defects and interfaces with semiconductors.

    PubMed

    Picozzi, Silvia; Freeman, Arthur J

    2007-08-01

    Half-metallic full-Heusler alloys represent a promising class of materials for spintronic applications. However, (i) intrinsic point defects in Heusler compounds can be detrimental with respect to their predicted 100% spin polarization at the Fermi level and (ii) when joined to mainstream semiconductors the presence of interface states-which destroys half-metallicity-can degrade their performance. Here, we present an overview of recent first-principles calculations performed to explore both these issues. In particular, we focus on ab initio FLAPW calculations performed for Co(2)MnGe and Co(2)MnSi in the presence of intrinsic defects (such as stoichiometric atomic swaps as well as non-stoichiometric antisites) and when interfaced with GaAs and Ge. Our findings show that Mn antisites, due to their low formation energies, can easily occur, in excellent consistency with experimental observations, and that they do not destroy half-metallicity. On the other hand, Co antisites, which also show a modest formation energy, give rise to defect states at the Fermi level. As for the [001]-ordered interfaces, we show that the strong hybridization in proximity to the junction gives rise to rather broad interface states that locally destroy half-metallicity. However, the bulk gaps (both in the minority spin channel for the Heusler alloy and for both spin channels in the semiconducting side) are fully recovered within a few layers away from the junction.

  3. The role of point defects and defect complexes in silicon device processing. Summary report and papers

    SciTech Connect

    Sopori, B.; Tan, T.Y.

    1994-08-01

    This report is a summary of a workshop hold on August 24--26, 1992. Session 1 of the conference discussed characteristics of various commercial photovoltaic silicon substrates, the nature of impurities and defects in them, and how they are related to the material growth. Session 2 on point defects reviewed the capabilities of theoretical approaches to determine equilibrium structure of defects in the silicon lattice arising from transitional metal impurities and hydrogen. Session 3 was devoted to a discussion of the surface photovoltaic method for characterizing bulk wafer lifetimes, and to detailed studies on the effectiveness of various gettering operations on reducing the deleterious effects of transition metals. Papers presented at the conference are also included in this summary report.

  4. First-principles calculations for point defects in MAX phases Ti2AlN

    NASA Astrophysics Data System (ADS)

    Zhang, Yaowen; Yang, Shutong; Wang, Canglong

    2016-04-01

    This paper outlines general physical issues associated with performing computational numerical simulations of primary point defects in MAX phases Ti2AlN. First-principles solutions are possible due to the development of computational resources of software and hardware. The calculation accuracy is a good agreement with the experimental results. As an important application of our simulations, the results could provide a theoretical guidance for future experiments and application of Ti2AlN. For example, the N mono-vacancy is the most difficult to form. On the contrary, the mono-vacancy formation in Ti2AlN is energetically most favorable for the Al atom. The essence of the phenomena is explained by the calculated density of state (DOS).

  5. Synergetic effects of Mn and Si in the interaction with point defects in bcc Fe

    NASA Astrophysics Data System (ADS)

    Bakaev, A.; Terentyev, D.; He, X.; Van Neck, D.

    2014-12-01

    The interaction of Mn, Si and Cr with a vacancy and self-interstitial defects in BCC Fe has been analyzed using ab initio calculations. While the interaction of the considered solute clusters with a single vacancy is linearly additive, there is a considerable synergetic effect in the case of self-interstitial atoms, found to bind strongly with Mn-Si pairs. The latter therefore act as deep trapping configurations for self-interstitials. At the same time, the presence of the point defects nearby weakly attractive Mn-Si pairs significantly enhances the solute-solute binding. The revealed effects are rationalized on the basis of charge density and local magnetic moment distributions.

  6. Controlled excitation of electromagnetic band-gap line and point defect modes at microwave frequencies

    NASA Astrophysics Data System (ADS)

    Schuster, M.; Klein, N.

    2003-03-01

    We report on the controlled excitation of line and point defect modes in a two-dimensional hexagonal electromagnetic band-gap structure made of rods of dielectric material (aluminium oxide). We compared simulation performed with a numerical field simulation software and experimental measurements at microwave frequencies with regard to coupling from external waveguides to line defects and subsequent coupling to resonant modes. We observed that for a line defect in the photonic crystal the impedance matching to a waveguide is strongly dependent on the defect width. We furthermore demonstrated that the coupling to a localized defect resonance can be strongly influenced by the variation of certain single lattice elements, affecting transmission behavior and quality factor of the resonant modes.

  7. Use of the point defect model to interpret the iron oxidation kinetics under proton irradiation

    SciTech Connect

    Lapuerta, S.; Moncoffre, N.; Jaffrezic, H.; Millard-Pinard, N.; Bererd, N.; Esnouf, C.; Crusset, D.

    2007-03-15

    This article concerns the study of iron corrosion in wet air under mega-electron-volt proton irradiation for different fluxes at room temperature and with a relative humidity fixed to 45%. Oxidized iron sample surfaces are characterized by ion beam analysis (Rutherford backscattering spectrometry and elastic recoil detection analysis), for the elemental analysis. The structural and physicochemical characterization is performed using the x-ray photoelectron spectroscopy and transmission electron microscopy techniques. We have also measured the iron oxidation kinetics. Radiation enhanced diffusion and transport processes have been evidenced. The modeling of the experimental data shows that the apparent oxygen diffusion coefficient increases whereas the oxygen transport velocity decreases as function of flux. Finally, the point defect model has been used to determine the electric field value in the samples. Results have shown that the transport process can be attributed to the presence of an electrical potential gradient.

  8. Point defect-induced magnetic properties in CuAlO2 films without magnetic impurities

    NASA Astrophysics Data System (ADS)

    Luo, Jie; Lin, Yow-Jon

    2016-03-01

    The magnetic properties of the undoped CuAlO2 thin films with different compositions are examined. In order to understand this phenomenon and to determine the correlation between the magnetic and electrical properties and point defects, the X-ray photoelectron spectroscopy and Hall effect measurements are performed. Combining with Hall effect, X-ray photoelectron spectroscopy and alternating gradient magnetometer measurements, a direct link between the hole concentration, magnetism, copper vacancy (VCu), oxygen vacancy, and interstitial oxygen (Oi) is established. It is shown that an increase in the number of acceptors (VCu and Oi) leads to an increase in the hole concentration. Based on theoretical and experimental investigations, the authors confirmed that both acceptors (VCu and Oi) in CuAlO2 could induce the ferromagnetic behavior at room temperature.

  9. Self-regulation mechanism for charged point defects in hybrid halide perovskites

    SciTech Connect

    Walsh, Aron; Scanlon, David O.; Chen, Shiyou; Gong, X. G.; Wei, Su -Huai

    2014-12-11

    Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) exhibit unusually low free-carrier concentrations despite being processed at low-temperatures from solution. We demonstrate, through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self-regulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature. Furthermore, this behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.

  10. The interaction of HVEM-generated point-defects with dissociated dislocations

    SciTech Connect

    King, S.L.; Jenkins, M.L. . Dept. of Materials); Kirk, M.A. ); English, C.A. . Materials and Chemistry)

    1991-05-01

    This paper describes experiments we have performed to investigate the mechanisms of interaction of IIVEM-generated point-defects with dissociated dislocations in a series of austenitic Fe-Ni-Cr alloys and review earlier work in Cu-Al alloys and in Ag. In the Fe-Ni-Cr alloys interstitial climb was observed only at favorable sites such as pre-existing jogs, while vacancies clustered near dislocations to form stacking-fault tetrahedra. These observations are similar to those in Ag; the complex climb mechanisms seen in Cu-Al alloys were not found. The differences between materials is believed to be due to differences in the case of interstitial pipe diffusion. 28 refs., 9 figs.

  11. Fourth Workshop on the Role of Point Defects and Defect Complexes in Silicon Device Processing

    NASA Astrophysics Data System (ADS)

    Tan, T.; Jastrzebski, L.; Sopori, B.

    1994-07-01

    The 4th Point Defect Workshop was aimed at reviewing recent new understanding of the defect engineering techniques that can improve the performance of solar cells fabricated on low-cost silicon substrates. The theme of the workshop was to identify approaches that can lead to 18 percent commercial silicon solar cells in the near future. These approaches also define the research tasks for the forthcoming new DOE/NREL silicon materials research program. It was a consensus of the workshop attendees that the goal of 18 percent-efficient multicrystalline silicon solar cells is right on target, and the payoff for the investment by DOE will manifest itself in the next few years as reduced costs for high-efficiency cell fabrication.

  12. Point defect concentrations and solid solution hardening in NiAl with Fe additions

    SciTech Connect

    Pike, L.M.; Chang, Y.A.; Liu, C.T.

    1997-08-01

    The solid solution hardening behavior exhibited when Fe is added to NiAl is investigated. This is an interesting problem to consider since the ternary Fe additions may choose to occupy either the Ni or the Al sublattice, affecting the hardness at differing rates. Moreover, the addition of Fe may affect the concentrations of other point defects such as vacancies and Ni anti-sites. As a result, unusual effects ranging from rapid hardening to solid solution softening are observed. Alloys with varying amounts of Fe were prepared in Ni-rich (40 at. % Al) and stoichiometric (50 at. % Al) compositions. Vacancy concentrations were measured using lattice parameter and density measurements. The site occupancy of Fe was determined using ALCHEMI. Using these two techniques the site occupancies of all species could be uniquely determined. Significant differences in the defect concentrations as well as the hardening behavior were encountered between the Ni-rich and stoichiometric regimes.

  13. Electronic Band Structures and Native Point Defects of Ultrafine ZnO Nanocrystals.

    PubMed

    Zeng, Yu-Jia; Schouteden, Koen; Amini, Mozhgan N; Ruan, Shuang-Chen; Lu, Yang-Fan; Ye, Zhi-Zhen; Partoens, Bart; Lamoen, Dirk; Van Haesendonck, Chris

    2015-05-20

    Ultrafine ZnO nanocrystals with a thickness down to 0.25 nm are grown by a metalorganic chemical vapor deposition method. Electronic band structures and native point defects of ZnO nanocrystals are studied by a combination of scanning tunneling microscopy/spectroscopy and first-principles density functional theory calculations. Below a critical thickness of ∼1 nm ZnO adopts a graphitic-like structure and exhibits a wide band gap similar to its wurtzite counterpart. The hexagonal wurtzite structure, with a well-developed band gap evident from scanning tunneling spectroscopy, is established for a thickness starting from ∼1.4 nm. With further increase of the thickness to 2 nm, VO-VZn defect pairs are easily produced in ZnO nanocrystals due to the self-compensation effect in highly doped semiconductors. PMID:25923131

  14. Electron spin resonance probing of fundamental point defects in nanometer-sized silica particles

    NASA Astrophysics Data System (ADS)

    Stesmans, A.; Clémer, K.; Afanas'Ev, V. V.

    2005-10-01

    Point defects in fumed ˜7-nm -sized silica nanoparticles have been studied by K - and Q -band electron spin resonance (ESR) following 10-eV irradiation used to photodissociate H from passivated defects. Various types of ESR-active point defects are revealed, including the familiar E' center (generic entity •SiO3 ), EX, the peroxy radical (POR), the methyl radical, and an unknown closely axially symmetric center ( g‖=2.0041 , g⊥=2.0027 ). The possible atomic nature of the latter is addressed. The E' defects, occurring in a maximum density of ˜1×10-3 per nanoparticle, are monitored as a function of thermal treatment in vacuum in the range 850-1115°C in order to assess specific physicochemical structural aspects of the particles. Experimental evidence is presented for the presence of two different systems of E' centers. The specific ESR parameters of the E' centers of one bath are found to be very similar to those of the E'γ center in bulk fused silica, while the second bath exhibits a different zero crossing g value and line shape, attributed to variations in local structure. It is inferred that the latter E' system pertains to the outer SiO2 layers, exposing a structural nature different from bulk glassy SiO2 . Besides PORs, large numbers of other oxygen-hole type defects appear to be present also. The exhaustive number of all oxygen-hole centers, including the PORs, is determined at ˜0.07 defects/nanoparticle, making this kind of defect highly unlikely as playing a substantial role in narrowing of the optical bandgap, in contrast with previous suggestion.

  15. Ab initio study of point defects in PbSe and PbTe: Bulk and nanowire

    SciTech Connect

    Wrasse, E. O.; Venezuela, P.; Baierle, R. J.

    2014-11-14

    First principles investigations, within the spin-polarized density functional theory, are performed to study energetic stability and electronic properties of point defects (vacancies and antisites) in PbSe and PbTe: bulk and nanowire (NW). Our results show that the energetic stability of these defects is ruled by relaxation process. These defects have lower formation energies in the nanowire structures as compared to the bulk, being more stable in the surface of the NWs. We also show that in the bulk system only one charge state is stable, otherwise, due to the larger band gaps, more than one charge state may be stable in the NWs. In addition, we have investigated how the presence of intrinsic defects affects the electronic properties of bulk and NW systems. Vacancies give rise to new electronic states near to the edges of the valence and conduction bands while the energetic position of the electronic states from antisites depends on the charge state, being localized inside the band gap or near the edges of the valence or conduction bands. We discuss how these changes in the electronic properties due to intrinsic defects may affect the thermoelectric properties of PbSe and PbTe NWs.

  16. Magnetic properties of point defect interaction with impurity atoms in Fe-Cr alloys

    NASA Astrophysics Data System (ADS)

    Nguyen-Manh, D.; Lavrentiev, M. Yu.; Dudarev, S. L.

    2009-04-01

    An integrated ab initio and statistical Monte Carlo investigation has been recently carried out to model the thermodynamic and kinetic properties of Fe-Cr alloys. We found that the conventional Fe-Cr phase diagram is not adequate at low temperature region where the magnetic contribution to the free energy plays an important role in the prediction of an ordered Fe 15Cr phase and its negative enthalpy of formation. The origin of the anomalous thermodynamic and magnetic properties of Fe-Cr alloys can be understood using a tight-binding Stoner model combined with the charge neutrality condition. We investigate the environmental dependence of magnetic moment distributions for various self-interstitial atom <1 1 0> dumbbells configurations using spin density maps found using density functional theory calculations. The mixed dumbbell Fe-Cr and Fe-Mn binding energies are found to be positive due to magnetic interactions. Finally, we discuss the relationship between the migration energy of vacancy in Fe-Cr alloys and magnetism at the saddle point configuration.

  17. Oxygen Point Defect Chemistry in Ruddlesden-Popper Oxides (La1-xSrx)2MO4±δ (M = Co, Ni, Cu).

    PubMed

    Xie, Wei; Lee, Yueh-Lin; Shao-Horn, Yang; Morgan, Dane

    2016-05-19

    Stability of oxygen point defects in Ruddlesden-Popper oxides (La1-xSrx)2MO4±δ (M = Co, Ni, Cu) is studied with density functional theory calculations to determine their stable sites, charge states, and energetics as functions of Sr content (x), transition metal (M), and defect concentration (δ). We demonstrate that the dominant O point defects can change between oxide interstitials, peroxide interstitials, and vacancies. In general, increasing x and atomic number of M stabilizes peroxide over oxide interstitials as well as vacancies over both peroxide and oxide interstitials; increasing δ destabilizes both oxide interstitials and vacancies but barely affects peroxide interstitials. We also demonstrate that the O 2p-band center is a powerful descriptor for these materials and correlates linearly with the formation energy of all defects. The trends of formation energy versus x, M, and δ and the correlation with O 2p-band center are explained in terms of oxidation chemistry and electronic structure.

  18. First principles studies on the impact of point defects on the phase stability of (AlxCr1-x)2O3 solid solutions

    NASA Astrophysics Data System (ADS)

    Koller, C. M.; Koutná, N.; Ramm, J.; Kolozsvári, S.; Paulitsch, J.; Holec, D.; Mayrhofer, P. H.

    2016-02-01

    Density Functional Theory applying the generalised gradient approximation is used to study the phase stability of (AlxCr1-x)2O3 solid solutions in the context of physical vapour deposition (PVD). Our results show that the energy of formation for the hexagonal α phase is lower than for the metastable cubic γ and B1-like phases-independent of the Al content x. Even though this suggests higher stability of the α phase, its synthesis by physical vapour deposition is difficult for temperatures below 800 °C. Aluminium oxide and Al-rich oxides typically exhibit a multi-phased, cubic-dominated structure. Using a model system of (Al0.69Cr0.31)2O3 which experimentally yields larger fractions of the desired hexagonal α phase, we show that point defects strongly influence the energetic relationships. Since defects and in particular point defects, are unavoidably present in PVD coatings, they are important factors and can strongly influence the stability regions. We explicitly show that defects with low formation energies (e.g. metal Frenkel pairs) are strongly preferred in the cubic phases, hence a reasonable factor contributing to the observed thermodynamically anomalous phase composition.

  19. Confining energy migration in upconversion nanoparticles towards deep ultraviolet lasing

    PubMed Central

    Chen, Xian; Jin, Limin; Kong, Wei; Sun, Tianying; Zhang, Wenfei; Liu, Xinhong; Fan, Jun; Yu, Siu Fung; Wang, Feng

    2016-01-01

    Manipulating particle size is a powerful means of creating unprecedented optical properties in metals and semiconductors. Here we report an insulator system composed of NaYbF4:Tm in which size effect can be harnessed to enhance multiphoton upconversion. Our mechanistic investigations suggest that the phenomenon stems from spatial confinement of energy migration in nanosized structures. We show that confining energy migration constitutes a general and versatile strategy to manipulating multiphoton upconversion, demonstrating an efficient five-photon upconversion emission of Tm3+ in a stoichiometric Yb lattice without suffering from concentration quenching. The high emission intensity is unambiguously substantiated by realizing room-temperature lasing emission at around 311 nm after 980-nm pumping, recording an optical gain two orders of magnitude larger than that of a conventional Yb/Tm-based system operating at 650 nm. Our findings thus highlight the viability of realizing diode-pumped lasing in deep ultraviolet regime for various practical applications. PMID:26739352

  20. Role of point defects and additives in kinetics of hydrogen storage materials

    NASA Astrophysics Data System (ADS)

    van de Walle, Chris

    2010-03-01

    First-principles computational studies of hydrogen interactions with storage materials can provide direct insight into the processes of H uptake and release, and may help in developing guidelines for designing storage media with improved storage capacity and kinetics. One important conclusion is that the defects involved in kinetics of semiconducting or insulating H-storage materials are charged, and hence their formation energy is Fermi-level dependent and can be affected by the presence of impurities that change the Fermi level [1,2]. This provides an explanation for the role played by transition-metal impurities in the kinetics of NaAlH4 and related materials. Desorption of H and decomposition of NaAlH4 requires not only mass transport of H but also of Al and/or Na. This process is mediated by native defects. We have investigated the structure, stability, and migration enthalpy of native defects based on density functional theory. The results allow us to estimate diffusion activation energies for the defects that may be involved in mass transport. Most of the relevant defects exist in charge states other than neutral, and consideration of these charge states is essential for a proper description of kinetics. We propose specific new mechanisms to explain the observed activation energies and their dependence on the presence of impurities. We have also expanded our studies to materials other than NaAlH4. In the case of LiBH4 and Li4BN3H10 we have found that the calculations have predictive power in terms of identifying which impurities will actually enhance kinetics. Other complex hydrides that we are currently investigating include Li2NH and LiNH2. [4pt] [1] A. Peles and C. G. Van de Walle, Phys. Rev. B 76, 214101 (2007). [0pt] [2] C. G. Van de Walle, A. Peles, A. Janotti, and G. B. Wilson-Short, Physica B 404, 793 (2009).

  1. Effect of intrinsic point defects on copper precipitation in large-diameter Czochralski silicon

    NASA Astrophysics Data System (ADS)

    Xi, Zhenqiang; Yang, Deren; Xu, Jin; Ji, Yujie; Que, Duanlin; Moeller, H. J.

    2003-10-01

    Effect of intrinsic point defects on copper precipitation in large-diameter Czochralski silicon annealed at 1100 °C under air cooling was studied by means of scanning infrared microscopy (SIRM), optical microscopy (OM), and transmission electron microscopy (TEM). The SIRM images showed that, in the A-defect zone of the Cu-contaminated silicon wafers, the copper-precipitate colonies with larger size were observed, while in the D-defect zone almost no copper precipitates could be observed. However, the OM results revealed that the density of etching pits in the D-defect zone was higher than that in the A-defect zone, indicating that the copper precipitates with smaller size and higher density formed in the D-defect zone. The TEM investigation showed that the size of copper precipitate colonies in the A-defect zone was about 300 nm, while that in the D-defect zone was about 50 nm. It is considered that as-grown vacancies in the D-defect zone enhanced the nucleation of copper precipitates but hindered their growth, whereas the role of as-grown interstitial silicon on copper precipitation was inverse.

  2. Effects of extrinsic point defects in phosphorene: B, C, N, O, and F adatoms

    SciTech Connect

    Wang, Gaoxue E-mail: pandey@mtu.edu Pandey, Ravindra E-mail: pandey@mtu.edu; Karna, Shashi P. E-mail: pandey@mtu.edu

    2015-04-27

    Phosphorene is emerging as a promising 2D semiconducting material with a direct band gap and high carrier mobility. In this paper, we examine the role of the extrinsic point defects including surface adatoms in modifying the electronic properties of phosphorene using density functional theory. The surface adatoms considered are B, C, N, O, and F with a [He] core electronic configuration. Our calculations show that B and C, with electronegativity close to P, prefer to break the sp{sup 3} bonds of phosphorene and reside at the interstitial sites in the 2D lattice by forming sp{sup 2} like bonds with the native atoms. On the other hand, N, O, and F, which are more electronegative than P, prefer the surface sites by attracting the lone pairs of phosphorene. B, N, and F adsorption will also introduce local magnetic moment to the lattice. Moreover, B, C, N, and F adatoms will modify the band gap of phosphorene, yielding metallic transverse tunneling characters. Oxygen does not modify the band gap of phosphorene, and a diode like tunneling behavior is observed. Our results therefore offer a possible route to tailor the electronic and magnetic properties of phosphorene by the adatom functionalization and provide the physical insights of the environmental sensitivity of phosphorene, which will be helpful to experimentalists in evaluating the performance and aging effects of phosphorene-based electronic devices.

  3. Effect of point defects on the thermal conductivity of UO2: molecular dynamics simulations

    SciTech Connect

    Liu, Xiang-Yang; Stanek, Christopher Richard; Andersson, Anders David Ragnar

    2015-07-21

    The thermal conductivity of uranium dioxide (UO2) fuel is an important materials property that affects fuel performance since it is a key parameter determining the temperature distribution in the fuel, thus governing, e.g., dimensional changes due to thermal expansion, fission gas release rates, etc. [1] The thermal conductivity of UO2 nuclear fuel is also affected by fission gas, fission products, defects, and microstructural features such as grain boundaries. Here, molecular dynamics (MD) simulations are carried out to determine quantitatively, the effect of irradiation induced point defects on the thermal conductivity of UO2, as a function of defect concentrations, for a range of temperatures, 300 – 1500 K. The results will be used to develop enhanced continuum thermal conductivity models for MARMOT and BISON by INL. These models express the thermal conductivity as a function of microstructure state-variables, thus enabling thermal conductivity models with closer connection to the physical state of the fuel [2].

  4. Bright UV Single Photon Emission at Point Defects in h-BN.

    PubMed

    Bourrellier, Romain; Meuret, Sophie; Tararan, Anna; Stéphan, Odile; Kociak, Mathieu; Tizei, Luiz H G; Zobelli, Alberto

    2016-07-13

    To date, quantum sources in the ultraviolet (UV) spectral region have been obtained only in semiconductor quantum dots. Color centers in wide bandgap materials may represent a more effective alternative. However, the quest for UV quantum emitters in bulk crystals faces the difficulty of combining an efficient UV excitation/detection optical setup with the capability of addressing individual color centers in potentially highly defective materials. In this work we overcome this limit by employing an original experimental setup coupling cathodoluminescence within a scanning transmission electron microscope to a Hanbury-Brown-Twiss intensity interferometer. We identify a new extremely bright UV single photon emitter (4.1 eV) in hexagonal boron nitride. Hyperspectral cathodoluminescence maps show a high spatial localization of the emission (∼80 nm) and a typical zero-phonon line plus phonon replica spectroscopic signature, indicating a point defect origin, most likely carbon substitutional at nitrogen sites. An additional nonsingle-photon broad emission may appear in the same spectral region, which can be attributed to intrinsic defects related to electron irradiation.

  5. Electron spin resonance study of point defects in thermal GaAs/GaAs-oxide structures

    NASA Astrophysics Data System (ADS)

    Nguyen, S.; Afanas'ev, V. V.; Stesmans, A.

    2012-12-01

    In an attempt to atomically assess interface and oxide-related point defects, a first basic multifrequency low-temperature electron spin resonance study has been carried out on semi-insulating (Fe compensated) GaAs/oxide structures, implying both powders and (100)GaAs/oxide slices, thermally grown in the range Tox=350-615 °C. Various spectra are observed: As for powders, this includes the 4-line EL2 defect spectrum centered at g~2.043 and characterized by the isotropic hyperfine constant Aiso~ 910 G, ascribed to the 75AsGa+ antisite defect. Observed in freshly crushed powder, it substantially increases in density with oxidation, in line with theoretical expectation; A maximum appears reached for at Tox~440 °C. It is not observed in the parent c-GaAs wafer. A second isotropic signal is observed at g~1.937 in powders for Tox in the range 510-615 °C, but only after additional VUV irradiation; it may correspond to As clusters. In bulk (100)GaAs, we observe the 5-branch spectrum of substitutional Fe impurities (spin S=5/2) in GaAs, with inferred crystal field constant a ≈ 360 G, well in line with previous observations. The results are discussed within the framework of advanced theoretical interface and defect models and previous experimental assessment.

  6. Point defect formation in optical materials expos ed to the space environment

    NASA Technical Reports Server (NTRS)

    Allen, J. L.; Seifert, N.; Yao, Y.; Albridge, R. G.; Barnes, A. V.; Tolk, N. H.; Strauss, A. M.; Linton, Roger C.; Kamenetzky, R. R.; Vaughn, Jason A.

    1995-01-01

    Point defect formation associated with early stages of optical damage was observed unexpectedly in two, and possibly three, different optical materials subjected to short-duration space exposure. Three calcium fluoride, two lithium fluoride, and three magnesium fluoride samples were flown on Space Shuttle flight STS-46 as part of the Evaluation of Oxygen Interactions with Materials - Third Phase experiment. One each of the calcium and magnesium fluoride samples was held at a fixed temperature of 60 C during the space exposure, while the temperatures of the other samples were allowed to vary with the ambient temperature of the shuttle cargo bay. Pre-flight and post-flight optical absorption measurements were performed on all of the samples. With the possible exception of the magnesium fluoride samples, every sample clearly showed the formation of F-centers in that section of the sample that was exposed to the low earth orbit environment. Solar vacuum ultraviolet radiation is the most probable primary cause of the defect formation; however, the resulting surface metallization may be synergistically altered by the atomic oxygen environment.

  7. Theoretical characterisation of point defects on a MoS2 monolayer by scanning tunnelling microscopy

    NASA Astrophysics Data System (ADS)

    González, C.; Biel, B.; Dappe, Y. J.

    2016-03-01

    Different S and Mo vacancies as well as their corresponding antisite defects in a free-standing MoS2 monolayer are analysed by means of scanning tunnelling microscopy (STM) simulations. Our theoretical methodology, based on the Keldysh nonequilibrium Green function formalism within the density functional theory (DFT) approach, is applied to simulate STM images for different voltages and tip heights. Combining the geometrical and electronic effects, all features of the different STM images can be explained, providing a valuable guide for future experiments. Our results confirm previous reports on S atom imaging, but also reveal a strong dependence on the applied bias for vacancies and antisite defects that include extra S atoms. By contrast, when additional Mo atoms cover the S vacancies, the MoS2 gap vanishes and a bias-independent bright protrusion is obtained in the STM image. Finally, we show that the inclusion of these point defects promotes the emergence of reactive dangling bonds that may act as efficient adsorption sites for external adsorbates.

  8. Quantum Monte Carlo models of substitutional point defects in zinc oxide and zinc selenide

    NASA Astrophysics Data System (ADS)

    Yu, Jaehyung; Ertekin, Elif

    2015-03-01

    Introducing dopants into semiconductors allows manipulation of electrical and optical properties, useful for applications such as optoelectronics and photovoltaics. While first principles quantitative descriptions of the defects properties in semiconductors are critical to understanding and engineering dopants in semiconductors, obtaining accurate descriptions has proven challenging in the past. Here we demonstrate the use of quantum Monte Carlo (QMC) methods to describing the properties of point defects in zinc oxide and zinc selenide. Due to its direct treatment of electron correlation, the QMC method is capable of accurate calculation of band gaps and defect behaviors. We describe the energetics and potential barrier to forming gallium DX-center defects according to QMC in zinc selenide, and compare the description to those of conventional and hybrid DFT. We also use QMC to determine the defect transition levels for nitrogen defects in zinc oxide, and show that QMC obtains descriptions that are in good agreement with GW and beyond-DFT approaches. Our results demonstrate the importance of accurate descriptions of electron correlation in the calculation of defect properties of semiconductors.

  9. Theoretical characterisation of point defects on a MoS2 monolayer by scanning tunnelling microscopy.

    PubMed

    González, C; Biel, B; Dappe, Y J

    2016-03-11

    Different S and Mo vacancies as well as their corresponding antisite defects in a free-standing MoS2 monolayer are analysed by means of scanning tunnelling microscopy (STM) simulations. Our theoretical methodology, based on the Keldysh nonequilibrium Green function formalism within the density functional theory (DFT) approach, is applied to simulate STM images for different voltages and tip heights. Combining the geometrical and electronic effects, all features of the different STM images can be explained, providing a valuable guide for future experiments. Our results confirm previous reports on S atom imaging, but also reveal a strong dependence on the applied bias for vacancies and antisite defects that include extra S atoms. By contrast, when additional Mo atoms cover the S vacancies, the MoS2 gap vanishes and a bias-independent bright protrusion is obtained in the STM image. Finally, we show that the inclusion of these point defects promotes the emergence of reactive dangling bonds that may act as efficient adsorption sites for external adsorbates. PMID:26862020

  10. Role of Point Defects on the Reactivity of Reconstructed Anatase Titanium Dioxide (001) Surface

    SciTech Connect

    Wang, Yang; Sun, Huijuan; Tan, Shijing; Feng, Hao; Cheng, Zhengwang; Zhao, Jin; Zhao, Aidi; Wang, Bing; Luo, Yi; Yang, Jinlong; Hou, J. G.

    2013-07-30

    The chemical reactivity of different surfaces of titanium dioxide (TiO2) has been the subject of extensive studies in recent decades. The anatase TiO2(001) and its (1x4) reconstructed surfaces were theoretically considered to be the most reactive and have been heavily pursued by synthetic chemists. However, the lack of direct experimental verification or determination of the active sites on these surfaces has caused controversy and debate. Here we report a systematic study on an anatase TiO2(001)-(1x4) surface by means of microscopic and spectroscopic techniques in combination with first-principles calculations. Two types of intrinsic point defects are identified, among which only the Ti3+ defect site on the reduced surface demonstrates considerable chemical activity. The perfect surface itself can be fully oxidized, but shows no obvious activity. Our findings suggest that the reactivity of the anatase TiO2(001) surface should depend on its reduction status, similar to that of rutile TiO2 surfaces.

  11. Structural and Optical Properties of Point Defects in α-SiO2 Cluster

    NASA Astrophysics Data System (ADS)

    Liu, Meng; Lu, Peng-Fei; Yang, Yang; Wu, Li-Yuan; Su, Rui; Chen, Jun

    2015-08-01

    First-principles methods based on the density functional theory (DFT) are used in order to calculate the structural and optical properties of α-SiO2 cluster with the non-bridging oxygen hole centers (NBOHC) and NBOHC-E' defects. We clarify the stable structure of the NBOHC-E' point defects for the first time using the functional B3LYP, which is also tested to investigate the influence of electronic properties. The calculation is carried out for cluster configurations extracted from supercell. The results of optical absorption peak for Si2O7H6 and Si5O16H12 with NBOHC-E' defects are found at 2.66 eV, which is higher than the often observed OA peak at 2.0 eV for the NBOHC defect in α-SiO2. The overall absorption spectra are in qualitative agreement with the experiment. Supported by the National Basic Research Program of China (973 Program) under Grant No. 2014CB643900, the Open Fund of IPOC (BUPT), the Open Program of State Key Laboratory of Functional Materials for Informatics, and the National Natural Science Foundation of China under Grant No. 61440061

  12. A study of void size growth in nonequilibrium stochastic systems of point defects

    NASA Astrophysics Data System (ADS)

    Kharchenko, Dmitrii O.; Kharchenko, Vasyl O.; Bashtova, Anna I.

    2016-05-01

    We study properties of voids growth dynamics in a stochastic system of point defects in solids under nonequilibrium conditions (sustained irradiation). It is shown that fluctuations of defect production rate (external noise) increase the critical void radius comparing to a deterministic system. An automodel regime of void size growth in a stochastic system is studied in detail. Considering a homogeneous system, it is found that external noise does not change the universality of the void size distribution function; the mean void size evolves according to classical nucleation theory. The noise increases the mean void size and spreads the void size distribution. Studying dynamics of spatially extended systems it was shown that vacancies remaining in a matrix phase are able to organize into vacancy enriched domains due to an instability caused by an elastic lattice deformation. It is shown that dynamics of voids growth is defined by void sinks strength with void size growth exponent varying from 1/3 up to 1/2.

  13. Theoretical characterisation of point defects on a MoS2 monolayer by scanning tunnelling microscopy.

    PubMed

    González, C; Biel, B; Dappe, Y J

    2016-03-11

    Different S and Mo vacancies as well as their corresponding antisite defects in a free-standing MoS2 monolayer are analysed by means of scanning tunnelling microscopy (STM) simulations. Our theoretical methodology, based on the Keldysh nonequilibrium Green function formalism within the density functional theory (DFT) approach, is applied to simulate STM images for different voltages and tip heights. Combining the geometrical and electronic effects, all features of the different STM images can be explained, providing a valuable guide for future experiments. Our results confirm previous reports on S atom imaging, but also reveal a strong dependence on the applied bias for vacancies and antisite defects that include extra S atoms. By contrast, when additional Mo atoms cover the S vacancies, the MoS2 gap vanishes and a bias-independent bright protrusion is obtained in the STM image. Finally, we show that the inclusion of these point defects promotes the emergence of reactive dangling bonds that may act as efficient adsorption sites for external adsorbates.

  14. Role of point defects on the reactivity of reconstructed anatase titanium dioxide (001) surface

    PubMed Central

    Wang, Yang; Sun, Huijuan; Tan, Shijing; Feng, Hao; Cheng, Zhengwang; Zhao, Jin; Zhao, Aidi; Wang, Bing; Luo, Yi; Yang, Jinlong; Hou, J. G.

    2013-01-01

    The chemical reactivity of different surfaces of titanium dioxide (TiO2) has been the subject of extensive studies in recent decades. The anatase TiO2(001) and its (1 × 4) reconstructed surfaces were theoretically considered to be the most reactive and have been heavily pursued by synthetic chemists. However, the lack of direct experimental verification or determination of the active sites on these surfaces has caused controversy and debate. Here we report a systematic study on an anatase TiO2(001)-(1 × 4) surface by means of microscopic and spectroscopic techniques in combination with first-principles calculations. Two types of intrinsic point defects are identified, among which only the Ti3+ defect site on the reduced surface demonstrates considerable chemical activity. The perfect surface itself can be fully oxidized, but shows no obvious activity. Our findings suggest that the reactivity of the anatase TiO2(001) surface should depend on its reduction status, similar to that of rutile TiO2 surfaces. PMID:23896829

  15. Effect of hole doping on the magnetism of point defects in graphene: A theoretical study

    NASA Astrophysics Data System (ADS)

    Yndurain, Felix

    2014-12-01

    Using ab initio methods based on the density functional theory, we study the magnetic properties of different point defects in graphene. We consider separately, atomic hydrogen, atomic fluorine, and single vacancies. The three defects have completely different magnetic properties. A local spin one-half magnetic moment is well defined at a hydrogen impurity, while single fluorine adatoms do not induce a well-defined magnetic moment unless there is a fluorine concentration of at least of a 0.5%. In this case, the induced magnetic moment is of the order of 0.45 μB per defect. This behavior is interpreted as being due to the charge transfer between fluorine and graphene. The case of magnetic moments localized at π electrons near vacancies is different from both previous cases; the size of the induced magnetic moment decreases with the dilution of defects and it is compatible with zero in the isolated vacancy. The effect of hole doping on these magnetic behaviors is studied and compared with the available experimental data. In the three cases, hole doping inhibits the formation of π states magnetic moments.

  16. Anticorrelation between surface and subsurface point defects and the impact on the redox chemistry of TiO2(110).

    PubMed

    Yoon, Yeohoon; Du, Yingge; Garcia, Juan C; Zhu, Zihua; Wang, Zhi-Tao; Petrik, Nikolay G; Kimmel, Gregory A; Dohnalek, Zdenek; Henderson, Michael A; Rousseau, Roger; Deskins, N Aaron; Lyubinetsky, Igor

    2015-02-01

    By using a combination of scanning tunneling microscopy (STM), density functional theory (DFT), and secondary-ion mass spectroscopy (SIMS), we explored the interplay and relative impact of surface versus subsurface defects on the surface chemistry of rutile TiO2 . STM results show that surface O vacancies (VO ) are virtually absent in the vicinity of positively charged subsurface point defects. This observation is consistent with DFT calculations of the impact of subsurface defect proximity on VO formation energy. To monitor the influence of such lateral anticorrelation on surface redox chemistry, a test reaction of the dissociative adsorption of O2 was employed and was observed to be suppressed around them. DFT results attribute this to a perceived absence of intrinsic (Ti), and likely extrinsic interstitials in the nearest subsurface layer beneath inhibited areas. We also postulate that the entire nearest subsurface region could be devoid of any charged point defects, whereas prevalent surface defects (VO ) are largely responsible for mediation of the redox chemistry at the reduced TiO2 (110).

  17. Anticorrelation between Surface and Subsurface Point Defects and the Impact on the Redox Chemistry of TiO2(110)

    SciTech Connect

    Yoon, Yeohoon; Du, Yingge; Garcia, Juan C.; Zhu, Zihua; Wang, Zhitao; Petrik, Nikolay G.; Kimmel, Gregory A.; Dohnalek, Zdenek; Henderson, Michael A.; Rousseau, Roger J.; Deskins, N. Aaron; Lyubinetsky, Igor

    2015-02-02

    Using combination of STM, DFT and SIMS, we explored the interplay and relative impact of surface vs. subsurface defects on the surface chemistry of rutile TiO2. STM results show that surface O vacancies (VO’s) are virtually absent in the vicinity of positively-charged subsurface point-defects. This observation is consistent with DFT calculations of impact of subsurface defect proximity on VO formation energy. To monitor the influence of such lateral anticorrelation on surface redox chemistry, a test reaction of the dissociative adsorption of O2 is employed, which is observed to be suppressed around them. DFT results attribute this to a perceived absence of the intrinsic (Ti) (and likely extrinsic) interstitials in the nearest subsurface layer beneath “inhibited” areas. We also postulate that the entire nearest subsurface region could be voided of any charged point-defects, whereas prevalent VO’s are largely responsible for mediation of the redox chemistry at reduced TiO2(110) surface.

  18. Research on the tunability of point defect modes in a two-dimensional magneto-elastic phononic crystal

    NASA Astrophysics Data System (ADS)

    Gu, Chunlong; Jin, Feng

    2016-05-01

    Point defect modes in a 2D phononic crystal with giant magnetostrictive material tuned by a magnetic field and compressive stress are investigated theoretically in this study. The 3D magnetostrictive constitutive model proposed by Liu and Zheng (2005 Acta Mech. Sin. 21 278-85) is adopted to develop effective elastic, piezomagnetic, and magnetic permeability constants. The finite element method, in combination with a supercell technique, is then applied to obtain the band structures and transmission spectra of the point defect modes in a 2D phononic crystal composed of Terfenol-D rods of circular cross section embedded in a polymethyl methacrylate matrix with a square lattice. The magnetic field not only enlarges the first band gap (FBG) but also opens up a new band gap of XY modes. New point defect modes are simultaneously trapped in the band gaps. The width of the FBG and the frequencies of the point defects of the Z mode decrease as the magnetic field increases.

  19. Optical spectroscopy and microscopy of radiation-induced light-emitting point defects in lithium fluoride crystals and films

    NASA Astrophysics Data System (ADS)

    Montereali, R. M.; Bonfigli, F.; Menchini, F.; Vincenti, M. A.

    2012-08-01

    Broad-band light-emitting radiation-induced F2 and F3+ electronic point defects, which are stable and laser-active at room temperature in lithium fluoride crystals and films, are used in dosimeters, tuneable color-center lasers, broad-band miniaturized light sources and novel radiation imaging detectors. A brief review of their photoemission properties is presented, and their behavior at liquid nitrogen temperatures is discussed. Some experimental data from optical spectroscopy and fluorescence microscopy of these radiation-induced point defects in LiF crystals and thin films are used to obtain information about the coloration curves, the efficiency of point defect formation, the effects of photo-bleaching processes, etc. Control of the local formation, stabilization, and transformation of radiation-induced light-emitting defect centers is crucial for the development of optically active micro-components and nanostructures. Some of the advantages of low temperature measurements for novel confocal laser scanning fluorescence microscopy techniques, widely used for spatial mapping of these point defects through the optical reading of their visible photoluminescence, are highlighted.

  20. Transition metal solute interactions with point defects in fcc iron from first principles

    NASA Astrophysics Data System (ADS)

    Hepburn, D. J.; MacLeod, E.; Ackland, G. J.

    2015-07-01

    We present a comprehensive set of first-principles electronic structure calculations of the properties of substitutional transition metal solutes and point defects in austenite (face-centered cubic, paramagnetic Fe). Clear trends were observed in these quantities across the transition metal series, with solute-defect interactions strongly related to atomic size, and only weakly related to more subtle details of magnetic or electronic structure. Oversized solutes act as strong traps for both vacancy and self-interstitial defects and as nucleation sites for the development of protovoids and small self-interstitial loops. The consequent reduction in defect mobility and net defect concentrations in the matrix explains the observation of reduced swelling and radiation-induced segregation. Our analysis of vacancy-mediated solute diffusion demonstrates that below about 400 K Ni and Co will be dragged by vacancies and their concentrations should be enhanced at defect sinks. Cr and Cu show opposite behavior and are depleted at defect sinks. The stable configuration of some oversized solutes is neither interstitial nor substitutional; rather they occupy two adjacent lattice sites. The diffusion of these solutes proceeds by a novel mechanism, which has important implications for the nucleation and growth of complex oxide nanoparticles contained in oxide dispersion strengthened steels. Interstitial-mediated solute diffusion is negligible for all except the magnetic solutes (Cr, Mn, Co, and Ni). Our results are consistent across several antiferromagnetic states and surprising qualitative similarities with ferromagnetic (body-centered cubic) Fe were observed; this implies that our conclusions will be valid for paramagnetic iron.

  1. Physicochemical characterization of point defects in fluorine doped tin oxide films

    SciTech Connect

    El Akkad, Fikry; Joseph, Sudeep

    2012-07-15

    The physical and chemical properties of spray deposited FTO films are studied using FESEM, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), electrical and optical measurements. The results of XRD measurements showed that the films are polycrystalline (grain size 20-50 nm) with Rutile structure and mixed preferred orientation along the (200) and (110) planes. An angular shift of the XRD peaks after F-doping is observed and interpreted as being due to the formation of substitutional fluorine defects (F{sub O}) in presence of high concentration of oxygen vacancies (V{sub O}) that are electrically neutral. The electrical neutrality of oxygen vacancies is supported by the observation that the electron concentration n is two orders of magnitude lower than the V{sub O} concentration calculated from chemical analyses using XPS measurements. It is shown that an agreement between XPS, XRD, and Hall effect results is possible provided that the degree of deviation from stoichiometry is calculated with the assumption that the major part of the bulk carbon content is involved in O-C bonds. High temperature thermal annealing is found to cause an increase in the F{sub O} concentration and a decrease in both n and V{sub O} concentrations with the increase of the annealing temperature. These results could be interpreted in terms of a high temperature chemical exchange reaction between the SnO{sub 2} matrix and a precipitated fluoride phase. In this reaction, fluorine is released to the matrix and Sn is trapped by the fluoride phase, thus creating substitutional fluorine F{sub O} and tin vacancy V{sub Sn} defects. The enthalpy of this reaction is determined to be approximately 2.4 eV while the energy of formation of a V{sub Sn} through the migration of Sn{sub Sn} host atom to the fluoride phase is approximately 0.45 eV.

  2. Physicochemical characterization of point defects in fluorine doped tin oxide films

    NASA Astrophysics Data System (ADS)

    Akkad, Fikry El; Joseph, Sudeep

    2012-07-01

    The physical and chemical properties of spray deposited FTO films are studied using FESEM, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), electrical and optical measurements. The results of XRD measurements showed that the films are polycrystalline (grain size 20-50 nm) with Rutile structure and mixed preferred orientation along the (200) and (110) planes. An angular shift of the XRD peaks after F-doping is observed and interpreted as being due to the formation of substitutional fluorine defects (FO) in presence of high concentration of oxygen vacancies (VO) that are electrically neutral. The electrical neutrality of oxygen vacancies is supported by the observation that the electron concentration n is two orders of magnitude lower than the VO concentration calculated from chemical analyses using XPS measurements. It is shown that an agreement between XPS, XRD, and Hall effect results is possible provided that the degree of deviation from stoichiometry is calculated with the assumption that the major part of the bulk carbon content is involved in O-C bonds. High temperature thermal annealing is found to cause an increase in the FO concentration and a decrease in both n and VO concentrations with the increase of the annealing temperature. These results could be interpreted in terms of a high temperature chemical exchange reaction between the SnO2 matrix and a precipitated fluoride phase. In this reaction, fluorine is released to the matrix and Sn is trapped by the fluoride phase, thus creating substitutional fluorine FO and tin vacancy VSn defects. The enthalpy of this reaction is determined to be approximately 2.4 eV while the energy of formation of a VSn through the migration of SnSn host atom to the fluoride phase is approximately 0.45 eV.

  3. Effects of point defect healing on phosphorus implanted germanium n+/p junction and its thermal stability

    NASA Astrophysics Data System (ADS)

    Shim, Jaewoo; Shin, Jeong-hun; Lee, In-Yeal; Choi, Daebeom; Woo Baek, Jung; Heo, Jonggon; Park, Wonkyu; Woo Leem, Jung; Su Yu, Jae; Jung, Woo-Shik; Saraswat, Krishna; Park, Jin-Hong

    2013-09-01

    In this work, the effect of Ge point defect healing process between 550 °C and 650 °C is investigated, in the aspect of leakage (off) current and junction depth of Ge n+/p junction diodes using ECV, TEM, J-V, and SIMS analyses. After 600 °C anneal, off-current density (2 × 10-4 A/cm2) is dramatically reduced due to the defect healing phenomenon that decreases the number of point defects, subsequently providing a higher on/off-current ratio of 5 × 103. In spite of the high healing temperature, junction diodes seem not to suffer from the deep diffusion of phosphorus (P) in Ge because those diffuse mostly through VGe. In addition, it is also confirmed that Ti is an appropriate material in terms of diffusion barrier and diffusivity for Ge n+/p junction contact metal.

  4. Interplay of point defects, biaxial strain, and thermal conductivity in homoepitaxial SrTiO{sub 3} thin films

    SciTech Connect

    Wiedigen, S.; Kramer, T.; Knorr, I.; Nee, N.; Hoffmann, J.; Volkert, C. A.; Jooss, Ch.; Feuchter, M.; Kamlah, M.

    2012-02-06

    Separating out effects of point defects and lattice strain on thermal conductivity is essential for improvement of thermoelectric properties of SrTiO{sub 3}. We study relations between defects generated during deposition, induced lattice strain, and their impact on thermal conductivity {kappa} in homoepitaxial SrTiO{sub 3} films prepared by ion-beam sputtering. Lowering the deposition temperature gives rise to lattice expansion by enhancement of point defect density which increases the hardness of the films. Due to a fully coherent substrate-film interface, the lattice misfit induces a large biaxial strain. However, we can show that the temperature dependence of {kappa} is mainly sensitive on the defect concentration.

  5. Modeling H, Na, and K diffusion in plagioclase feldspar by relating point defect parameters to bulk properties

    NASA Astrophysics Data System (ADS)

    Zhang, Baohua; Shan, Shuangming; Wu, Xiaoping

    2016-02-01

    Hydrogen and alkali ion diffusion in plagioclase feldspars is important to study the evolution of the crust and the kinetics of exsolution and ion-exchange reactions in feldspars. Using the available PVT equation of state of feldspars, we show that the diffusivities of H and alkali in plagioclase feldspars as a function of temperature can be successfully reproduced in terms of the bulk elastic and expansivity data through a thermodynamic model that interconnects point defect parameters with bulk properties. Our calculated diffusion coefficients of H, Na, and K well agree with experimental ones when uncertainties are considered. Additional point defect parameters such as activation enthalpy, activation entropy, and activation volume are also predicted. Furthermore, the electrical conductivity of feldspars inferred from our predicted diffusivities of H, Na, and K through the Nernst-Einstein equation is compared with previous experimental data.

  6. Effect of point defects on the optical and transport properties of MoS2 and WS2

    NASA Astrophysics Data System (ADS)

    Yuan, Shengjun; Roldán, Rafael; Katsnelson, M. I.; Guinea, Francisco

    2014-07-01

    Imperfections in the crystal structure, such as point defects, can strongly modify the optical and transport properties of materials. Here, we study the effect of point defects on the optical and dc conductivities of single layers of semiconducting transition metal dichalcogenides with the form MS2, where M=Mo or W. The electronic structure is considered within a six band tight-binding model, which accounts for the relevant combination of d orbitals of the metal M and p orbitals of the chalcogen S. We use the Kubo formula for the calculation of the conductivity in samples with different distributions of disorder. We find that M and/or S defects create midgap states that localize charge carriers around the defects and which modify the optical and transport properties of the material, in agreement with recent experiments. Furthermore, our results indicate a much higher mobility for p-doped WS2 in comparison to MoS2.

  7. Ab initio simulations of the structure, energetics and mobility of radiation-induced point defects in bcc Nb

    NASA Astrophysics Data System (ADS)

    Cerdeira, M. A.; Palacios, S. L.; González, C.; Fernández-Pello, D.; Iglesias, R.

    2016-09-01

    The formation, binding and migration energetics of helium clusters inside a niobium crystal have been analysed via ab initio simulations. The effect of placing several He atoms within an n-vacancy previously formed or as interstitials inside the initial perfect bulk matrix has been studied. DFT-based results show that He atoms prefer to aggregate forming small clusters at n-vacancy sites rather than at interstitial positions in the perfect crystal. The minimum formation energy is found when NHe is equal to the number of vacancies, n. It follows that vacancies act as almost perfect traps for He atoms, as is well known for other metals. The migration barriers of He atoms inside vacancies increase considerably when compared to what happens for vacancies alone. A secondary consequence is that the full set of energies obtained will be highly relevant as an input for new approaches to KMC simulations of defects in Nb.

  8. Preorganized Chromophores Facilitate Triplet Energy Migration, Annihilation and Upconverted Singlet Energy Collection.

    PubMed

    Mahato, Prasenjit; Yanai, Nobuhiro; Sindoro, Melinda; Granick, Steve; Kimizuka, Nobuo

    2016-05-25

    Photon upconversion (UC) based on triplet-triplet annihilation (TTA) has the potential to enhance significantly photovoltaic and photocatalytic efficiencies by harnessing sub-bandgap photons, but the progress of this field is held back by the chemistry problem of how to preorganize multiple chromophores for efficient UC under weak solar irradiance. Recently, the first maximization of UC quantum yield at solar irradiance was achieved using fast triplet energy migration (TEM) in metal-organic frameworks (MOFs) with ordered acceptor arrays, but at the same time, a trade-off between fast TEM and high fluorescence efficiency was also found. Here, we provide a solution for this trade-off issue by developing a new strategy, triplet energy migration, annihilation and upconverted singlet energy collection (TEM-UPCON). The porous structure of acceptor-based MOF crystals allows triplet donor molecules to be accommodated without aggregation. The surface of donor-doped MOF nanocrystals is modified with highly fluorescent energy collectors through coordination bond formation. Thanks to the higher fluorescence quantum yield of surface-bound collectors than parent MOFs, the implementation of the energy collector greatly improves the total UC quantum yield. The UC quantum yield maximization behavior at ultralow excitation intensity was retained because the TTA events take place only in the MOF acceptors. The TEM-UPCON concept may be generalized to collectors with various functions and would lead to quantitative harvesting of upconverted energy, which is difficult to achieve in common molecular diffusion-based systems.

  9. Preorganized Chromophores Facilitate Triplet Energy Migration, Annihilation and Upconverted Singlet Energy Collection.

    PubMed

    Mahato, Prasenjit; Yanai, Nobuhiro; Sindoro, Melinda; Granick, Steve; Kimizuka, Nobuo

    2016-05-25

    Photon upconversion (UC) based on triplet-triplet annihilation (TTA) has the potential to enhance significantly photovoltaic and photocatalytic efficiencies by harnessing sub-bandgap photons, but the progress of this field is held back by the chemistry problem of how to preorganize multiple chromophores for efficient UC under weak solar irradiance. Recently, the first maximization of UC quantum yield at solar irradiance was achieved using fast triplet energy migration (TEM) in metal-organic frameworks (MOFs) with ordered acceptor arrays, but at the same time, a trade-off between fast TEM and high fluorescence efficiency was also found. Here, we provide a solution for this trade-off issue by developing a new strategy, triplet energy migration, annihilation and upconverted singlet energy collection (TEM-UPCON). The porous structure of acceptor-based MOF crystals allows triplet donor molecules to be accommodated without aggregation. The surface of donor-doped MOF nanocrystals is modified with highly fluorescent energy collectors through coordination bond formation. Thanks to the higher fluorescence quantum yield of surface-bound collectors than parent MOFs, the implementation of the energy collector greatly improves the total UC quantum yield. The UC quantum yield maximization behavior at ultralow excitation intensity was retained because the TTA events take place only in the MOF acceptors. The TEM-UPCON concept may be generalized to collectors with various functions and would lead to quantitative harvesting of upconverted energy, which is difficult to achieve in common molecular diffusion-based systems. PMID:27163784

  10. A thermodynamic analysis of native point defect and dopant solubilities in zinc-blende III-V semiconductors

    SciTech Connect

    Hurle, D. T. J.

    2010-06-15

    A thermodynamic model is used to analyze available experimental data relevant to point defects in the binary zinc-blende III-V compounds (Ga,In)-(P,As,Sb). The important point defects and their complexes in each of the materials are identified and included in the model. Essentially all of the available experimental data on dopant solubility, crystal density, and lattice parameter of melt and solution grown crystals and epilayers are reproduced by the model. It extends an earlier study [Hurle, J. Appl. Phys. 85, 6957 (1999)] devoted solely to GaAs. Values for the enthalpy and entropy of formation of both native and dopant related point defects are obtained by fitting to experimental data. In undoped material, vacancies, and interstitials on the Group V sublattice dominate in the vicinity of the melting point (MP) in both the phosphides and arsenides, whereas, in the antimonides, vacancies on both sublattices dominate. The calculated concentrations of the native point defects are used to construct the solidus curves of all the compounds. The charged native point defect concentrations at the MP in four of the six materials are significantly higher than their intrinsic carrier concentrations. Thus the usually assumed high temperature 'intrinsic' electroneutrality condition for undoped material (n=p) is not valid for these materials. In GaSb, the Ga{sub Sb} antisite defect appears to be grown-in from the melt. This contrasts with the As{sub Ga} defect in GaAs for which the concentration grown-in at the MP is negligibly small. Compensation of donor-doped material by donor-Group III vacancy complexes is shown to exist in all the compounds except InP where Group VI doped crystals are uncompensated and in InSb where there is a lack of experimental data. The annealing effects in n{sup +} GaAs, including lattice superdilation, which were shown in the earlier paper to be due to Group III vacancy undersaturation during cooling, are found to be present also in GaSb and In

  11. Native point defect formation in flash sintered ZnO studied by depth-resolved cathodoluminescence spectroscopy

    NASA Astrophysics Data System (ADS)

    Gao, Hantian; Asel, Thaddeus J.; Cox, Jon W.; Zhang, Yuanyao; Luo, Jian; Brillson, L. J.

    2016-09-01

    Depth-resolved cathodoluminescence spectroscopy studies of flash sintered ZnO reveal that thermal runaway induces the formation of native point defects inside individual grains. Defects associated with oxygen vacancies (VO) form preferentially, contributing additional donors that increase conductivity within the grains of the polycrystalline material. Hyperspectral imaging of the granular cross sections shows filaments of increased VO following thermal runaway between the capacitor anode and cathode, supporting a heating mechanism localized on a granular scale. Within the grains, these defects form preferentially inside rather than at their boundaries, further localizing the dominant heating mechanism.

  12. Bats on a budget: torpor-assisted migration saves time and energy.

    PubMed

    McGuire, Liam P; Jonasson, Kristin A; Guglielmo, Christopher G

    2014-01-01

    Bats and birds must balance time and energy budgets during migration. Migrating bats face similar physiological challenges to birds, but nocturnality creates special challenges for bats, such as a conflict between travelling and refueling, which many birds avoid by feeding in daylight and flying at night. As endothermic animals, bats and birds alike must expend substantial amounts of energy to maintain high body temperatures. For migratory birds refueling at stopovers, remaining euthermic during inactive periods reduces the net refuelling rate, thereby prolonging stopover duration and delaying subsequent movement. We hypothesized that bats could mitigate similar ambient-temperature dependent costs by using a torpor-assisted migration strategy. We studied silver-haired bats Lasionycteris noctivagans during autumn migration using a combination of respirometry and temperature-sensitive radiotelemetry to estimate energy costs incurred under ambient temperature conditions, and the energy that bats saved by using torpor during daytime roosting periods. All bats, regardless of sex, age, or body condition used torpor at stopover and saved up to 91% of the energy they would have expended to remain euthermic. Furthermore, bats modulated use of torpor depending on ambient temperature. By adjusting the time spent torpid, bats achieved a rate of energy expenditure independent of the ambient temperature encountered at stopover. By lowering body temperature during inactive periods, fuel stores are spared, reducing the need for refuelling. Optimal migration models consider trade-offs between time and energy. Heterothermy provides a physiological strategy that allows bats to conserve energy without paying a time penalty as they migrate. Although uncommon, some avian lineages are known to use heterothermy, and current theoretical models of migration may not be appropriate for these groups. We propose that thermoregulatory strategies should be an important consideration of future

  13. Bats on a Budget: Torpor-Assisted Migration Saves Time and Energy

    PubMed Central

    McGuire, Liam P.; Jonasson, Kristin A.; Guglielmo, Christopher G.

    2014-01-01

    Bats and birds must balance time and energy budgets during migration. Migrating bats face similar physiological challenges to birds, but nocturnality creates special challenges for bats, such as a conflict between travelling and refueling, which many birds avoid by feeding in daylight and flying at night. As endothermic animals, bats and birds alike must expend substantial amounts of energy to maintain high body temperatures. For migratory birds refueling at stopovers, remaining euthermic during inactive periods reduces the net refuelling rate, thereby prolonging stopover duration and delaying subsequent movement. We hypothesized that bats could mitigate similar ambient-temperature dependent costs by using a torpor-assisted migration strategy. We studied silver-haired bats Lasionycteris noctivagans during autumn migration using a combination of respirometry and temperature-sensitive radiotelemetry to estimate energy costs incurred under ambient temperature conditions, and the energy that bats saved by using torpor during daytime roosting periods. All bats, regardless of sex, age, or body condition used torpor at stopover and saved up to 91% of the energy they would have expended to remain euthermic. Furthermore, bats modulated use of torpor depending on ambient temperature. By adjusting the time spent torpid, bats achieved a rate of energy expenditure independent of the ambient temperature encountered at stopover. By lowering body temperature during inactive periods, fuel stores are spared, reducing the need for refuelling. Optimal migration models consider trade-offs between time and energy. Heterothermy provides a physiological strategy that allows bats to conserve energy without paying a time penalty as they migrate. Although uncommon, some avian lineages are known to use heterothermy, and current theoretical models of migration may not be appropriate for these groups. We propose that thermoregulatory strategies should be an important consideration of future

  14. Point defect structures of YA12 and ZrCo2 Laves phase compounds by first-principles calculations

    SciTech Connect

    Krcmar, Maja; Fu, Chong Long

    2007-01-01

    In Laves phase alloys with prominent size mismatch between constituent atoms and/or large negative enthalpy of formation, the existence of vacancies as the dominant point defect type is often suggested. However, there are not enough experimental data to prove or disprove these arguments. Employing first-principles calculations, we study the point defect structures of YAl{sub 2} and ZrCo{sub 2} C15 Laves phases, as both compounds exhibit large size mismatch between constituent atoms, and large negative enthalpy of formation. We find that one must go beyond the simple geometrical or enthalpy arguments in determining the point defect structures of these alloys. In both compounds, the point defect structure is found to be dominated by the anti-site defects on the larger atom-rich side of the stoichiometry.

  15. Theoretical model of dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide

    NASA Astrophysics Data System (ADS)

    Ivády, Viktor; Szász, Krisztián; Falk, Abram L.; Klimov, Paul V.; Christle, David J.; Janzén, Erik; Abrikosov, Igor A.; Awschalom, David D.; Gali, Adam

    2015-09-01

    Dynamic nuclear spin polarization (DNP) mediated by paramagnetic point defects in semiconductors is a key resource for both initializing nuclear quantum memories and producing nuclear hyperpolarization. DNP is therefore an important process in the field of quantum-information processing, sensitivity-enhanced nuclear magnetic resonance, and nuclear-spin-based spintronics. DNP based on optical pumping of point defects has been demonstrated by using the electron spin of nitrogen-vacancy (NV) center in diamond, and more recently, by using divacancy and related defect spins in hexagonal silicon carbide (SiC). Here, we describe a general model for these optical DNP processes that allows the effects of many microscopic processes to be integrated. Applying this theory, we gain a deeper insight into dynamic nuclear spin polarization and the physics of diamond and SiC defects. Our results are in good agreement with experimental observations and provide a detailed and unified understanding. In particular, our findings show that the defect electron spin coherence times and excited state lifetimes are crucial factors in the entire DNP process.

  16. Point-defect properties in HCP rare earth metals with analytic modified embedded atom potentials

    NASA Astrophysics Data System (ADS)

    Hu, Wangyu; Deng, Huiqiu; Yuan, Xiaojian; Fukumoto, Masahiro

    2003-08-01

    The analytic embedded atom method (EAM) type many-body potentials of hcp rare earth metals (Dy, Er, Gd, Ho, Nd, Pr, and Tb) have been constructed. The hcp lattice is shown to be energetically most stable when compared with the fcc and bcc structure, and the hcp lattice with ideal c/a. The mechanical stability of the corresponding hcp lattice with respect to large change of density and c/a ratio is examined. The phonon spectra, stacking fault and surface energy are calculated. The activation energy for vacancy diffusion in these metals has been calculated and the most possible diffusion paths are predicted. Finally, the self-interstitial atom (SIA) formation energy and volume have been evaluated for eight possible sites. This calculation suggests that the crowdion and basal split are the most stable configurations. The SIA formation energy increases linearly with the increase of the melting temperature.

  17. The influence of dynamical structural relaxation of point defect clusters on void formation in irradiated copper

    NASA Astrophysics Data System (ADS)

    Shimomura, Y.; Mukouda, I.; Sugio, K.

    1997-11-01

    that voids nucleate as a metastable defects at coalesced vacancy clusters at high temperature. The nucleation of voids is not affected by the influence of gas atoms dissolved in the material. Micro-voids migrate in the specimens after their nucleation. During their movement, gas atoms are trapped in the voids. The trapping of a larger number of gas atoms limits the movement of voids. This leads to a higher number density of voids in the as-received copper than in residual-gas-free specimens at the high fluence. Voids form uniformly in specimens at the low flunce and they migrate to dislocation lines. Dislocations are also trapped at voids during climbing by absorbing interstitial clusters. These finally lead to the preferential formation of voids along dislocation lines.

  18. Point defects in twisted bilayer graphene: A density functional theory study

    NASA Astrophysics Data System (ADS)

    Ulman, Kanchan; Narasimhan, Shobhana

    2014-06-01

    We have used ab initio density functional theory, incorporating van der Waals corrections, to study twisted bilayer graphene (TBLG) where Stone-Wales defects or monovacancies are introduced in one of the layers. We compare these results to those for defects in single-layer graphene or Bernal stacked graphene. The energetics of defect formation is not very sensitive to the stacking of the layers or the specific site at which the defect is created, suggesting a weak interlayer coupling. However, signatures of the interlayer coupling are manifested clearly in the electronic band structure. For the "γγ" Stone-Wales defect in TBLG, we observe two Dirac cones that are shifted in both momentum space and energy. This up/down shift in energy results from the combined effect of a charge transfer between the two graphene layers and a chemical interaction between the layers, which mimics the effects of a transverse electric field. Charge density plots show that states near the Dirac points have significant admixture between the two layers. For Stone-Wales defects at other sites in TBLG, this basic structure is modified by the creation of minigaps at energy crossings. For a monovacancy, the Dirac cone of the pristine layer is shifted up in energy by ˜0.25 eV due to a combination of the requirement of the equilibration of Fermi energy between the two layers with different numbers of electrons, charge transfer, and chemical interactions. Both kinds of defects increase the density of states at the Fermi level. The monovacancy also results in spin polarization, with magnetic moments on the defect of 1.2-1.8 μB.

  19. Two distinctive energy migration pathways of monolayer molecules on metal nanoparticle surfaces

    PubMed Central

    Li, Jiebo; Qian, Huifeng; Chen, Hailong; Zhao, Zhun; Yuan, Kaijun; Chen, Guangxu; Miranda, Andrea; Guo, Xunmin; Chen, Yajing; Zheng, Nanfeng; Wong, Michael S.; Zheng, Junrong

    2016-01-01

    Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. Here we report two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces investigated with ultrafast vibrational spectroscopy. On a 5 nm platinum particle, within a few picoseconds the vibrational energy of a carbon monoxide adsorbate rapidly dissipates into the particle through electron/hole pair excitations, generating heat that quickly migrates on surface. In contrast, the lack of vibration-electron coupling on approximately 1 nm particles results in vibrational energy migration among adsorbates that occurs on a twenty times slower timescale. Further investigations reveal that the rapid carbon monoxide energy relaxation is also affected by the adsorption sites and the nature of the metal but to a lesser extent. These findings reflect the dependence of electron/vibration coupling on the metallic nature, size and surface site of nanoparticles and its significance in mediating energy relaxations and migrations on nanoparticle surfaces. PMID:26883665

  20. Point-defect recombination efficiency at grain boundaries in irradiated SiC

    SciTech Connect

    Moriani, Andrea; Cleri, Fabrizio

    2006-06-01

    We studied the atomic-scale mechanisms of radiation damage recovery, by molecular dynamics simulations of irradiation cascades in a {beta}-SiC model system, containing one general (001) twist grain boundary in the direction approximately perpendicular to the cascade. The (001) grain boundary has a disordered atomic structure, representative of high-angle, high-energy boundaries in cubic silicon carbide. Compared to the perfect crystal model system, we find a relevant effect of grain boundaries on the annealing of cascade defects, both in terms of localization of defects, which are preferentially concentrated around the grain boundary, and of relative defect recovery efficiency. In general, C interstitials are the prevalent type of defect over the whole range of energies explored. A slight grain boundary expansion is observed, accompanied by a broadening of the central atomic planes.

  1. Atomistic simulation study of the effects of point defects on the inception of plastic deformation in metals

    NASA Astrophysics Data System (ADS)

    Salehinia, Iman

    Atomistic simulations have been used to study the effect of various types of point defects on the mechanical response of FCC single crystals in nanoindentation and uniaxial tests. To study the effect of spatial distribution of defects in nanoindentation testing, various point defects were located in different relative positions to the indenter. When the defect position was close to the regions of high shear stresses the nucleation of dislocations was related to the location of the defect; however homogeneous nucleation of dislocations was also observed for defect-containing crystals. The effect of the point defects was independent of the indenter size, and the applied pressure needed to initiate plasticity, when compared to defect-free crystals, was a reduction of approximately 10%, 20%, 20% and 50% for a single vacancy, di-vacancy, self-interstitial atom and stacking fault tetrahedron (SFT), respectively. The stochastic nature of the pop-in loads was further explored for different orientations using molecular dynamics and complementary nanoindentation experiments on (100), (101) and (111) single crystals of copper and Ni200. The sensitivity of the crystal to the presence of internal structural defects depends strongly on its crystallographic orientation. The simulations suggest that the first event observed experimentally may not correspond to the first plastic deformation event. Anisotropy effects were also studied for various orientations in uniaxial tests in the presence of a centered SFT. Both the normal stresses to the slip plane and the relative values of Schmid factor in compression and tension affect the observed compression/tension yield asymmetry. The reduction in yield stress was found to be larger in compression than in tension for almost all orientations. The simulations suggest that compression is a more reliable experimental tool for studying the effect of structural defects on the mechanical behavior of the FCC crystals, while tension may be more

  2. Native point defects and doping in ZnGeN2

    NASA Astrophysics Data System (ADS)

    Skachkov, Dmitry; Punya Jaroenjittichai, Atchara; Huang, Ling-yi; Lambrecht, Walter R. L.

    2016-04-01

    A computational study within the framework of density functional theory in the local density approximation (LDA) is presented for native defects and doping in ZnGeN2. Gap corrections are taken into account using an LDA+U approach and finite size corrections for charged defects are evaluated in terms of an effective charge model, introduced in this paper. The donor or acceptor characteristics of each of the cation and N vacancies and the two cation antisite defects are determined as well as their energies of formation under different chemical potential conditions. These are then used to determine defect concentrations and Fermi level pinning self-consistently. The cation antisite defects are found to have significantly lower formation energy than the cation vacancies. At a typical growth temperature of 1200 K, the charge neutrality condition pins the Fermi level close to the crossing of the formation energies of the ZnGe-1 acceptor with the GeZn2 + shallow donor. Since this point lies closer to the valence-band maximum (VBM), intrinsic p -type doping would result at the growth temperature and will persist at room temperature if the defect concentrations are frozen in. It is the highest and of order 1016cm-3 for the most Ge-poor condition. On the other hand, for the most Ge-poor condition, it drops to 1013cm-3 at 1200 K and to almost zero at 300 K because then the Fermi level is too close to the middle of the gap. Oxygen impurities are found to strongly prefer the ON substitutional site and are found to be shallow donors with a very low energy of formation. It can only be suppressed by strongly reducing the oxygen partial pressure relative to that of nitrogen. At high temperatures, however, introduction of oxygen will be accompanied by compensating ZnGe-2 acceptors and would lead to negligible net doping. The prospects for Ga base p -type doping are evaluated. While good solubility is expected, site competition between Zn and Ge sites is found to lead to a

  3. Recrystallization of atomically balanced amorphous pockets in Si: A source of point defects

    SciTech Connect

    Marques, Luis A.; Pelaz, Lourdes; Lopez, Pedro; Santos, Ivan; Aboy, Maria

    2007-10-15

    We use classical molecular dynamics simulation techniques to study the regrowth behavior of amorphous pockets in Si. We demonstrate that crystallization depends on the morphology of the pocket-crystal interface. Although our simulated amorphous pockets had not any excess nor deficit of atoms with respect to perfect crystal, after regrowth we found residual defects. Most of them are single Si interstitials and vacancies, but also larger defects have been encountered. We have determined their atomic structures and calculated their formation energies. These complexes are more stable than amorphous pockets, and may trigger the formation of extended defects or favor damage accumulation.

  4. Excitation energy migration between elongated fluorophores in uniaxially oriented polyvinyl alcohol films

    NASA Astrophysics Data System (ADS)

    Bojarski, P.; Gryczyński, I.; Kułak, L.; Synak, A.; Barnett, A.

    2006-11-01

    Nonradiative multistep energy migration is studied for 3,3'-diethylthiacarbocyanine iodide (DTCI) in uniaxially stretched and unstretched poly(vinyl alcohol) films. The results of fluorescence anisotropy decay measurements were found to be extremely different for unstretched and (stretched) systems. For disordered systems, fast depolarization was observed due to effective energy migration between randomly distributed fluorophores. However, for partly ordered systems, much slower depolarization was found as a result of preferential angular distribution of transition dipole moments of identical fluorophores. These results and properties of energy transport in uniaxially stretched polymer films are analyzed using the technique of Monte-Carlo simulation.

  5. Quasi-reversible point defect relaxation in amorphous In-Ga-Zn-O thin films by in situ electrical measurements

    NASA Astrophysics Data System (ADS)

    Adler, Alexander U.; Yeh, Ted C.; Bruce Buchholz, D.; Chang, Robert P. H.; Mason, Thomas O.

    2013-03-01

    Quasi-reversible oxygen exchange/point defect relaxation in an amorphous In-Ga-Zn-O thin film was monitored by in situ electrical property measurements (conductivity, Seebeck coefficient) at 200 °C subjected to abrupt changes in oxygen partial pressure (pO2). By subtracting the long-term background decay from the conductivity curves, time-independent conductivity values were obtained at each pO2. From these values, a log-log "Brouwer" plot of conductivity vs. pO2 of approximately -1/2 was obtained, which may indicate co-elimination (filling) of neutral and charged oxygen vacancies. This work demonstrates that Brouwer analysis can be applied to the study of defect structure in amorphous oxide thin films.

  6. Kinetically evolving irradiation-induced point defect clusters in UO{sub 2} by molecular dynamics simulation

    SciTech Connect

    Aidhy, Dilpuneet S.; Millett, Paul C.; Desai, Tapan; Wolf, Dieter; Phillpot, Simon R.

    2009-09-01

    The evolution of irradiation-induced point defects in UO{sub 2} is captured in molecular dynamics simulations. The approach used circumvents their creation during the ballistic phase of a traditional collision-cascade molecular dynamics simulation but rather focuses on their kinetic evolution. The simulations reveal that in the absence of defects on the cation sublattice, the defects initially present on the anion sublattice recombine and annihilate completely during equilibration. However, in the simultaneous presence of defects on both sublattices, Schottky defects are formed, thereby sequestering the oxygen vacancies. The resulting excess oxygen interstitials form cuboctahedral clusters, whose existence has previously been identified experimentally but whose generation mechanism has not been determined. It is concluded that the cation sublattice is primarily responsible for the radiation tolerance or intolerance of the material.

  7. Point Defect Distributions in ZnSe Crystals: Effects of Gravity Vector Orientation during Physical Vapor Transport Growth

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Feth, S.; Hirschfeld, D.; Smith, T. M.; Wang, Ling Jun; Volz, M. P.; Lehoczky, S. L.

    1999-01-01

    ZnSe crystals were grown by the physical vapor transport technique under horizontal and vertical (stabilized and destabilized) configurations. Secondary ion mass spectroscopy and photoluminescence measurements were performed on the ZnSe samples to map the distributions of [Si], [Fe], [Cu], [Al] and [Li or Na] impurities as well as Zn vacancy, [V(zn)]. The annealings of ZnSe under controlled Zn pressures were studied to correlate the measured photoluminescence emission intensity to the equilibrium Zn partial pressure. In the horizontal grown crystals the segregations of [Si], [Fe], [Al] and [V(zn)] along the gravity vector direction were observed whereas in the vertically stabilized grown crystal the segregations of these point defects were radially symmetrical. No apparent pattern was observed on the measured distributions in the vertically destabilized grown crystal. The observed segregations in the three growth configurations were interpreted based on the possible buoyancy-driven convection in the vapor phase.

  8. Fourth workshop on the role of point defects/defect complexes in silicon device processing. Summary report

    SciTech Connect

    Tan, T.; Jastrzebski, L.; Sopori, B.

    1994-07-01

    The 4th Point Defect Workshop was aimed at reviewing recent new understanding of the defect engineering techniques that can improve the performance of solar cells fabricated on low-cost silicon substrates. The theme of the workshop was to identify approaches that can lead to 18% commercial silicon solar cells in the near future. These approaches also define the research tasks for the forthcoming new DOE/NREL silicon materials research program. It was a consensus of the workshop attendees that the goal of 18%-efficient multicrystalline silicon solar cells is right on target, and the payoff for the investment by DOE will manifest itself in the next few years as reduced costs for high-efficiency cell fabrication.

  9. Point Defect Distributions in ZnSe Crystals: Effects of Gravity Vector Orientation During Physical Vapor Transport Growth

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Feth, S.; Hirschfeld, D.; Smith, T. M.; Wang, Ling Jun; Volz, M. P.; Lehoczky, S. L.

    1999-01-01

    ZnSe crystals were grown by the physical vapor transport technique under horizontal and vertical (stabilized and destabilized) configurations. Secondary ion mass spectroscopy and photoluminescence measurements were performed on the grown ZnSe samples to map the distributions of [Si], [Fe], [Cu], [Al] and [Li or Na] impurities as well as Zn vacancy, [V (sub Zn)]. Annealings of ZnSe under controlled Zn pressures were studied to correlate the measured photoluminescence emission intensity to the equilibrium Zn partial pressure. In the horizontal grown crystals the segregations of [Si], [Fe], [Al] and [V (sub Zn)] were observed along the gravity vector direction whereas in the vertically stabilized grown crystal the segregation of these point defects was radially symmetrical. No apparent pattern was observed on the measured distributions in the vertically destabilized grown crystal. The observed segregations in the three growth configurations were interpreted based on the possible buoyancy-driven convection in the vapor phase.

  10. Three-dimensional interaction force and tunneling current spectroscopy of point defects on rutile TiO2(110)

    NASA Astrophysics Data System (ADS)

    Baykara, Mehmet Z.; Mönig, Harry; Schwendemann, Todd C.; Ünverdi, Ã.-zhan; Altman, Eric I.; Schwarz, Udo D.

    2016-02-01

    The extent to which point defects affect the local chemical reactivity and electronic properties of an oxide surface was evaluated with picometer resolution in all three spatial dimensions using simultaneous atomic force/scanning tunneling microscopy measurements performed on the (110) face of rutile TiO2. Oxygen atoms were imaged as protrusions in both data channels, corresponding to a rarely observed imaging mode for this prototypical metal oxide surface. Three-dimensional spectroscopy of interaction forces and tunneling currents was performed on individual surface and subsurface defects as a function of tip-sample distance. An interstitial defect assigned to a subsurface hydrogen atom is found to have a distinct effect on the local density of electronic states on the surface, but no detectable influence on the tip-sample interaction force. Meanwhile, spectroscopic data acquired on an oxygen vacancy highlight the role of the probe tip in chemical reactivity measurements.

  11. Demonstrating Energy Migration in Coupled Oscillators: A Central Concept in the Theory of Unimolecular Reactions

    ERIC Educational Resources Information Center

    Marcotte, Ronald E.

    2005-01-01

    This physical chemistry lecture demonstration is designed to aid the understanding of intramolecular energy transfer processes as part of the presentation of the theory of unimolecular reaction rates. Coupled pendulums are used to show the rate of migration of energy between oscillators under resonant and nonresonant conditions with varying…

  12. Native point defects in LaAlO3: A hybrid functional study

    NASA Astrophysics Data System (ADS)

    Choi, Minseok; Janotti, Anderson; Van de Walle, Chris G.

    2013-12-01

    We investigate the electronic structure of defects in LaAlO3 (LAO) and their effects on electronic properties of bulk and heterostructures. Our calculations indicate that vacancies have lower formation energies than interstitials and antisites. The La vacancy (VLa) and the Al vacancy (VAl) are deep acceptors, while the oxygen vacancy (VO) is a deep donor. The impact of these defects on the performance of metal-oxide-semiconductor devices is analyzed by placing the LAO band edges and defect levels with respect to the band edges of GaN, InGaAs, and Si. VO introduces levels in the gap or in the vicinity of the semiconductor conduction band, resulting in carrier traps and/or leakage current through the gate oxide, while VLa and VAl are sources of negative fixed charges. We also discuss how oxygen vacancies in LAO can influence the observed two-dimensional electron gas (2DEG) in LaAlO3/SrTiO3 heterostructures. We conclude that VO in the LAO layer may provide electrons that fill compensating surface states, resulting in higher 2DEG densities, at least for modest LAO layer thicknesses.

  13. Fluorescence depolarization in the presence of excitation energy migration in partly ordered polymer films

    NASA Astrophysics Data System (ADS)

    Synak, A.; Gondek, G.; Bojarski, P.; Kułak, L.; Kubicki, A.; Szabelski, M.; Kwiek, P.

    2004-11-01

    Excitation energy migration is studied for 3,3'-diethylthiacyanine iodide in uniaxially stretched and unstretched poly(vinyl alcohol) films. Time-resolved and steady-state measurements of fluorescence depolarization yield quite different results for disordered and partly ordered systems. Contrary to disordered systems, for which strong concentration depolarization was observed, partly ordered systems exhibit much weaker depolarization as a result of preferential angular distribution of transition dipole moments of identical fluorophores participating in energy migration. Difference of the same origin was found for time-resolved emission anisotropy measurements. These results are in agreement with those obtained from the Monte-Carlo simulation.

  14. A heuristic placement selection of live virtual machine migration for energy-saving in cloud computing environment.

    PubMed

    Zhao, Jia; Hu, Liang; Ding, Yan; Xu, Gaochao; Hu, Ming

    2014-01-01

    The field of live VM (virtual machine) migration has been a hotspot problem in green cloud computing. Live VM migration problem is divided into two research aspects: live VM migration mechanism and live VM migration policy. In the meanwhile, with the development of energy-aware computing, we have focused on the VM placement selection of live migration, namely live VM migration policy for energy saving. In this paper, a novel heuristic approach PS-ES is presented. Its main idea includes two parts. One is that it combines the PSO (particle swarm optimization) idea with the SA (simulated annealing) idea to achieve an improved PSO-based approach with the better global search's ability. The other one is that it uses the Probability Theory and Mathematical Statistics and once again utilizes the SA idea to deal with the data obtained from the improved PSO-based process to get the final solution. And thus the whole approach achieves a long-term optimization for energy saving as it has considered not only the optimization of the current problem scenario but also that of the future problem. The experimental results demonstrate that PS-ES evidently reduces the total incremental energy consumption and better protects the performance of VM running and migrating compared with randomly migrating and optimally migrating. As a result, the proposed PS-ES approach has capabilities to make the result of live VM migration events more high-effective and valuable. PMID:25251339

  15. A heuristic placement selection of live virtual machine migration for energy-saving in cloud computing environment.

    PubMed

    Zhao, Jia; Hu, Liang; Ding, Yan; Xu, Gaochao; Hu, Ming

    2014-01-01

    The field of live VM (virtual machine) migration has been a hotspot problem in green cloud computing. Live VM migration problem is divided into two research aspects: live VM migration mechanism and live VM migration policy. In the meanwhile, with the development of energy-aware computing, we have focused on the VM placement selection of live migration, namely live VM migration policy for energy saving. In this paper, a novel heuristic approach PS-ES is presented. Its main idea includes two parts. One is that it combines the PSO (particle swarm optimization) idea with the SA (simulated annealing) idea to achieve an improved PSO-based approach with the better global search's ability. The other one is that it uses the Probability Theory and Mathematical Statistics and once again utilizes the SA idea to deal with the data obtained from the improved PSO-based process to get the final solution. And thus the whole approach achieves a long-term optimization for energy saving as it has considered not only the optimization of the current problem scenario but also that of the future problem. The experimental results demonstrate that PS-ES evidently reduces the total incremental energy consumption and better protects the performance of VM running and migrating compared with randomly migrating and optimally migrating. As a result, the proposed PS-ES approach has capabilities to make the result of live VM migration events more high-effective and valuable.

  16. A Heuristic Placement Selection of Live Virtual Machine Migration for Energy-Saving in Cloud Computing Environment

    PubMed Central

    Zhao, Jia; Hu, Liang; Ding, Yan; Xu, Gaochao; Hu, Ming

    2014-01-01

    The field of live VM (virtual machine) migration has been a hotspot problem in green cloud computing. Live VM migration problem is divided into two research aspects: live VM migration mechanism and live VM migration policy. In the meanwhile, with the development of energy-aware computing, we have focused on the VM placement selection of live migration, namely live VM migration policy for energy saving. In this paper, a novel heuristic approach PS-ES is presented. Its main idea includes two parts. One is that it combines the PSO (particle swarm optimization) idea with the SA (simulated annealing) idea to achieve an improved PSO-based approach with the better global search's ability. The other one is that it uses the Probability Theory and Mathematical Statistics and once again utilizes the SA idea to deal with the data obtained from the improved PSO-based process to get the final solution. And thus the whole approach achieves a long-term optimization for energy saving as it has considered not only the optimization of the current problem scenario but also that of the future problem. The experimental results demonstrate that PS-ES evidently reduces the total incremental energy consumption and better protects the performance of VM running and migrating compared with randomly migrating and optimally migrating. As a result, the proposed PS-ES approach has capabilities to make the result of live VM migration events more high-effective and valuable. PMID:25251339

  17. The influence of sodium on the point defect characteristics in off stoichiometric CuInSe2

    NASA Astrophysics Data System (ADS)

    Stephan, Christiane; Greiner, Dieter; Schorr, Susan; Kaufmann, Christian A.

    2016-11-01

    The device performance of polycrystalline chalcogenide thin film solar cells is strongly influenced by different kinds of defects within the material. The presence of sodium or other alkali metals like potassium during the deposition process is well known to influence the electronic properties of the solar cell and thus to improve the efficiency of the final device. Structural analysis of neutron powder diffraction data collected at low temperatures and subsequent profile analysis by the LeBail and Rietveld method demonstrates the impact of sodium on the point defect characteristics in off stoichiometric CuInSe2. The analyzed materials are powder and thin film solar absorber material with addition of NaF and free of sodium. It is illustrated, the so called "sodium effect" cannot be reduced to one single origin. A range of effects, the reduction of InCu donors with a followed increase of VCu acceptors is possible. The main effect is an increased ordered character of the chalcopyrite crystal structure at off stoichiometric composition, when containing sodium.

  18. Carbon, oxygen and their interaction with intrinsic point defects in solar silicon ribbon material: A speculative approach

    NASA Technical Reports Server (NTRS)

    Goesele, U.; Ast, D. G.

    1983-01-01

    Some background information on intrinsic point defects is provided and on carbon and oxygen in silicon in so far as it may be relevant for the efficiency of solar cells fabricated from EFG ribbon material. The co-precipitation of carbon and oxygen and especially of carbon and silicon self interstitials are discussed. A simple model for the electrical activity of carbon-self-interstitial agglomerates is presented. The self-interstitial content of these agglomerates is assumed to determine their electrical activity and that both compressive stresses (high self-interstitial content) and tensile stresses (low self-interstitial content) give rise to electrical activity of the agglomerates. The self-interstitial content of these carbon-related agglomerates may be reduced by an appropriate high temperature treatment and enhanced by a supersaturation of self-interstitials generated during formation of the p-n junction of solar cells. Oxygen present in supersaturation in carbon-rich silicon may be induced to form SiO, precipitates by self-interstitials generated during phosphorus diffusion. It is proposed that the SiO2-Si interface of the precipates gives rise to a continuum of donor stables and that these interface states are responsible for at least part of the light inhancement effects observed in oxygen containing EFG silicon after phosphorus diffusion.

  19. The interaction of point defects with line dislocations in HVEM (high voltage electron microscope) irradiated Fe-Ni-Cr alloys

    SciTech Connect

    King, S.L.; Jenkins, M.L. . Dept. of Materials); Kirk, M.A. ); English, C.A. . Materials Development Div.)

    1990-05-01

    This paper presents results of a study of the interaction of point defects produced by high voltage electron microscope (HVEM) irradiation with pre-existing dislocations in austenitic Fe-15% 25%Ni-17%Cr alloys, aimed at the determination of the mechanisms of climb of dissociated dislocations. Dislocations were initially characterized at sub-threshold voltages (here 200kV) using the weak-beam technique. These dislocations were then irradiated with 1MeV electrons in the Argonne HVEM before being returned to a lower voltage microscope for post-irradiation characterization. Interstitial climb was seen only at particularly favorable sites, such as pre-existing jogs, whilst vacancies clustered near dislocations, forming stacking fault tetrahedra (SFT). Partial separations were also observed to have decreased after irradiation. The post-irradiation configuration was found to depend strongly on both dislocation character and pre-irradiation dislocation configuration. These results, and their relevance to the void swelling problem, are discussed. 52 refs., 8 figs.

  20. Line and Point Defects in MoSe2 Bilayer Studied by Scanning Tunneling Microscopy and Spectroscopy.

    PubMed

    Liu, Hongjun; Zheng, Hao; Yang, Fang; Jiao, Lu; Chen, Jinglei; Ho, Wingkin; Gao, Chunlei; Jia, Jinfeng; Xie, Maohai

    2015-06-23

    Bilayer (BL) MoSe2 films grown by molecular-beam epitaxy (MBE) are studied by scanning tunneling microscopy and spectroscopy (STM/S). Similar to monolayer (ML) films, networks of inversion domain boundary (DB) defects are observed both in the top and bottom layers of BL MoSe2, and often they are seen spatially correlated such that one is on top of the other. There are also isolated ones in the bottom layer without companion in the top-layer and are detected by STM/S through quantum tunneling of the defect states through the barrier of the MoSe2 ML. Comparing the DB states in BL MoSe2 with that of ML film reveals some common features as well as differences. Quantum confinement of the defect states is indicated. Point defects in BL MoSe2 are also observed by STM/S, where ionization of the donor defect by the tip-induced electric field is evidenced. These results are of great fundamental interests as well as practical relevance of devices made of MoSe2 ultrathin layers. PMID:26051223

  1. Giant electric-field induced strain in ferroelectric crystals by point-defect mediated reversible domain switching

    NASA Astrophysics Data System (ADS)

    Ren, Xiaobing

    2004-03-01

    Ferroelectric crystals are characterized by their asymmetric or polar structures. In electric field, ions undergo asymmetric displacement and result in a small change in crystal dimension, which is proportional to the applied field. Such electric-field induced strain (or piezoelectricity) has found extensive applications in actuators and sensors. However, the effect is generally very small and thus limits its usefulness. Here I show that with a different mechanism, an aged BaTiO3 single crystal can generate a large recoverable non-linear strain of 0.75% at a low field of 200V/mm. At the same field this value is about 40 times higher than piezoelectric PZT ceramics and more than 10 times higher than the high strain PZN-PT single crystals. This giant electro-strain stems from an unusual reversible domain switching (most importantly the switching of non-180^o domains) in which the restoring force is provided by a general symmetry-conforming property of point defects. This mechanism provides a general method to achieve large electro-strain effect in a wide range of ferroelectric systems and the effect may lead to novel applications in ultra large stroke and non-linear actuators.

  2. Large electric-field-induced strain in ferroelectric crystals by point-defect-mediated reversible domain switching

    NASA Astrophysics Data System (ADS)

    Ren, Xiaobing

    2004-02-01

    Ferroelectric crystals are characterized by their asymmetric or polar structures. In an electric field, ions undergo asymmetric displacement and result in a small change in crystal dimension, which is proportional to the applied field. Such electric-field-induced strain (or piezoelectricity) has found extensive applications in actuators and sensors. However, the effect is generally very small and thus limits its usefulness. Here I show that with a different mechanism, an aged BaTiO3 single crystal can generate a large recoverable nonlinear strain of 0.75% at a low field of 200 V mm-1. At the same field this value is about 40 times higher than piezoelectric Pb(Zr, Ti)O3 (PZT) ceramics and more than 10 times higher than the high-strain Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) single crystals. This large electro-strain stems from an unusual reversible domain switching (most importantly the switching of non-180° domains) in which the restoring force is provided by a general symmetry-conforming property of point defects. This mechanism provides a general method to achieve large electro-strain effect in a wide range of ferroelectric systems and the effect may lead to novel applications in ultra-large stroke and nonlinear actuators.

  3. All-optical XOR and OR logic gates based on line and point defects in 2-D photonic crystal

    NASA Astrophysics Data System (ADS)

    Goudarzi, Kiyanoosh; Mir, Ali; Chaharmahali, Iman; Goudarzi, Dariush

    2016-04-01

    In this paper, we have proposed an all-optical logic gate structure based on line and point defects created in the two dimensional square lattice of silicon rods in air photonic crystals (PhCs). Line defects are embedded in the DX and DZ directions of the momentum space. The device has two input and two output ports. It has been shown analytically whether the initial phase difference between the two input beams is π/2, they interfere together constructively or destructively to realize the logical functions. The simulation results show that the device can acts as a XOR and an OR logic gate. It is applicable in the frequency range of 0-0.45 (a/λ), however we set it at (a/λ=) 0.419 for low dispersion condition, correspondingly the lambda is equal to 1.55 μm. The maximum delay time to response to the input signals is about 0.4 ps, hence the speed of the device is about 2.5 THz. Also 6.767 dB is the maximum contrast ratio of the device.

  4. Detection of one-dimensional migration of single self-interstitial atoms in tungsten using high-voltage electron microscopy

    PubMed Central

    Amino, T.; Arakawa, K.; Mori, H.

    2016-01-01

    The dynamic behaviour of atomic-size disarrangements of atoms—point defects (self-interstitial atoms (SIAs) and vacancies)—often governs the macroscopic properties of crystalline materials. However, the dynamics of SIAs have not been fully uncovered because of their rapid migration. Using a combination of high-voltage transmission electron microscopy and exhaustive kinetic Monte Carlo simulations, we determine the dynamics of the rapidly migrating SIAs from the formation process of the nanoscale SIA clusters in tungsten as a typical body-centred cubic (BCC) structure metal under the constant-rate production of both types of point defects with high-energy electron irradiation, which must reflect the dynamics of individual SIAs. We reveal that the migration dimension of SIAs is not three-dimensional (3D) but one-dimensional (1D). This result overturns the long-standing and well-accepted view of SIAs in BCC metals and supports recent results obtained by ab-initio simulations. The SIA dynamics clarified here will be one of the key factors to accurately predict the lifetimes of nuclear fission and fusion materials. PMID:27185352

  5. Detection of one-dimensional migration of single self-interstitial atoms in tungsten using high-voltage electron microscopy.

    PubMed

    Amino, T; Arakawa, K; Mori, H

    2016-05-17

    The dynamic behaviour of atomic-size disarrangements of atoms-point defects (self-interstitial atoms (SIAs) and vacancies)-often governs the macroscopic properties of crystalline materials. However, the dynamics of SIAs have not been fully uncovered because of their rapid migration. Using a combination of high-voltage transmission electron microscopy and exhaustive kinetic Monte Carlo simulations, we determine the dynamics of the rapidly migrating SIAs from the formation process of the nanoscale SIA clusters in tungsten as a typical body-centred cubic (BCC) structure metal under the constant-rate production of both types of point defects with high-energy electron irradiation, which must reflect the dynamics of individual SIAs. We reveal that the migration dimension of SIAs is not three-dimensional (3D) but one-dimensional (1D). This result overturns the long-standing and well-accepted view of SIAs in BCC metals and supports recent results obtained by ab-initio simulations. The SIA dynamics clarified here will be one of the key factors to accurately predict the lifetimes of nuclear fission and fusion materials.

  6. Detection of one-dimensional migration of single self-interstitial atoms in tungsten using high-voltage electron microscopy

    NASA Astrophysics Data System (ADS)

    Amino, T.; Arakawa, K.; Mori, H.

    2016-05-01

    The dynamic behaviour of atomic-size disarrangements of atoms—point defects (self-interstitial atoms (SIAs) and vacancies)—often governs the macroscopic properties of crystalline materials. However, the dynamics of SIAs have not been fully uncovered because of their rapid migration. Using a combination of high-voltage transmission electron microscopy and exhaustive kinetic Monte Carlo simulations, we determine the dynamics of the rapidly migrating SIAs from the formation process of the nanoscale SIA clusters in tungsten as a typical body-centred cubic (BCC) structure metal under the constant-rate production of both types of point defects with high-energy electron irradiation, which must reflect the dynamics of individual SIAs. We reveal that the migration dimension of SIAs is not three-dimensional (3D) but one-dimensional (1D). This result overturns the long-standing and well-accepted view of SIAs in BCC metals and supports recent results obtained by ab-initio simulations. The SIA dynamics clarified here will be one of the key factors to accurately predict the lifetimes of nuclear fission and fusion materials.

  7. Detection of one-dimensional migration of single self-interstitial atoms in tungsten using high-voltage electron microscopy.

    PubMed

    Amino, T; Arakawa, K; Mori, H

    2016-01-01

    The dynamic behaviour of atomic-size disarrangements of atoms-point defects (self-interstitial atoms (SIAs) and vacancies)-often governs the macroscopic properties of crystalline materials. However, the dynamics of SIAs have not been fully uncovered because of their rapid migration. Using a combination of high-voltage transmission electron microscopy and exhaustive kinetic Monte Carlo simulations, we determine the dynamics of the rapidly migrating SIAs from the formation process of the nanoscale SIA clusters in tungsten as a typical body-centred cubic (BCC) structure metal under the constant-rate production of both types of point defects with high-energy electron irradiation, which must reflect the dynamics of individual SIAs. We reveal that the migration dimension of SIAs is not three-dimensional (3D) but one-dimensional (1D). This result overturns the long-standing and well-accepted view of SIAs in BCC metals and supports recent results obtained by ab-initio simulations. The SIA dynamics clarified here will be one of the key factors to accurately predict the lifetimes of nuclear fission and fusion materials. PMID:27185352

  8. Energy Migration Upconversion in Manganese(II)-Doped Nanoparticles.

    PubMed

    Li, Xiyan; Liu, Xiaowang; Chevrier, Daniel M; Qin, Xian; Xie, Xiaoji; Song, Shuyan; Zhang, Hongjie; Zhang, Peng; Liu, Xiaogang

    2015-11-01

    We report the synthesis and characterization of cubic NaGdF4:Yb/Tm@NaGdF4:Mn core-shell structures. By taking advantage of energy transfer through Yb→Tm→Gd→Mn in these core-shell nanoparticles, we have realized upconversion emission of Mn(2+) at room temperature in lanthanide tetrafluoride based host lattices. The upconverted Mn(2+) emission, enabled by trapping the excitation energy through a Gd(3+) lattice, was validated by the observation of a decreased lifetime from 941 to 532 μs in the emission of Gd(3+) at 310 nm ((6)P(7/2)→(8)S(7/2)). This multiphoton upconversion process can be further enhanced under pulsed laser excitation at high power densities. Both experimental and theoretical studies provide evidence for Mn(2+) doping in the lanthanide-based host lattice arising from the formation of F(-) vacancies around Mn(2+) ions to maintain charge neutrality in the shell layer. PMID:26358961

  9. Current-induced changes of migration energy barriers in graphene and carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Obodo, J. T.; Rungger, I.; Sanvito, S.; Schwingenschlögl, U.

    2016-05-01

    An electron current can move atoms in a nanoscale device with important consequences for the device operation and breakdown. We perform first principles calculations aimed at evaluating the possibility of changing the energy barriers for atom migration in carbon-based systems. In particular, we consider the migration of adatoms and defects in graphene and carbon nanotubes. Although the current-induced forces are large for both the systems, in graphene the force component along the migration path is small and therefore the barrier height is little affected by the current flow. In contrast, the same barrier is significantly reduced in carbon nanotubes as the current increases. Our work also provides a real-system numerical demonstration that current-induced forces within density functional theory are non-conservative.An electron current can move atoms in a nanoscale device with important consequences for the device operation and breakdown. We perform first principles calculations aimed at evaluating the possibility of changing the energy barriers for atom migration in carbon-based systems. In particular, we consider the migration of adatoms and defects in graphene and carbon nanotubes. Although the current-induced forces are large for both the systems, in graphene the force component along the migration path is small and therefore the barrier height is little affected by the current flow. In contrast, the same barrier is significantly reduced in carbon nanotubes as the current increases. Our work also provides a real-system numerical demonstration that current-induced forces within density functional theory are non-conservative. Electronic supplementary information (ESI) available. See DOI: 10.1039/C6NR00534A

  10. Native point defects in CuIn(1-x)Ga(x)Se2: hybrid density functional calculations predict the origin of p- and n-type conductivity.

    PubMed

    Bekaert, J; Saniz, R; Partoens, B; Lamoen, D

    2014-10-28

    We have performed a first-principles study of the p- and n-type conductivity in CuIn(1-x)Ga(x)Se2 due to native point defects, based on the HSE06 hybrid functional. Band alignment shows that the band gap becomes larger with x due to the increasing conduction band minimum, rendering it hard to establish n-type conductivity in CuGaSe2. From the defect formation energies, we find that In/GaCu is a shallow donor, while V(Cu), V(In/Ga) and Cu(In/Ga) act as shallow acceptors. Using the total charge neutrality of ionized defects and intrinsic charge carriers to determine the Fermi level, we show that under In-rich growth conditions InCu causes strongly n-type conductivity in CuInSe2. Under increasingly In-poor growth conditions, the conductivity type in CuInSe2 alters to p-type and compensation of the acceptors by In(Cu) reduces, as also observed in photoluminescence experiments. In CuGaSe2, the native acceptors pin the Fermi level far away from the conduction band minimum, thus inhibiting n-type conductivity. On the other hand, CuGaSe2 shows strong p-type conductivity under a wide range of Ga-poor growth conditions. Maximal p-type conductivity in CuIn(1-x)Ga(x)Se2 is reached under In/Ga-poor growth conditions, in agreement with charge concentration measurements on samples with In/Ga-poor stoichiometry, and is primarily due to the dominant acceptor Cu(In/Ga).

  11. Thermocapillary Bubble Migration - An Oseen-Like Analysis of the Energy Equation

    NASA Technical Reports Server (NTRS)

    Balasubramaniam, R.; Dill, L. H.

    1992-01-01

    The thermocapillary migration of a bubble in a liquid possessing a temperature gradient is analyzed in the limit of large Reynolds and Marangoni numbers. Crespo and Manuel (1983) performed an analysis in this limit wherein energy conduction is completely neglected and obtained the bubble migration velocity using energy dissipation arguments. In the present analysis, performed in a coordinate system moving with the bubble, the velocity field in the convection term in the energy equation is approximated in an Oseen-like manner by replacing it with the velocity field far away from the bubble (i.e., the migration velocity of the bubble). Conduction is retained to satisfy the zero conductive heat flux boundary condition on the bubble surface. An approximate solution has been obtained for the Oseen-like energy equation. The bubble velocity obtained using energy dissipation considerations is in agreement with the result of Crespo and Manuel. The solution shows the thermal boundary layer and wake structure in the vicinity of the bubble. The Oseen-like analysis, however, has inherent limitations, as the flow penetrates the bubble surface. These issues are discussed and the result are compared to those in the literature.

  12. Inherent point defects at the thermal higher-Miller index (211)Si/SiO{sub 2} interface

    SciTech Connect

    Iacovo, S.; Stesmans, A.

    2014-12-29

    Electron spin resonance (ESR) studies were carried out on the higher-Miller index (211)Si/SiO{sub 2} interface thermally grown in the temperature range T{sub ox} = 400–1066 °C to assess interface quality in terms of inherently incorporated point defects. This reveals the presence predominantly of two species of a P{sub b}-type interface defect (interfacial Si dangling bond), which, based on pertinent ESR parameters, is typified as P{sub b0}{sup (211)} variant, close to the P{sub b0} center observed in standard (100)Si/SiO{sub 2}—known as utmost detrimental interface trap. T{sub ox} ≳ 750 °C is required to minimize the P{sub b0}{sup (211)} defect density (∼4.2 × 10{sup 12 }cm{sup −2}; optimized interface). The data clearly reflect the non-elemental nature of the (211)Si face as an average of (100) and (111) surfaces. It is found that in oxidizing (211)Si at T{sub ox} ≳ 750 °C, the optimum Si/SiO{sub 2} interface quality is retained for the two constituent low-index (100) and (111) faces separately, indicating firm anticipating power for higher-index Si/SiO{sub 2} interfaces in general. It implies that, as a whole, the quality of a thermal higher-index Si/SiO{sub 2} interface can never surmount that of the low-index (100)Si/SiO{sub 2} structure.

  13. Light Harvesting in Microscale Metal-Organic Frameworks by Energy Migration and Interfacial Electron Transfer Quenching

    SciTech Connect

    Kent, Caleb A.; Liu, Demin; Ma, Liqing; Papanikolas, John M.; Meyer, Thomas J.; Lin, Wenbin

    2011-08-24

    Microscale metal–organic frameworks (MOFs) were synthesized from photoactive Ru(II)-bpy building blocks with strong visible light absorption and long-lived triplet metal-to-ligand charge transfer (³MLCT) excited states. These MOFs underwent efficient luminescence quenching in the presence of either oxidative or reductive quenchers. Up to 98% emission quenching was achieved with either an oxidative quencher (1,4-benzoquinone) or a reductive quencher (N,N,N',N'-tetramethylbenzidine), as a result of rapid energy migration over several hundred nanometers followed by efficient electron transfer quenching at the MOF/solution interface. The photoactive MOFs act as an excellent light-harvesting system by combining intraframework energy migration and interfacial electron transfer quenching.

  14. Nonstoichiometry, point defects and magnetic properties in Sr{sub 2}FeMoO{sub 6-{delta}} double perovskites

    SciTech Connect

    Kircheisen, R.; Toepfer, J.

    2012-01-15

    The phase stability, nonstoichiometry and point defect chemistry of polycrystalline Sr{sub 2}FeMoO{sub 6-{delta}} (SFMO) was studied by thermogravimety at 1000, 1100, and 1200 Degree-Sign C. Single-phase SFMO exists between -10.2{<=}log pO{sub 2}{<=}-13.7 at 1200 Degree-Sign C. At lower oxygen partial pressure a mass loss signals reductive decomposition. At higher pO{sub 2} a mass gain indicates oxidative decomposition into SrMoO{sub 4} and SrFeO{sub 3-x}. The nonstoichiometry {delta} at 1000, 1100, and 1200 Degree-Sign C was determined as function of pO{sub 2}. SFMO is almost stoichiometric at the upper phase boundary (e.g. {delta}=0.006 at 1200 Degree-Sign C and log pO{sub 2}=-10.2) and becomes more defective with decreasing oxygen partial pressure (e.g. {delta}=0.085 at 1200 Degree-Sign C and log pO{sub 2}=-13.5). Oxygen vacancies are shown to represent majority defects. From the temperature dependence of the oxygen vacancy concentration the defect formation enthalpy was estimated ({Delta}H{sub OV}=253{+-}8 kJ/mol). Samples of different nonstoichiometry {delta} were prepared by quenching from 1200 Degree-Sign C at various pO{sub 2}. An increase of the unit cell volume with increasing defect concentration {delta} was found. The saturation magnetization is reduced with increasing nonstoichiometry {delta}. This demonstrates that in addition to Fe/Mo site disorder, oxygen nonstoichiometry is another source of reduced magnetization values. - Graphical abstract: Nonstoichiometry {delta} of Sr{sub 2}FeMoO{sub 6-{delta}} as function of oxygen partial pressure at 1000, 1100, and 1200 Degree-Sign C. Highlights: Black-Right-Pointing-Pointer Sr{sub 2}FeMoO{sub 6-{delta}} is stable at T=1200 Degree-Sign C at low pO{sub 2} only. Black-Right-Pointing-Pointer Nonstoichiometry {delta} measured at 1200, 1100, and 1000 Degree-Sign C. Black-Right-Pointing-Pointer Increase of oxygen vacancy concentration with lower pO{sub 2}. Black-Right-Pointing-Pointer Reduction of magnetization

  15. Eel migration to the Sargasso: remarkably high swimming efficiency and low energy costs.

    PubMed

    van Ginneken, Vincent; Antonissen, Erik; Müller, Ulrike K; Booms, Ronald; Eding, Ep; Verreth, Johan; van den Thillart, Guido

    2005-04-01

    One of the mysteries of the animal kingdom is the long-distance migration (5000-6000 km) of the European eel Anguilla anguilla L. from the coasts of Europe to its spawning grounds in the Sargasso Sea. The only evidence for the location of the spawning site of the European eel in the Sargasso Sea is the discovery by Johannes Schmidt at the beginning of the previous century of the smallest eel larvae (leptocephali) near the Sargasso Sea. For years it has been questioned whether the fasting eels have sufficient energy reserves to cover this enormous distance. We have tested Schmidt's theory by placing eels in swim tunnels in the laboratory and allowing them to make a simulated migration of 5500 km. We find that eels swim 4-6 times more efficiently than non-eel-like fish. Our findings are an important advance in this field because they remove a central objection to Schmidt's theory by showing that their energy reserves are, in principle, sufficient for the migration. Conclusive proof of the Sargasso Sea theory is likely to come from satellite tracking technology.

  16. An efficient algorithm for finding the minimum energy path for cation migration in ionic materials.

    PubMed

    Rong, Ziqin; Kitchaev, Daniil; Canepa, Pieremanuele; Huang, Wenxuan; Ceder, Gerbrand

    2016-08-21

    The Nudged Elastic Band (NEB) is an established method for finding minimum-energy paths and energy barriers of ion migration in materials, but has been hampered in its general application by its significant computational expense when coupled with density functional theory (DFT) calculations. Typically, an NEB calculation is initialized from a linear interpolation of successive intermediate structures (also known as images) between known initial and final states. However, the linear interpolation introduces two problems: (1) slow convergence of the calculation, particularly in cases where the final path exhibits notable curvature; (2) divergence of the NEB calculations if any intermediate image comes too close to a non-diffusing species, causing instabilities in the ensuing calculation. In this work, we propose a new scheme to accelerate NEB calculations through an improved path initialization and associated energy estimation workflow. We demonstrate that for cation migration in an ionic framework, initializing the diffusion path as the minimum energy path through a static potential built upon the DFT charge density reproduces the true NEB path within a 0.2 Å deviation and yields up to a 25% improvement in typical NEB runtimes. Furthermore, we find that the locally relaxed energy barrier derived from this initialization yields a good approximation of the NEB barrier, with errors within 20 meV of the true NEB value, while reducing computational expense by up to a factor of 5. Finally, and of critical importance for the automation of migration path calculations in high-throughput studies, we find that the new approach significantly enhances the stability of the calculation by avoiding unphysical image initialization. Our algorithm promises to enable efficient calculations of diffusion pathways, resolving a long-standing obstacle to the computational screening of intercalation compounds for Li-ion and multivalent batteries.

  17. An efficient algorithm for finding the minimum energy path for cation migration in ionic materials.

    PubMed

    Rong, Ziqin; Kitchaev, Daniil; Canepa, Pieremanuele; Huang, Wenxuan; Ceder, Gerbrand

    2016-08-21

    The Nudged Elastic Band (NEB) is an established method for finding minimum-energy paths and energy barriers of ion migration in materials, but has been hampered in its general application by its significant computational expense when coupled with density functional theory (DFT) calculations. Typically, an NEB calculation is initialized from a linear interpolation of successive intermediate structures (also known as images) between known initial and final states. However, the linear interpolation introduces two problems: (1) slow convergence of the calculation, particularly in cases where the final path exhibits notable curvature; (2) divergence of the NEB calculations if any intermediate image comes too close to a non-diffusing species, causing instabilities in the ensuing calculation. In this work, we propose a new scheme to accelerate NEB calculations through an improved path initialization and associated energy estimation workflow. We demonstrate that for cation migration in an ionic framework, initializing the diffusion path as the minimum energy path through a static potential built upon the DFT charge density reproduces the true NEB path within a 0.2 Å deviation and yields up to a 25% improvement in typical NEB runtimes. Furthermore, we find that the locally relaxed energy barrier derived from this initialization yields a good approximation of the NEB barrier, with errors within 20 meV of the true NEB value, while reducing computational expense by up to a factor of 5. Finally, and of critical importance for the automation of migration path calculations in high-throughput studies, we find that the new approach significantly enhances the stability of the calculation by avoiding unphysical image initialization. Our algorithm promises to enable efficient calculations of diffusion pathways, resolving a long-standing obstacle to the computational screening of intercalation compounds for Li-ion and multivalent batteries. PMID:27544092

  18. An efficient algorithm for finding the minimum energy path for cation migration in ionic materials

    NASA Astrophysics Data System (ADS)

    Rong, Ziqin; Kitchaev, Daniil; Canepa, Pieremanuele; Huang, Wenxuan; Ceder, Gerbrand

    2016-08-01

    The Nudged Elastic Band (NEB) is an established method for finding minimum-energy paths and energy barriers of ion migration in materials, but has been hampered in its general application by its significant computational expense when coupled with density functional theory (DFT) calculations. Typically, an NEB calculation is initialized from a linear interpolation of successive intermediate structures (also known as images) between known initial and final states. However, the linear interpolation introduces two problems: (1) slow convergence of the calculation, particularly in cases where the final path exhibits notable curvature; (2) divergence of the NEB calculations if any intermediate image comes too close to a non-diffusing species, causing instabilities in the ensuing calculation. In this work, we propose a new scheme to accelerate NEB calculations through an improved path initialization and associated energy estimation workflow. We demonstrate that for cation migration in an ionic framework, initializing the diffusion path as the minimum energy path through a static potential built upon the DFT charge density reproduces the true NEB path within a 0.2 Å deviation and yields up to a 25% improvement in typical NEB runtimes. Furthermore, we find that the locally relaxed energy barrier derived from this initialization yields a good approximation of the NEB barrier, with errors within 20 meV of the true NEB value, while reducing computational expense by up to a factor of 5. Finally, and of critical importance for the automation of migration path calculations in high-throughput studies, we find that the new approach significantly enhances the stability of the calculation by avoiding unphysical image initialization. Our algorithm promises to enable efficient calculations of diffusion pathways, resolving a long-standing obstacle to the computational screening of intercalation compounds for Li-ion and multivalent batteries.

  19. Effect of ultrasonic cavitation on the diffusivity of a point defect in the passive film on formed Nb in 0.5 M HCl solution.

    PubMed

    Li, D G

    2015-11-01

    This work primarily focused on the influence of ultrasonic cavitation on the transport property of the point defect in the passive film on formed Nb in 0.5M HCl solution via electrochemical techniques based on the point defect model (PDM). The influence of ultrasonic cavitation on the composition and structure of the passive film was detected by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The transport property of a point defect in the passive film was characterized by the diffusivity of the point defect (D0). The influences of the ultrasonic cavitation power, passivated time and the distance between horn bottom and sample surface on D0 were analyzed. The results demonstrated that the passive film formed on Nb was an n-type semiconductor with a donor density (ND) ranging from 10(19) cm(-3) to 10(20) cm(-3) in the case of static state, while the order of ND increased one to two times by applying ultrasonic cavitation during film formation. The diffusivity of the point defect (D0) in the passive film formed on Nb at 0.5 V for 1 h in a 0.5 M HCl solution in the static state was calculated to be 9.704×10(-18) cm(2) s(-1), and it increased to 1.255×10(-16) cm(2) s(-1), 7.259×10(-16) cm(2) s(-1) and 7.296×10(-15) cm(2) s(-1) when applying the 180 W, 270 W and 450 W ultrasonic cavitation powers during film formation. D0 increased with the increment of the ultrasonic cavitation power, and decreased with the increased in formation time and distance between the horn bottom and sample surface. AES results showed the film structure and composition were changed by applying the ultrasonic cavitation. XPS results revealed that the passive film was mainly composed of Nb2O5 in the static state, and the low valence Nb-oxide (NbO) appeared in the passive film except Nb2O5 in the case of applying a 270 W ultrasonic cavitation power.

  20. Surviving utopia: Energy, social capital, and international migration in Ixcan, Guatemala

    NASA Astrophysics Data System (ADS)

    Taylor, Matthew John

    Mounting peasant impoverishment in Guatemala comes face to face with growing ecological impoverishment. Abysmal living standards for Guatemala's majority results from highly skewed land distribution, rapid population growth, and a brutal civil war, which lasted almost four decades and laid waste to many rural communities and fields. In the face of such adversity, Guatemalans migrate to remaining forested frontiers and make longer journeys to North America in search of work. In an attempt to understand and improve natural resource use, especially firewood, I uncover how networks of social relations (social capital) and international migration influence livelihoods in agricultural communities along a forested frontier. I used both qualitative and quantitative methods to gather information about the lives of residents in four agricultural villages in Ixcan, Guatemala. The results from extended fieldwork illustrate how high levels of social capital can benefit the lives of rural residents. I argue that development programs can take advantage of existing high levels of social capital and take measures to create social capital where it is lacking to ensure the successful implementation of development programs. I also discuss firewood management in each community and demonstrate the disjuncture between local firewood use and national energy plans. Finally, I show how migrants and the money they send home from North America radically alter land use and land distribution in this part of rural Guatemala. My study reveals the need to examine the linkages between large-scale international migration, social capital, and the environment in communities that rely on the land for survival.

  1. Using a biocultural approach to examine migration/globalization, diet quality, and energy balance.

    PubMed

    Himmelgreen, David A; Cantor, Allison; Arias, Sara; Romero Daza, Nancy

    2014-07-01

    The aim of this paper is to examine the role and impact that globalization and migration (e.g., intra-/intercontinental, urban/rural, and circular) have had on diet patterns, diet quality, and energy balance as reported on in the literature during the last 20 years. Published literature from the fields of anthropology, public health, nutrition, and other disciplines (e.g., economics) was collected and reviewed. In addition, case studies from the authors' own research are presented in order to elaborate on key points and dietary trends identified in the literature. While this review is not intended to be comprehensive, the findings suggest that the effects of migration and globalization on diet quality and energy balance are neither lineal nor direct, and that the role of social and physical environments, culture, social organization, and technology must be taken into account to better understand this relationship. Moreover, concepts such as acculturation and the nutrition transition do not necessarily explain or adequately describe all of the global processes that shape diet quality and energy balance. Theories from nutritional anthropology and critical bio-cultural medical anthropology are used to tease out some of these complex interrelationships.

  2. O₂migration rates in [NiFe] hydrogenases. A joint approach combining free-energy calculations and kinetic modeling.

    PubMed

    Topin, Jérémie; Diharce, Julien; Fiorucci, Sébastien; Antonczak, Serge; Golebiowski, Jérôme

    2014-01-23

    Hydrogenases are promising candidates for the catalytic production of green energy by means of biological ways. The major impediment to such a production is rooted in their inhibition under aerobic conditions. In this work, we model dioxygen migration rates in mutants of a hydrogenase of Desulfovibrio fructusovorans. The approach relies on the calculation of the whole potential of mean force for O2 migration within the wild-type as well as in V74M, V74F, and V74Q mutant channels. The three free-energy barriers along the entire migration pathway are converted into chemical rates through modeling based on Transition State Theory. The use of such a model recovers the trend of O2 migration rates among the series.

  3. Meta-ecosystems and biological energy transport from ocean to coast: the ecological importance of herring migration.

    PubMed

    Varpe, Oystein; Fiksen, Oyvind; Slotte, Aril

    2005-12-01

    Ecosystems are not closed, but receive resource subsidies from other ecosystems. Energy, material and organisms are moved between systems by physical vectors, but migrating animals also transport resources between systems. We report on large scale energy transport from ocean to coast by a migrating fish population, the Norwegian spring-spawning (NSS) herring Clupea harengus. We observe a rapid body mass increase during parts of the annual, oceanic feeding migration and we use a bioenergetics model to quantify energy consumption. The model predicts strong seasonal variation in food consumption with a marked peak in late May to July. The copepod Calanus finmarchicus is the most important prey and 23 x 10(6) tones (wet weight) of C. finmarchicus is consumed annually. The annual consumption-biomass ratio is 5.2. During the feeding migration 17% of consumed energy is converted to body mass. The biomass transported to the coast and left as reproductive output is estimated from gonad weight and is about 1.3 x 10(6) tones for the current population. This transport is to our knowledge the world's largest flux of energy caused by a single population. We demonstrate marked temporal variation in transport during the last century and discuss the effects of NSS herring in the ocean, as a major consumer, and at the coast, where eggs and larvae are important for coastal predators. In particular, we suggest that the rapid decline of lobster Homarus gammarus landings in Western Norway during the 1960s was related to the collapse of NSS herring. We also discuss spatial variation in energy transport caused by changed migration patterns. Both climate and fisheries probably triggered historical changes in the migration patterns of NSS herring. New migration routes emerge at the level of individuals, which in turn determines where resources are gathered and delivered, and therefore, how meta-ecosystems function. PMID:16195881

  4. Meta-ecosystems and biological energy transport from ocean to coast: the ecological importance of herring migration.

    PubMed

    Varpe, Oystein; Fiksen, Oyvind; Slotte, Aril

    2005-12-01

    Ecosystems are not closed, but receive resource subsidies from other ecosystems. Energy, material and organisms are moved between systems by physical vectors, but migrating animals also transport resources between systems. We report on large scale energy transport from ocean to coast by a migrating fish population, the Norwegian spring-spawning (NSS) herring Clupea harengus. We observe a rapid body mass increase during parts of the annual, oceanic feeding migration and we use a bioenergetics model to quantify energy consumption. The model predicts strong seasonal variation in food consumption with a marked peak in late May to July. The copepod Calanus finmarchicus is the most important prey and 23 x 10(6) tones (wet weight) of C. finmarchicus is consumed annually. The annual consumption-biomass ratio is 5.2. During the feeding migration 17% of consumed energy is converted to body mass. The biomass transported to the coast and left as reproductive output is estimated from gonad weight and is about 1.3 x 10(6) tones for the current population. This transport is to our knowledge the world's largest flux of energy caused by a single population. We demonstrate marked temporal variation in transport during the last century and discuss the effects of NSS herring in the ocean, as a major consumer, and at the coast, where eggs and larvae are important for coastal predators. In particular, we suggest that the rapid decline of lobster Homarus gammarus landings in Western Norway during the 1960s was related to the collapse of NSS herring. We also discuss spatial variation in energy transport caused by changed migration patterns. Both climate and fisheries probably triggered historical changes in the migration patterns of NSS herring. New migration routes emerge at the level of individuals, which in turn determines where resources are gathered and delivered, and therefore, how meta-ecosystems function.

  5. Enhancement of quantum well intermixing on InP/InGaAs/InGaAsP heterostructures using titanium oxide surface stressors to induce forced point defect diffusion

    SciTech Connect

    Francois, A.; Aimez, V.; Beauvais, J.; Gendry, M.; Regreny, P.

    2006-10-16

    Quantum well intermixing was studied on InP/InGaAs/InGaAsP heterostructures under stress induced by a TiO{sub x} surface stressor. Results provide a comparison of thermal emission wavelength shift and effective emission wavelength shift for samples intermixed with and without applied stress. It is shown that TiO{sub x} decreases the measured thermal shift depending on the amplitude of the induced stress. It is also shown that the diffusion of point defects created during ion implantation prior to TiO{sub x} stressor deposition is significantly enhanced. This results in an increase of the effective wavelength shift by up to 300%.

  6. Computer simulation of energy migration in the C-phycocyanin of the blue-green algae Agmenellum Quadruplicatum

    PubMed Central

    Demidov, Andrey A.; Borisov, Alexander Yu.

    1993-01-01

    Two methods for simulation of energy migration in the C-phycocyanin fragments of PBS were developed. Both methods are based on the statistical analysis of an excitation behavior in modeling complexes with a limited number (up to hundreds) of chromophores using the Monte-Carlo approach and calculation of migration rates for the system of linear balance equations. Energy migration rates were calculated in the case of C-phycocyanin of the blue-green algae Agmenellum quadruplicatum. The main channels of energy migration were determined in a monomer, trimer, hexamer, and in the rods consisting of 2-4 hexamers. The influence of the “screw” angle between two adjoining trimers of hexamer on the rates of energy migration and on its efficiencies in 1-4 hexamers was also estimated. The analysis was made for the average (random) and real orientation of chromophores in the C-phycocyanin. For both cases the optimal angle values were determined and the one for real C-phycocyanin structure was found to be very close (Δø ≤ 5°) to the optimal angle calculated. PMID:19431892

  7. Control of point defects in the Cu(In,Ga)Se2 film synthesized at low temperature from a Cu/In2Se3 stacked precursor

    NASA Astrophysics Data System (ADS)

    Jung, Gwang Sun; Kim, Seungtae; Ko, Young Min; Moon, Sun Hong; Choi, Yong Woo; Ahn, Byung Tae

    2016-07-01

    Low-temperature fabrication of Cu(In,Ga)Se2 (CIGS) film is essential for flexible CIGS solar cells. A large-grained CIGS film was synthesized with a Se-deficient Cu/In,Ga)2Se3 stacked precursor by reacting at 500 °C in a vacuum and was then subsequently annealing in Se environment. The CIGS solar cell with the as-prepared CIGS film had a poor cell performance due to a very low Ga composition at the CIGS surface. The surface Ga composition was controlled to 0.2 by supplying In, Ga, and Se in a temperature range of 350 to 500 °C. From an analysis of the photoluminescence spectra, we found that the point defects, Se vacancy and In-in-Cu antisite, in the CIGS film were greatly reduced by annealing below 450 °C. The short-circuit current was pronouncedly increased in the CIGS cells. The open-circuit voltage depended on both the Ga composition and Cu composition at the CIGS surface. In particular, a low Cu composition at the CIGS surface was essential for the higher performance solar cells. Our results indicated that CIGSs film synthesized at high temperature must be annealed at 350 °C or below to reduce undesirable point defects. [Figure not available: see fulltext.

  8. Intrinsic point defects in CuInSe2 and CuGaSe2 as seen via screened-exchange hybrid density functional theory

    NASA Astrophysics Data System (ADS)

    Pohl, Johan; Albe, Karsten

    2013-06-01

    A fully self-contained study of the thermodynamic and electronic properties of intrinsic point defects in the solar absorber materials CuInSe2 and CuGaSe2 based on screened-exchange hybrid density functional theory is presented. The results are partly at odds with data obtained within local density functional theory in former studies. GaCu electron traps as well as CuIn and CuGa hole traps are found to be the dominant intrinsic recombination centers. In contrast to the accepted view, complex formation of antisites with copper vacancies is not decisive for explaining the favorable properties of CuInSe2, since InCu is already a shallow defect by itself. The localization of holes is observed on CuIn and CuGa as well as on VIn and VGa when supercells of 216 atoms are used. Furthermore, the results raise doubts about the relevance of selenium vacancies and DX centers for experimentally observed metastabilities. Finally, a guide to the optimal preparation conditions in terms of the point defect physics of CuInSe2 and CuGaSe2 for their application as solar cell absorbers is provided.

  9. Modulation of contact resistance between metal and graphene by controlling the graphene edge, contact area, and point defects: An ab initio study

    SciTech Connect

    Ma, Bo; Wen, Yanwei E-mail: bshan@mail.hust.edu.cn; Gong, Cheng; Cho, Kyeongjae; Chen, Rong; Shan, Bin E-mail: bshan@mail.hust.edu.cn

    2014-05-14

    A systematic first-principles non-equilibrium Green's function study is conducted on the contact resistance between a series of metals (Au, Ag, Pt, Cu, Ni, and Pd) and graphene in the side contact geometry. Different factors such as the termination of the graphene edge, contact area, and point defect in contacted graphene are investigated. Notable differences are observed in structural configurations and electronic transport characteristics of these metal-graphene contacts, depending on the metal species and aforementioned influencing factors. It is found that the enhanced chemical reactivity of the graphene due to dangling bonds from either the unsaturated graphene edge or point defects strengthens the metal-graphene bonding, leading to a considerable contact resistance reduction for weakly interacting metals Au and Ag. For stronger interacting metals Pt and Cu, a slightly reduced contact resistance is found due to such influencing factors. However, the wetting metals Ni and Pd most strongly hybridize with graphene, exhibiting negligible dependence on the above influencing factors. This study provides guidance for the optimization of metal-graphene contacts at an atomic scale.

  10. Formation, migration, and clustering energies of interstitial He in α-quartz and β-cristobalite

    NASA Astrophysics Data System (ADS)

    Lin, Kan-Ju; Ding, Hepeng; Demkowicz, Michael J.

    2016-10-01

    Precipitation of implanted helium (He) is detrimental to many nuclear materials. A solid in which implanted He does not precipitate, but rather remains in solution and diffuses readily is potentially of interest for applications requiring resistance to He-induced damage. We use density functional theory (DFT) calculations to examine He interstitial formation, migration, and clustering energies in two SiO2 polymorphs: α-quartz and β-cristobalite. Our findings show greater He solubility and mobility in the latter than in the former. This difference appears to be due primarily to the unlike atomic-level structures of α-quartz and β-cristobalite, rather than their differing densities. Our findings also suggest that He is unlikely to cluster in either material. The behavior of He in α-quartz and β-cristobalite, and similar forms of silica make them promising materials for further investigation for potential use in applications requiring resistance to He-induced damage.

  11. Fundamental Studies of Charge Migration and Delocalization Relevant to Solar Energy Conversion

    SciTech Connect

    Michael J. Therien

    2012-06-01

    This program aimed to understand the molecular-level principles by which complex chemical systems carry out photochemical charge separation, transport, and storage, and how these insights could impact the design of practical solar energy conversion and storage devices. Towards these goals, this program focused on: (1) carrying out fundamental mechanistic and transient dynamical studies of proton-coupled electron-transfer (PCET) reactions; (2) characterizing and interrogating via electron paramagnetic resonance (EPR) spectroscopic methods novel conjugated materials that feature large charge delocalization lengths; and (3) exploring excitation delocalization and migration, as well as polaron transport properties of meso-scale assemblies that are capable of segregating light-harvesting antennae, nanoscale wire-like conduction elements, and distinct oxidizing and reducing environments.

  12. Energy Expenditure and Metabolic Changes of Free-Flying Migrating Northern Bald Ibis.

    PubMed

    Bairlein, Franz; Fritz, Johannes; Scope, Alexandra; Schwendenwein, Ilse; Stanclova, Gabriela; van Dijk, Gertjan; Meijer, Harro A J; Verhulst, Simon; Dittami, John

    2015-01-01

    Many migrating birds undertake extraordinary long flights. How birds are able to perform such endurance flights of over 100-hour durations is still poorly understood. We examined energy expenditure and physiological changes in Northern Bald Ibis Geronticus eremite during natural flights using birds trained to follow an ultra-light aircraft. Because these birds were tame, with foster parents, we were able to bleed them immediately prior to and after each flight. Flight duration was experimentally designed ranging between one and almost four hours continuous flights. Energy expenditure during flight was estimated using doubly-labelled-water while physiological properties were assessed through blood chemistry including plasma metabolites, enzymes, electrolytes, blood gases, and reactive oxygen compounds. Instantaneous energy expenditure decreased with flight duration, and the birds appeared to balance aerobic and anaerobic metabolism, using fat, carbohydrate and protein as fuel. This made flight both economic and tolerable. The observed effects resemble classical exercise adaptations that can limit duration of exercise while reducing energetic output. There were also in-flight benefits that enable power output variation from cruising to manoeuvring. These adaptations share characteristics with physiological processes that have facilitated other athletic feats in nature and might enable the extraordinary long flights of migratory birds as well.

  13. Energy Expenditure and Metabolic Changes of Free-Flying Migrating Northern Bald Ibis.

    PubMed

    Bairlein, Franz; Fritz, Johannes; Scope, Alexandra; Schwendenwein, Ilse; Stanclova, Gabriela; van Dijk, Gertjan; Meijer, Harro A J; Verhulst, Simon; Dittami, John

    2015-01-01

    Many migrating birds undertake extraordinary long flights. How birds are able to perform such endurance flights of over 100-hour durations is still poorly understood. We examined energy expenditure and physiological changes in Northern Bald Ibis Geronticus eremite during natural flights using birds trained to follow an ultra-light aircraft. Because these birds were tame, with foster parents, we were able to bleed them immediately prior to and after each flight. Flight duration was experimentally designed ranging between one and almost four hours continuous flights. Energy expenditure during flight was estimated using doubly-labelled-water while physiological properties were assessed through blood chemistry including plasma metabolites, enzymes, electrolytes, blood gases, and reactive oxygen compounds. Instantaneous energy expenditure decreased with flight duration, and the birds appeared to balance aerobic and anaerobic metabolism, using fat, carbohydrate and protein as fuel. This made flight both economic and tolerable. The observed effects resemble classical exercise adaptations that can limit duration of exercise while reducing energetic output. There were also in-flight benefits that enable power output variation from cruising to manoeuvring. These adaptations share characteristics with physiological processes that have facilitated other athletic feats in nature and might enable the extraordinary long flights of migratory birds as well. PMID:26376193

  14. Energy Expenditure and Metabolic Changes of Free-Flying Migrating Northern Bald Ibis

    PubMed Central

    Bairlein, Franz; Fritz, Johannes; Scope, Alexandra; Schwendenwein, Ilse; Stanclova, Gabriela; van Dijk, Gertjan; Meijer, Harro A. J.; Verhulst, Simon

    2015-01-01

    Many migrating birds undertake extraordinary long flights. How birds are able to perform such endurance flights of over 100-hour durations is still poorly understood. We examined energy expenditure and physiological changes in Northern Bald Ibis Geronticus eremite during natural flights using birds trained to follow an ultra-light aircraft. Because these birds were tame, with foster parents, we were able to bleed them immediately prior to and after each flight. Flight duration was experimentally designed ranging between one and almost four hours continuous flights. Energy expenditure during flight was estimated using doubly-labelled-water while physiological properties were assessed through blood chemistry including plasma metabolites, enzymes, electrolytes, blood gases, and reactive oxygen compounds. Instantaneous energy expenditure decreased with flight duration, and the birds appeared to balance aerobic and anaerobic metabolism, using fat, carbohydrate and protein as fuel. This made flight both economic and tolerable. The observed effects resemble classical exercise adaptations that can limit duration of exercise while reducing energetic output. There were also in-flight benefits that enable power output variation from cruising to manoeuvring. These adaptations share characteristics with physiological processes that have facilitated other athletic feats in nature and might enable the extraordinary long flights of migratory birds as well. PMID:26376193

  15. The effect of surface coating on energy migration-mediated upconversion.

    PubMed

    Su, Qianqian; Han, Sanyang; Xie, Xiaoji; Zhu, Haomiao; Chen, Hongyu; Chen, Chih-Kai; Liu, Ru-Shi; Chen, Xueyuan; Wang, Feng; Liu, Xiaogang

    2012-12-26

    Lanthanide-doped upconversion nanoparticles have been the focus of a growing body of investigation because of their promising applications ranging from data storage to biological imaging and drug delivery. Here we present the rational design, synthesis, and characterization of a new class of core-shell upconversion nanoparticles displaying unprecedented optical properties. Specifically, we show that the epitaxial growth of an optically inert NaYF(4) layer around a lanthanide-doped NaGdF(4)@NaGdF(4) core-shell nanoparticle effectively prevents surface quenching of excitation energy. At room temperature, the energy migrates over Gd sublattices and is adequately trapped by the activator ions embedded in host lattices. Importantly, the NaYF(4) shell-coating strategy gives access to tunable upconversion emissions from a variety of activators (Dy(3+), Sm(3+), Tb(3+), and Eu(3+)) doped at very low concentrations (down to 1 mol %). Our mechanistic investigations make possible, for the first time, the realization of efficient emissions from Tb(3+) and Eu(3+) activators that are doped homogeneously with Yb(3+)/Tm(3+) ions. The advances on these luminescent nanomaterials offer exciting opportunities for important biological and energy applications.

  16. Point defects diagrams for pure and doped copper oxide Cu{sub 2{+-}{delta}}O in the temperature range of 873-1473 K

    SciTech Connect

    Stoklosa, A.

    2012-10-15

    Point defects diagrams for the Cu{sub 2{+-}{delta}}O, both pure and doped with M{sup 2+} metal ions with all the types of defects in the cation sublattice considered are presented in this work. The calculations of the diagrams were performed by a novel method. The calculations were based on the data from the results of the studies of the deviation from stoichiometry and the electrical conductivity in the temperature range of 873-1473 K. The values of {Delta}G{sup o} of the formation of Frenkel-type defects, of copper vacancies, and of interstitial copper atoms were determined and their temperature dependence. It was shown that character of the dependence of the sum of concentrations of electronic defects ([h{sup Bullet }]+b[e Prime ]) on p{sub O{sub 2}} is fully consistent with its dependence of the electrical conductivity. Their mobility ({mu}{sub e}/{mu}{sub h}=b), vary from 1300 to 30. The dope M{sup 2+} increases the concentration of electrons and shifts the minimum of electrical conductivity toward higher oxygen pressures. - Graphical abstract: This work presents point defects diagrams for the Cu{sub 2{+-}{delta}}O, with all the types of defects in the cation sublattice considered. Highlights: Black-Right-Pointing-Pointer The point defects diagrams. Copper oxide Cu{sub 2{+-}{delta}}O, for pure and M{sup 2+} doped. Black-Right-Pointing-Pointer The values of {Delta}G{sup o} of the formation of Frenkel-type defects were determined. Black-Right-Pointing-Pointer The values of {Delta}G{sup o} of the formation QUOTE QUOTE of singly-ionized copper vacancies were determined. Black-Right-Pointing-Pointer The values of {Delta}G{sup o} of the formation of electroneutral copper vacancies were determined. Black-Right-Pointing-Pointer The values of {Delta}G{sup o} of the formation and of interstitial copper atoms were determined.

  17. Some notes on hydrogen-related point defects and their role in the isotope exchange and electrical conductivity in olivine

    NASA Astrophysics Data System (ADS)

    Karato, Shun-ichiro

    2015-11-01

    Nominally anhydrous minerals such as olivine dissolve hydrogen in a variety of forms including free (or interstitial) proton (Hrad) and two protons trapped at the M-site ((2 H)M×). The strength of chemical bonding between protons and the surrounding atoms are different among different species, and consequently protons belonging to different species likely have different mobility (diffusion coefficients). I discuss the role of diffusion of protons in different species in the isotope exchange and hydrogen-assisted electrical conductivity adding a few notes to the previous work by Karato (2013) including a new way to test the model. I conclude that in the case of isotope exchange, the interaction among these species is strong because diffusion is heterogeneous, whereas there is no strong interaction among different species in electrical conduction where diffusion is homogeneous (in an infinite crystal). Consequently, the slowest diffusing species controls the rate of isotope exchange, whereas the fastest diffusing species controls electrical conductivity leading to a different temperature dependence of activation energy and anisotropy. This model explains the differences in the activation energy and anisotropy between isotope diffusion and electrical conductivity, and predicts that the mechanism of electrical conductivity changes with temperature providing an explanation for most of the discrepancies among different experimental observations at different temperatures except for those by Poe et al. (2010) who reported anomalously high water content dependence and highly anisotropic activation energy. When the results obtained at high temperatures are used, most of the geophysically observed high and highly anisotropic electrical conductivity in the asthenosphere can be explained without invoking partial melting.

  18. Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4

    NASA Astrophysics Data System (ADS)

    Chen, Shiyou; Yang, Ji-Hui; Gong, X. G.; Walsh, Aron; Wei, Su-Huai

    2010-06-01

    Current knowledge of the intrinsic defect properties of Cu2ZnSnS4 (CZTS) is limited, which is hindering further improvement of the performance of CZTS-based solar cells. Here, we have performed first-principles calculations for a series of intrinsic defects and defect complexes in CZTS, from which we have the following observations. (i) It is important to control the elemental chemical potentials during crystal growth to avoid the formation of secondary phases such as ZnS, CuS, and Cu2SnS3 . (ii) The intrinsic p -type conductivity is attributed to the CuZn antisite which has a lower formation energy and relatively deeper acceptor level compared to the Cu vacancy. (iii) The low formation energy of many of the acceptor defects will lead to the intrinsic p -type character, i.e., n -type doping is very difficult in this system. (iv) The role of electrically neutral defect complexes is predicted to be important, because they have remarkably low formation energies and electronically passivate deep levels in the band gap. For example, [CuZn-+ZnCu+] , [VCu-+ZnCu+] , and [ZnSn2-+2ZnCu+] may form easily in nonstoichiometric samples. The band alignment between Cu2ZnSnS4 , CuInSe2 and the solar-cell window layer CdS has also been calculated, revealing that a type-II band alignment exists for the CdS/Cu2ZnSnS4 heterojunction. The fundamental differences between CZTS and CuInSe2 for use in thin-film photovoltaics are discussed. The results are expected to be relevant to other I2-II-IV-VI4 semiconductors.

  19. Understanding Historical Human Migration Patterns and Interbreeding (JGI Seventh Annual User Meeting 2012: Genomics of Energy and Environment)

    SciTech Connect

    Willerslev, Eske

    2012-03-21

    Eske Willerslev from the University of Copenhagen on "Understanding Historical Human Migration Patterns and Interbreeding Using the Ancient Genomes of a Palaeo-Eskimo and an Aboriginal Australian" at the 7th Annual Genomics of Energy & Environment Meeting on March 21, 2012 in Walnut Creek, California.

  20. Understanding Historical Human Migration Patterns and Interbreeding (JGI Seventh Annual User Meeting 2012: Genomics of Energy and Environment)

    ScienceCinema

    Willerslev, Eske [University of Copenhagen

    2016-07-12

    Eske Willerslev from the University of Copenhagen on "Understanding Historical Human Migration Patterns and Interbreeding Using the Ancient Genomes of a Palaeo-Eskimo and an Aboriginal Australian" at the 7th Annual Genomics of Energy & Environment Meeting on March 21, 2012 in Walnut Creek, California.

  1. Nondestructive quantitative mapping of impurities and point defects in thin films: Ga and V{sub Zn} in ZnO:Ga

    SciTech Connect

    Look, David C.; Leedy, Kevin D.; Agresta, Donald L.

    2014-06-16

    Ga-doped ZnO (GZO) films grown by pulsed-laser deposition on quartz and other lattice-mismatched substrates can routinely attain resistivities of 2 × 10{sup −4} Ω·cm and thus compete with Sn-doped In{sub 2}O{sub 3} (ITO) in large-area transparent-electrode applications. Nondestructive, high-resolution (1-mm) maps of thickness d, concentration n, and mobility μ on such films can be obtained automatically from commercial spectroscopic ellipsometers. From n and μ, degenerate-electron scattering theory yields donor N{sub D} and acceptor N{sub A} concentrations at each point. Finally, N{sub D} and N{sub A} can be identified as [Ga] and [V{sub Zn}], respectively, demonstrating high-density mapping of impurities and point defects in a semiconductor thin film.

  2. Calculating Effect of Point Defects on Optical Absorption Spectra of III-V Semiconductor Superlattices Based on (8x8) k-dot-p Band Structures

    NASA Astrophysics Data System (ADS)

    Huang, Danhong; Iurov, Andrii; Gumbs, Godfrey; Cardimona, David; Krishna, Sanjay

    For a superlattice which is composed of layered zinc-blende structure III-V semiconductor materials, its realistic anisotropic band structures around the Gamma-point are calculated by using the (8x8)k-dot-p method with the inclusion of the self-consistent Hartree potential and the spin-orbit coupling. By including the many-body screening effect, the obtained band structures are further employed to calculate the optical absorption coefficient which is associated with the interband electron transitions. As a result of a reduced quasiparticle lifetime due to scattering with point defects in the system, the self-consistent vertex correction to the optical response function is also calculated with the help of the second-order Born approximation.

  3. Highly bright multicolour emission through energy migration in core/shell nanotubes.

    PubMed

    Liu, Lu; Zhang, Nannan; Leng, Zhihua; Liang, Yimai; Li, Ruiqing; Zou, Lianchun; Gan, Shucai

    2015-04-14

    This paper describes a simple and environmentally-friendly approach that allowed for the facile synthesis of a gadolinium-based core/shell/shell nanotube structure with a set of lanthanide ions incorporated into separated layers. In addition, by the rational design of a core/shell structure we systematically investigated the luminescence properties of different lanthanide ions in NaGdF4 host, and efficient down-conversion emission can be realized through gadolinium sublattice-mediated energy migration. The Gd(3+) ions play an intermediate role in this process. By changing the doped lanthanide ions, we generated multicolour emissions from the luminescent Ln(3+) centers via energy transfer of Ce(3+)→Gd(3+)→Ln(3+) and Ce(3+)→Ln(3+) (Ln = Eu, Tb, Dy and Sm) in separated layers. Due to the strong absorption of ultraviolet (UV) irradiation by Ce(3+) ions, the luminescence efficiency could be enhanced after doping Ce(3+) ions in the shell. In NaGdF4:5% Eu(3+)@NaGdF4@NaGdF4:5% Ce(3+) core/shell/shell nanotubes, with increasing the NaGdF4 interlayer thickness, a gradual decrease in emission intensity was observed for the Eu(3+) activator.

  4. Electronic and magnetic structure of 3d-transition-metal point defects in silicon calculated from first principles

    NASA Astrophysics Data System (ADS)

    Beeler, F.; Andersen, O. K.; Scheffler, M.

    1990-01-01

    We describe spin-unrestricted self-consistent linear muffin-tin-orbital (LMTO) Green-function calculations for Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu transition-metal impurities in crystalline silicon. Both defect sites of tetrahedral symmetry are considered. All possible charge states with their spin multiplicities, magnetization densities, and energy levels are discussed and explained with a simple physical picture. The early transition-metal interstitial and late transition-metal substitutional 3d ions are found to have low spin. This is in conflict with the generally accepted crystal-field model of Ludwig and Woodbury, but not with available experimental data. For the interstitial 3d ions, the calculated deep donor and acceptor levels reproduce all experimentally observed transitions. For substitutional 3d ions, a large number of predictions is offered to be tested by future experimental studies.

  5. The Effect of VMoS3 Point Defect on the Elastic Properties of Monolayer MoS2 with REBO Potentials.

    PubMed

    Li, Minglin; Wan, Yaling; Tu, Liping; Yang, Yingchao; Lou, Jun

    2016-12-01

    Structural defects in monolayer molybdenum disulfide (MoS2) have significant influence on the electric, optical, thermal, chemical, and mechanical properties of the material. Among all the types of structural defects of the chemical vapor phase-grown monolayer MoS2, the VMoS3 point defect (a vacancy complex of Mo and three nearby S atoms) is another type of defect preferentially generated by the extended electron irradiation. Here, using the classical molecular dynamics simulation with reactive empirical bond-order (REBO) potential, we first investigate the effect of VMoS3 point defects on the elastic properties of monolayer MoS2 sheets. Under the constrained uniaxial tensile test, the elastic properties of monolayer MoS2 sheets containing VMoS3 vacancies with defect fraction varying from 0.01 to 0.1 are obtained based on the plane anisotropic constitutive relations of the material. It is found that the increase of VMoS3 vacancy concentration leads to the noticeable decrease in the elastic modulus but has a slight effect on Poisson's ratio. The maximum decrease of the elastic modulus is up to 25 %. Increasing the ambient temperature from 10 K to 500 K has trivial influences on the elastic modulus and Poisson's ratio for the monolayer MoS2 without defect and with 5 % VMoS3 vacancies. However, an anomalous parabolic relationship between the elastic modulus and the temperature is found in the monolayer MoS2 containing 0.1 % VMoS3 vacancy, bringing a crucial and fundamental issue to the application of monolayer MoS2 with defects.

  6. Multistep energy migration between 3,3‧-diethyl-9-methylthiacarbocyanine iodide monomers in uniaxially oriented polymer films

    NASA Astrophysics Data System (ADS)

    Bojarski, Piotr; Gryczyński, Ignacy; Kułak, Leszek; Synak, Anna; Bharill, Shashank; Rangełowa, Simeonika; Szabelski, Mariusz

    2007-05-01

    Multistep energy migration was studied for 3,3'-diethyl-9-methylthiacarbocyanine iodide (MDTCI) in uniaxially stretched and unstretched poly(vinyl alcohol) films. At low and intermediate concentrations fluorescence anisotropy decay measurements yield completely different results for disordered and ordered systems due to strong redistribution and angular correlation of fluorophores transition moments in ordered polymer matrix. These results and other selected properties of energy transport in uniaxially stretched polymer films were analyzed using the technique of Monte-Carlo simulation.

  7. Defect diffusion during annealing of low-energy ion-implanted silicon

    SciTech Connect

    Bedrossian, P J; Caturla, M-J; Diaz de la Rubia, T

    2000-03-08

    We present a new approach for investigating the kinetics of defect migration during annealing of low-energy, ion-implanted silicon, employing a combination of computer simulations and atomic-resolution tunneling microscopy. Using atomically-clean Si(111)-7x7 as a sink for bulk point defects created by 5 keV Xe and Ar irradiation, we observe distinct, temperature-dependent surface arrival rates for vacancies and interstitials. A combination of simulation tools provides a detailed description of the processes that underly the observed temperature-dependence of defect segregation, and the predictions of the simulations agree closely with the experimental observations.

  8. Application of donor-donor energy migration (DDEM) for examining protein structure and function

    NASA Astrophysics Data System (ADS)

    Bergstrom, Fredrik; Hagglof, Peter; Karolin, Jan; Ny, Tor; Johansson, Lennart B.

    1999-05-01

    Donor-Donor Energy Migration (DDEM) and fluorescence anisotropy experiments can be utilized as a versatile tool for examining protein structure and function. For this, pairs of identical fluorescent probes (D) are attached to unique residues created by means of site specific mutagenesis. Present work illustrates the applicability of the method on the latent form of Plasminogen Activator Inhibitor-1 (PAI-1). Different DD-pairs of mutated PAI-1 were prepared and studied, namely; V106C-H185C, H185C-M266C and M266C-V106C. The Cys residues were labelled with a sulfhydryl specific derivative of BODIPYR [N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a- diaza-s-indacene-3-yl)methyl iodo-acetamide]. To determine the rate of DDEM within such a pair, intramolecular order and dynamics must be considered (Biophys. J., 74, 11-21, 1997). For analysis of data, additonal information was obtained from experiments with the corresponding D-labelled single Cys mutants, that is, V106C, H185C and M266C. The stability of values determined was tested by generating and re-analyzing synthetic data. The intramolecular distances obtained agree, reasonably well, with those determined from the X-ray structure of latent PAI-1.

  9. Gas Diffusion in Metals: Fundamental Study of Helium-Point Defect Interactions in Iron and Kinetics of Hydrogen Desorption from Zirconium Hydride

    NASA Astrophysics Data System (ADS)

    Hu, Xunxiang

    The behavior of gaseous foreign species (e.g., helium and hydrogen), which are either generated, adsorbed or implanted within the structural materials (e.g., iron and zirconium) exposed to irradiation environments, is an important and largely unsolved topic, as they intensively interact with the irradiation-induced defects, or bond with the lattice atoms to form new compounds, and impose significant effects on their microstructural and mechanical properties in fission and fusion reactors. This research investigates two cases of gas diffusion in metals (i.e., the helium-point defect interactions in iron and kinetics of hydrogen desorption from zirconium hydride) through extensive experimental and modeling studies, with the objective of improving the understanding of helium effects on the microstructures of iron under irradiation and demonstrating the kinetics of hydrogen diffusion and precipitation behavior in zirconium that are crucial to predict cladding failures and hydride fuel performance. The study of helium effects in structural materials aims to develop a self-consistent, experimentally validated model of helium---point defect, defect cluster and intrinsic defects through detailed inter-comparisons between experimental measurements on helium ion implanted iron single crystals and computational models. The combination of thermal helium desorption spectrometry (THDS) experiment with the cluster dynamic model helps to reveal the influence of impurities on the energetics and kinetics of the He-defect interactions and to realize the identification of possible mechanisms governing helium desorption peaks. Positron annihilation spectroscopy is employed to acquire additional information on He-vacancy cluster evolution, which provides an opportunity to validate the model qualitatively. The inclusion of He---self-interstitial clusters extends the cluster dynamic model while MD simulations explore the effects of dislocation loops on helium clustering. In addition, the

  10. Energy Migration Engineering of Bright Rare-Earth Upconversion Nanoparticles for Excitation by Light-Emitting Diodes.

    PubMed

    Zhong, Yeteng; Rostami, Iman; Wang, Zihua; Dai, Hongjie; Hu, Zhiyuan

    2015-11-01

    A novel Nd(3+) -sensitized upconversion nanoparticle (UCNP) that can be excited by near-infrared 740 nm light-emitting diode (LED) lamps with bright upconversion luminescence is designed. Yb(3+) ion distribution is engineered to increase the energy migration efficiency. The benefit of the novel LED-excited UCNPs is demonstrated by imaging of breast cancer cells and enabling an economic handheld semiquantitative visual measurement device. PMID:26393770

  11. Direct quantification of energy intake in an apex marine predator suggests physiology is a key driver of migrations

    PubMed Central

    Whitlock, Rebecca E.; Hazen, Elliott L.; Walli, Andreas; Farwell, Charles; Bograd, Steven J.; Foley, David G.; Castleton, Michael; Block, Barbara A.

    2015-01-01

    Pacific bluefin tuna (Thunnus orientalis) are highly migratory apex marine predators that inhabit a broad thermal niche. The energy needed for migration must be garnered by foraging, but measuring energy intake in the marine environment is challenging. We quantified the energy intake of Pacific bluefin tuna in the California Current using a laboratory-validated model, the first such measurement in a wild marine predator. Mean daily energy intake was highest off the coast of Baja California, Mexico in summer (mean ± SD, 1034 ± 669 kcal), followed by autumn when Pacific bluefin achieve their northernmost range in waters off northern California (944 ± 579 kcal). Movements were not always consistent with maximizing energy intake: the Pacific bluefin move out of energy rich waters both in late summer and winter, coincident with rising and falling water temperatures, respectively. We hypothesize that temperature-related physiological constraints drive migration and that Pacific bluefin tuna optimize energy intake within a range of optimal aerobic performance. PMID:26601248

  12. Point defects and p-type conductivity in Zn{sub 1–x}Mn{sub x}GeAs{sub 2}

    SciTech Connect

    Kilanski, L.; Podgórni, A.; Dynowska, E.; Dobrowolski, W.

    2014-07-14

    Positron annihilation spectroscopy is used to study point defects in Zn{sub 1–x}Mn{sub x}GeAs{sub 2} crystals with low Mn content 0≤x≤0.042 with disordered zincblende and chalcopyrite structure. The role of negatively charged vacancies and non-open-volume defects is discussed with respect to the high p-type conductivity with carrier concentration 10{sup 19}≤p≤10{sup 21}cm{sup −3} in our samples. Neutral As vacancies, together with negatively charged Zn vacancies and non-open-volume defects with concentrations around 10{sup 16}−10{sup 18}cm{sup −3}, are observed to increase with increasing Mn content in the alloy. The observed concentrations of defects are not sufficient to be responsible for the strong p-type conductivity of our crystals. Therefore, we suggest that other types of defects, such as extended defects, have a strong influence on the conductivity of Zn{sub 1–x}Mn{sub x}GeAs{sub 2} crystals.

  13. Direct correlation and strong reduction of native point defects and microwave dielectric loss in air-annealed (Ba,Sr)TiO3

    NASA Astrophysics Data System (ADS)

    Zeng, Z. Q.; Podpirka, A.; Kirchoefer, S. W.; Asel, T. J.; Brillson, L. J.

    2015-05-01

    We report on the native defect and microwave properties of 1 μm thick Ba0.50Sr0.50TiO3 (BST) films grown on MgO (100) substrates by molecular beam epitaxy (MBE). Depth-resolved cathodoluminescence spectroscopy (DRCLS) showed high densities of native point defects in as-deposited BST films, causing strong subgap emission between 2.0 eV and 3.0 eV due to mixed cation VC and oxygen Vo vacancies. Post growth air anneals reduce these defects with 2.2, 2.65, and 3.0 eV VO and 2.4 eV VC intensities decreasing with increasing anneal temperature and by nearly two orders of magnitude after 950 °C annealing. These low-defect annealed BST films exhibited high quality microwave properties, including room temperature interdigitated capacitor tunability of 13% under an electric bias of 40 V and tan δ of 0.002 at 10 GHz and 40 V bias. The results provide a feasible route to grow high quality BST films by MBE through post-air annealing guided by DRCLS.

  14. Direct correlation and strong reduction of native point defects and microwave dielectric loss in air-annealed (Ba,Sr)TiO{sub 3}

    SciTech Connect

    Zeng, Z. Q.; Podpirka, A.; Kirchoefer, S. W.; Asel, T. J.; Brillson, L. J.

    2015-05-04

    We report on the native defect and microwave properties of 1 μm thick Ba{sub 0.50}Sr{sub 0.50}TiO{sub 3} (BST) films grown on MgO (100) substrates by molecular beam epitaxy (MBE). Depth-resolved cathodoluminescence spectroscopy (DRCLS) showed high densities of native point defects in as-deposited BST films, causing strong subgap emission between 2.0 eV and 3.0 eV due to mixed cation V{sub C} and oxygen Vo vacancies. Post growth air anneals reduce these defects with 2.2, 2.65, and 3.0 eV V{sub O} and 2.4 eV V{sub C} intensities decreasing with increasing anneal temperature and by nearly two orders of magnitude after 950 °C annealing. These low-defect annealed BST films exhibited high quality microwave properties, including room temperature interdigitated capacitor tunability of 13% under an electric bias of 40 V and tan δ of 0.002 at 10 GHz and 40 V bias. The results provide a feasible route to grow high quality BST films by MBE through post-air annealing guided by DRCLS.

  15. New insights on P-related paramagnetic point defects in irradiated phosphate glasses: Impact of glass network type and irradiation dose

    SciTech Connect

    Pukhkaya, V.; Ollier, N.; Trompier, F.

    2014-09-28

    P-related paramagnetic point defects were studied in irradiated Yb-doped phosphate glasses by electron paramagnetic resonance spectroscopy (X and Q-bands). A strong impact of the glass network type on the defect nature is shown. In all glasses, r-POHC defects formation is in strong correlation with Q{sup 2} tetrahedra amount supporting the structure of r-POHC. Ultra-phosphate glasses contain the larger defect type: Peroxy radicals, P{sub 1}, P{sub 2}, and P{sub 4} defects whose formation is linked to Q{sup 3} tetrahedra presence. In meta-phosphate and poly-phosphate glasses, peroxy radicals appear with r-POHC thermal recovery. In meta-phosphate glasses, a combination of P{sub 1} and P{sub 3} defects was evidenced for the first time, whereas in poly-phosphate glasses, only P{sub 3} defects were identified. Dose effect as well as defect recovery were analyzed.

  16. Migration by soaring or flapping: numerical atmospheric simulations reveal that turbulence kinetic energy dictates bee-eater flight mode.

    PubMed

    Sapir, Nir; Horvitz, Nir; Wikelski, Martin; Avissar, Roni; Mahrer, Yitzhak; Nathan, Ran

    2011-11-22

    Aerial migrants commonly face atmospheric dynamics that may affect their movement and behaviour. Specifically, bird flight mode has been suggested to depend on convective updraught availability and tailwind assistance. However, this has not been tested thus far since both bird tracks and meteorological conditions are difficult to measure in detail throughout extended migratory flyways. Here, we applied, to our knowledge, the first comprehensive numerical atmospheric simulations by mean of the Regional Atmospheric Modeling System (RAMS) to study how meteorological processes affect the flight behaviour of migrating birds. We followed European bee-eaters (Merops apiaster) over southern Israel using radio telemetry and contrasted bird flight mode (flapping, soaring-gliding or mixed flight) against explanatory meteorological variables estimated by RAMS simulations at a spatial grid resolution of 250 × 250 m(2). We found that temperature and especially turbulence kinetic energy (TKE) determine bee-eater flight mode, whereas, unexpectedly, no effect of tailwind assistance was found. TKE during soaring-gliding was significantly higher and distinct from TKE during flapping. We propose that applying detailed atmospheric simulations over extended migratory flyways can elucidate the highly dynamic behaviour of air-borne organisms, help predict the abundance and distribution of migrating birds, and aid in mitigating hazardous implications of bird migration.

  17. Luminescence and photosensitization properties of ensembles of silicon nanocrystals in terms of an exciton migration model

    SciTech Connect

    Demin, V. A.; Konstantinova, E. A. Kashkarov, P. K.

    2010-11-15

    The relaxation processes that occur in ensembles of coupled silicon nanocrystals are described by a quantitative model that takes into account the energy transfer between them during exciton migration. This model is used to study the formation of singlet oxygen during the photoexcitation of silicon nanocrystals in porous silicon layers under various external conditions. It is experimentally found that, upon fine milling of as-deposited porous silicon films, the photoluminescence decay time increases despite an increase in the concentration of point defects. The photosensitized activity of ensembles of silicon nanocrystals degrades monotonically during their photostimulated oxidation. These experimental results agree completely with the conclusions of the model and are explained by a decrease in the number of exciton migration ways between nanocrystals when the granule size of a porous silicon powder decreases and by an increase in the efficiency of nonradiative recombination during the photostimulated oxidation of the nanocrystals. Our data indicate that nanocrystalline silicon is a promising material for the methods of nontoxic photodynamic therapy of oncological diseases.

  18. Return migration.

    PubMed

    Gmelch, G

    1980-01-01

    The author reviews the findings of the growing literature on return migration. Topics covered include typologies of return migrants, reasons for return, adaptation and readjustment of returnees, and the impact of return migration on the migrants' home societies. The focus of the study is on international return migration, migration to Northern Europe and northeastern North America, and return migration to the southern and eastern fringes of Europe and the Caribbean

  19. Fluorescence resonance energy transfer measured by spatial photon migration in CdSe-ZnS quantum dots colloidal systems as a function of concentration

    SciTech Connect

    Azevedo, G.; Monte, A. F. G.; Reis, A. F.; Messias, D. N.

    2014-11-17

    The study of the spatial photon migration as a function of the concentration brings into attention the problem of the energy transfer in quantum dot embedded systems. By measuring the photon propagation and its spatial dependence, it is possible to understand the whole dynamics in a quantum dot system, and also improve their concentration dependence to maximize energy propagation due to radiative and non-radiative processes. In this work, a confocal microscope was adapted to scan the spatial distribution of photoluminescence from CdSe-ZnS core-shell quantum dots in colloidal solutions. The energy migration between the quantum dots was monitored by the direct measurement of the photon diffusion length, according to the diffusion theory. We observed that the photon migration length decreases by increasing the quantum dot concentration, this kind of behavior has been regarded as a signature of Förster resonance energy transfer in the system.

  20. Utilization Probability Map for Migrating Bald Eagles in Northeastern North America: A Tool for Siting Wind Energy Facilities and Other Flight Hazards

    PubMed Central

    Mojica, Elizabeth K.; Watts, Bryan D.; Turrin, Courtney L.

    2016-01-01

    Collisions with anthropogenic structures are a significant and well documented source of mortality for avian species worldwide. The bald eagle (Haliaeetus leucocephalus) is known to be vulnerable to collision with wind turbines and federal wind energy guidelines include an eagle risk assessment for new projects. To address the need for risk assessment, in this study, we 1) identified areas of northeastern North America utilized by migrating bald eagles, and 2) compared these with high wind-potential areas to identify potential risk of bald eagle collision with wind turbines. We captured and marked 17 resident and migrant bald eagles in the northern Chesapeake Bay between August 2007 and May 2009. We produced utilization distribution (UD) surfaces for 132 individual migration tracks using a dynamic Brownian bridge movement model and combined these to create a population wide UD surface with a 1 km cell size. We found eagle migration movements were concentrated within two main corridors along the Appalachian Mountains and the Atlantic Coast. Of the 3,123 wind turbines ≥100 m in height in the study area, 38% were located in UD 20, and 31% in UD 40. In the United States portion of the study area, commercially viable wind power classes overlapped with only 2% of the UD category 20 (i.e., the areas of highest use by migrating eagles) and 4% of UD category 40. This is encouraging because it suggests that wind energy development can still occur in the study area at sites that are most viable from a wind power perspective and are unlikely to cause significant mortality of migrating eagles. In siting new turbines, wind energy developers should avoid the high-use migration corridors (UD categories 20 & 40) and focus new wind energy projects on lower-risk areas (UD categories 60–100). PMID:27336482

  1. Utilization Probability Map for Migrating Bald Eagles in Northeastern North America: A Tool for Siting Wind Energy Facilities and Other Flight Hazards.

    PubMed

    Mojica, Elizabeth K; Watts, Bryan D; Turrin, Courtney L

    2016-01-01

    Collisions with anthropogenic structures are a significant and well documented source of mortality for avian species worldwide. The bald eagle (Haliaeetus leucocephalus) is known to be vulnerable to collision with wind turbines and federal wind energy guidelines include an eagle risk assessment for new projects. To address the need for risk assessment, in this study, we 1) identified areas of northeastern North America utilized by migrating bald eagles, and 2) compared these with high wind-potential areas to identify potential risk of bald eagle collision with wind turbines. We captured and marked 17 resident and migrant bald eagles in the northern Chesapeake Bay between August 2007 and May 2009. We produced utilization distribution (UD) surfaces for 132 individual migration tracks using a dynamic Brownian bridge movement model and combined these to create a population wide UD surface with a 1 km cell size. We found eagle migration movements were concentrated within two main corridors along the Appalachian Mountains and the Atlantic Coast. Of the 3,123 wind turbines ≥100 m in height in the study area, 38% were located in UD 20, and 31% in UD 40. In the United States portion of the study area, commercially viable wind power classes overlapped with only 2% of the UD category 20 (i.e., the areas of highest use by migrating eagles) and 4% of UD category 40. This is encouraging because it suggests that wind energy development can still occur in the study area at sites that are most viable from a wind power perspective and are unlikely to cause significant mortality of migrating eagles. In siting new turbines, wind energy developers should avoid the high-use migration corridors (UD categories 20 & 40) and focus new wind energy projects on lower-risk areas (UD categories 60-100). PMID:27336482

  2. Utilization Probability Map for Migrating Bald Eagles in Northeastern North America: A Tool for Siting Wind Energy Facilities and Other Flight Hazards.

    PubMed

    Mojica, Elizabeth K; Watts, Bryan D; Turrin, Courtney L

    2016-01-01

    Collisions with anthropogenic structures are a significant and well documented source of mortality for avian species worldwide. The bald eagle (Haliaeetus leucocephalus) is known to be vulnerable to collision with wind turbines and federal wind energy guidelines include an eagle risk assessment for new projects. To address the need for risk assessment, in this study, we 1) identified areas of northeastern North America utilized by migrating bald eagles, and 2) compared these with high wind-potential areas to identify potential risk of bald eagle collision with wind turbines. We captured and marked 17 resident and migrant bald eagles in the northern Chesapeake Bay between August 2007 and May 2009. We produced utilization distribution (UD) surfaces for 132 individual migration tracks using a dynamic Brownian bridge movement model and combined these to create a population wide UD surface with a 1 km cell size. We found eagle migration movements were concentrated within two main corridors along the Appalachian Mountains and the Atlantic Coast. Of the 3,123 wind turbines ≥100 m in height in the study area, 38% were located in UD 20, and 31% in UD 40. In the United States portion of the study area, commercially viable wind power classes overlapped with only 2% of the UD category 20 (i.e., the areas of highest use by migrating eagles) and 4% of UD category 40. This is encouraging because it suggests that wind energy development can still occur in the study area at sites that are most viable from a wind power perspective and are unlikely to cause significant mortality of migrating eagles. In siting new turbines, wind energy developers should avoid the high-use migration corridors (UD categories 20 & 40) and focus new wind energy projects on lower-risk areas (UD categories 60-100).

  3. On the analyses of fluorescence depolarisation data in the presence of electronic energy migration. Part II: Applying and evaluating two-photon excited fluorescence.

    PubMed

    Opanasyuk, Oleg; Mikaelsson, Therese; Ryderfors, Linus; Mukhtar, Emad; Johansson, Lennart B-Å

    2012-02-14

    Electronic energy migration within a bifluorophoric molecule has been studied by time-resolved two-photon excited (TPE) fluorescence depolarisation experiments. Data were analysed by using a recently developed quantitative approach [O. Opanasyuk and L. B.-Å. Johansson, On the Analyses of Fluorescence Depolarisation Data in the Presence of Electronic Energy Migration. Part I: Theory and General Description, Phys. Chem. Chem. Phys., submitted]. The energy migration occurs between the 9-anthrylmethyl groups of the bifluorophoric molecule, bis-(9-anthrylmethylphosphonate) bisteroid. These groups undergo local reorientations, while overall tumbling of the bisteroid is strongly hampered in the used viscous solvent, 1,2-propanediol. To solely obtain information about local reorientations of the 9-anthrylmethyl group, also the mono-(9-anthrylmethylphosphonate) bisteroid was studied, which enabled modelling of the ordering potential shape. The analysis of data is partly performed in the Fourier domain and the best-fit parameters are determined by using an approach based on a Genetic Algorithm. The energy migration process was described by an extended Förster theory (EFT). A reasonable value of the distance between the 9-anthrylmethyl groups, as well as for the mutual orientation of the ordering potentials, is found. Furthermore, values of the two-photon tensor components were obtained.

  4. The cancer cell 'energy grid': TGF-β1 signaling coordinates metabolism for migration.

    PubMed

    Jiang, Lei; Deberardinis, Ralph; Boothman, David A

    2015-01-01

    Cancer cells have an increased reliance on lipogenesis, which is required for uncontrolled cell division. We recently reported transcriptional and functional 'reprogramming' of the cellular energy grid, allowing cancer cells to divert metabolism from biosynthesis to bioenergetic pathways and thus supplying enhanced mobility during epithelial-mesenchymal transition (EMT) induced by transforming growth factor β (TGF-β1) (Fig. 1). PMID:27308459

  5. Cell Migration

    PubMed Central

    Trepat, Xavier; Chen, Zaozao; Jacobson, Ken

    2015-01-01

    Cell migration is fundamental to establishing and maintaining the proper organization of multicellular organisms. Morphogenesis can be viewed as a consequence, in part, of cell locomotion, from large-scale migrations of epithelial sheets during gastrulation, to the movement of individual cells during development of the nervous system. In an adult organism, cell migration is essential for proper immune response, wound repair, and tissue homeostasis, while aberrant cell migration is found in various pathologies. Indeed, as our knowledge of migration increases, we can look forward to, for example, abating the spread of highly malignant cancer cells, retarding the invasion of white cells in the inflammatory process, or enhancing the healing of wounds. This article is organized in two main sections. The first section is devoted to the single-cell migrating in isolation such as occurs when leukocytes migrate during the immune response or when fibroblasts squeeze through connective tissue. The second section is devoted to cells collectively migrating as part of multicellular clusters or sheets. This second type of migration is prevalent in development, wound healing, and in some forms of cancer metastasis. PMID:23720251

  6. Long-Range Energy Transfer and Singlet-Exciton Migration in Working Organic Light-Emitting Diodes

    NASA Astrophysics Data System (ADS)

    Ingram, Grayson L.; Nguyen, Carmen; Lu, Zheng-Hong

    2016-06-01

    Rapid industrialization of organic light-emitting devices for flat-panel displays and solid-state lighting makes a deep understanding of device physics more desirable than ever. Developing reliable experimental techniques to measure fundamental physical properties such as exciton diffusion lengths is a vital part of developing device physics. In this paper, we present a study of exciton diffusion and long-range energy transfer in working organic light-emitting devices, and a study of the interplay between these two tangled processes through both experimental probes and simulations. With the inclusion of multiple factors including long-range energy transfer, exciton boundary conditions, and the finite width of the exciton generation zone, we quantify exciton migration based on emission characteristics from rubrene sensing layers placed in working organic light-emitting devices. This comprehensive analysis is found to be essential to accurately measuring exciton diffusion length, and in the present case the measured singlet-exciton diffusion length in the archetype material 4' -bis(carbazol-9-yl)biphenyl is 4.3 ±0.3 nm with a corresponding diffusivity of (2.6 ±0.3 )×10-4 cm2/s .

  7. Short range atomic migration in amorphous silicon

    NASA Astrophysics Data System (ADS)

    Strauß, F.; Jerliu, B.; Geue, T.; Stahn, J.; Schmidt, H.

    2016-05-01

    Experiments on self-diffusion in amorphous silicon between 400 and 500 °C are presented, which were carried out by neutron reflectometry in combination with 29Si/natSi isotope multilayers. Short range diffusion is detected on a length scale of about 2 nm, while long range diffusion is absent. Diffusivities are in the order of 10-19-10-20 m2/s and decrease with increasing annealing time, reaching an undetectable low value for long annealing times. This behavior is strongly correlated to structural relaxation and can be explained as a result of point defect annihilation. Diffusivities for short annealing times of 60 s follow the Arrhenius law with an activation enthalpy of (0.74 ± 0.21) eV, which is interpreted as the activation enthalpy of Si migration.

  8. Evaluation of energy expenditure in adult spring Chinook salmon migrating upstream in the Columbia River Basin: an assessment based on sequential proximate analysis

    USGS Publications Warehouse

    Mesa, M.G.; Magie, C.D.

    2006-01-01

    The upstream migration of adult anadromous salmonids in the Columbia River Basin (CRB) has been dramatically altered and fish may be experiencing energetically costly delays at dams. To explore this notion, we estimated the energetic costs of migration and reproduction of Yakima River-bound spring Chinook salmon Oncorhynchus tshawytscha using a sequential analysis of their proximate composition (i.e., percent water, fat, protein, and ash). Tissues (muscle, viscera, and gonad) were sampled from fish near the start of their migration (Bonneville Dam), at a mid point (Roza Dam, 510 km upstream from Bonneville Dam) and from fresh carcasses on the spawning grounds (about 100 km above Roza Dam). At Bonneville Dam, the energy reserves of these fish were remarkably high, primarily due to the high percentage of fat in the muscle (18-20%; energy content over 11 kJ g-1). The median travel time for fish from Bonneville to Roza Dam was 27 d and ranged from 18 to 42 d. Fish lost from 6 to 17% of their energy density in muscle, depending on travel time. On average, fish taking a relatively long time for migration between dams used from 5 to 8% more energy from the muscle than faster fish. From the time they passed Bonneville Dam to death, these fish, depending on gender, used 95-99% of their muscle and 73-86% of their viscera lipid stores. Also, both sexes used about 32% of their muscular and very little of their visceral protein stores. However, we were unable to relate energy use and reproductive success to migration history. Our results suggest a possible influence of the CRB hydroelectric system on adult salmonid energetics.

  9. Formation and Migration Energies of Interstitials in Silicon Under Strain Conditions

    NASA Technical Reports Server (NTRS)

    Halicioglu, Timur; Barnett, David M.

    1999-01-01

    Simulation calculations are conducted for Si substrates to analyze formation and diffusion energies of interstitials under strain condition using statics methods .based on a Stillinger-Weber type potential function. Defects in the vicinity of the surface region and in the bulk are examined, and the role played by compressive and tensile strains on the energetics of interstitials is investigated. Results indicate that strain alters defect energetics which, in turn, modifies their diffusion characteristics.

  10. Theory of point-defects, non-stoichiometry, and solute additions in SmCo{sub 5+x}-Sm{sub 2}Co{sub 17{minus}y} and related compounds

    SciTech Connect

    Welch, D.O.

    1998-08-01

    There is considerable interest in the possibility of producing Sm-Co-based nanocomposite magnets by rapid solidification and other far-from-equilibrium processing methods. Thermodynamic and kinetic models are quite valuable in understanding and optimizing such methods. This paper describes a method of estimation, utilizing tight-binding-based bond-order interatomic interaction potentials, of the thermodynamic properties of point defects such as vacancies, interstitials, antisite defects, and solute additions in the SmCo{sub 5+x} and Sm{sub 2}Co{sub 17{minus}y} phases and related rare-earth-transition metal compounds. Illustrative calculations for point defects in SmCo{sub 5} will be presented. The results suggest a unified model of the thermodynamic properties of the SmCo{sub 5+x}-Sm{sub 2}Co{sub 17{minus}y} region of the phase diagram, based on the 1-5 structure and the replacement of Sm by interacting dumb-bell interstitials to form the 2-17 structure; the model is similar in nature to theories of the thermodynamics of metal hydrides.

  11. Theory of point-defects, non-stoichiometry, and solute additions in SmCo{sub 5+x}-Sm{sub 2}Co{sub 17{minus}y} and related compounds

    SciTech Connect

    WELCH,D.O.

    1998-09-03

    There is considerable interest in the possibility of producing Sm-Co-based nanocomposite magnets by rapid solidification and other far-from-equilibrium processing methods. Thermodynamic and kinetic models are quite valuable in understanding and optimizing such methods. This paper describes a method of estimation, utilizing tight-binding-based bond-order interatomic interaction potentials, of the thermodynamic properties of point defects such as vacancies, interstitials, antisite defects, and solute additions in the SmCo{sub 5+x} and Sm{sub 2}Co {sub 17}-y phases and related rare-earth-transition metal compounds. Illustrative calculations for point defects in SmCo{sub 5} will be presented. The results suggest a unified model of the thermodynamic properties of the SmCo{sub 5+x} -- Sm{sub 2} Co{sub 17{minus}y} region of the phase diagram, based on the 1-5 structure and the replacement of Sm by interacting dumb-bell interstitials to form the 2-17 structure; the model is similar in nature to theories of the thermodynamics of metal hydrides.

  12. One-dimensional fast migration of vacancy clusters in metals

    SciTech Connect

    Matsukawa, Yoshitaka; Zinkle, Steven J

    2007-01-01

    The migration of point defects, e.g. crystal lattice vacancies and self-interstitial atoms (SIAs), typically occurs through three-dimensional (3-D) random walk. However, when vacancies and SIAs agglomerate with like defects forming clusters, the migration mode may change. Recently, atomic-scale computer simulations using molecular dynamics (MD) codes have reported that nanometer-sized two-dimensional (2-D) clusters of SIAs exhibit one-dimensional (1-D) fast migration1-7. The 1-D migration mode transports the entire cluster containing several tens of SIAs with a mobility comparable to single SIAs3. This anisotropic migration of SIA clusters can have a significant impact on the evolution of a material fs neutron-irradiation damage microstructure, which dominates the material fs lifetime in nuclear reactor environments8-9. This is also proposed to be a key physical mechanism for the self-organization of nanometer-sized sessile vacancy cluster arrays10-13. Given these findings for SIA clusters, a fundamental question is whether the 1-D migration mode is also possible for 2-D clusters of vacancies. Preceding MD results predicted that 1-D migration of vacancy clusters is possible in body-centered cubic (bcc) iron, but not in face-centered cubic (fcc) copper2. Previous experimental studies have reported 1-D migration of SIA clusters14, but there have been no observations of 1-D vacancy cluster migration. Here we present the first experimental transmission electron microscopy (TEM) dynamic observation demonstrating the 1-D migration of vacancy clusters in fcc gold. It was found that the mobility of the vacancy clusters via the 1-D migration is much higher than single vacancies via 3-D random walk and comparable to single SIAs via 3-D random walk. Hence, the mobility of the glissile clusters is not associated with the character of their constituent point defects. Dynamic conversion of a planar vacancy loop into a 3-D stacking fault tetrahedron geometry was also observed.

  13. The model case of an oxygen storage catalyst - non-stoichiometry, point defects and electrical conductivity of single crystalline CeO2-ZrO2-Y2O3 solid solutions.

    PubMed

    Eufinger, Jens-Peter; Daniels, Maximilian; Schmale, Kerstin; Berendts, Stefan; Ulbrich, Gregor; Lerch, Martin; Wiemhöfer, Hans-Dieter; Janek, Jürgen

    2014-12-14

    The ternary solid solution CeO2-ZrO2 is known for its superior performance as an oxygen storage catalyst in exhaust gas catalysis (e.g. TWC), although the defect chemical background of these outstanding properties is not fully understood quantitatively. Here, a comprehensive experimental study is reported regarding defects and defect-related transport properties of cubic stabilized single crystalline (CexZr1-x)0.8Y0.2O1.9-δ (0 ≤x≤ 1) solid solutions as a model system for CeO2-ZrO2. The constant fraction of yttria was chosen in order to fix a defined concentration of oxygen vacancies and to stabilize the cubic fluorite-type lattice for all Ce/Zr ratios. Measurements of the total electrical conductivity, the partial electronic conductivity, the ionic transference number and the non-stoichiometry (oxygen deficiency, oxygen storage capacity) were performed in the oxygen partial pressure range -25 < lg pO2/bar < 0 and for temperatures between 500 °C and 750 °C. The total conductivity at low pO2 is dominated by electronic transport. A strong deviation from the widely accepted ideal solution based point defect model was observed. An extended point defect model was developed using defect activities rather than concentrations in order to describe the point defect reactions in CeO2-ZrO2-Y2O3 properly. It served to obtain good quantitative agreement with the measured data. By a combination of values for non-stoichiometries and for electronic conductivities, the electron mobility could be calculated as a function of pO2, ranging between 10(-2) cm(2) V(-1) s(-1) and 10(-5) cm(2) V(-1) s(-1). Finally, the origin of the high oxygen storage capacity and superior catalytic promotion performance at a specific ratio of n(Ce)/n(Zr) ≈ 1 was attributed to two main factors: (1) a strongly enhanced electronic conductivity in the high and medium pO2 range qualifies the material to be a good mixed conductor, which is essential for a fast oxygen exchange and (2) the equilibrium

  14. [Internal migration].

    PubMed

    Borisovna, L

    1991-06-01

    Very few studies have been conducted that truly permit explanation of internal migration and it repercussions on social and economic structure. It is clear however that a profound knowledge of the determinants and consequences of internal migration will be required as a basis for economic policy decisions that advance the goal of improving the level of living of the population. the basic supposition of most studies of the relationship of population and development is that socioeconomic development conditions demographic dynamics. The process of development in Mexico, which can be characterized by great heterogeneity, consequently produces great regional disparities. At the national level various studies have estimated the volume of internal migration in Mexico, but they have usually been limited to interstate migration because the main source of data, the census, is classified by states. But given the great heterogeneity within states in all the elements related to internal migration, it is clear that studies of internal migration within states are also needed. Such studies are almost nonexistent because of their technical difficulty. National level studies show that interstate migration increased significantly between 1940-80. The proportion of Mexicans living outside their states of birth increased by 558% in those years, compared to the 342% increase in the total Mexican population. Although Puebla has a high rate of increase, migration has kept it below Mexico's national growth rate. Migration between Puebla and other states and within Puebla has led to an increasing unevenness of spatial distribution. Between 1970-80, 57 of Puebla's municipios had growth rates above the state average of 2.8%/year, 6 had growth rates equal to the average, and 129 had growth rates that were below the average but not negative. 25 states with negative growth rates that were considered strongly expulsive. In 1980, 51.7% of the population was concentrated in the 57 municipios

  15. Point defects as a test ground for the local density approximation +U theory: Mn, Fe, and V{sub Ga} in GaN

    SciTech Connect

    Volnianska, O.; Zakrzewski, T.; Boguslawski, P.

    2014-09-21

    Electronic structure of the Mn and Fe ions and of the gallium vacancy V{sub Ga} in GaN was analysed within the GGA + U approach. First, the +U term was treated as a free parameter, and applied to p(N), d(Mn), and d(Fe). The band gap of GaN is reproduced for U(N) ≈ 4 eV. The electronic structure of defect states was found to be more sensitive to the value of U than that of the bulk states. Both the magnitude and the sign of the U-induced energy shifts of levels depend on occupancies, and thus on the defect charge state. The energy shifts also depend on the hybridization between defect and host states, and thus are different for different level symmetries. In the case of V{sub Ga}, these effects lead to stabilization of spin polarization and the “negative-U{sub eff}” behavior. The values of Us were also calculated using the linear response approach, which gives U(Fe) ≈ U(Mn) ≈ 4 eV. This reproduces well the results of previous hybrid functionals calculations. However, the best agreement with the experimental data is obtained for vanishing or even negative U(Fe) and U(Mn)

  16. Influence of grown-in defects on final oxygen precipitates during heat treatment of Cz-Si wafer analyzed by a coupled model with the interaction of point defects, oxygen precipitates, and dislocation loops

    NASA Astrophysics Data System (ADS)

    Gao, Bing; Juel, Mari; Mhamdi, Mohammed

    2016-11-01

    To illuminate the role of crystal growth process on final oxygen precipitates during heat treatment of Cz-Si wafer, a coupled model, including the interaction of oxygen precipitates, point defects, and dislocation loops, has been used to test the influence of grown-in defects generated during crystal growth process. Several grown-in defect parameters such as density and size of oxygen precipitates and concentration of net silicon interstitials were checked. Results show that it is essential to control grown-in oxygen precipitate size and density, and net Si vacancy. By well controlling the three parameters less than some values, it is possible to remove the influence of crystal growth process on the final oxygen precipitates after heat treatment of Cz-Si wafer. Simple 1D results clearly demonstrates that it is feasible to control grown-in oxygen precipitates during crystal growth process.

  17. Impact of Mg content on native point defects in Mg{sub x}Zn{sub 1−x}O (0 ≤ x ≤ 0.56)

    SciTech Connect

    Perkins, J.; Foster, G. M.; Myer, M.; Mehra, S.; Chauveau, J. M.; Hierro, A.; Windl, W.; Brillson, L. J.

    2015-06-01

    We used depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to measure the densities, energy levels, and spatial distributions of zinc/magnesium cation and oxygen vacancies in isostructural, single-phase, non-polar Mg{sub x}Zn{sub 1−x}O alloys over a wide (0 ≤ x ≤ 0.56) range. Within this wide range, both defect types exhibit strong Mg content-dependent surface segregation and pronounced bulk density minima corresponding to unit cell volume minima, which can inhibit defect formation due to electrostatic repulsion. Mg in ZnO significantly reduces native defect densities and their non-polar surface segregation, both major factors in carrier transport and doping of these oxide semiconductors.

  18. Linear-scaling density-functional simulations of charged point defects in Al2O3 using hierarchical sparse matrix algebra.

    PubMed

    Hine, N D M; Haynes, P D; Mostofi, A A; Payne, M C

    2010-09-21

    We present calculations of formation energies of defects in an ionic solid (Al(2)O(3)) extrapolated to the dilute limit, corresponding to a simulation cell of infinite size. The large-scale calculations required for this extrapolation are enabled by developments in the approach to parallel sparse matrix algebra operations, which are central to linear-scaling density-functional theory calculations. The computational cost of manipulating sparse matrices, whose sizes are determined by the large number of basis functions present, is greatly improved with this new approach. We present details of the sparse algebra scheme implemented in the ONETEP code using hierarchical sparsity patterns, and demonstrate its use in calculations on a wide range of systems, involving thousands of atoms on hundreds to thousands of parallel processes.

  19. Point defects introduced by InN alloying into In{sub x}Ga{sub 1-x}N probed using a monoenergetic positron beam

    SciTech Connect

    Uedono, A.; Tsutsui, T.; Watanabe, T.; Kimura, S.; Zhang, Y.; Lozac'h, M.; Sang, L. W.; Sumiya, M.; Ishibashi, S.

    2013-03-28

    Native defects in In{sub x}Ga{sub 1-x}N (x = 0.06-0.14) grown by metal organic chemical vapor deposition were studied using a monoenergetic positron beam. Measurements of Doppler broadening spectra of the annihilation radiation as a function of incident positron energy for In{sub x}Ga{sub 1-x}N showed that vacancy-type defects were introduced with increasing InN composition, and the major defect species was identified as complexes between a cation vacancy and a nitrogen vacancy. The concentration of the divacancy, however, was found to be suppressed by Mg doping. The momentum distribution of electrons at the In{sub x}Ga{sub 1-x}N/GaN interface was close to that in defect-free GaN or In{sub x}Ga{sub 1-x}N, which was attributed to localization of positrons at the interface due to the built-in electric field, and to suppression of positron trapping by vacancy-type defects. We have also shown that the diffusion property of positrons is sensitive to an electric field near the In{sub x}Ga{sub 1-x}N/GaN interface.

  20. Migration Theories

    NASA Astrophysics Data System (ADS)

    Crida, Aurélien

    2015-08-01

    The great variety of the architectures of the extra-solar planetary systems has revealed the fundamental role played by planetary migration: the interactions between the planets and the gaseous disk in which they form leads to a modification of their orbits. Here, I will review the basic processes and the most recent results in this area.Planets up to ~50 Earth masses are prone to so-called type I migration.I will describe the processes at play, namely the Lindblad and corotation torques, and explain how the total torque depends on the planet mass and the local disk structure. Application to realistic disks shows one or two sweet spot(s) for outward migration of planets roughly between 5 and 30 Earth masses around the snowline ; this is confirmed by dedicated 3D numerical simulations. This has strong consequences on the formation of hot Super-Earths or mini-Neptunes.For smaller mass planets, it has been recently proposed that the heating of the neighboring gas by the luminous planet can lead to a positive torque, hence promoting outward migration. On the other hand, if the planet is not a heat source, a cold finger appears, whose resulting torque is negative. Applications of these two recent results should be discussed.Giant planets open gaps in the proto-planetary disk, and then are supposedly subject to type II migration, following the viscous accretion of the disk. This standard picture has been questioned recently, as gas appears to drift through the gap. Although the gap opening process is well understood in 2D for a planet on a fixed orbit, recent results on 3D simulations or migrating planets make the picture more accurate.Our ever better understanding of planet-disk interactions is of crucial importance as the statistics on extra solar systems keep growing and the results of these interactions are now imaged.

  1. North Atlantic blue and fin whales suspend their spring migration to forage in middle latitudes: building up energy reserves for the journey?

    PubMed

    Silva, Mónica A; Prieto, Rui; Jonsen, Ian; Baumgartner, Mark F; Santos, Ricardo S

    2013-01-01

    The need to balance energy reserves during migration is a critical factor for most long-distance migrants and an important determinant of migratory strategies in birds, insects and land mammals. Large baleen whales migrate annually between foraging and breeding sites, crossing vast ocean areas where food is seldom abundant. How whales respond to the demands and constraints of such long migrations remains unknown. We applied a behaviour discriminating hierarchical state-space model to the satellite tracking data of 12 fin whales and 3 blue whales tagged off the Azores, to investigate their movements, behaviour (transiting and area-restricted search, ARS) and daily activity cycles during the spring migration. Fin and blue whales remained at middle latitudes for prolonged periods, spending most of their time there in ARS behaviour. While near the Azores, fin whale ARS behaviour occurred within a restricted area, with a high degree of overlap among whales. There were noticeable behavioural differences along the migratory pathway of fin whales tracked to higher latitudes: ARS occurred only in the Azores and north of 56°N, whereas in between these areas whales travelled at higher overall speeds while maintaining a nearly direct trajectory. This suggests fin whales may alternate periods of active migration with periods of extended use of specific habitats along the migratory route. ARS behaviour in blue whales occurred over a much wider area as whales slowly progressed northwards. The tracks of these whales terminated still at middle latitudes, before any behavioural switch was detected. Fin whales exhibited behavioural-specific diel rhythms in swimming speed but these varied significantly between geographic areas, possibly due to differences in the day-night cycle across areas. Finally, we show a link between fin whales seen in the Azores and those summering in eastern Greenland-western Iceland along a migratory corridor located in central Atlantic waters. PMID:24116112

  2. North Atlantic blue and fin whales suspend their spring migration to forage in middle latitudes: building up energy reserves for the journey?

    PubMed

    Silva, Mónica A; Prieto, Rui; Jonsen, Ian; Baumgartner, Mark F; Santos, Ricardo S

    2013-01-01

    The need to balance energy reserves during migration is a critical factor for most long-distance migrants and an important determinant of migratory strategies in birds, insects and land mammals. Large baleen whales migrate annually between foraging and breeding sites, crossing vast ocean areas where food is seldom abundant. How whales respond to the demands and constraints of such long migrations remains unknown. We applied a behaviour discriminating hierarchical state-space model to the satellite tracking data of 12 fin whales and 3 blue whales tagged off the Azores, to investigate their movements, behaviour (transiting and area-restricted search, ARS) and daily activity cycles during the spring migration. Fin and blue whales remained at middle latitudes for prolonged periods, spending most of their time there in ARS behaviour. While near the Azores, fin whale ARS behaviour occurred within a restricted area, with a high degree of overlap among whales. There were noticeable behavioural differences along the migratory pathway of fin whales tracked to higher latitudes: ARS occurred only in the Azores and north of 56°N, whereas in between these areas whales travelled at higher overall speeds while maintaining a nearly direct trajectory. This suggests fin whales may alternate periods of active migration with periods of extended use of specific habitats along the migratory route. ARS behaviour in blue whales occurred over a much wider area as whales slowly progressed northwards. The tracks of these whales terminated still at middle latitudes, before any behavioural switch was detected. Fin whales exhibited behavioural-specific diel rhythms in swimming speed but these varied significantly between geographic areas, possibly due to differences in the day-night cycle across areas. Finally, we show a link between fin whales seen in the Azores and those summering in eastern Greenland-western Iceland along a migratory corridor located in central Atlantic waters.

  3. State transition analysis of spontaneous branch migration of the Holliday junction by photon-based single-molecule fluorescence resonance energy transfer.

    PubMed

    Okamoto, Kenji; Sako, Yasushi

    2016-02-01

    Branch migration of Holliday junction (HJ) DNA in solution is a spontaneous conformational change between multiple discrete states. We applied single-molecule fluorescence resonance energy transfer (smFRET) measurement to three-state branch migration. The photon-based variational Bayes-hidden Markov model (VB-HMM) method was applied to fluorescence signals to reproduce the state transition trajectories and evaluate the transition parameters, such as transition rate. The upper limit of time resolution suggested in simulation was nearly achieved for the state dynamics with relatively small FRET changes, and the distinctions in the populations of different states were successfully retrieved. We also discuss the suitability of the HJ as a standard sample for smFRET dynamics measurements and data analysis.

  4. State transition analysis of spontaneous branch migration of the Holliday junction by photon-based single-molecule fluorescence resonance energy transfer.

    PubMed

    Okamoto, Kenji; Sako, Yasushi

    2016-02-01

    Branch migration of Holliday junction (HJ) DNA in solution is a spontaneous conformational change between multiple discrete states. We applied single-molecule fluorescence resonance energy transfer (smFRET) measurement to three-state branch migration. The photon-based variational Bayes-hidden Markov model (VB-HMM) method was applied to fluorescence signals to reproduce the state transition trajectories and evaluate the transition parameters, such as transition rate. The upper limit of time resolution suggested in simulation was nearly achieved for the state dynamics with relatively small FRET changes, and the distinctions in the populations of different states were successfully retrieved. We also discuss the suitability of the HJ as a standard sample for smFRET dynamics measurements and data analysis. PMID:26687325

  5. Topological Point Defects in Relaxor Ferroelectrics.

    PubMed

    Nahas, Y; Prokhorenko, S; Kornev, I; Bellaiche, L

    2016-03-25

    First-principles-based effective Hamiltonian simulations are used to reveal the hidden connection between topological defects (hedgehogs and antihedgehogs) and relaxor behavior. Such defects are discovered to predominantly lie at the border of polar nanoregions in both Ba(Zr_{0.5}Ti_{0.5})O_{3} (BZT) and Pb(Sc_{0.5}Nb_{0.5})O_{3} (PSN) systems, and the temperature dependency of their density allows us to distinguish between noncanonical (PSN) and canonical (BZT) relaxor behaviors (via the presence or absence of a crossing of a percolation threshold). This density also possesses an inflection point at precisely the temperature for which the dielectric response peaks. Moreover, hedgehogs and antihedgehogs are found to be mobile excitations, and the dynamical nature of their annihilation is demonstrated (using simple hydrodynamical arguments) to follows laws, such as those of Vogel-Fulcher and Arrhenius, that are characteristic of dipolar relaxation kinetics of relaxor ferroelectrics. PMID:27058101

  6. Topological Point Defects in Relaxor Ferroelectrics.

    PubMed

    Nahas, Y; Prokhorenko, S; Kornev, I; Bellaiche, L

    2016-03-25

    First-principles-based effective Hamiltonian simulations are used to reveal the hidden connection between topological defects (hedgehogs and antihedgehogs) and relaxor behavior. Such defects are discovered to predominantly lie at the border of polar nanoregions in both Ba(Zr_{0.5}Ti_{0.5})O_{3} (BZT) and Pb(Sc_{0.5}Nb_{0.5})O_{3} (PSN) systems, and the temperature dependency of their density allows us to distinguish between noncanonical (PSN) and canonical (BZT) relaxor behaviors (via the presence or absence of a crossing of a percolation threshold). This density also possesses an inflection point at precisely the temperature for which the dielectric response peaks. Moreover, hedgehogs and antihedgehogs are found to be mobile excitations, and the dynamical nature of their annihilation is demonstrated (using simple hydrodynamical arguments) to follows laws, such as those of Vogel-Fulcher and Arrhenius, that are characteristic of dipolar relaxation kinetics of relaxor ferroelectrics.

  7. Topological Point Defects in Relaxor Ferroelectrics

    NASA Astrophysics Data System (ADS)

    Nahas, Y.; Prokhorenko, S.; Kornev, I.; Bellaiche, L.

    2016-03-01

    First-principles-based effective Hamiltonian simulations are used to reveal the hidden connection between topological defects (hedgehogs and antihedgehogs) and relaxor behavior. Such defects are discovered to predominantly lie at the border of polar nanoregions in both Ba (Zr0.5 Ti0.5 )O3 (BZT) and Pb (Sc0.5 Nb0.5 )O3 (PSN) systems, and the temperature dependency of their density allows us to distinguish between noncanonical (PSN) and canonical (BZT) relaxor behaviors (via the presence or absence of a crossing of a percolation threshold). This density also possesses an inflection point at precisely the temperature for which the dielectric response peaks. Moreover, hedgehogs and antihedgehogs are found to be mobile excitations, and the dynamical nature of their annihilation is demonstrated (using simple hydrodynamical arguments) to follows laws, such as those of Vogel-Fulcher and Arrhenius, that are characteristic of dipolar relaxation kinetics of relaxor ferroelectrics.

  8. Point defects diffusion in $alpha;-Ti

    NASA Astrophysics Data System (ADS)

    Fernández, J. R.; Monti, A. M.; Pasianot, R. C.

    1996-04-01

    A research on the statics and dynamics of vacancies and self-interstitials in model α-Ti lattices is carried out by means of computer simulation techniques. A comprehensive study beginning with the development of an appropriate interatomic potential up to the final evaluation of the anisotropy of the self-diffusion by both vacancy and self-interstitial mechanisms is undertaken. Experimental results on self-diffusion in α-Ti single-crystals are analyzed within the framework of the calculated diffusion constants for a vacancy mechanism. A strongly dominating basal diffusion for self-interstitials is predicted.

  9. [Theories on migration and migration policy].

    PubMed

    Waldrauch, H

    1995-01-01

    "In its first part the article gives a short historical overview of theories on migration.... The author tries to clarify the term[s]...'migration policy' and...'migration' itself and assesses the usefulness of various migration typologies. The final chapter analyses determinants and trends of migration policies in Europe in the 1990s: the continuing pressures for migration in developing countries, the end of numerous barriers to emigration, the revival of nationalistic concepts of immigration and exclusionary tendencies founded on culturalistic arguments, the process of harmonizing control mechanisms in the European Union, and the influence of international human rights declarations on the formulation of migration policies." (SUMMARY IN ENG AND FRE)

  10. Chemo-responsive shape memory effect in shape memory polyurethane triggered by inductive release of mechanical energy storage undergoing copper (II) chloride migration

    NASA Astrophysics Data System (ADS)

    Lu, Haibao; Lu, Chunrui; Huang, Wei Min; Leng, Jinsong

    2015-03-01

    In this study, 10% weight fraction of copper (II) chloride (CuCl2) was embedded into shape memory polyurethane (SMPU) by dissolving it in a solvent mixture of tetrahydrofuran and N,N-dimethyl formamide. It is found that CuCl2 particles migrate; they are released from the polymer in the water-driven shape recovery process of SMPU composites. SMPU composites, after various immersion times in water, were characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Experimental results support that hydrogen bonding between polyurethane macromolecules and water molecules is the driving force, resulting from the inductive decrease in the glass transition temperature. Furthermore, the release of the stored mechanical energy in SMPU is demonstrated by means of tracking the migration of CuCl2 particles via x-ray diffraction and scanning electron microscopy tests. This study focuses on the mechanism of release of the stored mechanical energy of a polymer, which is identified as the driving force for the chemo-responsive shape memory effect and inductive decrease in glass transition temperature of SMPU in response to the water.

  11. Migration strategies of insects.

    PubMed

    Dingle, H

    1972-03-24

    transports genotypes across long distances with considerable mixing of populations. An understanding of its operation therefore carries with it implications for population genetics, zoogeography, and evolutionary theory. Finally, at least parts of the above general theory would seem to be applicable to forms other than insects. Bird and insect migrations, for example, are in many respects ecologically and physiologically similar. Birds, like insects, emphasize locomotory. as opposed to vegetative functions during long-distance flight; the well-known Zugenruhe or migratory restlessness is a case in point. Further, many birds migrateat nigt at a time when they would ordinarily roost(vegetative activity). Because their life spans exceed single seasons, bird migrants are not prereproductive in the same sense that insect migrants are, and hence reproductive values do not have the same meaning(but note that some insects are also interreproductive migrants). The situaion is complicated further by the fact that in many birds adult survivorship is virtually independent of age so that colonizing ability tends to be also (10, 54). Nevertheless, birds arrive on their nesting grounds in reproductive condition with the result that migration is a colonizing episode. It is also phenotypically modifiable by environmental factors, some of which, for example, photoperiod, influence insects as well (55). The similarities between birds and insects thus seem sufficient to indicate, at least provisionally, that the theory developed for insects applies also to birds with appropriate modifications for longer life span and more complex social behavior; comparisons between insects and fish (56) lead to the same conclusion. In birds especially, and also in other forms, various functions accessory to migration such as reproductive endocrinology, energy budgets, and orientation mechanisms have been studied extensively (55, 56). But there is need in vertebrates for more data andtheoy on the ecology and

  12. A Pluridisciplinary Study of the Impact of Future Ice Sheets Instability on Sea Level Rise, Climate Changes, Migrations and Energy Supply

    NASA Astrophysics Data System (ADS)

    Ramstein, G.; Defrance, D.; Dumas, C.; Charbit, S.; Gemenne, F.; Vanderlinden, J. P.; Bouneau, S.; David, S.

    2014-12-01

    From paleoclimate data and GCM models simulations, we learnt that, when ice sheets are unstable, they produce large surges of icebergs that cover North Atlantic and produce global climate instability through atmosphere and ocean dynamics. Indeed, these instabilities are associated with a cold (glacial) context. In a warming world, it appears that the two remaining ice sheets (Antarctica and Greenland) are becoming unstable. The probability of abrupt ice sheet changes is therefore to be investigated. In comparison with predicted AR5 sea level rise (SLR) (28 to 82 cm), such a fast melting could add a contribution of 2 to 5 meters, which corresponds to the sea level rise estimate of glacial instability (Heinrich events). The aim of this presentation is to describe consistent sea level and climate simulations. We define 3 scenarios, the sea level rise is produced by the melting of about third of the Greenland, or the disappearance of West Antarctica, or finally a mixture of both ice sheets melting. We first analyze, using IPSL OAGCM, the climatic impact. Indeed, superimposed to direct sea level rise, climate changes has drastic consequences in North Atlantic (Europe and east of North America) due to the thermohaline circulation breakdown. Moreover, when freshwater is injected into North Atlantic, far field effects on the location and amplitude of Asian Monsoon have been shown with its drastic decrease. The second step of this multidisciplinary study is to quantify the impact of the sea level rise and the climatic changes on populations migration, with a focus on Southern Asia with a prospective migration of several hundreds of people. In a third step, we intend to simulate the reorganization of power supply to adapt to the expected new distribution of population, using a quantitative energy model (COSIME). Here, we show the climate response to the 3 investigated climate scenarios and the response in terms of migration, which is huge due to the very large part of the

  13. Migration of dispersive GPR data

    USGS Publications Warehouse

    Powers, M.H.; Oden, C.P.; ,

    2004-01-01

    Electrical conductivity and dielectric and magnetic relaxation phenomena cause electromagnetic propagation to be dispersive in earth materials. Both velocity and attenuation may vary with frequency, depending on the frequency content of the propagating energy and the nature of the relaxation phenomena. A minor amount of velocity dispersion is associated with high attenuation. For this reason, measuring effects of velocity dispersion in ground penetrating radar (GPR) data is difficult. With a dispersive forward model, GPR responses to propagation through materials with known frequency-dependent properties have been created. These responses are used as test data for migration algorithms that have been modified to handle specific aspects of dispersive media. When either Stolt or Gazdag migration methods are modified to correct for just velocity dispersion, the results are little changed from standard migration. For nondispersive propagating wavefield data, like deep seismic, ensuring correct phase summation in a migration algorithm is more important than correctly handling amplitude. However, the results of migrating model responses to dispersive media with modified algorithms indicate that, in this case, correcting for frequency-dependent amplitude loss has a much greater effect on the result than correcting for proper phase summation. A modified migration is only effective when it includes attenuation recovery, performing deconvolution and migration simultaneously.

  14. Internationalization and migration pressure.

    PubMed

    Kultalahti, O

    1994-01-01

    The author first develops the concept of migration pressure, which is defined as the growth in the number of people wishing to migrate and the barriers preventing them from so doing. Both macro- and micro-level factors affecting migration pressure are identified. Historical trends in migration pressure in Finland are then discussed. The author then applies this concept to the analysis of current Finnish migration trends. The primary focus is on international migration.

  15. Return migration to Italy and labour migration.

    PubMed

    Calvaruso, C

    1983-01-01

    The problems caused by large-scale return migration to Italy in recent years are considered. The importance of the additional skills and capital acquired by these migrants while abroad is stressed. Extensive data on the volume of return migration in the 1970s are included.

  16. Population, migration and urbanization.

    PubMed

    1982-06-01

    Despite recent estimates that natural increase is becoming a more important component of urban growth than rural urban transfer (excess of inmigrants over outmigrants), the share of migration in the total population growth has been consistently increasing in both developed and developing countries. From a demographic perspective, the migration process involves 3 elements: an area of origin which the mover leaves and where he or she is considered an outmigrant; the destination or place of inmigration; and the period over which migration is measured. The 2 basic types of migration are internal and international. Internal migration consists of rural to urban migration, urban to urban migration, rural to rural migration, and urban to rural migration. Among these 4 types of migration various patterns or processes are followed. Migration may be direct when the migrant moves directly from the village to the city and stays there permanently. It can be circular migration, meaning that the migrant moves to the city when it is not planting season and returns to the village when he is needed on the farm. In stage migration the migrant makes a series of moves, each to a city closer to the largest or fastest growing city. Temporary migration may be 1 time or cyclical. The most dominant pattern of internal migration is rural urban. The contribution of migration to urbanization is evident. For example, the rapid urbanization and increase in urban growth from 1960-70 in the Republic of Korea can be attributed to net migration. In Asia the largest component of the population movement consists of individuals and groups moving from 1 rural location to another. Recently, because urban centers could no longer absorb the growing number of migrants from other places, there has been increased interest in the urban to rural population redistribution. This reverse migration also has come about due to slower rates of employment growth in the urban centers and improved economic opportunities

  17. Migration and Adult Education

    ERIC Educational Resources Information Center

    Gois, William

    2007-01-01

    The objective of this paper is to highlight the role of adult education as a tool in addressing labour migration issues, specifically those concerning the protection of migrant workers' rights and the transformation of the impact of migration into positive holistic developmental gains. The view of labour migration as a means to forge the economic…

  18. Method for reducing energy losses in laser crystals

    DOEpatents

    Atherton, L.J.; DeYoreo, J.J.; Roberts, D.H.

    1992-03-24

    A process for reducing energy losses in crystals is disclosed which comprises: a. heating a crystal to a temperature sufficiently high as to cause dissolution of microscopic inclusions into the crystal, thereby converting said inclusions into point-defects, and b. maintaining said crystal at a given temperature for a period of time sufficient to cause said point-defects to diffuse out of said crystal. Also disclosed are crystals treated by the process, and lasers utilizing the crystals as a source of light. 12 figs.

  19. Method for reducing energy losses in laser crystals

    DOEpatents

    Atherton, L. Jeffrey; DeYoreo, James J.; Roberts, David H.

    1992-01-01

    A process for reducing energy losses in crystals is disclosed which comprises: a. heating a crystal to a temperature sufficiently high as to cause dissolution of microscopic inclusions into the crystal, thereby converting said inclusions into point-defects, and b. maintaining said crystal at a given temperature for a period of time sufficient to cause said point-defects to diffuse out of said crystal. Also disclosed are crystals treated by the process, and lasers utilizing the crystals as a source of light.

  20. Can migration mitigate the effects of ecosystem change? Patterns of dispersal, energy acquisition and allocation in Great Lakes lake whitefish (Coregonus clupeaformis)

    USGS Publications Warehouse

    Rennie, Michael D.; Ebener, Mark P.; Wagner, Tyler

    2012-01-01

    Migration can be a behavioural response to poor or declining home range habitat quality and can occur when the costs of migration are overcome by the benefi ts of encountering higher-quality resources elsewhere. Despite dramatic ecosystem-level changes in the benthic food web of the Laurentian Great Lakes since the colonization of dreissenid mussels, coincident changes in condition and growth rates among benthivorous lake whitefi sh populations have been variable. We hypothesized that this variation could be in part mitigated by differences in migratory habits among populations, where increased migration distance can result in an increased probability of encountering high-quality habitat (relative to the home range). Results from four Great Lakes populations support this hypothesis; relative growth rates increased regularly with migration distance. The population with the largest average migration distance also had the least reduction in size-at-age during a period of signifi cant ecosystem change and among the highest estimated consumption and activity rates. In comparison, the population with the greatest declines in size-at-age was among the least mobile, demonstrating only moderate rates of consumption and activity. The least mobile population of lake whitefi sh was supported by a remnant Diporeia population and has experienced only moderate temporal growth declines. Our study provides evidence for the potential role of migration in mitigating the effects of ecosystem change on lake whitefi sh populations.

  1. Aquaporins and cell migration.

    PubMed

    Papadopoulos, M C; Saadoun, S; Verkman, A S

    2008-07-01

    Aquaporin (AQP) water channels are expressed primarily in cell plasma membranes. In this paper, we review recent evidence that AQPs facilitate cell migration. AQP-dependent cell migration has been found in a variety of cell types in vitro and in mice in vivo. AQP1 deletion reduces endothelial cell migration, limiting tumor angiogenesis and growth. AQP4 deletion slows the migration of reactive astrocytes, impairing glial scarring after brain stab injury. AQP1-expressing tumor cells have enhanced metastatic potential and local infiltration. Impaired cell migration has also been seen in AQP1-deficient proximal tubule epithelial cells, and AQP3-deficient corneal epithelial cells, enterocytes, and skin keratinocytes. The mechanisms by which AQPs enhance cell migration are under investigation. We propose that, as a consequence of actin polymerization/depolymerization and transmembrane ionic fluxes, the cytoplasm adjacent to the leading edge of migrating cells undergoes rapid changes in osmolality. AQPs could thus facilitate osmotic water flow across the plasma membrane in cell protrusions that form during migration. AQP-dependent cell migration has potentially broad implications in angiogenesis, tumor metastasis, wound healing, glial scarring, and other events requiring rapid, directed cell movement. AQP inhibitors may thus have therapeutic potential in modulating these events, such as slowing tumor growth and spread, and reducing glial scarring after injury to allow neuronal regeneration. PMID:17968585

  2. Migration to Windows NT.

    ERIC Educational Resources Information Center

    Doles, Daniel T.

    In the constantly changing world of technology, migration is not only inevitable but many times necessary for survival, especially when the end result is simplicity for both users and IT support staff. This paper describes the migration at Franklin College (Indiana). It discusses the reasons for selecting Windows NT, the steps taken to complete…

  3. The Future of Migration.

    ERIC Educational Resources Information Center

    Organisation for Economic Cooperation and Development, Paris (France).

    This book comprises papers delivered at a conference of National Experts on Migration. The principle objective of the conference was twofold: to examine significant trends that will affect the future of migration in countries in the Organization for Economic Co-operation and Development (OCED), and to identify the relevant issues that will have to…

  4. The Great Migration.

    ERIC Educational Resources Information Center

    Trotter, Joe William, Jr.

    2002-01-01

    Describes the migration of African Americans in the United States and the reasons why African Americans migrated from the south. Focuses on issues, such as the effect of World War I, the opportunities offered in the north, and the emergence of a black industrial working class. (CMK)

  5. Migration and Environmental Hazards

    PubMed Central

    Hunter, Lori M.

    2011-01-01

    Losses due to natural hazards (e.g., earthquakes, hurricanes) and technological hazards (e.g., nuclear waste facilities, chemical spills) are both on the rise. One response to hazard-related losses is migration, with this paper offering a review of research examining the association between migration and environmental hazards. Using examples from both developed and developing regional contexts, the overview demonstrates that the association between migration and environmental hazards varies by setting, hazard types, and household characteristics. In many cases, however, results demonstrate that environmental factors play a role in shaping migration decisions, particularly among those most vulnerable. Research also suggests that risk perception acts as a mediating factor. Classic migration theory is reviewed to offer a foundation for examination of these associations. PMID:21886366

  6. Migration and Environmental Hazards.

    PubMed

    Hunter, Lori M

    2005-03-01

    Losses due to natural hazards (e.g., earthquakes, hurricanes) and technological hazards (e.g., nuclear waste facilities, chemical spills) are both on the rise. One response to hazard-related losses is migration, with this paper offering a review of research examining the association between migration and environmental hazards. Using examples from both developed and developing regional contexts, the overview demonstrates that the association between migration and environmental hazards varies by setting, hazard types, and household characteristics. In many cases, however, results demonstrate that environmental factors play a role in shaping migration decisions, particularly among those most vulnerable. Research also suggests that risk perception acts as a mediating factor. Classic migration theory is reviewed to offer a foundation for examination of these associations.

  7. Migration of health workers.

    PubMed

    Buchan, James

    2008-01-01

    The discussion and debate stimulated by these papers focused across a range of issues but there were four main areas of questioning: "measuring" and monitoring migration (issues related to comparability, completeness and accuracy of data sets on human resources); the impact of migration of health workers on health systems; the motivations of individual health workers to migrate (the "push" and "pull" factors) and the effect of policies designed either to reduce migration (e.g "self ufficiency") or to stimulate it (e.g active international recruitment). It was recognised that there was a critical need to examine migratory flows within the broader context of all health care labour market dynamics within a country, that increasing migration of health workers was an inevitable consequence of globalisation, and that there was a critical need to improve monitoring so as to better inform policy formulation and policy testing in this area. PMID:18561695

  8. Migration of health workers.

    PubMed

    Buchan, James

    2008-01-01

    The discussion and debate stimulated by these papers focused across a range of issues but there were four main areas of questioning: "measuring" and monitoring migration (issues related to comparability, completeness and accuracy of data sets on human resources); the impact of migration of health workers on health systems; the motivations of individual health workers to migrate (the "push" and "pull" factors) and the effect of policies designed either to reduce migration (e.g "self ufficiency") or to stimulate it (e.g active international recruitment). It was recognised that there was a critical need to examine migratory flows within the broader context of all health care labour market dynamics within a country, that increasing migration of health workers was an inevitable consequence of globalisation, and that there was a critical need to improve monitoring so as to better inform policy formulation and policy testing in this area.

  9. On marriage and migration.

    PubMed

    Stark, O

    1988-09-01

    Marriage, migration, and related phenomena such as marital stability, fertility, and investment in human capital may be better explained by studying marriage and migration jointly. This paper examines the role of migration in obtaining joint labor market and marriage market equilibrium. When broadly interpreted, marriage and migration share a number of common features. Both involve search and its resolution (pairing of mates in the former and matching of labor and firms in the latter). In both cases, success in finding a partner is sensitive to the availability of partners and to the distribution of their endowments and traits. Almost always, and along with separation and divorce, marriage mandates spatial relocation which may translate into migration. Both involve a movement that is associated with adjustment costs from 1 state into another. The decisions to enter marriage and undertake employment or the decisions to divorce and quit a job depend on exogenous parameters, some of which are determined by the marriage market and the labor market. Since both marriage and divorce take place in the context of broadly defined markets, they may and often are analyzed applying market concepts, theorems, and solutions. Yet the authors could not pinpoint 1 single, systematic attempt that checks through the interactions between marriage and migration, so this paper attempts to rectify this state of research. Essentially, this paper 1) discusses individual decision making pending possible migration prior to or following marriage, 2) examines whether it is easier for a married couple or a single person to migrate, and 3) considers whether marriage dissolution could cause migration when marriage is the only reason that has kept a spouse from moving. This integrated research agenda for both marriage and migration can delineate interesting new implications to examine.

  10. Nanoclay migration from food packaging materials.

    PubMed

    Echegoyen, Yolanda; Rodríguez, Silvia; Nerín, Cristina

    2016-01-01

    A recent trend is to use nanocomposites materials for food-packaging applications. Different kinds of nanoparticles are incorporated into the polymers to improve their characteristics, and, among them, nanoclay is used to improve their barrier properties to gases. In this work, the results of migration studies with different food simulants (ethanol 10% and acetic acid 3%), temperatures and times (40 ºC for 10 days and 70 ºC for 2 h) from two commercialised LDPE nanocomposite bags are presented. The migration solutions thus obtained were analysed by ICP-MS to evaluate the amount of aluminium which migrated into the solutions both in dissolved form and as a part of nanoparticles. Aluminium migration was observed for both samples with a maximum migration value of 51.65 ng cm(-)(2) for the Aisaika bags and 24.14 ng cm(-)(2) for the Debbie Meyer bags. The presence of spikes working in single-particle mode using ICP-MS indicated that part of this aluminium was present as nanoparticles. The size and morphology of the nanoclay, in both the original material and the migration solutions, was studied by scanning electron microscopy coupled to energy-dispersive X-ray diffraction (SEM-EDX). In this manner, nanoparticles of different morphologies and sizes were found to migrate into the food simulants. PMID:26751017

  11. The Use of Electromyogram (EMG) Telemetry to Assess Swimming Activity and Energy Use of Adult Spring Chinook Salmon Migrating through the Tailraces, Fishways, and Forebays of Bonneville Dam, 2000 and 2001

    SciTech Connect

    Brown, Richard S.; Geist, David R.; Mesa, Matthew G.

    2002-10-16

    In 2000, PNNL conducted a two-year study for the U.S. Army Corps of Engineers to investigate energy use and swimming performance of adult spring chinook salmon (Oncorhynchus tshawystcha) migrating upstream through a large hydropower dam on the Columbia River. The investigation involved one year of laboratory study and one year of field study at Bonneville Dam. The objectives of the laboratory study were to 1) measure active rates of oxygen consumption of adult spring chinook salmon at three water temperatures over a range of swimming speeds; 2) estimate the upper critical swimming speed (Ucrit) of adult spring chinook salmon; and 3) monitor electromyograms (EMGs) of red and white muscle in the salmon over a range of swimming speeds. Laboratory results showed rate of oxygen consumption and red and white muscle activity in adult spring chinook salmon were strongly correlated with swimming speed over a range of fish sizes and at three different temperatures. In the field studies at Bonneville Dam, EMG radiotelemetry was used to examine the amount of energy spring chinook salmon expend while migrating upstream past the dam?s tailraces, fishways, and forebays. Aerobic and anaerobic energy use rates were determined. Energy use was estimated for different specific sections of each fishway also. The rates of energy used (kcal/kg/h) by spring chinook salmon were significantly higher in the tailraces (2.80 kcal/kg/h) than in other parts of the dam. Among all fishway areas, Cascade Island fishway appears to be more energetically costly than other fishways. Also, section 12 of the Washington shore fishway appears costly. Energy used during fallouts was substantial (11.5% to 18.8% of the amount of energy used for successful fishway passages).

  12. [Determinants of migration to Warsaw].

    PubMed

    Kucinski, K; Rakowski, W

    1990-01-01

    The authors examine trends and determinants of internal migration to Warsaw, Poland. Consideration is given to occupational and socioeconomic status of migrants, rural-urban migration, and effects of migration on marriage. (SUMMARY IN ENG AND RUS)

  13. Neuronal migration illuminated

    PubMed Central

    Trivedi, Niraj

    2011-01-01

    During vertebrate brain development, migration of neurons from the germinal zones to their final laminar positions is essential to establish functional neural circuits.1–3 Whereas key insights into neuronal migration initially came from landmark studies identifying the genes mutated in human cortical malformations,4 cell biology has recently greatly advanced our understanding of how cytoskeletal proteins and molecular motors drive the morphogenic cell movements that build the developing brain. This Commentary & View reviews recent studies examining the role of the molecular motors during neuronal migration and critically examines current models of acto-myosin function in the two-step neuronal migration cycle. Given the apparent emerging diversity of neuronal sub-type cytoskeletal organizations, we propose that two approaches must be taken to resolve differences between the current migration models: the mechanisms of radial and tangential migration must be compared, and the loci of tension generation, migration substrates and sites of adhesion dynamics must be precisely examined in an integrated manner. PMID:20935494

  14. Migration without migraines

    SciTech Connect

    Lines, L.; Burton, A.; Lu, H.X.

    1994-12-31

    Accurate velocity models are a necessity for reliable migration results. Velocity analysis generally involves the use of methods such as normal moveout analysis (NMO), seismic traveltime tomography, or iterative prestack migration. These techniques can be effective, and each has its own advantage or disadvantage. Conventional NMO methods are relatively inexpensive but basically require simplifying assumptions about geology. Tomography is a more general method but requires traveltime interpretation of prestack data. Iterative prestack depth migration is very general but is computationally expensive. In some cases, there is the opportunity to estimate vertical velocities by use of well information. The well information can be used to optimize poststack migrations, thereby eliminating some of the time and expense of iterative prestack migration. The optimized poststack migration procedure defined here computes the velocity model which minimizes the depth differences between seismic images and formation depths at the well by using a least squares inversion method. The optimization methods described in this paper will hopefully produce ``migrations without migraines.``

  15. Women in migration.

    PubMed

    Morokvasic, M

    1984-01-01

    This special issue reflects the belated but growing scholarly appreciation of the specificity and importance of women in migration. Aside from the sheer numerical significance of female migration documented in this issue, women migrants encounter problems and make special contributions which render comprehension of their specificity critical to an understanding of international migration in general. In an introductory essay, Morokvasic surveys the state of knowledge concerning women in migration. The focus then shifts, in Part II, to regional and national case studies which collectively elucidate the multifaceted dimensions of the women in migration research issue through time and space. In Part III, an international comparison of female immigrants and their labor market characteristics reveals striking similarities but also important differences. The US Canada and Australia can be discretely compared through 5 census-based quantitative analyses. The role of migrant women in the labor market is also the theme of Part IV. But the 5 studies comprising this section are based on survey research or on discernible global trends in migration and employment. Part V is devoted to the theme of female rural to urban migration in the Third World.

  16. Labor migration in Asia.

    PubMed

    Martin, P L

    1991-01-01

    "A recent conference sponsored by the United Nations Center for Regional Development (UNCRD) in Nagoya, Japan examined the growing importance of labor migration for four major Asian labor importers (Japan, Hong Kong, Malaysia, and Singapore) and five major labor exporters (Bangladesh, Korea, Pakistan, Philippines, and Thailand).... The conference concluded that international labor migration would increase within Asia because the tight labor markets and rising wages which have stimulated Japanese investment in other Asian nations, for example, have not been sufficient to eliminate migration push and pull forces...."

  17. SUPER-ECCENTRIC MIGRATING JUPITERS

    SciTech Connect

    Socrates, Aristotle; Katz, Boaz; Dong Subo; Tremaine, Scott

    2012-05-10

    An important class of formation theories for hot Jupiters involves the excitation of extreme orbital eccentricity (e = 0.99 or even larger) followed by tidal dissipation at periastron passage that eventually circularizes the planetary orbit at a period less than 10 days. In a steady state, this mechanism requires the existence of a significant population of super-eccentric (e > 0.9) migrating Jupiters with long orbital periods and periastron distances of only a few stellar radii. For these super-eccentric planets, the periastron is fixed due to conservation of orbital angular momentum and the energy dissipated per orbit is constant, implying that the rate of change in semi-major axis a is a-dot {proportional_to}a{sup 1/2} and consequently the number distribution satisfies dN/d log a{proportional_to}a{sup 1/2}. If this formation process produces most hot Jupiters, Kepler should detect several super-eccentric migrating progenitors of hot Jupiters, allowing for a test of high-eccentricity migration scenarios.

  18. First principle-based AKMC modelling of the formation and medium-term evolution of point defect and solute-rich clusters in a neutron irradiated complex Fe-CuMnNiSiP alloy representative of reactor pressure vessel steels

    NASA Astrophysics Data System (ADS)

    Ngayam-Happy, R.; Becquart, C. S.; Domain, C.

    2013-09-01

    The formation and medium-term evolution of point defect and solute-rich clusters under neutron irradiation have been modelled in a complex Fe-CuMnNiSiP alloy representative of RPV steels, by means of first principle-based atomistic kinetic Monte Carlo simulations. The results obtained reproduce most features observed in available experimental studies, highlighting the very good agreement between both series. According to simulation, solute-rich clusters form and develop via an induced segregation mechanism on either the vacancy or interstitial clusters, and these point defect clusters are efficiently generated only in cascade debris and not Frenkel pair flux. The results have revealed the existence of two distinct populations of clusters with different characteristic features. Solute-rich clusters in the first group are bound essentially to interstitial clusters and they are enriched in Mn mostly, but also Ni to a lesser extent. Over the low dose regime, their density increases in the alloy as a result of the accumulation of highly stable interstitial clusters. In the second group, the solute-rich clusters are merged with vacancy clusters, and they contain mostly Cu and Si, but also substantial amount of Mn and Ni. The formation of a sub-population of pure solute clusters has been observed, which results from annihilation of the low stable vacancy clusters on sinks. The results indicate finally that the Mn content in clusters is up to 50%, Cu, Si, and Ni sharing the other half in more or less equivalent amounts. This composition has not demonstrated any noticeable modification with increasing dose over irradiation.

  19. 3D grain boundary migration

    NASA Astrophysics Data System (ADS)

    Becker, J. K.; Bons, P. D.

    2009-04-01

    Microstructures of rocks play an important role in determining rheological properties and help to reveal the processes that lead to their formation. Some of these processes change the microstructure significantly and may thus have the opposite effect in obliterating any fabrics indicative of the previous history of the rocks. One of these processes is grain boundary migration (GBM). During static recrystallisation, GBM may produce a foam texture that completely overprints a pre-existing grain boundary network and GBM actively influences the rheology of a rock, via its influence on grain size and lattice defect concentration. We here present a new numerical simulation software that is capable of simulating a whole range of processes on the grain scale (it is not limited to grain boundary migration). The software is polyhedron-based, meaning that each grain (or phase) is represented by a polyhedron that has discrete boundaries. The boundary (the shell) of the polyhedron is defined by a set of facets which in turn is defined by a set of vertices. Each structural entity (polyhedron, facets and vertices) can have an unlimited number of parameters (depending on the process to be modeled) such as surface energy, concentration, etc. which can be used to calculate changes of the microstructre. We use the processes of grain boundary migration of a "regular" and a partially molten rock to demonstrate the software. Since this software is 3D, the formation of melt networks in a partially molten rock can also be studied. The interconnected melt network is of fundamental importance for melt segregation and migration in the crust and mantle and can help to understand the core-mantle differentiation of large terrestrial planets.

  20. Indonesia's migration transition.

    PubMed

    Hugo, G

    1995-01-01

    This article describes population movements in Indonesia in the context of rapid and marked social and economic change. Foreign investment in Indonesia is increasing, and global mass media is available to many households. Agriculture is being commercialized, and structural shifts are occurring in the economy. Educational levels are increasing, and women's role and status are shifting. Population migration has increased over the decades, both short and long distance, permanent and temporary, legal and illegal, and migration to and between urban areas. This article focuses specifically on rural-to-urban migration and international migration. Population settlements are dense in the agriculturally rich inner areas of Java, Bali, and Madura. Although the rate of growth of the gross domestic product was 6.8% annually during 1969-94, the World Bank ranked Indonesia as a low-income economy in 1992 because of the large population size. Income per capita is US $670. Indonesia is becoming a large exporter of labor to the Middle East, particularly women. The predominance of women as overseas contract workers is changing women's role and status in the family and is controversial due to the cases of mistreatment. Malaysia's high economic growth rate of over 8% per year means an additional 1.3 million foreign workers and technicians are needed. During the 1980s urban growth increased at a very rapid rate. Urban growth tended to occur along corridors and major transportation routes around urban areas. It is posited that most of the urban growth is due to rural-to-urban migration. Data limitations prevent an exact determination of the extent of rural-to-urban migration. More women are estimated to be involved in movements to cities during the 1980s compared to the 1970s. Recruiters and middlemen have played an important role in rural-to-urban migration and international migration. PMID:12347370

  1. Migration and malaria.

    PubMed

    Jitthai, Nigoon

    2013-01-01

    Migration is an important global issue as poorly managed migration can result in a diversity of problems, including an increase in the transmission of diseases such as malaria. There is evidence to suggest that malaria is no longer a forest-dependent disease and may largely be affected by population movements, mostly to agricultural areas. While internal and transnational migration has different legal implications in most countries, both types of migration occur for the same reasons; economic and/ or safety. Although migration in itself is not a definitive risk for malaria, several factors can put, migrants and local communities alike, in vulnerable situations. In particular, infrastructure and rural development, deforestation for logging and economic farming, political movements, and natural disasters are some of the major factors that push and pull people in and out of malaria-endemic areas. Therefore, understanding the changing socio-environmental situation as well as population movements and their associated risks for malaria infection, is critical for malaria control, containment, and elimination. Efforts to address these issues should include advocacy, mapping exercises and expanded/ strengthened surveillance to also include migrant health information systems. Malaria related information, prevention measures, and early diagnosis and appropriate treatment should be made easily accessible for migrants regardless of their migration status; not only to ensure that they are equipped with appropriate knowledge and devices to protect themselves, but also to ensure that they are properly diagnosed and treated, to prevent further transmission, and to ensure that they are captured by the surveillance system. PMID:24159832

  2. Timescales of DNAPL migration

    NASA Astrophysics Data System (ADS)

    Kueper, B.; Gerhard, J.; Reynolds, D.

    2003-04-01

    Dense, non-aqueous phase liquids such as chlorinated solvents, PCB oils, creosote, and coal tar are common soil and groundwater contaminants at sites throughout the world. Current source zone remediation approaches typically assume that the residual and pooled DNAPL of interest is no longer migrating. The motivation for partial mass removal from a DNAPL source zone varies from site to site, but is often motivated by the belief that mass removal will lead to shorter steady-state plumes, shorter longevity of the source zone, and possibly aquifer restoration in a reasonable period of time. This talk addresses the issue of DNAPL migration timescales, and illustrates that certain types of DNAPL in certain geological environments are likely still migrating at some sites. The implication of this is that remedial strategies may need to be aimed at source zone stabilization in the short term, not partial mass removal for the reasons outlined above. The timescales of DNAPL migration at a site are influenced by many factors, including fluid properties, capillary properties, relative permeability characteristics, boundary conditions, and the volume and nature of release. Accurate prediction of DNAPL migration timescales requires a model that properly accounts for both the entry and terminal pressures in the capillary pressure -- saturation constitutive relationship, and properly accounts for relative permeability characteristics. This talk will address the above issues, and will present the results of laboratory experiments and numerical simulations to illustrate the timescale of DNAPL migration in a variety of environments including fractured rock, fractured clay, and unconsolidated porous media.

  3. Migration of helium-pair in metals

    NASA Astrophysics Data System (ADS)

    Cao, J. L.; Geng, W. T.

    2016-09-01

    We have carried out a first-principles density functional theory investigation into the migration of both a single interstitial He and an interstitial He-pair in Fe, Mo, W, Cu, Pd, and Pt. We find the migration trajectories and barriers are determined predominantly by low-energy He-pair configurations which depend mainly on the energy state of a single He in different interstices. The migration barrier for a He-pair in bcc metals is always slightly higher than for a single He. Configurations of a He-pair in fcc metals are very complicated, due to the existence of interstitial sites with nearly identical energy for a single He. The migration barrier for a He-pair is slightly lower than (in Cu), or similar to (in Pd and Pt) a single He. The collective migrations of a He-pair are ensured by strong Hesbnd He interactions with strength-versus-distance forms resembling chemical bonds and can be described with Morse potentials.

  4. Curvature capillary migration of microspheres.

    PubMed

    Sharifi-Mood, Nima; Liu, Iris B; Stebe, Kathleen J

    2015-09-14

    We address the question: how does capillarity propel microspheres along curvature gradients? For a particle on a fluid interface, there are two conditions that can apply at the three phase contact line: either the contact line adopts an equilibrium contact angle, or it can be pinned by kinetic trapping, e.g. at chemical heterogeneities, asperities, or other pinning sites on the particle surface. We formulate the curvature capillary energy for both scenarios for particles smaller than the capillary length and far from any pinning boundaries. The scale and range of the distortion made by the particle are set by the particle radius; we use singular perturbation methods to find the distortions and to rigorously evaluate the associated capillary energies. For particles with equilibrium contact angles, contrary to the literature, we find that the capillary energy is negligible, with the first contribution bounded to fourth order in the product of the particle radius and the deviatoric curvature of the host interface. For pinned contact lines, we find curvature capillary energies that are finite, with a functional form investigated previously by us for disks and microcylinders on curved interfaces. In experiments, we show microspheres migrate along deterministic trajectories toward regions of maximum deviatoric curvature with curvature capillary energies ranging from 6 × 10(3)-5 × 10(4)kBT. These data agree with the curvature capillary energy for the case of pinned contact lines. The underlying physics of this migration is a coupling of the interface deviatoric curvature with the quadrupolar mode of nanometric disturbances in the interface owing to the particle's contact line undulations. This work is an example of the major implications of nanometric roughness and contact line pinning for colloidal dynamics.

  5. Underground radionuclide migration at the Nevada Test Site

    SciTech Connect

    Nimz, G.J. ); Thompson, J.L. )

    1992-06-22

    This document reviews results from a number of studies concerning underground migration of radionuclides from nuclear test cavities at the Nevada Test Site (NTS). Discussed are all cases known to the Department of Energy's Hydrology and Radionuclide Migration Program where radionuclides have been detected outside of the immediate vicinity of nuclear test cavities that are identifiable as the-source of the nuclides, as well as cases where radionuclides might have been expected and were intentionally sought but not fixed. There are nine locations where source-identifiable radionuclide migration has been detected, one where migration was purposely induced by pumping, and three where migration might be expected but was not found. In five of the nine cases of non-induced migration, the inferred migration mechanism is prompt fracture injection during detonation. In the other four cases, the inferred migration mechanism is water movement. In only a few of the reviewed cases can the actual migration mechanism be stated with confidence, and the attempt has been made to indicate the level of confidence for each case. References are cited where more information may be obtained. As an aid to future study, this document concludes with a brief discussion of the aspects of radionuclide migration that, as the present review indicates, are not yet understood. A course of action is suggested that would produce a better understanding of the phenomenon of radionuclide migration.

  6. Determination of the activation enthalpy for migration of dislocations in plastically deformed 8006 Al-alloy by positron annihilation lifetime technique

    NASA Astrophysics Data System (ADS)

    Salah, Mohammed; Abdel-Rahman, M.; Badawi, Emad A.; Abdel-Rahman, M. A.

    2016-06-01

    The activation enthalpy for migration of dislocations of plastically deformed 8006 Al-alloy was investigated by positron annihilation lifetime technique. Plastic deformation using a hydraulic press produces mainly dislocations and may produce point defects. The type of defect was studied by isochronal annealing which determines the temperature range of recovery of each type. Only one type of defect (dislocations) was observed for the investigated sample and was found to be recovered within the range 455-700 K. Isothermal annealing by slow cooling was performed through this range and used in determination of the activation enthalpy of migration of dislocations which was found to be 0.26 ± 0.01 eV.

  7. Human rights and migration policies.

    PubMed

    Marmora, L

    1990-01-01

    This paper concerns the history of migration, migration policies, and the rights of migrants in Latin America from 1500 to the present. In the first part of the article, the author identifies and discusses the basic rights of migrants. In the second part, migration policies, migration flows, and the treatment of migrants are examined over time.

  8. Optimal migration energetics of humpback whales and the implications of disturbance

    PubMed Central

    Braithwaite, Janelle E.; Meeuwig, Jessica J.; Hipsey, Matthew R.

    2015-01-01

    Whales migrate long distances and reproduce on a finite store of energy. Budgeting the use of this limited energy reserve is an important factor to ensure survival over the period of migration and to maximize reproductive investment. For some whales, migration routes are closely associated with coastal areas, exposing animals to high levels of human activity. It is currently unclear how various forms of human activity may disturb whales during migration, how this might impact their energy balance and how this could translate into long-term demographic changes. Here, we develop a theoretical bioenergetic model for migrating humpback whales to investigate the optimal migration strategy that minimizes energy use. The average migration velocity was an important driver of the total energy used by a whale, and an optimal velocity of 1.1 m s−1 was determined. This optimal velocity is comparable to documented observed migration speeds, suggesting that whales migrate at a speed that conserves energy. Furthermore, the amount of resting time during migration was influenced by both transport costs and feeding rates. We simulated hypothetical disturbances to the optimal migration strategy in two ways, by altering average velocity to represent changes in behavioural activity and by increasing total travelled distance to represent displacement along the migration route. In both cases, disturbance increased overall energy use, with implications for the growth potential of calves. PMID:27293686

  9. Environmental concerns and international migration.

    PubMed

    Hugo, G

    1996-01-01

    "This article focuses on international migration occurring as a result of environmental changes and processes. It briefly reviews attempts to conceptualize environment-related migration and then considers the extent to which environmental factors have been and may be significant in initiating migration. Following is an examination of migration as an independent variable in the migration-environment relationship. Finally, ethical and policy dimensions are addressed."

  10. Environmental concerns and international migration.

    PubMed

    Hugo, G

    1996-01-01

    "This article focuses on international migration occurring as a result of environmental changes and processes. It briefly reviews attempts to conceptualize environment-related migration and then considers the extent to which environmental factors have been and may be significant in initiating migration. Following is an examination of migration as an independent variable in the migration-environment relationship. Finally, ethical and policy dimensions are addressed." PMID:12291410

  11. Biometrics and international migration.

    PubMed

    Redpath, Jillyanne

    2007-01-01

    This paper will focus on the impact of the rapid expansion in the use of biometric systems in migration management on the rights of individuals; it seeks to highlight legal issues for consideration in implementing such systems, taking as the starting point that the security interests of the state and the rights of the individual are not, and should not be, mutually exclusive. The first part of this paper briefly describes the type of biometric applications available, how biometric systems function, and those used in migration management. The second part examines the potential offered by biometrics for greater security in migration management, and focuses on developments in the use of biometrics as a result of September 11. The third part discusses the impact of the use of biometrics in the management of migration on the individual's right to privacy and ability to move freely and lawfully. The paper highlights the increasing need for domestic and international frameworks to govern the use of biometric applications in the migration/security context, and proposes a number of issues that such frameworks could address. PMID:17536151

  12. Synthetic seismic monitoring using reverse-time migration and Kirchhoff migration for CO2 sequestration in Korea

    NASA Astrophysics Data System (ADS)

    Kim, W.; Kim, Y.; Min, D.; Oh, J.; Huh, C.; Kang, S.

    2012-12-01

    vertical CO2 migration at injection point, the reverse time migration yields better images than Kirchhoff migration does. On the other hand, Kirchhoff migration images horizontal CO2 migration clearer than the reverse time migration does. From these results, we can conclude that the reverse-time migration and Kirchhoff migration can complement with each other to describe the behavior of CO2 in the subsurface. Acknowledgement This work was financially supported by the Brain Korea 21 project of Energy Systems Engineering, the "Development of Technology for CO2 Marine Geological Storage" program funded by the Ministry of Land, Transport and Maritime Affairs (MLTM) of Korea and the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Education, Science and Technology) (No. 2012-0008926).

  13. Migration from Packaging Materials

    NASA Astrophysics Data System (ADS)

    Meulenaer, B. De

    Various chemical compounds can be present in foodstuffs which may induce health problems in humans. The origin of these compounds can be very diverse. Mathematical modeling can sometimes be used to predict the concentration of these chemicals in the food. Particularly for compounds which are produced in the food during, e.g., processing and for compounds which migrate from a food contact material this technique can be very fruitful. For the former type of compounds, classical chemical kinetics can be applied. In this contribution, the modeling of the migration from polymeric food contact materials is considered. This migration phenomenon can be modeled mathematically since the physical processes which govern this process are very well studied and understood. Therefore, initially some of these fundamentals will be discussed in more detail.

  14. More myths of migration.

    PubMed

    Basch, L; Lerner, G

    1986-01-01

    This paper discusses some of the myths of migration. The 5 myths presented are: 1) racism has little to do with the causes of migration and does not necessarily impede the adjustment or success of migrants; 2) in areas where there is a strong feminist movement and trade unions, migrant women receive their support and can count on the solidarity of these organizations; 3) transnational corporations are positive forces in the developing countries where they operate--not only do they provide these states with new sources of capital, but they also impart new industrial skills to the labor force; 4) migration today is essentially short-term in nature--it therefore does not have a strong impact on family life; and 5) most migrants cluster together in ethnic enclaves which provide a strong source of support and diminish dislocation inherent in the migrant process.

  15. [Migration, climate and health].

    PubMed

    Tellier, Siri; Carballo, Manuel; Calballo, Manuel

    2009-10-26

    Many tentative connections have been postulated between migration and climate. This article points to rural-urban migration, particularly into low elevation urban slums prone to flooding as an issue needing urgent attention by health professionals. It also notes the no-man's land in which environmental refugees find themselves and the consequences this may have. Finally, it points to the urgent need to reform health systems in both developing and developed countries to adapt to rapidly changing disease patterns and to become more responsive to them.

  16. What's driving migration?

    PubMed

    Kane, H

    1995-01-01

    During the 1990s investment in prevention of international or internal migration declined, and crisis intervention increased. The budgets of the UN High Commissioner for Refugees and the UN Development Program remained about the same. The operating assumption is that war, persecution, famine, and environmental and social disintegration are inevitable. Future efforts should be directed to stabilizing populations through investment in sanitation, public health, preventive medicine, land tenure, environmental protection, and literacy. Forces pushing migration are likely to increase in the future. Forces include depletion of natural resources, income disparities, population pressure, and political disruption. The causes of migration are not constant. In the past, migration occurred during conquests, settlement, intermarriage, or religious conversion and was a collective movement. Current migration involves mass movement of individuals and the struggle to survive. There is new pressure to leave poor squatter settlements and the scarcities in land, water, and food. The slave trade between the 1500s and the 1800s linked continents, and only 2-3 million voluntarily crossed national borders. Involuntary migration began in the early 1800s when European feudal systems were in a decline, and people sought freedom. Official refugees, who satisfy the strict 1951 UN definition, increased from 15 million in 1980 to 23 million in 1990 but remained a small proportion of international migrants. Much of the mass movement occurs between developing countries. Migration to developed countries is accompanied by growing intolerance, which is misinformed. China practices a form of "population transfer" in Tibet in order to dilute Tibetan nationalism. Colonization of countries is a new less expensive form of control over territory. Eviction of minorities is another popular strategy in Iraq. Public works projects supported by foreign aid displace millions annually. War and civil conflicts

  17. [Migration and health].

    PubMed

    Litvinjenko, S

    1997-01-01

    In the last decades of this century we are witnesses of frequent crises in different parts of the world produced by internal disturbance and wars. These crises, together with natural disasters, poverty and hunger, follow the history of mankind often forcing huge population groups to leave their homes. The harmful health consequences are among negative effects of migrations. While stable populations have well-tried routines for maintaining health, migrations mean abandoning such support systems. The increased exposure to harmful factors contributes more to the bad health condition of the migrant population. Setting of newcomers and local people together in the same homes, reduction in food and heating resources, drug shortage as well as importation of new infectious agents, may also endanger health of the native population. These observations have also been confirmed by Yugoslav experience. Depending on the fact whether a migration is elemental or organized i.e. dependent on its place in the large scale between these two extreme endpoints, the size of risk is also dependent on the consequences and degree of their difficulty. Mass health disturbances occur during migrations of the population from war regions, migrations from areas of natural disasters, mass pilgrimage, migrations of seasonal workers and migrations of armies during wars. However, even in these difficult times and conditions, a good organization can contribute to the mitigation of harmful consequences caused by these migrations. For instance, in 1942 there was an epidemic of typhus fever in Bosnia when many refugees crossed the Drina river on the way to Serbia escaping from Ustasha terrorism. At the Serbian side there were checkpoints where the refugees could taka a bath and where their laundry and clothing were depediculated with dry air, and after a two-week quarantine they could continue to Serbian provinces without making new foci of typhus fever. The most vulnerable and numerous group of refugees

  18. Atomistic models of point defects in plutonium metal.

    SciTech Connect

    Valone, S. M.; Baskes, M. I.; Uberuaga, B. P.; Voter, A. F.

    2003-01-01

    The aging properties of plutonium (Pu) metal and alloys are. driven by a combination of materials composit ion, p rocessing history, and self-irradiat ion effects . Understanding these driving forces requires a knowledge of both t h ermodynamic and defect properties of the material . The multiplicity of phases and the small changes in tempe rat u re, pressure, and/or stress that can induce phase changes lie at the heart of these properties . In terms of radiation damage, Pu metal represents a unique situation because of the large volume chan ges that accompany the phase changes . The most workable form of the meta l is the fcc (S-) phase, which in practice is stabi l ized by addit io n of a ll oying el eme n ts s u c h as Ga or Al. The thermodynamically stable phase at ambient conditions is the monoclinic (a-) phase, which, however, is 2 0 % lower i n volume th an the S phase . In stabilized Pu metal, there is an in t er play between th e n atu ral swe l li n g tendencies of fcc metals and the volume-contraction tendency of the u n d erlyin g thermodynamicall y stable phase. This study exp lores the point d efect pr operties that are necessary to model the long-term outcome of this interplay.

  19. Scanning tip measurement for identification of point defects.

    PubMed

    Dózsa, László; Molnár, György; Raineri, Vito; Giannazzo, Filippo; Ferencz, János; Lányi, Stefan

    2011-01-01

    Self-assembled iron-silicide nanostructures were prepared by reactive deposition epitaxy of Fe onto silicon. Capacitance-voltage, current-voltage, and deep level transient spectroscopy (DLTS) were used to measure the electrical properties of Au/silicon Schottky junctions. Spreading resistance and scanning probe capacitance microscopy (SCM) were applied to measure local electrical properties. Using a preamplifier the sensitivity of DLTS was increased satisfactorily to measure transients of the scanning tip semiconductor junction. In the Fe-deposited area, Fe-related defects dominate the surface layer in about 0.5 μm depth. These defects deteriorated the Schottky junction characteristic. Outside the Fe-deposited area, Fe-related defect concentration was identified in a thin layer near the surface. The defect transients in this area were measured both in macroscopic Schottky junctions and by scanning tip DLTS and were detected by bias modulation frequency dependence in SCM. PMID:21711635

  20. Transforming graphene nanoribbons into nanotubes by use of point defects.

    PubMed

    Sgouros, A; Sigalas, M M; Papagelis, K; Kalosakas, G

    2014-03-26

    Using molecular dynamics simulations with semi-empirical potentials, we demonstrate a method to fabricate carbon nanotubes (CNTs) from graphene nanoribbons (GNRs), by periodically inserting appropriate structural defects into the GNR crystal structure. We have found that various defect types initiate the bending of GNRs and eventually lead to the formation of CNTs. All kinds of carbon nanotubes (armchair, zigzag, chiral) can be produced with this method. The structural characteristics of the resulting CNTs, and the dependence on the different type and distribution of the defects, were examined. The smallest (largest) CNT obtained had a diameter of ∼ 5 Å (∼ 39 Å). Proper manipulation of ribbon edges controls the chirality of the CNTs formed. Finally, the effect of randomly distributed defects on the ability of GNRs to transform into CNTs is considered.

  1. Point defect states in Sb-doped germanium

    SciTech Connect

    Patel, Neil S. Monmeyran, Corentin; Agarwal, Anuradha; Kimerling, Lionel C.

    2015-10-21

    Defect states in n-type Sb-doped germanium were investigated by deep-level transient spectroscopy. Cobalt-60 gamma rays were used to generate isolated vacancies and interstitials which diffuse and react with impurities in the material to form four defect states (E{sub 37}, E{sub 30}, E{sub 22}, and E{sub 21}) in the upper half of the bandgap. Irradiations at 77 K and 300 K as well as isothermal anneals were performed to characterize the relationships between the four observable defects. E{sub 37} is assigned to the Sb donor-vacancy associate (E-center) and is the only vacancy containing defect giving an estimate of 2 × 10{sup 11 }cm{sup −3} Mrad{sup −1} for the uncorrelated vacancy-interstitial pair introduction rate. The remaining three defect states are interstitial associates and transform among one another. Conversion ratios between E{sub 22}, E{sub 21}, and E{sub 30} indicate that E{sub 22} likely contains two interstitials.

  2. Current understanding of point defects and diffusion processes in silicon

    NASA Technical Reports Server (NTRS)

    Tan, T. Y.; Goesele, U.

    1985-01-01

    The effects of oxidation of Si which established that vacancies (V) and Si self interstitials (I) coexist in Si at high temperatures under thermal equilibrium and oxidizing conditions are discussed. Some essential points associated with Au diffusion in Si are then discussed. Analysis of Au diffusion results allowed a determination of the I component and an estimate of the V component of the Si self diffusion coefficient. A discussion of theories on high concentration P diffusion into Si is then presented. Although presently there still is no theory that is completely satisfactory, significant progresses are recently made in treating some essential aspects of this subject.

  3. STM fingerprints of point defects in graphene: a theoretical prediction

    SciTech Connect

    Amara, Hakim; Latil, Sylvain; Meunier, Vincent; Lambin, Philippe; Charlier, Jean Christophe

    2007-01-01

    Scanning tunneling microscopy (STM) is one of the most appropriate techniques to investigate the atomic structure of carbon nanomaterials. However, the experimental identification of topological and nontopological modifications of the hexagonal network of sp{sup 2} carbon nanostructures remains a great challenge. The goal of the present theoretical work is to predict the typical electronic features of a few defects that are likely to occur in sp{sup 2} carbon nanostructures, such as atomic vacancy, divacancy, adatom, and Stone-Wales defect. The modifications induced by those defects in the electronic properties of the graphene sheet are investigated using first-principles calculations. In addition, computed constant-current STM images of these defects are calculated within a tight-binding approach in order to facilitate the interpretation of STM images of defected carbon nanostructures.

  4. Green`s function implementation of common-offset, wave-equation migration

    SciTech Connect

    Ehinger, A.; Lailly, P.; Marfurt, K.J.

    1996-11-01

    Common-offset migration is extremely important in the context of migration velocity analysis (MVA) since it generates geologically interpretable migrated images. However, only a wave-equation-based migration handles multipathing of energy in contrast to the popular Kirchhoff migration with first-arrival traveltimes. The authors have combined the superior treatment of multipathing of energy by wave-equation-based migration with the advantages of the common-offset domain for MVA by implementing wave-equation migration algorithms via the use of finite-difference Green`s functions. With this technique, the authors are able to apply wave-equation migration in measurement configurations that are usually considered to be of the realm of Kirchhoff migration. In particular, wave-equation migration of common offset sections becomes feasible. The application of the wave-equation, common-offset migration algorithm to the Marmousi data set confirms the large increase in interpretability of individual migrated sections, for about twice the cost of standard wave-equation common-shot migration. The implementation of wave-equation migration via the Green`s functions is based on wavefield extrapolation via paraxial one-way wave equations. For these equations, theoretical results allow one to perform exact inverse extrapolation of wavefields.

  5. Method of migrating seismic records

    DOEpatents

    Ober, Curtis C.; Romero, Louis A.; Ghiglia, Dennis C.

    2000-01-01

    The present invention provides a method of migrating seismic records that retains the information in the seismic records and allows migration with significant reductions in computing cost. The present invention comprises phase encoding seismic records and combining the encoded seismic records before migration. Phase encoding can minimize the effect of unwanted cross terms while still allowing significant reductions in the cost to migrate a number of seismic records.

  6. The commercialization of migration.

    PubMed

    Abrera-mangahas, M A

    1989-01-01

    International migration is not new to the Philippines. In the recent outflow of contract workers to the Middle East, there is a shift from individual and family initiated migrations to the more organized, highly commercial variety. While profit-taking intermediaries have played some role in the past, the increase in the number and influence of these intermediaries has altered the story of migration decision-making. In 1975, the signing of the bilateral labor agreement between the governments of Iran and the Philippines signalled the rising demand for Filipino contract workers. From 1970 to 1975, the number of Asian migrant workers in the Gulf countries rose from about 120,000 to 370,000. These figures rose dramatically to 3.3 million in 1985. The growing share of organized and commercialized migration has altered migration decision making. Primarily, intermediaries are able to broaden access to foreign job and high wage opportunities. Commercialization effectively raises the transaction costs for contract migration. Studies on recruitment costs and fees show that self-solicited foreign employment costs less than employment obtained through recruitment agents and intermediaries. The difference in the 2 prices is due, not only to overhead costs of intermediation, but more importantly to the rent exacted by agents from having job information and placement rights. In the Philippines in October 1987 the average placement fee was P8000, greatly exceeding the mandated maximum fee level of P5000. This average is understated because the computation includes the 17% who do not pay any fees. The widespread and popular view of recruitment intermediaries is negative, dominated by images of abuses and victims. Private intermediaries and the government bureaucracy need each other. Intermediaries need government; their consistent demand for incentives and protection is indicative. On the other hand, government expands its supervision of control of overseas employment via the

  7. [Return migrations in the Italian migration system: a reexamination].

    PubMed

    Bonifazi, C; Heins, F

    1996-06-01

    The authors discuss trends in return migration in Italy, with a focus on regional differences. "On a regional level, the effects of return migration are very much connected with socio-economic structure. Only highly developed and autonomous regions can absorb productive investments and changes brought about by return migration. Several southern regions--especially those with greater migration experience--have shown in the last decades a very poor socio-economic development." The study is concerned with both internal and international migration. (SUMMARY IN ENG AND FRE)

  8. Forced Migration: Refugee Populations

    PubMed Central

    Boyle, Joyceen S.

    2015-01-01

    Undocumented migration is a global phenomenon that manifests in various contexts. This article describes the impact of the movement of large numbers of people in several African countries, producing a unique type of migrant—the refugee. We describe issues that refugee movements create on fragile health care systems, situations that precipitate refugee movements, certain human rights violations that are of particular concern such as gender based violence (GBV) and child soldiers, and lastly, implications for nursing practice and policy. We use examples from several countries in Sub-Saharan Africa, including the Democratic Republic of the Congo, Rwanda, Liberia, Sierra Leone, and Mozambique. Drawing on key documents from the United Nations High Commissioner for Refugees, current literature, as well as the international experience of the authors, this article presents an overview of forced migration and discusses opportunities for nurses to impact research, practice and policy related to refugee health. PMID:25645484

  9. Migration with fiscal externalities.

    PubMed

    Hercowitz, Z; Pines, D

    1991-11-01

    "This paper analyses the distribution of a country's population among regions when migration involves fiscal externalities. The main question addressed is whether a decentralized decision making [by] regional governments can produce an optimal population distribution...or a centralized intervention is indispensable, as argued before in the literature.... It turns out that, while with costless mobility the fiscal externality is fully internalized by voluntary interregional transfers, with costly mobility, centrally coordinated transfers still remain indispensable for achieving the socially optimal allocation."

  10. IAPs and cell migration.

    PubMed

    Dubrez, Laurence; Rajalingam, Krishnaraj

    2015-03-01

    Inhibitors of apoptosis (IAPs) constitute a family of cell signaling regulators controlling several fundamental biological processes such as innate immunity, inflammation, cell death, cell proliferation, and cell differentiation. Increasing evidence from in vivo and in vitro studies indicate a function for IAPs in the modulation of invasive and migratory properties of cells. Here, we present and discuss the mechanisms whereby IAPs can control cell migration.

  11. Migration and stratification

    PubMed Central

    Jasso, Guillermina

    2011-01-01

    Migration and stratification are increasingly intertwined. One day soon it will be impossible to understand one without the other. Both focus on life chances. Stratification is about differential life chances - who gets what and why - and migration is about improving life chances - getting more of the good things of life. To examine the interconnections of migration and stratification, we address a mix of old and new questions, carrying out analyses newly enabled by a unique new data set on recent legal immigrants to the United States (the New Immigrant Survey). We look at immigrant processing and lost documents, depression due to the visa process, presentation of self, the race-ethnic composition of an immigrant cohort (made possible by the data for the first time since 1961), black immigration from Africa and the Americas, skin-color diversity among couples formed by U.S. citizen sponsors and immigrant spouses, and English fluency among children age 8–12 and their immigrant parents. We find, inter alia, that children of previously illegal parents are especially more likely to be fluent in English, that native-born U.S. citizen women tend to marry darker, that immigrant applicants who go through the visa process while already in the United States are more likely to have their documents lost and to suffer visa depression, and that immigration, by introducing accomplished black immigrants from Africa (notably via the visa lottery), threatens to overturn racial and skin color associations with skill. Our analyses show the mutual embeddedness of migration and stratification in the unfolding of the immigrants' and their children's life chances and the impacts on the stratification structure of the United States. PMID:26321771

  12. Pathologic tooth migration.

    PubMed

    Brunsvold, Michael A

    2005-06-01

    Pathologic tooth migration (PTM) is a common complication of moderate to severe periodontitis and is often the motivation for patients to seek periodontal therapy. In this review of the literature, available information concerning prevalence, etiology, treatment, and prevention of pathologic tooth migration is summarized. Prevalence of PTM among periodontal patients has been reported to range from 30.03% to 55.8%. A survey of the literature regarding chief complaints of periodontal patients support these high prevalence findings. The etiology of PTM appears to be multifactorial. Periodontal bone loss appears to be a major factor in the etiology of PTM. Many aspects of occlusion can contribute to abnormal migration of teeth, and more than one of those factors may be present in an individual patient. Soft tissue forces of the tongue, cheeks, and lips are known to cause tooth movement and in some situations can cause PTM. Also considered important in the etiology of PTM is pressure produced from inflammatory tissues within periodontal pockets. Because extrusion is a common form of PTM, clinical observations support the theory that eruption forces sometimes play a role in the etiology of PTM. Many oral habits have been associated with PTM which are often difficult for the therapist to detect. Most cases of severe PTM require a team approach to achieve success. Periodontal, orthodontic, and prosthodontic treatment are often required. Many patient variables enter into the selection of treatment for PTM. In early stages of PTM, spontaneous correction of migrated teeth sometimes occurs after periodontal therapy. Light intrusive forces are used successfully to treat extrusion and flaring forms of PTM. Based on the literature reviewed, it appears that many cases of PTM could be prevented through the early diagnosis and treatment of periodontal disease, occlusal contributing factors, gingival enlargement, and oral habits. PMID:15948679

  13. Gender and Migration from Albania

    PubMed Central

    STECKLOV, GUY; CARLETTO, CALOGERO; AZZARRI, CARLO; DAVIS, BENJAMIN

    2010-01-01

    This article examines the dynamics and causes of the shift in the gender composition of migration, and more particularly, in women’s access to migration opportunities and decision-making. Our analysis focuses on Albania, a natural laboratory for studying international migration where out-migration was essentially nonexistent from the end of World War II to the end of the 1980s. Interest in the Albanian case is heightened because of the complex layers of inequality existing at the time when migration began: relatively low levels of inequality within the labor market and educational system—a product of the Communist era—while household relations remained heavily steeped in tradition and patriarchy. We use micro-level data from the Albania 2005 Living Standards Measurement Study, including migration histories for family members since migration began. Based on discrete-time hazard models, the analysis shows a dramatic increase in male migration and a gradual and uneven expansion of the female proportion of this international migration. Female migration, which is shown to be strongly associated with education, wealth, and social capital, appears responsive to economic incentives and constraints. Using information on the dependency of female migration to the household demographic structure as well as the sensitivity of female migration to household-level shocks, we show how household-level constraints and incentives affect male and female migration differently. Throughout this period, however, women’s migration behavior appears more directly aligned with household-level factors, and there is little evidence to suggest that increased female migration signals rising behavioral independence among Albanian women. PMID:21308565

  14. Conservation physiology of animal migration.

    PubMed

    Lennox, Robert J; Chapman, Jacqueline M; Souliere, Christopher M; Tudorache, Christian; Wikelski, Martin; Metcalfe, Julian D; Cooke, Steven J

    2016-01-01

    Migration is a widespread phenomenon among many taxa. This complex behaviour enables animals to exploit many temporally productive and spatially discrete habitats to accrue various fitness benefits (e.g. growth, reproduction, predator avoidance). Human activities and global environmental change represent potential threats to migrating animals (from individuals to species), and research is underway to understand mechanisms that control migration and how migration responds to modern challenges. Focusing on behavioural and physiological aspects of migration can help to provide better understanding, management and conservation of migratory populations. Here, we highlight different physiological, behavioural and biomechanical aspects of animal migration that will help us to understand how migratory animals interact with current and future anthropogenic threats. We are in the early stages of a changing planet, and our understanding of how physiology is linked to the persistence of migratory animals is still developing; therefore, we regard the following questions as being central to the conservation physiology of animal migrations. Will climate change influence the energetic costs of migration? Will shifting temperatures change the annual clocks of migrating animals? Will anthropogenic influences have an effect on orientation during migration? Will increased anthropogenic alteration of migration stopover sites/migration corridors affect the stress physiology of migrating animals? Can physiological knowledge be used to identify strategies for facilitating the movement of animals? Our synthesis reveals that given the inherent challenges of migration, additional stressors derived from altered environments (e.g. climate change, physical habitat alteration, light pollution) or interaction with human infrastructure (e.g. wind or hydrokinetic turbines, dams) or activities (e.g. fisheries) could lead to long-term changes to migratory phenotypes. However, uncertainty remains

  15. Conservation physiology of animal migration

    PubMed Central

    Lennox, Robert J.; Chapman, Jacqueline M.; Souliere, Christopher M.; Tudorache, Christian; Wikelski, Martin; Metcalfe, Julian D.; Cooke, Steven J.

    2016-01-01

    Migration is a widespread phenomenon among many taxa. This complex behaviour enables animals to exploit many temporally productive and spatially discrete habitats to accrue various fitness benefits (e.g. growth, reproduction, predator avoidance). Human activities and global environmental change represent potential threats to migrating animals (from individuals to species), and research is underway to understand mechanisms that control migration and how migration responds to modern challenges. Focusing on behavioural and physiological aspects of migration can help to provide better understanding, management and conservation of migratory populations. Here, we highlight different physiological, behavioural and biomechanical aspects of animal migration that will help us to understand how migratory animals interact with current and future anthropogenic threats. We are in the early stages of a changing planet, and our understanding of how physiology is linked to the persistence of migratory animals is still developing; therefore, we regard the following questions as being central to the conservation physiology of animal migrations. Will climate change influence the energetic costs of migration? Will shifting temperatures change the annual clocks of migrating animals? Will anthropogenic influences have an effect on orientation during migration? Will increased anthropogenic alteration of migration stopover sites/migration corridors affect the stress physiology of migrating animals? Can physiological knowledge be used to identify strategies for facilitating the movement of animals? Our synthesis reveals that given the inherent challenges of migration, additional stressors derived from altered environments (e.g. climate change, physical habitat alteration, light pollution) or interaction with human infrastructure (e.g. wind or hydrokinetic turbines, dams) or activities (e.g. fisheries) could lead to long-term changes to migratory phenotypes. However, uncertainty remains

  16. Conservation physiology of animal migration.

    PubMed

    Lennox, Robert J; Chapman, Jacqueline M; Souliere, Christopher M; Tudorache, Christian; Wikelski, Martin; Metcalfe, Julian D; Cooke, Steven J

    2016-01-01

    Migration is a widespread phenomenon among many taxa. This complex behaviour enables animals to exploit many temporally productive and spatially discrete habitats to accrue various fitness benefits (e.g. growth, reproduction, predator avoidance). Human activities and global environmental change represent potential threats to migrating animals (from individuals to species), and research is underway to understand mechanisms that control migration and how migration responds to modern challenges. Focusing on behavioural and physiological aspects of migration can help to provide better understanding, management and conservation of migratory populations. Here, we highlight different physiological, behavioural and biomechanical aspects of animal migration that will help us to understand how migratory animals interact with current and future anthropogenic threats. We are in the early stages of a changing planet, and our understanding of how physiology is linked to the persistence of migratory animals is still developing; therefore, we regard the following questions as being central to the conservation physiology of animal migrations. Will climate change influence the energetic costs of migration? Will shifting temperatures change the annual clocks of migrating animals? Will anthropogenic influences have an effect on orientation during migration? Will increased anthropogenic alteration of migration stopover sites/migration corridors affect the stress physiology of migrating animals? Can physiological knowledge be used to identify strategies for facilitating the movement of animals? Our synthesis reveals that given the inherent challenges of migration, additional stressors derived from altered environments (e.g. climate change, physical habitat alteration, light pollution) or interaction with human infrastructure (e.g. wind or hydrokinetic turbines, dams) or activities (e.g. fisheries) could lead to long-term changes to migratory phenotypes. However, uncertainty remains

  17. Gender and migration from Albania.

    PubMed

    Stecklov, Guy; Carletto, Calogero; Azzarri, Carlo; Davis, Benjamin

    2010-11-01

    This article examines the dynamics and causes of the shift in the gender composition of migration, and more particularly, in women's access to migration opportunities and decision-making. Our analysis focuses on Albania, a natural laboratory for studying international migration where out-migration was essentially nonexistent from the end of World War II to the end of the 1980s. Interest in the Albanian case is heightened because of the complex layers of inequality existing at the time when migration began: relatively low levels of inequality within the labor market and educational system-a product of the Communist era-while household relations remained heavily steeped in tradition and patriarchy. We use micro-level data from the Albania 2005 Living Standards Measurement Study, including migration histories for family members since migration began. Based on discrete-time hazard models, the analysis shows a dramatic increase in male migration and a gradual and uneven expansion of the female proportion of this international migration. Female migration, which is shown to be strongly associated with education, wealth, and social capital, appears responsive to economic incentives and constraints. Using information on the dependency of female migration to the household demographic structure as well as the sensitivity of female migration to household-level shocks, we show how household-level constraints and incentives affect male and female migration differently. Throughout this period, however, women's migration behavior appears more directly aligned with household-level factors, and there is little evidence to suggest that increased female migration signals rising behavioral independence among Albanian women.

  18. Hydrodynamics of pronuclear migration

    NASA Astrophysics Data System (ADS)

    Nazockdast, Ehssan; Needleman, Daniel; Shelley, Michael

    2014-11-01

    Microtubule (MT) filaments play a key role in many processes involved in cell devision including spindle formation, chromosome segregation, and pronuclear positioning. We present a direct numerical technique to simulate MT dynamics in such processes. Our method includes hydrodynamically mediated interactions between MTs and other cytoskeletal objects, using singularity methods for Stokes flow. Long-ranged many-body hydrodynamic interactions are computed using a highly efficient and scalable fast multipole method, enabling the simulation of thousands of MTs. Our simulation method also takes into account the flexibility of MTs using Euler-Bernoulli beam theory as well as their dynamic instability. Using this technique, we simulate pronuclear migration in single-celled Caenorhabditis elegans embryos. Two different positioning mechanisms, based on the interactions of MTs with the motor proteins and the cell cortex, are explored: cytoplasmic pulling and cortical pushing. We find that although the pronuclear complex migrates towards the center of the cell in both models, the generated cytoplasmic flows are fundamentally different. This suggest that cytoplasmic flow visualization during pronuclear migration can be utilized to differentiate between the two mechanisms.

  19. Migration of phospholipid vesicles in response to OH(-) stimuli.

    PubMed

    Kodama, Atsuji; Sakuma, Yuka; Imai, Masayuki; Oya, Yutaka; Kawakatsu, Toshihiro; Puff, Nicolas; Angelova, Miglena I

    2016-03-21

    We demonstrate migration of phospholipid vesicles in response to a pH gradient. Upon simple micro-injection of a NaOH solution, the vesicles linearly moved to the tip of the micro-pipette and the migration velocity was proportional to the gradient of OH(-) concentration. Vesicle migration was characteristic of OH(-) ions and no migration was observed for monovalent salts or nonionic sucrose solutions. The migration of vesicles is quantitatively described by the surface tension gradient model where the hydrolysis of the phospholipids by NaOH solution decreases the surface tension of the vesicle. The vesicles move toward a direction where the surface energy decreases. Thus the chemical modification of lipids produces a mechanical force to drive vesicles. PMID:26883729

  20. Population, desertification, and migration.

    PubMed

    Westing, A H

    1994-01-01

    When an imbalance develops between population numbers and the carrying capacity of the land, the persons thereby displaced are referred to as environmental refugees. The utilization of the land beyond sustainability leads to land degradation and ultimately, desertification. The social and political impacts of long-term environmental migration can be distinguished: a) at the site of origin of the displaced persons by the residual population; b) at rural sites of destination within the nation between the new arrivals and preestablished populations; c) in the cities within the nation; d) in the nonindustrialized foreign countries; and e) in the industrialized foreign countries. In the event that an area which had previously been devoted to pastoralism is converted to agriculture, the displaced pastoralists might respond through armed rebellion. In some instances, the disenchanted urban squatters become a politically restive and even a destabilizing force, as occurred in Sudan in the 1980s, especially in Khartoum and Port Sudan. The foreign countries to which many of the displaced persons are migrating are subjected to increasing levels of migrant-induced economic, cultural, and political strains. The growing problems associated with south-to-north migration across the Mediterranean Sea have recently led France, Italy, Portugal, and Spain to enter into a consultative arrangement with Algeria, Libya, Mauritania, Morocco, and Tunisia. All foreign aid to the nonindustrialized countries that attempts to ameliorate the problem of desertification must adopt integrated approaches that: a) address population issues; b) support environmental education; c) provide for the protection of biodiversity; d) encourage participatory forms of local and national government; e) provide opportunities for income generation outside the livestock sector; and f) foster political security and facilitate ecogeographical (subregional) cooperation.

  1. ILO - International Migration Programme.

    PubMed

    Boudraa, Miriam

    2011-01-01

    In a wide International Context characterised not only by the economical development but also by the social, cultural, political and individual development, we witness more and more to a exchange between the developed and the developing countries, which can be translated especially in the migration of the work force. In theory, all countries are either countries of origin either countries of transit or destination, and they are all responsible for the rights of migrant workers by promoting the rights, by monitoring and by preventing the abusive conditions. The process of migration of the workforce can be divided into three stages: the first coincides with the period prior to departure, the second is represented by the aftermath of the departure and the period of stay in the country of destination, the third stage corresponds to the return in the country of origin. The workers must be protected throughout this process by the international organizations that perform the catalytic role of communication and exchange between countries, for the only purpose of protecting the rights of immigrant and/or immigrants workers. The responsibility for the protection of workers is divided among the various players in the International Labour Organisation. Every country has to apply measures according to the international standards regarding workers' rights, standards that guide the various countries in the formulation and implementation of their policies and legislation. These standards are suggested by International Conventions, the ILO Conventions and other international instruments such as the human rights instrument. There has been a big step forward once the ILO Fundamental Conventions and Conventions on Migrant Workers where implemented and this implementation represented the use of the Guidelines "ILO Multilateral Framework on Labour Migration".

  2. ILO - International Migration Programme.

    PubMed

    Boudraa, Miriam

    2011-01-01

    In a wide International Context characterised not only by the economical development but also by the social, cultural, political and individual development, we witness more and more to a exchange between the developed and the developing countries, which can be translated especially in the migration of the work force. In theory, all countries are either countries of origin either countries of transit or destination, and they are all responsible for the rights of migrant workers by promoting the rights, by monitoring and by preventing the abusive conditions. The process of migration of the workforce can be divided into three stages: the first coincides with the period prior to departure, the second is represented by the aftermath of the departure and the period of stay in the country of destination, the third stage corresponds to the return in the country of origin. The workers must be protected throughout this process by the international organizations that perform the catalytic role of communication and exchange between countries, for the only purpose of protecting the rights of immigrant and/or immigrants workers. The responsibility for the protection of workers is divided among the various players in the International Labour Organisation. Every country has to apply measures according to the international standards regarding workers' rights, standards that guide the various countries in the formulation and implementation of their policies and legislation. These standards are suggested by International Conventions, the ILO Conventions and other international instruments such as the human rights instrument. There has been a big step forward once the ILO Fundamental Conventions and Conventions on Migrant Workers where implemented and this implementation represented the use of the Guidelines "ILO Multilateral Framework on Labour Migration". PMID:22073693

  3. Physical view on migration modes

    PubMed Central

    Mierke, Claudia Tanja

    2015-01-01

    Cellular motility is essential for many processes such as embryonic development, wound healing processes, tissue assembly and regeneration, immune cell trafficing and diseases such as cancer. The migration efficiency and the migratory potential depend on the type of migration mode. The previously established migration modes such as epithelial (non-migratory) and mesenchymal (migratory) as well as amoeboid (squeezing motility) relay mainly on phenomenological criteria such as cell morphology and molecular biological criteria such as gene expression. However, the physical view on the migration modes is still not well understood. As the process of malignant cancer progression such as metastasis depends on the migration of single cancer cells and their migration mode, this review focuses on the different migration strategies and discusses which mechanical prerequisites are necessary to perform a special migration mode through a 3-dimensional microenvironment. In particular, this review discusses how cells can distinguish and finally switch between the migration modes and what impact do the physical properties of cells and their microenvironment have on the transition between the novel migration modes such as blebbing and protrusive motility. PMID:26192136

  4. Migration of Asteroidal Dust

    NASA Technical Reports Server (NTRS)

    Ipatov, S. I.; Mather, J. C.

    2003-01-01

    Using the Bulirsh Stoer method of integration, we investigated the migration of dust particles under the gravitational influence of all planets, radiation pressure, Poynting Robertson drag and solar wind drag for equal to 0.01, 0.05, 0.1, 0.25, and 0.4. For silicate particles such values of correspond to diameters equal to about 40, 9, 4, 2, and 1 microns, respectively [1]. The relative error per integration step was taken to be less than 10sup-8. Initial orbits of the particles were close to the orbits of the first numbered mainbelt asteroids.

  5. Intermixing at the absorber-buffer layer interface in thin-film solar cells: The electronic effects of point defects in Cu(In,Ga)(Se,S){sub 2} and Cu{sub 2}ZnSn(Se,S){sub 4} devices

    SciTech Connect

    Varley, J. B.; Lordi, V.

    2014-08-14

    We investigate point defects in the buffer layers CdS and ZnS that may arise from intermixing with Cu(In,Ga)(S,Se){sub 2} (CIGS) or Cu{sub 2}ZnSn(S,Se){sub 4} (CZTS) absorber layers in thin-film photovoltaics. Using hybrid functional calculations, we characterize the electrical and optical behavior of Cu, In, Ga, Se, Sn, Zn, Na, and K impurities in the buffer. We find that In and Ga substituted on the cation site act as shallow donors in CdS and tend to enhance the prevailing n-type conductivity at the interface facilitated by Cd incorporation in CIGS, whereas they are deep donors in ZnS and will be less effective dopants. Substitutional In and Ga can favorably form complexes with cation vacancies (A-centers) which may contribute to the “red kink” effect observed in some CIGS-based devices. For CZTS absorbers, we find that Zn and Sn defects substituting on the buffer cation site are electrically inactive in n-type buffers and will not supplement the donor doping at the interface as in CIGS/CdS or ZnS devices. Sn may also preferentially incorporate on the S site as a deep acceptor in n-type ZnS, which suggests possible concerns with absorber-related interfacial compensation in CZTS devices with ZnS-derived buffers. Cu, Na, and K impurities are found to all have the same qualitative behavior, most favorably acting as compensating acceptors when substituting on the cation site. Our results suggest one beneficial role of K and Na incorporation in CIGS or CZTS devices is the partial passivation of vacancy-related centers in CdS and ZnS buffers, rendering them less effective interfacial hole traps and recombination centers.

  6. Identifying impediments to long-distance mammal migrations.

    PubMed

    Seidler, Renee G; Long, Ryan A; Berger, Joel; Bergen, Scott; Beckmann, Jon P

    2015-02-01

    In much of the world, the persistence of long-distance migrations by mammals is threatened by development. Even where human population density is relatively low, there are roads, fencing, and energy development that present barriers to animal movement. If we are to conserve species that rely on long-distance migration, then it is critical that we identify existing migration impediments. To delineate stopover sites associated with anthropogenic development, we applied Brownian bridge movement models to high-frequency locations of pronghorn (Antilocapra americana) in the Greater Yellowstone Ecosystem. We then used resource utilization functions to assess the threats to long-distance migration of pronghorn that were due to fences and highways. Migrating pronghorn avoided dense developments of natural gas fields. Highways with relatively high volumes of traffic and woven-wire sheep fence acted as complete barriers. At crossings with known migration bottlenecks, use of high-quality forage and shrub habitat by pronghorn as they approached the highway was lower than expected based on availability of those resources. In contrast, pronghorn consistently utilized high-quality forage close to the highway at crossings with no known migration bottlenecks. Our findings demonstrate the importance of minimizing development in migration corridors in the future and of mitigating existing pressure on migratory animals by removing barriers, reducing the development footprint, or installing crossing structures. PMID:25158993

  7. Identifying impediments to long-distance mammal migrations.

    PubMed

    Seidler, Renee G; Long, Ryan A; Berger, Joel; Bergen, Scott; Beckmann, Jon P

    2015-02-01

    In much of the world, the persistence of long-distance migrations by mammals is threatened by development. Even where human population density is relatively low, there are roads, fencing, and energy development that present barriers to animal movement. If we are to conserve species that rely on long-distance migration, then it is critical that we identify existing migration impediments. To delineate stopover sites associated with anthropogenic development, we applied Brownian bridge movement models to high-frequency locations of pronghorn (Antilocapra americana) in the Greater Yellowstone Ecosystem. We then used resource utilization functions to assess the threats to long-distance migration of pronghorn that were due to fences and highways. Migrating pronghorn avoided dense developments of natural gas fields. Highways with relatively high volumes of traffic and woven-wire sheep fence acted as complete barriers. At crossings with known migration bottlenecks, use of high-quality forage and shrub habitat by pronghorn as they approached the highway was lower than expected based on availability of those resources. In contrast, pronghorn consistently utilized high-quality forage close to the highway at crossings with no known migration bottlenecks. Our findings demonstrate the importance of minimizing development in migration corridors in the future and of mitigating existing pressure on migratory animals by removing barriers, reducing the development footprint, or installing crossing structures.

  8. Graphene Layer Growth: Collision of Migrating Five-MemberRings

    SciTech Connect

    Whitesides, Russell; Kollias, Alexander C.; Domin, Dominik; Lester Jr., William A.; Frenklach, Michael

    2005-12-02

    A reaction pathway is explored in which two cyclopenta groups combine on the zigzag edge of a graphene layer. The process is initiated by H addition to a five-membered ring, followed by opening of that ring and the formation of a six-membered ring adjacent to another five-membered ring. The elementary steps of the migration pathway are analyzed using density functional theory to examine the region of the potential energy surface associated with the pathway. The calculations are performed on a substrate modeled by the zigzag edge of tetracene. Based on the obtained energetics, the dynamics of the system are analyzed by solving the energy transfer master equations. The results indicate energetic and reaction-rate similarity between the cyclopenta combination and migration reactions. Also examined in the present study are desorption rates of migrating cyclopenta rings which are found to be comparable to cyclopenta ring migration.

  9. Process migration in UNIX environments

    NASA Technical Reports Server (NTRS)

    Lu, Chin; Liu, J. W. S.

    1988-01-01

    To support process migration in UNIX environments, the main problem is how to encapsulate the location dependent features of the system in such a way that a host independent virtual environment is maintained by the migration handlers on the behalf of each migrated process. An object-oriented approach is used to describe the interaction between a process and its environment. More specifically, environmental objects were introduced in UNIX systems to carry out the user-environment interaction. The implementation of the migration handlers is based on both the state consistency criterion and the property consistency criterion.

  10. Exciton migration and quenching in poly(propylene imine) dendrimers

    NASA Astrophysics Data System (ADS)

    Minevičiūtė, I.; Gulbinas, V.; Franckevičius, M.; Vaišnoras, R.; Marcos, M.; Serrano, J. L.

    2009-05-01

    Exciton migration between chromophore groups of the poly(propylene imine) dendrimer in chloroform solution and in solid state has been investigated by means of the time-resolved fluorescence measurements. Fluorescence decay kinetics, dynamic band shift and the depolarization rate have been analyzed. Exciton migration in a single dendrimer was found to be slow in comparison with temperature-dependent chromophore reorientation time of 150-600 ps. In a solid state chromophore groups form collective excitonic states responsible for the dendrimer film fluorescence. Exciton migration and localization to the lowest energy sites within the distributed density of states take place on a subnanosecond-nanosecond time scale.

  11. [International migrations in the Middle East and their economic effects].

    PubMed

    Tapinos, G

    1981-10-15

    A general review of migration patterns in the oil-producing countries of the Persian Gulf is presented. The inter-country differences in income, similarities in culture, and world energy demands are identified as the main factors responsible for this migration. The economic effects on countries of origin are considered, with particular reference to the negative impact that the substantial emigration from Yemen has had on that country's development. PMID:12339031

  12. DIRECTLY IMAGING TIDALLY POWERED MIGRATING JUPITERS

    SciTech Connect

    Dong Subo; Katz, Boaz; Socrates, Aristotle

    2013-01-10

    Upcoming direct-imaging experiments may detect a new class of long-period, highly luminous, tidally powered extrasolar gas giants. Even though they are hosted by {approx} Gyr-'old' main-sequence stars, they can be as 'hot' as young Jupiters at {approx}100 Myr, the prime targets of direct-imaging surveys. They are on years-long orbits and presently migrating to 'feed' the 'hot Jupiters'. They are expected from 'high-e' migration mechanisms, in which Jupiters are excited to highly eccentric orbits and then shrink semimajor axis by a factor of {approx}10-100 due to tidal dissipation at close periastron passages. The dissipated orbital energy is converted to heat, and if it is deposited deep enough into the atmosphere, the planet likely radiates steadily at luminosity L {approx} 100-1000 L{sub Jup}(2 Multiplication-Sign 10{sup -7}-2 Multiplication-Sign 10{sup -6} L{sub Sun }) during a typical {approx} Gyr migration timescale. Their large orbital separations and expected high planet-to-star flux ratios in IR make them potentially accessible to high-contrast imaging instruments on 10 m class telescopes. {approx}10 such planets are expected to exist around FGK dwarfs within {approx}50 pc. Long-period radial velocity planets are viable candidates, and the highly eccentric planet HD 20782b at maximum angular separation {approx}0.''08 is a promising candidate. Directly imaging these tidally powered Jupiters would enable a direct test of high-e migration mechanisms. Once detected, the luminosity would provide a direct measurement of the migration rate, and together with mass (and possibly radius) estimate, they would serve as a laboratory to study planetary spectral formation and tidal physics.

  13. Kirchhoff migration without phases

    NASA Astrophysics Data System (ADS)

    Bardsley, Patrick; Guevara Vasquez, Fernando

    2016-10-01

    We present a simple, frequency domain, preprocessing step to Kirchhoff migration that allows the method to image scatterers when the wave field phase information is lost at the receivers, and only intensities are measured. The resulting imaging method does not require knowing the phases of the probing field or manipulating the phase of the wave field at the receivers. In a regime where the scattered field is small compared to the probing field, the problem of recovering the full-waveform scattered field from intensity data can be formulated as an embarrassingly simple least-squares problem. Although this only recovers the projection (on a known subspace) of the full-waveform scattered field, we show that, for high frequencies, this projection gives Kirchhoff images asymptotically identical to the images obtained with full waveform data. Our method can also be used when the source is modulated by a Gaussian process and autocorrelations are measured at an array of receivers.

  14. Chandra Contaminant Migration Model

    NASA Technical Reports Server (NTRS)

    Swartz, Douglas A.; O'Dell, Steve L.

    2014-01-01

    High volatility cleans OBFs and low volatility produces a high build-up at OBF centers; only a narrow (factor of 2 or less) volatility range produces the observed spatial pattern. Simulations predict less accumulation above outer S-array CCDs; this may explain, in part, gratings/imaging C/MnL discrepancies. Simulations produce a change in center accumulation due solely to DH heater ON/OFF temperature change; but a 2nd contaminant and perhaps a change in source rate is also required. Emissivity E may depend on thickness; another model parameter. Additional physics, e.g., surface migration, is not warranted at this time. At t approx. 14 yrs, model produced 0.22 grams of contaminant, 0.085 grams remaining within ACIS cavity; 7 percent (6mg) on OBFs.

  15. Migration and women's health.

    PubMed

    Adanu, Richard M K; Johnson, Timothy R B

    2009-08-01

    Women have been migrating at similar rates to men for the past 40 years, and comprised about half of all migrants in 2005. Women and children are most affected by displacement as a result of wars and human trafficking. In some cases, the health of female migrants is improved via integration into better health systems in the host country. More often, however, the health of female migrants is affected negatively. Women are doubly disadvantaged because they are discriminated against as women and as migrants. Female migrants are also highly vulnerable to acts of sexual abuse, rape, and violence. This is especially true for women in refugee camps, whose reproductive health needs are often overlooked. To improve the health of female migrants it is important to develop and implement policies that recognize and insist on the respect of the rights of migrants. PMID:19539929

  16. Migration issues important -- Mongolia.

    PubMed

    1999-01-01

    Migration and urbanization are issues that require increasing attention in Mongolia. Mr. Sodov Sonin, Minister of Health and Social Welfare, stated at the Forum that fertility has declined, but mortality, in particular the mortality of children and mothers, is still too high. In addition, there is a significant gap between the knowledge of and behaviors concerning reproductive health, which is one of the causes of the country's high abortion rates. However, on the positive side, literacy is high among women--70% of the students in Mongolia's higher educational institutions are female and the State recognizes equal rights for women. Moreover, programs that promote health and education, including the National Program on Reproductive Health, are being implemented; but despite all these, Mongolia still lacks the human and financial resources to implement the ICPD Program of Action satisfactorily. The country also needs dramatic changes in mind-set and in terms of capacity building, given its ongoing socioeconomic transition. PMID:12295512

  17. Migration of defect clusters and xenon-vacancy clusters in uranium dioxide

    SciTech Connect

    Chen, Dong; Gao, Fei; Deng, Huiqiu; Hu, Wangyu; Sun, Xin

    2014-07-01

    The possible transition states, minimum energy paths and migration mechanisms of defect clusters and xenon-vacancy defect clusters in uranium dioxide have been investigated using the dimer and the nudged elastic-band methods. The nearby O atom can easily hop into the oxygen vacancy position by overcoming a small energy barrier, which is much lower than that for the migration of a uranium vacancy. A simulation for a vacancy cluster consisting of two oxygen vacancies reveals that the energy barrier of the divacancy migration tends to decrease with increasing the separation distance of divacancy. For an oxygen interstitial, the migration barrier for the hopping mechanism is almost three times larger than that for the exchange mechanism. Xe moving between two interstitial sites is unlikely a dominant migration mechanism considering the higher energy barrier. A net migration process of a Xe-vacancy pair containing an oxygen vacancy and a xenon interstitial is identified by the NEB method. We expect the oxygen vacancy-assisted migration mechanism to possibly lead to a long distance migration of the Xe interstitials in UO2. The migration of defect clusters involving Xe substitution indicates that Xe atom migrating away from the uranium vacancy site is difficult.

  18. Cohort Size Effects and Migration.

    ERIC Educational Resources Information Center

    Wilson, Franklin D.

    1983-01-01

    Explores whether changes in the size of cohorts entering the labor force affected the propensity within the U.S. labor force to migrate and socioeconomic circumstances of migrants at destination within 1965-76. Suggests that a significant reduction in the volume of migration among members of the baby boom cohort was the primary adjustment…

  19. Migration in asymmetric, random environments

    NASA Astrophysics Data System (ADS)

    Deem, Michael; Wang, Dong

    Migration is a key mechanism for expansion of communities. As a population migrates, it experiences a changing environment. In heterogeneous environments, rapid adaption is key to the evolutionary success of the population. In the case of human migration, environmental heterogeneity is naturally asymmetric in the North-South and East-West directions. We here consider migration in random, asymmetric, modularly correlated environments. Knowledge about the environment determines the fitness of each individual. We find that the speed of migration is proportional to the inverse of environmental change, and in particular we find that North-South migration rates are lower than East-West migration rates. Fast communication within the population of pieces of knowledge between individuals, similar to horizontal gene transfer in genetic systems, can help to spread beneficial knowledge among individuals. We show that increased modularity of the relation between knowledge and fitness enhances the rate of evolution. We investigate the relation between optimal information exchange rate and modularity of the dependence of fitness on knowledge. These results for the dependence of migration rate on heterogeneity, asymmetry, and modularity are consistent with existing archaeological facts.

  20. Africa: Setting for Human Migration

    ERIC Educational Resources Information Center

    Buuba, Babacar Diop

    2007-01-01

    Analysis of African migrations can help to understand prehistoric, historical, ancient modern and contemporaneous migrations. Movements of populations were and continue to be so intense that, for some analysts, they constitute one of the dominant trends of the history and destiny of the very old continent. African and non-African states, whether…

  1. Merlin's wizardry guides cohesive migration.

    PubMed

    Zoch, Ansgar; Morrison, Helen

    2015-03-01

    Cells often migrate in tightly connected groups with coordinated movement and polarity. The collective migration of epithelial cell sheets is now shown to be mediated by a signalling axis that involves the merlin tumour-suppressor protein, the tight-junction-associated angiomotin-Rich1 complex and the Rac1 small GTPase. PMID:25720961

  2. Ecological and scaling analysis of the energy expenditure of rest, activity, flight, and evaporative water loss in Passeriformes and non-Passeriformes in relation to seasonal migrations and to the occupation of boreal stations in high and moderate latitudes.

    PubMed

    Gavrilov, Valery M

    2014-06-01

    A unified system of bioenergetic parameters that describe thermal regulation and energy metabolism in many passerine and non-passerine species has been developed. These parameters have been analyzed as functions of ambient temperature, and bioenergetic models for various species have been developed. The level of maximum food energy or maximal existence metabolism (MPE) is 1.3 times higher in passerines than in non-passerines, which is consistent with the ratio of their basal metabolic rates (BMR). The optimal ambient temperature for maximizing productive processes (e.g., reproduction, molting) is lower for passerines than for non passerines, which allows passerines to have higher production rates at moderate ambient temperatures. This difference in the optimal ambient temperature may explain the variation in bioenergetic parameters along latitudinal gradients, such as the well-known ecological rule of clutch size (or mass) increase in the more northerly passerine birds. The increased potential for productive energy output in the north may also allow birds to molt faster there. This phenomenon allows passerine birds to occupy a habitat that fluctuates widely in ambient temperature compared with non-passerine birds of similar size. Passerines have a more effective system for maintaining heat balance at both high and low temperatures. The high metabolism and small body sizes of passerines are consistent with omnivore development and with ecological plasticity. Among large passerines, the unfavorable ratio of MPE to BMR should decrease the energy that is available for productive processes. This consequence limits both the reproductive output and the development of long migration (particularly in Corvus corax). The hypothesis regarding BMR increase in passerines was suggested based on an aerodynamic analysis of the flight speed and the wing characteristics. This allometric analysis shows that the flight velocity is approximately 20% lower in Passeriformes than in non

  3. Role of point defects in bipolar fatigue behavior of Bi(Mg{sub 1/2}Ti{sub 1/2})O{sub 3} modified (Bi{sub 1/2}K{sub 1/2})TiO{sub 3}-(Bi{sub 1/2}Na{sub 1/2})TiO{sub 3} relaxor ceramics

    SciTech Connect

    Kumar, Nitish Ansell, Troy Y.; Cann, David P.

    2014-04-21

    Lead-free Bi(Mg{sub 1/2}Ti{sub 1/2})O{sub 3}-(Bi{sub 1/2}K{sub 1/2})TiO{sub 3}-(Bi{sub 1/2}Na{sub 1/2})TiO{sub 3} (BMT-BKT-BNT) ceramics have been shown to exhibit large electromechanical strains under high electric fields along with negligible fatigue under strong electric fields. To investigate the role of point defects on the fatigue characteristics, the composition 5BMT-40BKT-55BNT was doped to incorporate acceptor and donor defects on the A and B sites by adjusting the Bi/Na and Ti/Mg stoichiometries. All samples had pseudo-cubic symmetries based on x-ray diffraction, typical of relaxors. Dielectric measurements showed that the high and low temperature phase transitions were largely unaffected by doping. Acceptor doping resulted in the observation of a typical ferroelectric-like polarization with a remnant polarization and strain hysteresis loops with significant negative strain. Donor-doped compositions exhibited characteristics that were indicative of an ergodic relaxor phase. Fatigue measurements were carried out on all of the compositions. While the A-site acceptor-doped composition showed a small degradation in maximum strain after 10{sup 6} cycles, the other compositions were essentially fatigue free. Impedance measurements were used to identify the important conduction mechanisms in these compositions. As expected, the presence of defects did not strongly influence the fatigue behavior in donor-doped compositions owing to the nature of their reversible field-induced phase transformation. Even for the acceptor-doped compositions, which had stable domains in the absence of an electric field at room temperature, there was negligible degradation in the maximum strain due to fatigue. This suggests that either the defects introduced through stoichiometric variations do not play a prominent role in fatigue in these systems or it is compensated by factors like decrease in coercive field, an increase in ergodicity, symmetry change, or other factors.

  4. Is feeding behaviour related to glass eel propensity to migrate?

    NASA Astrophysics Data System (ADS)

    Bureau du Colombier, Sarah; Lambert, Patrick; Bardonnet, Agnès

    2008-11-01

    Several studies have shown that eel diadromy is facultative and that migratory divergences may appear during glass eel estuarine migration. The origin of the differences in migratory behaviour among glass eels remains unclear but initial evidence supports the role of individual energetic and thyroidal status. Even if starvation is usually associated with glass eel migration, feeding does seem to occur in some glass eels. The aim of the present study was to investigate feeding behaviour and glass eel growth in relation to the propensity to migrate. Feeding rate and weight gain were higher in fish having a high propensity to migrate (M + fish) than in fish having a low propensity to migrate (M - fish) in fed glass eels, whereas no clear difference in the variation in body weight was observed among unfed fish (controls). M - fish initially had lower percent dry weight than M + fish, which suggests a link between appetite, propensity to migrate, and energy content. We discuss the role played by endocrine signals on these processes. In fish, thyroid hormones contribute to the control of growth and development. In addition, they play a role in flatfish and leptocephalus metamorphosis and appear to be involved in smolt and glass eel migratory behaviour. As such, they represent a good candidate which would promote the propensity to migrate as well as digestive system development. Their role in the hormonal control of food intake however remains vague. The large and sharp decline in glass eel abundances observed since the 1980s could partly be explained by changes in ocean productivity. If so, it could be accompanied by a decrease in glass eel energy stores. The ability to resume feeding in the course of the estuarine crossing would then represent a serious advantage to maintain energy levels compatible with migration.

  5. Migrating thermospheric tides

    NASA Astrophysics Data System (ADS)

    Hagan, M. E.; Roble, R. G.; Hackney, J.

    2001-07-01

    The capabilities of the global-scale wave model (GSWM) [Hagan et al., 1995, 1999] are extended to include migrating thermospheric solar tides. The GSWM thermospheric tidal forcing parameterization is based on neutral gas heating calculated from first principles in the National Center for Atmospheric Research (NCAR) thermosphere/ionosphere electrodynamics general circulation model (TIE-GCM). This is the first time that a physics-based thermospheric forcing scheme has been used in a model like GSWM. Previous two-dimensional steady state linear tidal models used exospheric temperature measurements to calibrate upper atmospheric tidal forcing. New GSWM results illustrate thermospheric tidal responses that are largely consistent with tides in the TIE-GCM. Diurnal temperature amplitudes increase with increasing solar activity, but there is no analogous diurnal wind response. The thermospheric semidiurnal tide is much weaker than the diurnal tide. Semidiurnal temperature perturbations peak in the lower thermosphere where the semidiurnal forcing maximizes. The new in situ results must be combined with the GSWM upward propagating tide in the lower thermosphere, because the upward propagating components dominate the semidiurnal response throughout the region and the diurnal response below ˜130 km. In situ forcing accounts for most of the diurnal response aloft. Our preliminary evaluation of the GSWM thermospheric predictions is inconclusive. More extensive evaluations are necessary to make a firm assessment of whether the model captures the salient features of the seasonal and solar cycle variability of thermospheric tides.

  6. Neuritin 1 promotes neuronal migration.

    PubMed

    Zito, Arianna; Cartelli, Daniele; Cappelletti, Graziella; Cariboni, Anna; Andrews, William; Parnavelas, John; Poletti, Angelo; Galbiati, Mariarita

    2014-01-01

    Neuritin 1 (Nrn1 or cpg15-1) is an activity-dependent protein involved in synaptic plasticity during brain development, a process that relies upon neuronal migration. By analyzing Nrn1 expression, we found that it is highly expressed in a mouse model of migrating immortalized neurons (GN11 cells), but not in a mouse model of non-migrating neurons (GT1-7 cells). We thus hypothesized that Nrn1 might control neuronal migration. By using complementary assays, as Boyden's microchemotaxis, scratch-wounding and live cell imaging, we found that GN11 cell migration is enhanced when Nrn1 is overexpressed and decreased when Nrn1 is silenced. The effects of Nrn1 in promoting neuronal migration have been then confirmed ex vivo, on rat cortical interneurons, by Boyden chamber assays and focal electroporation of acute embryonic brain slices. Furthermore, we found that Nrn1 level modulation affects GN11 cell morphology. The process is also paralleled by Nrn1-induced α-tubulin post-translational modifications, a well-recognized marker of microtubule stability. Altogether, the data demonstrate a novel function of Nrn1 in promoting migration of neuronal cells and indicate that Nrn1 levels impact on microtubule stability. PMID:23212301

  7. Vulnerable to HIV / AIDS. Migration.

    PubMed

    Fernandez, I

    1998-01-01

    This special report discusses the impact of globalization, patterns of migration in Southeast Asia, gender issues in migration, the links between migration and HIV/AIDS, and spatial mobility and social networks. Migrants are particularly marginalized in countries that blame migrants for transmission of infectious and communicable diseases and other social ills. Effective control of HIV/AIDS among migrant and native populations requires a multisectoral approach. Programs should critically review the privatization of health care services and challenge economic models that polarize the rich and the poor, men and women, North and South, and migrant and native. Programs should recognize the equality between locals and migrants in receipt of health services. Countermeasures should have input from migrants in order to reduce the conditions that increase vulnerability to HIV/AIDS. Gender-oriented research is needed to understand women's role in migration. Rapid assessment has obscured the human dimension of migrants' vulnerability to HIV. Condom promotion is not enough. Migration is a major consequence of globalization, which holds the promise, real or imagined, of prosperity for all. Mass migration can be fueled by explosive regional developments. In Southeast Asia, migration has been part of the process of economic development. The potential to emigrate increases with greater per capita income. "Tiger" economies have been labor importers. Safe sex is not practiced in many Asian countries because risk is not taken seriously. Migrants tend to be used as economic tools, without consideration of social adjustment and sex behavior among singles.

  8. Population commission discusses international migration.

    PubMed

    1997-01-01

    At the 30th session of the Commission on Population and Development during February 24-28, 1997, international migration was the main topic, with special linkages between migration and development and on gender issues and the family. New and emerging issues were also considered. Members stressed the need for more reliable data on migration, the direction of migrants flows, and the characteristics of migrants. The Commission requested a task force on basic social services to hold a technical symposium of experts on international migration in 1998. Its chair, Dr. Nafis Sadik, said that migration issues should based on the reality of choice not on coercive measures or quotas. Almost half of the migrants globally are women. The Commission was given a new impetus by the International Conference on Population and Development held at Cairo in 1994. Migration pressures intensified in the second half of the 1980s and in the early 1990s, creating areas of concern: the negative impact of short-term migration on working conditions in host countries; migration pressures emanating from climatic change; the protection of migrant women and their children; the right of receiving countries to regulate access to their territory; the adverse consequences of forced migration; the situation of persons whose asylum claims have been rejected; the trafficking in women and children, prostitution and coercive adoption; and the sudden and massive arrival of refugees in need of international protection. The 1998 session of the Commission will feature the theme of health and mortality, with special emphasis on the linkages between health and development and on gender and age.

  9. Environmental factors influence lesser scaup migration chronology and population monitoring

    USGS Publications Warehouse

    Finger, Taylor A.; Afton, Alan D.; Schummer, Michael L.; Petrie, Scott A.; Badzinski, Shannon S.; Johnson, Michael A.; Szymanski, Michael L.; Jacobs, Kevin J.; Olsen, Glenn H.; Mitchell, Mark

    2016-01-01

    Identifying environmental metrics specific to lesser scaup (Aythya affinis; scaup) spring migration chronology may help inform development of conservation, management and population monitoring. Our objective was to determine how environmental conditions influence spring migration of lesser scaup to assess the effectiveness of the Waterfowl Breeding Population and Habitat Survey in accurately estimating scaup populations. We first compared peak timing of mallard (Anas platyrhynchos) and scaup migration from weekly ground surveys in North Dakota, USA because the Waterfowl Breeding Population and Habitat Survey is designed to capture annual mallard migration. As predicted, we detected that peak timing of scaup and mallard migrations differed in 25 of 36 years investigated (1980–2010). We marked scaup with satellite transmitters (n = 78; 7,403 locations) at Long Point, Lake Erie, Ontario, Canada; Pool 19 of the Mississippi River, Iowa and Illinois, USA; and Presque Isle Bay, Lake Erie, Pennsylvania, USA. We tested the assumption that our marked scaup were representative of the continental population using the traditional survey area by comparing timing of migration of marked birds and scaup counted in the North Dakota Game and Fish Department survey. We detected a strong positive correlation between marked scaup and the survey data, which indicated that marked scaup were representative of the population. We subsequently used our validated sample of marked scaup to investigate the effects of annual variation in temperature, precipitation, and ice cover on spring migration chronology in the traditional and eastern survey areas of the Waterfowl Breeding Population and Habitat Survey, 2005–2010. We evaluated competing environmental models to explain variation in timing and rate of scaup migration at large-scale and local levels. Spring migration of scaup occurred earlier and faster during springs with warmer temperatures and greater precipitation, variables known

  10. Space Based Ornithology: On the Wings of Migration and Biophysics

    NASA Technical Reports Server (NTRS)

    Smith, James A.

    2005-01-01

    The study of bird migration on a global scale is one of the compelling and challenging problems of modern biology with major implications for human health and conservation biology. Migration and conservation efforts cross national boundaries and are subject to numerous international agreements and treaties. Space based technology offers new opportunities to shed understanding on the distribution and migration of organisms on the planet and their sensitivity to human disturbances and environmental changes. Migration is an incredibly diverse and complex behavior. A broad outline of space based research must address three fundamental questions: (1) where could birds be, i.e. what is their fundamental niche constrained by their biophysical limits? (2) where do we actually find birds, i.e. what is their realizable niche as modified by local or regional abiotic and biotic factors, and (3) how do they get there (and how do we know?), that is what are their migration patterns and associated mechanisms? Our working hypothesis is that individual organism biophysical models of energy and water balance, driven by satellite measurements of spatio-temporal gradients in climate and habitat, will help us to explain the variability in avian species richness and distribution. Dynamic state variable modeling provides one tool for studying bird migration across multiple scales and can be linked to mechanistic models describing the time and energy budget states of migrating birds. Such models yield an understanding of how a migratory flyway and its component habitats function as a whole and link stop-over ecology with biological conservation and management. Further these models provide an ecological forecasting tool for science and application users to address what are the possible consequences of loss of wetlands, flooding, drought or other natural disasters such as hurricanes on avian biodiversity and bird migration.

  11. NASTRAN migration to UNIX

    NASA Technical Reports Server (NTRS)

    Chan, Gordon C.; Turner, Horace Q.

    1990-01-01

    COSMIC/NASTRAN, as it is supported and maintained by COSMIC, runs on four main-frame computers - CDC, VAX, IBM and UNIVAC. COSMIC/NASTRAN on other computers, such as CRAY, AMDAHL, PRIME, CONVEX, etc., is available commercially from a number of third party organizations. All these computers, with their own one-of-a-kind operating systems, make NASTRAN machine dependent. The job control language (JCL), the file management, and the program execution procedure of these computers are vastly different, although 95 percent of NASTRAN source code was written in standard ANSI FORTRAN 77. The advantage of the UNIX operating system is that it has no machine boundary. UNIX is becoming widely used in many workstations, mini's, super-PC's, and even some main-frame computers. NASTRAN for the UNIX operating system is definitely the way to go in the future, and makes NASTRAN available to a host of computers, big and small. Since 1985, many NASTRAN improvements and enhancements were made to conform to the ANSI FORTRAN 77 standards. A major UNIX migration effort was incorporated into COSMIC NASTRAN 1990 release. As a pioneer work for the UNIX environment, a version of COSMIC 89 NASTRAN was officially released in October 1989 for DEC ULTRIX VAXstation 3100 (with VMS extensions). A COSMIC 90 NASTRAN version for DEC ULTRIX DECstation 3100 (with RISC) is planned for April 1990 release. Both workstations are UNIX based computers. The COSMIC 90 NASTRAN will be made available on a TK50 tape for the DEC ULTRIX workstations. Previously in 1988, an 88 NASTRAN version was tested successfully on a SiliconGraphics workstation.

  12. [Oxyuriasis and prehistoric migrations].

    PubMed

    Araújo, A; Ferreira, L F

    1995-01-01

    Parasite findings in archeological material have made it possible to trace the dispersion of infectious agents and their human hosts in ancient times. These findings allow us to re-examine theories proposed at the beginning of the century concerning transpacific contacts that Asian populations may have had with South America. This has been the case, for example, with hookworm eggs found in archeological material dating up to 7,000 years before present. Because of the increase in scientific production in this area, it has now become necessary to undertake syntheses that assess the state of the art and propose workable paleoepidemological models of the prehistoric dispersion of human parasitoses. Based on findings of Enterobius vermicularis eggs in archeological material in the Americas, the present study is an effort in this direction. Unlike the hookworm, the pinworm does not require a soil cycle in order to be transmitted from one host to another, thereby meaning that its persistence in a given human population does not depend on climatic conditions. Thus, it could have been brought from the old to the new continent, possibly by human migrations across the Bering Strait. This may explain the greater geographical dispersion and dissemination of these findings in North America from 10,000 yrs B.P. till today. In South America, on the other hand, archeological findings have only confirmed existence of Enterobius vermicularis eggs within the Andean region, with findings located specifically in Chile and northern Argentina. Although a large number of samples have been examined, no such eggs have been found in coprolites in Brazil. The paper discusses models that account for the known distribution of this parasitosis in prehistoric populations. PMID:11625244

  13. Nonequilibrium migration in human history.

    PubMed Central

    Wakeley, J

    1999-01-01

    A nonequilibrium migration model is proposed and applied to genetic data from humans. The model assumes symmetric migration among all possible pairs of demes and that the number of demes is large. With these assumptions it is straightforward to allow for changes in demography, and here a single abrupt change is considered. Under the model this change is identical to a change in the ancestral effective population size and might be caused by changes in deme size, in the number of demes, or in the migration rate. Expressions for the expected numbers of sites segregating at particular frequencies in a multideme sample are derived. A maximum-likelihood analysis of independent polymorphic restriction sites in humans reveals a decrease in effective size. This is consistent with a change in the rates of migration among human subpopulations from ancient low levels to present high ones. PMID:10581291

  14. Radiation damage in silicon exposed to high-energy protons

    SciTech Connect

    Davies, Gordon; Hayama, Shusaku; Murin, Leonid; Krause-Rehberg, Reinhard; Bondarenko, Vladimir; Sengupta, Asmita; Davia, Cinzia; Karpenko, Anna

    2006-04-15

    Photoluminescence, infrared absorption, positron annihilation, and deep-level transient spectroscopy (DLTS) have been used to investigate the radiation damage produced by 24 GeV/c protons in crystalline silicon. The irradiation doses and the concentrations of carbon and oxygen in the samples have been chosen to monitor the mobility of the damage products. Single vacancies (and self-interstitials) are introduced at the rate of {approx}1 cm{sup -1}, and divacancies at 0.5 cm{sup -1}. Stable di-interstitials are formed when two self-interstitials are displaced in one damage event, and they are mobile at room temperature. In the initial stages of annealing the evolution of the point defects can be understood mainly in terms of trapping at the impurities. However, the positron signal shows that about two orders of magnitude more vacancies are produced by the protons than are detected in the point defects. Damage clusters exist, and are largely removed by annealing at 700 to 800 K, when there is an associated loss of broad band emission between 850 and 1000 meV. The well-known W center is generated by restructuring within clusters, with a range of activation energies of about 1.3 to 1.6 eV, reflecting the disordered nature of the clusters. Comparison of the formation of the X centers in oxygenated and oxygen-lean samples suggests that the J defect may be interstitial related rather than vacancy related. To a large extent, the damage and annealing behavior may be factorized into point defects (monitored by sharp-line optical spectra and DLTS) and cluster defects (monitored by positron annihilation and broadband luminescence). Taking this view to the limit, the generation rates for the point defects are as predicted by simply taking the damage generated by the Coulomb interaction of the protons and Si nuclei.

  15. Correctness criteria for process migration

    NASA Technical Reports Server (NTRS)

    Lu, Chin; Liu, J. W. S.

    1987-01-01

    Two correctness criteria, the state consistency criterion and the property consistency criterion for process migration are discussed. The state machine approach is used to model the interactions between a user process and its environment. These criteria are defined in terms of the model. The idea of environment view was introduced to distinguish what a user process observes about its environment from what its environment state really is and argue that a consistent view of the environment must be maintained for every migrating process.

  16. Homeward bound: Yemeni return migration.

    PubMed

    Colton, N A

    1993-01-01

    The author discusses the return migration of Yemenis from Saudi Arabia during the period 1970-1989. "Through the use of original, empirical data collected in Yemen, this article sheds light on who these returning migrants are, where they have come from, and what sort of future awaits them.... The survey conducted on return migration was administered in the winter and spring of 1989 in a region of North Yemen called al-Hujariyya."

  17. Radar studies of bird migration

    NASA Technical Reports Server (NTRS)

    Williams, T. C.; Williams, J. M.

    1974-01-01

    Observations of bird migration with NASA radars were made at Wallops Island, Va. Simultaneous observations were made at a number of radar sites in the North Atlantic Ocean in an effort to discover what happened to those birds that were observed leaving the coast of North America headed toward Bermuda, the Caribbean and South America. Transatlantic migration, utilizing observations from a large number of radars is discussed. Detailed studies of bird movements at Wallops Island are presented.

  18. Hydrogen migration in polycrystalline silicon

    SciTech Connect

    Nickel, N.H.; Jackson, W.B.; Walker, J.

    1996-03-01

    Hydrogen migration in solid-state crystallized and low-pressure chemical-vapor-deposited (LPCVD) polycrystalline silicon (poly-Si) was investigated by deuterium diffusion experiments. The concentration profiles of deuterium, introduced into the poly-Si samples either from a remote D plasma or from a deuterated amorphous-silicon layer, were measured as a function of time and temperature. At high deuterium concentrations the diffusion was dispersive depending on exposure time. The dispersion is consistent with multiple trapping within a distribution of hopping barriers. The data can be explained by a two-level model used to explain diffusion in hydrogenated amorphous silicon. The energy difference between the transport level and the deuterium chemical potential was found to be about 1.2{endash}1.3 eV. The shallow levels for hydrogen trapping are about 0.5 eV below the transport level, while the deep levels are about 1.5{endash}1.7 eV below. The hydrogen chemical potential {mu}{sub H} decreases as the temperature increases. At lower concentrations, {mu}{sub H} was found to depend markedly on the method used to prepare the poly-Si, a result due in part to the dependence of crystallite size on the deposition process. Clear evidence for deuterium deep traps was found only in the solid-state crystallized material. The LPCVD-grown poly-Si, with columnar grains extending through the film thickness, displayed little evidence of deep trapping, and exhibited enhanced D diffusion. Many concentration profiles in the columnar LPCVD material indicated complex diffusion behavior, perhaps reflecting spatial variations of trap densities, complex formation, and/or multiple transport paths. Many aspects of the diffusion in poly-Si are consistent with diffusion data obtained in amorphous silicon. {copyright} {ital 1996 The American Physical Society.}

  19. Distance and Intrastate College Student Migration

    ERIC Educational Resources Information Center

    Alm, James; Winters, John V.

    2009-01-01

    Most studies of student migration focus on "interstate" migration of college students, largely because the aggregate data typically used are limited in geographic specificity to states. However, interstate migration is only a small part of the total student migration. Public institutions generally get most of their students from within their…

  20. Rural-Urban Migration in Colombia.

    ERIC Educational Resources Information Center

    Schultz, T. Paul

    The rural-urban migration pattern in Colombia during the last 25 years has resulted in a population increase in urban areas from 30 to 52 percent of the total population. This study explores the causes of internal migration. Migration rates are estimated for various groups in the population to clarify who migrates and to where. A model of…

  1. [On the theoretical basis of migration research].

    PubMed

    Soderling, I

    1980-01-01

    A critique of current theories of migration is first presented. The author suggests that a variety of theories is needed to explain different types of migration. The model used to study internal migration at the Institute of Migration in Turku, Finland, is then described. (summary in ENG) PMID:12265094

  2. International migration: a global challenge.

    PubMed

    Martin, P; Widgren, J

    1996-04-01

    Trends in international migration are presented in this multiregional analysis. Seven of the world's wealthiest countries have about 33% of the world's migrant population, but under 16% of the total world population. Population growth in these countries is substantially affected by the migrant population. The migration challenge is external and internal. The external challenge is to balance the need for foreign labor and the commitment to human rights for those migrants seeking economic opportunity and political freedom. The internal challenge is to assure the social adjustment of immigrants and their children and to integrate them into society as citizens and future leaders. Why people cross national borders and how migration flows are likely to evolve over the next decades are explained. This report also presents some ways that countries can manage migration or reduce the pressures which force people to migrate. It is recommended that receiving nations control immigration by accelerating global economic growth and reducing wars and human rights violations. This report examines the impact of immigration on international trade, aid, and direct intervention policies. Although migration is one of the most important international economic issues, it is not coordinated by an international group. The European experience indicates that it is not easy to secure international cooperation on issues that affect national sovereignty. It is suggested that countries desiring control of their borders should remember that most people never cross national borders to live or work in another country, that 50% of the world's migrants move among developing countries, and that countries can shift from being emigration to immigration countries. The author suggests that sustained reductions in migration pressure are a better alternative than the "quick fixes" that may invite the very much feared mass and unpredictable movements.

  3. International migration: a global challenge.

    PubMed

    Martin, P; Widgren, J

    1996-04-01

    Trends in international migration are presented in this multiregional analysis. Seven of the world's wealthiest countries have about 33% of the world's migrant population, but under 16% of the total world population. Population growth in these countries is substantially affected by the migrant population. The migration challenge is external and internal. The external challenge is to balance the need for foreign labor and the commitment to human rights for those migrants seeking economic opportunity and political freedom. The internal challenge is to assure the social adjustment of immigrants and their children and to integrate them into society as citizens and future leaders. Why people cross national borders and how migration flows are likely to evolve over the next decades are explained. This report also presents some ways that countries can manage migration or reduce the pressures which force people to migrate. It is recommended that receiving nations control immigration by accelerating global economic growth and reducing wars and human rights violations. This report examines the impact of immigration on international trade, aid, and direct intervention policies. Although migration is one of the most important international economic issues, it is not coordinated by an international group. The European experience indicates that it is not easy to secure international cooperation on issues that affect national sovereignty. It is suggested that countries desiring control of their borders should remember that most people never cross national borders to live or work in another country, that 50% of the world's migrants move among developing countries, and that countries can shift from being emigration to immigration countries. The author suggests that sustained reductions in migration pressure are a better alternative than the "quick fixes" that may invite the very much feared mass and unpredictable movements. PMID:12320315

  4. Seasonal Survival Probabilities Suggest Low Migration Mortality in Migrating Bats

    PubMed Central

    Giavi, Simone; Moretti, Marco; Bontadina, Fabio; Zambelli, Nicola; Schaub, Michael

    2014-01-01

    Migration is adaptive if survival benefits are larger than costs of residency. Many aspects of bat migration ecology such as migratory costs, stopover site use and fidelity are largely unknown. Since many migrating bats are endangered, such information is urgently needed to promote conservation. We selected the migrating Leisler's bat (Nyctalus leisleri) as model species and collected capture-recapture data in southern Switzerland year round during 6 years. We estimated seasonal survival and site fidelity with Cormack-Jolly-Seber models that accounted for the presence of transients fitted with Bayesian methods and assessed differences between sexes and seasons. Activity peaked in autumn and spring, whereas very few individuals were caught during summer. We hypothesize that the study site is a migratory stopover site used during fall and spring migration for most individuals, but there is also evidence for wintering. Additionally, we found strong clues for mating during fall. Summer survival that included two major migratory journeys was identical to winter survival in males and slightly higher in females, suggesting that the migratory journeys did not bear significant costs in terms of survival. Transience probability was in both seasons higher in males than in females. Our results suggest that, similarly to birds, Leisler's bat also use stopover sites during migration with high site fidelity. In contrast to most birds, the stopover site was also used for mating and migratory costs in terms of survival seemed to be low. Transients' analyses highlighted strong individual variation in site use which makes particularly challenging the study and modelling of their populations as well as their conservation. PMID:24454906

  5. Seasonal survival probabilities suggest low migration mortality in migrating bats.

    PubMed

    Giavi, Simone; Moretti, Marco; Bontadina, Fabio; Zambelli, Nicola; Schaub, Michael

    2014-01-01

    Migration is adaptive if survival benefits are larger than costs of residency. Many aspects of bat migration ecology such as migratory costs, stopover site use and fidelity are largely unknown. Since many migrating bats are endangered, such information is urgently needed to promote conservation. We selected the migrating Leisler's bat (Nyctalus leisleri) as model species and collected capture-recapture data in southern Switzerland year round during 6 years. We estimated seasonal survival and site fidelity with Cormack-Jolly-Seber models that accounted for the presence of transients fitted with Bayesian methods and assessed differences between sexes and seasons. Activity peaked in autumn and spring, whereas very few individuals were caught during summer. We hypothesize that the study site is a migratory stopover site used during fall and spring migration for most individuals, but there is also evidence for wintering. Additionally, we found strong clues for mating during fall. Summer survival that included two major migratory journeys was identical to winter survival in males and slightly higher in females, suggesting that the migratory journeys did not bear significant costs in terms of survival. Transience probability was in both seasons higher in males than in females. Our results suggest that, similarly to birds, Leisler's bat also use stopover sites during migration with high site fidelity. In contrast to most birds, the stopover site was also used for mating and migratory costs in terms of survival seemed to be low. Transients' analyses highlighted strong individual variation in site use which makes particularly challenging the study and modelling of their populations as well as their conservation.

  6. Thread Migration in the Presence of Pointers

    NASA Technical Reports Server (NTRS)

    Cronk, David; Haines, Matthew; Mehrotra, Piyush

    1996-01-01

    Dynamic migration of lightweight threads supports both data locality and load balancing. However, migrating threads that contain pointers referencing data in both the stack and heap remains an open problem. In this paper we describe a technique by which threads with pointers referencing both stack and non-shared heap data can be migrated such that the pointers remain valid after migration. As a result, threads containing pointers can now be migrated between processors in a homogeneous distributed memory environment.

  7. Ethical concerns in nurse migration.

    PubMed

    McElmurry, Beverly J; Solheim, Karen; Kishi, Rieko; Coffia, Marcia A; Woith, Wendy; Janepanish, Poolsuk

    2006-01-01

    International nurse migration is natural and to be expected. Recently, however, those who have fostered nurse migration believe that it will solve nursing shortages in developed countries and offer nurse migrants better working conditions and an improved quality of life. Whether natural or manipulated, migration flow patterns largely occur from developing to developed countries. In this article, nurse migration is examined using primary health care (PHC) as an ethical framework. The unmanaged flow of nurse migrants from developing to developed countries is inconsistent with "health for all" principles. Removing key health personnel from countries experiencing resource shortages is contrary to PHC equity. Often, nurse migrants are placed in vulnerable, inequitable work roles, and employing nurse migrants fails to address basic causes of nurse shortages in developed countries, such as dissatisfaction with work conditions and decreased funding for academic settings. Nurse migration policies and procedures can be developed to satisfy PHC ethics criteria if they (1) leave developing countries enhanced rather than depleted, (2) contribute to country health outcomes consistent with essential care for all people, (3) are based on community participation, (4) address common nursing labor issues, and (5) involve equitable and clear financial arrangements. PMID:16873045

  8. Sedimentary record of erg migration

    SciTech Connect

    Porter, M.L.

    1986-06-01

    The sedimentary record of erg (eolian sand sea) migration consists of an idealized threefold division of sand-sea facies sequences. The basal division, here termed the fore-erg, is composed of a hierarchy of eolian sand bodies contained within sediments of the flanking depositional environment. The fore-erg represents the downwind, leading edge of the erg and records the onset of eolian sedimentation. Basin subsidence coupled with erg migration places the medial division, termed the central erg, over the fore-erg strata. Eolian influence on regional sedimentation patterns is greatest in the central erg, and most of the sand transported and deposited in the erg is contained within this region. Reduction in sand supply and continued erg migration will cover the central-erg deposits with a veneer of back-erg deposits. This upper division of the erg facies sequence resembles closely the fore-erg region. Back-erg deposits may be thin due to limited eolian influence on sedimentation or incomplete erg migration, or they may be completely absent because of great susceptibility to postdepositional erosion. Tectonic, climatic, and eustatic influences on sand-sea deposition will produce distinctive variations or modifications of the idealized erg facies sequence. The resulting variants in the sedimentary record of erg migration are illustrated with ancient examples from western North America, Europe, southern Africa, and South America. 38 references, 2 figures, 2 tables.

  9. Controlled-aperture wave-equation migration

    SciTech Connect

    Huang, L.; Fehler, Michael C.; Sun, H.; Li, Z.

    2003-01-01

    We present a controlled-aperture wave-equation migration method that no1 only can reduce migration artiracts due to limited recording aperlurcs and determine image weights to balance the efl'ects of limited-aperture illumination, but also can improve thc migration accuracy by reducing the slowness perturbations within thc controlled migration regions. The method consists of two steps: migration aperture scan and controlled-aperture migration. Migration apertures for a sparse distribution of shots arc determined using wave-equation migration, and those for the other shots are obtained by interpolation. During the final controlled-aperture niigration step, we can select a reference slowness in c;ontrollecl regions of the slowness model to reduce slowncss perturbations, and consequently increase the accuracy of wave-equation migration inel hods that makc use of reference slownesses. In addition, the computation in the space domain during wavefield downward continuation is needed to be conducted only within the controlled apertures and therefore, the computational cost of controlled-aperture migration step (without including migration aperture scan) is less than the corresponding uncontrolled-aperture migration. Finally, we can use the efficient split-step Fourier approach for migration-aperture scan, then use other, more accurate though more expensive, wave-equation migration methods to perform thc final controlled-apertio.ee migration to produce the most accurate image.

  10. Neurobiology of Monarch Butterfly Migration.

    PubMed

    Reppert, Steven M; Guerra, Patrick A; Merlin, Christine

    2016-01-01

    Studies of the migration of the eastern North American monarch butterfly (Danaus plexippus) have revealed mechanisms behind its navigation. The main orientation mechanism uses a time-compensated sun compass during both the migration south and the remigration north. Daylight cues, such as the sun itself and polarized light, are processed through both eyes and integrated through intricate circuitry in the brain's central complex, the presumed site of the sun compass. Monarch circadian clocks have a distinct molecular mechanism, and those that reside in the antennae provide time compensation. Recent evidence shows that migrants can also use a light-dependent inclination magnetic compass for orientation in the absence of directional daylight cues. The monarch genome has been sequenced, and genetic strategies using nuclease-based technologies have been developed to edit specific genes. The monarch butterfly has emerged as a model system to study the neural, molecular, and genetic basis of long-distance animal migration. PMID:26473314

  11. Focus: Asian migration to Canada.

    PubMed

    Kobayashi, A

    1988-01-01

    This collection of 5 short essays on Asian migration to Canada focuses on the relationships between individual migrants and their social contexts, both Asian and Canadian. Papers by Anderson and Kobayashi adopt research perspectives of outsider and insider, respectively. Vibert provides a historical overview against which the substantive issues introduced in the other 3 papers can be understood, and he illustrates the links between circumstances of migration and the larger issues by which the course of Canadian social progress has been steered. Mercer provides an introduction to issues that dominate the agenda of contemporary research, to show that Canadian communities of Asian heritage continue to grow in size, diversity, and complexity, as they become more established on the Canadian landscape. This collection is as much about the geography of racism as it is about migration.

  12. Neocortical neurogenesis and neuronal migration

    PubMed Central

    Tan, Xin; Shi, Song-Hai

    2012-01-01

    The neocortex, the evolutionarily newest part of the cerebral cortex, controls nearly all aspects of behavior, including perception, language and decision-making. It contains an immense number of neurons that can be broadly divided into two groups, excitatory neurons and inhibitory interneurons. These neurons are predominantly produced through extensive progenitor cell divisions during the embryonic stages. Moreover, they are not randomly dispersed, but spatially organized into horizontal layers that are essential for neocortex function. The formation of this laminar structure requires exquisite control of neuronal migration from their birthplace to their final destination. Extensive research over the past decade has greatly advanced our understanding of the production and migration of both excitatory neurons and inhibitory interneurons in the developing neocortex. In this review, we aim to give an overview on the molecular and cellular processes of neocortical neurogenesis and neuronal migration. PMID:24014417

  13. Migration, refugees, and health risks.

    PubMed Central

    Carballo, M.; Nerukar, A.

    2001-01-01

    Migration both voluntary and forced is increasing all over the world. People are moving in larger numbers faster and further than at any other time in history. This is happening at a time when many countries are ill-prepared to deal with a changing demography and when policies and attitudes to population movement and immigration are hardening. The health implications of this are many, and, in some cases, illness and death rates associated with migration are exacerbated by a lack of policies needed to make migration a healthy and socially productive process. From a public health point of view, this is having and will continue to have serious ramifications for the people that move, the family they leave behind, and the communities that host the newcomers. PMID:11485671

  14. Migration, refugees, and health risks.

    PubMed

    Carballo, M; Nerukar, A

    2001-01-01

    Migration both voluntary and forced is increasing all over the world. People are moving in larger numbers faster and further than at any other time in history. This is happening at a time when many countries are ill-prepared to deal with a changing demography and when policies and attitudes to population movement and immigration are hardening. The health implications of this are many, and, in some cases, illness and death rates associated with migration are exacerbated by a lack of policies needed to make migration a healthy and socially productive process. From a public health point of view, this is having and will continue to have serious ramifications for the people that move, the family they leave behind, and the communities that host the newcomers.

  15. Climate Migration and Moral Responsibility

    PubMed Central

    Nawrotzki, Raphael

    2016-01-01

    Even though anthropogenic climate change is largely caused by industrialized nations, its burden is distributed unevenly with poor developing countries suffering the most. A common response to livelihood insecurities and destruction is migration. Using Peter Singer’s “historical principle” this paper argues that a morally just evaluation requires taking causality between climate change and migration under consideration. The historical principle is employed to emphasize shortcomings in commonly made philosophical arguments to oppose immigration. The article concludes that none of these arguments is able to override the moral responsibility of industrialized countries to compensate for harms that their actions have caused.

  16. The migration transition in Malaysia.

    PubMed

    Lim, L L

    1996-01-01

    "Exploring the unique experience of migration transition in Malaysia, this paper identifies the turning points in relation to the level and nature of economic and labor market developments in Malaysia. Examining the development dynamics that mark the passage from exporting labor to depending on foreign labor, the paper concludes that such dynamics are influenced not only by economic but also sociocultural, demographic and policy factors. Several lessons from the Malaysian experience are drawn at the end to be utilized by other countries that still have to reach the turning points of the migration transition." PMID:12320775

  17. Rural migration in southern Nevada

    SciTech Connect

    Mosser, D.; Soden, D.L.

    1993-08-01

    This study reviews the history of migration in two rural counties in Southern Nevada. It is part of a larger study about the impact of a proposed high-level nuclear waste repository on in- and out-migration patterns in the state. The historical record suggests a boom and bust economic cycle has predominated in the region for the past century creating conditions that should be taken into account by decision makers when ascertaining the long-term impacts of the proposed repository.

  18. Climate Migration and Moral Responsibility

    PubMed Central

    Nawrotzki, Raphael

    2016-01-01

    Even though anthropogenic climate change is largely caused by industrialized nations, its burden is distributed unevenly with poor developing countries suffering the most. A common response to livelihood insecurities and destruction is migration. Using Peter Singer’s “historical principle” this paper argues that a morally just evaluation requires taking causality between climate change and migration under consideration. The historical principle is employed to emphasize shortcomings in commonly made philosophical arguments to oppose immigration. The article concludes that none of these arguments is able to override the moral responsibility of industrialized countries to compensate for harms that their actions have caused. PMID:27668124

  19. [Family reunification, family-formation migration, and return migration of Turks and Moroccans].

    PubMed

    De Beer, J; Kuijper, H; Noordam, R

    1991-01-01

    The authors analyze migration patterns to the Netherlands from Turkey and Morocco for the period 1977-1989. Three distinct phases emerge: labor migration, family-reunification migration, and family-formation migration. The rise in migration from Turkey and Morocco during the second half of the 1980s is attributed to family-formation migration, which is defined as a member of the second generation marrying a partner from the country of origin. (SUMMARY IN ENG)

  20. Migration cost externality and interregional equilibrium.

    PubMed

    Shin, C

    1994-01-01

    "This paper will investigate the characteristics of population allocation between two regions in the presence of migration cost. It will also examine both populations and the non-migration range of the initial population in which migration does not occur, in social optimum and market equilibrium with central government intervention, to reveal migration cost externality, and to propose a remedy for it." The author finds that "migration cost gives the social planner an additional burden of population reallocation, and it has an important effect upon an individual's decisions on migration in a decentralized market mechanism."

  1. Zebrafish germ cells: motility and guided migration.

    PubMed

    Paksa, Azadeh; Raz, Erez

    2015-10-01

    In the course of embryonic development, the process of cell migration is critical for establishment of the embryonic body plan, for morphogenesis and for organ function. Investigating the molecular mechanisms underlying cell migration is thus crucial for understanding developmental processes and clinical conditions resulting from abnormal cell migration such as cancer metastasis. The long-range migration of primordial germ cells toward the region at which the gonad develops occurs in embryos of various species and thus constitutes a useful in vivo model for single-cell migration. Recent studies employing zebrafish embryos have greatly contributed to the understanding of the mechanisms facilitating the migration of these cells en route to their target.

  2. A Discrete Cell Migration Model

    SciTech Connect

    Nutaro, James J; Kruse, Kara L; Ward, Richard C; O'Quinn, Elizabeth; Woerner, Matthew M; Beckerman, Barbara G

    2007-01-01

    Migration of vascular smooth muscle cells is a fundamental process in the development of intimal hyperplasia, a precursor to development of cardiovascular disease and a potential response to injury of an arterial wall. Boyden chamber experiments are used to quantify the motion of cell populations in response to a chemoattractant gradient (i.e., cell chemotaxis). We are developing a mathematical model of cell migration within the Boyden chamber, while simultaneously conducting experiments to obtain parameter values for the migration process. In the future, the model and parameters will be used as building blocks for a detailed model of the process that causes intimal hyperplasia. The cell migration model presented in this paper is based on the notion of a cell as a moving sensor that responds to an evolving chemoattractant gradient. We compare the results of our three-dimensional hybrid model with results from a one-dimensional continuum model. Some preliminary experimental data that is being used to refine the model is also presented.

  3. Externality, migration, and urban crises.

    PubMed

    Kanemoto, Y

    1980-01-01

    The author analyzes a special aspect of the short-run dynamics of an urban area which arises when there is a contrast between different types of households. The possibility of cumulative or catastrophic processes because of migration by poor families and then by rich families in or out of an urban area is discussed

  4. The OECD and International Migration.

    ERIC Educational Resources Information Center

    Organisation for Economic Cooperation and Development, Paris (France).

    The document focuses on various aspects of the social, economic, and policy implications of migration in Europe based on the actions undertaken by the Organization for Economic Cooperation and Development (OECD). A discussion of issues which may remedy the disequilibrium between the relative portions of the factors of production is presented: (1)…

  5. Migration and circulation in Ecuador.

    PubMed

    Brea, J A

    1991-01-01

    "This study investigates labor mobility in Ecuador as a function of the socioeconomic structure of places and personal attributes of migrants and circulators (short-term movers).... Particular attention is given to the role of agrarian change in altering established migration and circulation patterns." The author finds that "circulation may significantly alter family organization and the role of women in many Ecuadorian communities."

  6. Les questions de migrations internationales

    NASA Astrophysics Data System (ADS)

    Samman, Mouna Liliane

    1993-03-01

    International migrations have growing implications for both countries of origin and countries of destination. In the latter, the presence of foreigners and of members of their families today creates problems of integration, causes argument and brings mounting xenophobia. Paralleling political, economic and social measures taken by public authorities to respond to these difficulties, education needs to assist in defusing the resulting social tensions by preparing the minds of learners and helping to develop new attitudes. In particular, when educational programmes address questions of international migration, these should be treated in the framework of historical evolution so that their real significance and their true temporal and spatial dimensions become apparent. It is also important that the growing interdependence between countries should be made plain, that national history should be placed in its international context, and that the true consequences of these developments should be made clear. In this context, learners need to be acquainted with Human Rights, thereby stressing universal moral values and the role of the individual. Lastly, questions relating to international migration are usually presented in the media in a selective and partial manner, and the young people who take in this information often accept the hasty judgments which are made of situations as proven facts. This is why all teaching about international migration needs to be considered or reconsidered in the light of the complementary or competing actions of the media.

  7. Youth Migration from Rural Areas.

    ERIC Educational Resources Information Center

    Haller, Emil J.; Monk, David H.

    The persistent net loss of young people from rural areas has potentially contradictory implications for educational policy. Believing that youth migration to urban areas is inevitable, one school board might feel obligated to prepare students for urban jobs. Another board might view such actions as community suicide and attempt to slow…

  8. Job Migration: A Collaborative Effort

    ERIC Educational Resources Information Center

    Wagoner, Cynthia L.

    2012-01-01

    Music teachers often change jobs several times during their careers. Reasons for job changes vary, but regardless, these changes bring a different set of challenges. Sharing knowledge and learning are part and parcel of collaboration. So what if, as education professionals, music teachers decided to collaborate during job migrations? For all music…

  9. [Haitian migration to Santo Domingo].

    PubMed

    Latortue, P R

    1985-01-01

    This work examines the history of Haitian migration to the Dominican Republic, the central role of Haitian migration in Dominican society, working conditions of Haitian migrants in the Dominican Republic, and the relationship of the migration to economic development on the island of Hispaniola. Lack of data, the difficulty of measuring illegal movement, and the problem of defining Haitians in Santo Domingo have impeded understanding of migration to the Dominican Republic. It is believed by many authorities that Haitian migration to Santo Domingo is considerable and perhaps exceeds that to the US. Haitian migration to the Dominican Republic began after 1915 with the fall of the Haitian president, a worsening of economic conditions partly caused by stagnation in the agricultural sector, and the newly dominant role of the US in Haitian economic affairs. The Great Depression of the 1930s was a direct antecedent of the massacre of Haitians by Dominican police in which some 30 thousand persons were killed; the economic recession of the early 1980s has also caused an outburst of antiHaitian feeling in the Dominican Republic although 80% of laborers in the sugar industry are Haitians. Sugar is extremely important to the Dominican economy: in 1974, sugar covered 12% of cultivated land, produced 40% of foreign exchange earnings, and was responsable for 21% of taxable income. Dominicans however refuse to work in sugar plantations under the current technological. conditions and wage system. Although the government periodically demands the Dominicanization of the sugar work force, no such changes have been made. Sugar will probably continue to play a decisive role in the generation of foreign exchange despite introduction of more technologically advanced sectors which benefit from better prices in the international market. Possibilities of mechanizing sugar production in the Dominican Republic appear remote, and failure to modernize an important sector of the economy has

  10. [Haitian migration to Santo Domingo].

    PubMed

    Latortue, P R

    1985-01-01

    This work examines the history of Haitian migration to the Dominican Republic, the central role of Haitian migration in Dominican society, working conditions of Haitian migrants in the Dominican Republic, and the relationship of the migration to economic development on the island of Hispaniola. Lack of data, the difficulty of measuring illegal movement, and the problem of defining Haitians in Santo Domingo have impeded understanding of migration to the Dominican Republic. It is believed by many authorities that Haitian migration to Santo Domingo is considerable and perhaps exceeds that to the US. Haitian migration to the Dominican Republic began after 1915 with the fall of the Haitian president, a worsening of economic conditions partly caused by stagnation in the agricultural sector, and the newly dominant role of the US in Haitian economic affairs. The Great Depression of the 1930s was a direct antecedent of the massacre of Haitians by Dominican police in which some 30 thousand persons were killed; the economic recession of the early 1980s has also caused an outburst of antiHaitian feeling in the Dominican Republic although 80% of laborers in the sugar industry are Haitians. Sugar is extremely important to the Dominican economy: in 1974, sugar covered 12% of cultivated land, produced 40% of foreign exchange earnings, and was responsable for 21% of taxable income. Dominicans however refuse to work in sugar plantations under the current technological. conditions and wage system. Although the government periodically demands the Dominicanization of the sugar work force, no such changes have been made. Sugar will probably continue to play a decisive role in the generation of foreign exchange despite introduction of more technologically advanced sectors which benefit from better prices in the international market. Possibilities of mechanizing sugar production in the Dominican Republic appear remote, and failure to modernize an important sector of the economy has

  11. [Agricultural migration has changed face].

    PubMed

    Ouedraogo, D

    1991-04-01

    Movements related to colonization of new lands for cultivation or pasturing have constituted the dominant form of migration in the Sahel countries since the colonial period. the relative importance of such movements declined with the development of labor migration, but geographic mobility continues to be an integral part of Sahel life. A principal strategy during crises of agricultural production was the vast movement of population toward new lands, but such movements had little macroeconomic or macrosocial importance given the low population density and technical development of the time; the family subsistence enterprise was merely displaced. The artificial division into separate countries in the colonial era brought some control of migratory movements, and especially those across international borders, but such migrations increased again after independence and especially during the prolonged drought. Rural migration has been encouraged by development of transportation and communication facilities and by progress in controlling endemic diseases such as river blindness and sleeping sickness. Contemporary migration differs fundamentally from agricultural migration of the past. Migration has become, in addition to a survival strategy, a strategy of economic and social advancement. The change of residence is often accompanied by a restructuring of economic activities and substantial increases in the household's resources. Migrants attempt to produce enough for their own consumption, with some left for sale. They may also take on secondary employment, especially in the dry season: sale of firewood, petty trading, artisanal production. Spontaneous population movements seem to benefit the migrants, improving family and national agricultural production and contributing to a better distribution of rural population, but they have a high social and ecological cost and should receive more attention from planners and researchers in the context of the current campaign against

  12. Wind assistance: A requirement for migration of shorebirds?

    USGS Publications Warehouse

    Butler, R.W.; Williams, T.D.; Warnock, N.; Bishop, M.A.

    1997-01-01

    We investigated the importance of wind-assisted flight for northward (spring) migration by Western Sandpipers (Calidris mauri) along the Pacific Coast of North America. Using current models of energy costs of flight and recent data on the phenology of migration, we estimated the energy (fat) requirements for migration in calm winds and with wind-assisted flight for different rates of fat deposition: (1) a variable rate, assuming that birds deposit the minimum amount of fat required to reach the next stopover site; (2) a constant maximum rate of 1.0 g/day; and (3) a lower constant rate of 0.4 g/day. We tested these models by comparing conservative estimates of predicted body mass along the migration route with empirical data on body mass of Western Sandpipers at different stopover sites and upon arrival at the breeding grounds. In calm conditions, birds would have to deposit unrealistically high amounts of fat (up to 330% of observed values) to maintain body mass above absolute lean mass values. Fat-deposition rates of 1.0 g/day and 0.4 g/day, in calm conditions, resulted in a steady decline in body mass along the migration route, with predicted body masses on arrival in Alaska of only 60% (13.6 g) and 26% (5.9 g) of average lean mass (22.7 g). Conversely, birds migrating with wind assistance would be able to complete migration with fat-deposition rates as low as 0.4 g/day, similar to values reported for this size bird from field studies. Our results extend the conclusion of the importance of winds for large, long-distance migrants to a small, short-distance migrant. We suggest that the migratory decisions of birds are more strongly influenced by the frequency and duration of winds aloft, i.e. by events during the flight phase, than by events during the stopover phase of migration, such as fat-deposition rate, that have been the focus of much recent migration theory.

  13. The circular migration of smallholders in Kenya.

    PubMed

    Bigsten, A

    1996-01-01

    Circular migration is a central phenomenon in the lives of smallholders in East Africa. Many migration decisions are not individual decisions, but rather household decisions in which the household allocates its labor force among activities to maximize household utility. A probit model which incorporates circular migration and takes into account contacts, information, and indivisibilities is used to analyze migration among 763 farm households in the Central and Nyanza provinces of Kenya. Study data are from a 1982 survey. The pull of high urban wages appears to be a far more important determinant of migration decision outcomes than the push of land scarcity, while a strong local nonagricultural economy does not seem to restrict migration. Networks of personal contacts were found to be highly significant determinants of migration. These findings suggest that rural development will probably not reduce the flow of migration.

  14. Individual Decisions to Migrate During Civil Conflict

    PubMed Central

    Bohra-Mishra, Pratikshya; Massey, Douglas S.

    2012-01-01

    The existing literature on forced migration limits our understanding of how violence affects migration to competing destinations. This article adds to the literature on forced migration by studying how armed violence during a period of civil conflict in south-central Nepal influenced the likelihood of local, internal, and international migration. We find that violence has a nonlinear effect on migration, such that low to moderate levels of violence reduce the odds of movement, but when violence reaches high levels, the odds of movement increase. We also find that the effect of violence on mobility increases as the distance of the move increases. When we consider the influence of violence on microlevel decision-making, we find that the effects of individual and household-level determinants were mostly consistent with hypotheses derived from contemporary theories of voluntary migration and that no predictor of migration influenced the decision to migrate differently in the presence of violence. PMID:21541805

  15. Migration policies and return migration with particular reference to Yugoslavia.

    PubMed

    Chepulis, R L

    1981-09-01

    "The essay analyses some of the most recent migratory trends in Europe in the light of labour turnover, integration and assimilation. The author states that [since] World War II the expansion of the European and international capitalistic system has encouraged or discouraged, according to the various circumstances, the transferral of workers to different countries. "Particular attention has been given to the case of Yugoslavia. The changing of its migration policies has often coincided with the different modalities of flows." (summary in FRE)

  16. [Urban employment and internal migration in Peru].

    PubMed

    Cotlear, D

    1984-06-01

    The relationship between internal migration and employment problems in Peru is examined. The author argues that regional differences in income distribution are the primary causes of migration, particularly to urban areas. A model of the migration process is developed and tested using data from official sources, surveys, and the published literature. PMID:12313840

  17. [Urban employment and internal migration in Peru].

    PubMed

    Cotlear, D

    1984-06-01

    The relationship between internal migration and employment problems in Peru is examined. The author argues that regional differences in income distribution are the primary causes of migration, particularly to urban areas. A model of the migration process is developed and tested using data from official sources, surveys, and the published literature.

  18. Japanese Migration and the Americas: An Introduction to the Study of Migration.

    ERIC Educational Resources Information Center

    Mukai, Gary; Brunette, Rachel

    This curriculum module introduces students to the study of migration, including a brief overview of some categories of migration and reasons why people migrate. As a case study, the module uses the Japanese migration experience in the United States, Peru, Brazil, Canada, Mexico, Argentina, Bolivia, and Paraguay. The module introduces students to…

  19. Resonance locking as the source of rapid tidal migration in the Jupiter and Saturn moon systems

    NASA Astrophysics Data System (ADS)

    Fuller, Jim; Luan, Jing; Quataert, Eliot

    2016-10-01

    The inner moons of Jupiter and Saturn migrate outwards due to tidal energy dissipation within the planets, the details of which remain poorly understood. We demonstrate that resonance locking between moons and internal oscillations of the planet can produce rapid tidal migration. Resonance locking arises due to the structural evolution of the planet and typically produces an outward migration rate comparable to the age of the solar system. Resonance locking predicts a similar migration timescale but a different effective tidal quality factor Q governing the migration of each moon. It also predicts nearly constant migration timescales a function of semi-major axis, such that effective Q values were larger in the past. Recent measurements of Jupiter and Saturn's moon systems find effective Q values that are much smaller than expected (and are different between moons), and which correspond to migration timescales of ~10 Gyr. If confirmed, the measurements are broadly consistent with resonance locking as the dominant source of tidal dissipation in Jupiter and Saturn. Resonance locking also provides solutions to several problems posed by current measurements: it naturally explains the exceptionally small Q governing Rhea's migration, it allows the large heating rate of Enceladus to be achieved in an equilibrium eccentricity configuration, and it resolves evolutionary problems arising from present-day migration/heating rates.

  20. Resonance locking as the source of rapid tidal migration in the Jupiter and Saturn moon systems

    NASA Astrophysics Data System (ADS)

    Fuller, Jim; Luan, Jing; Quataert, Eliot

    2016-06-01

    The inner moons of Jupiter and Saturn migrate outwards due to tidal energy dissipation within the planets, the details of which remain poorly understood. We demonstrate that resonance locking between moons and internal oscillation modes of the planet can produce rapid tidal migration. Resonance locking arises due to the internal structural evolution of the planet and typically produces an outward migration rate comparable to the age of the Solar system. Resonance locking predicts a similar migration time-scale but a different effective tidal quality factor Q governing the migration of each moon. The theory also predicts nearly constant migration time-scales a function of semimajor axis, such that effective Q values were larger in the past. Recent measurements of Jupiter and Saturn's moon systems find effective Q values that are smaller than expected (and are different between moons), and which correspond to migration time-scales of ˜10 Gyr. If confirmed, the measurements are broadly consistent with resonance locking as the dominant source of tidal dissipation in Jupiter and Saturn. Resonance locking also provides solutions to several problems posed by current measurements: it naturally explains the exceptionally small Q governing Rhea's migration, it allows the large heating rate of Enceladus to be achieved in an equilibrium eccentricity configuration, and it resolves evolutionary problems arising from present-day migration/heating rates.

  1. Nuclide-migration field experiments

    SciTech Connect

    Erdal, B.R.; Wolfsberg, K.; Johnstone, J.K.; Erickson, K.L.; Friedman, A.M.; Fried, S.; Hines, J.J.

    1981-03-01

    When considering groundwater flow and radionuclide retention in the complex flow systems that can occur in geologic formations, one has a serious problem in determining if laboratory studies are being performed under conditions appropriate to natural systems. This document is the project plan for a program designed to begin to address these problems. The project is being carried out jointly by the Los Alamos National Laboratory, Sandia National Laboratories, and Argonne National Laboratory. The work has three principal objectives: (1) to develop the experimental, instrumental, and safety techniques necessary to conduct controlled, small-scale radionuclide migration field experiments, including those involving actinides; (2) to use these techniques to define radionuclide migration through rock by performing generic, at-depth experiments under closely monitored conditions; and (3) to determine whether available lithologic, geochemical, and hydrologic properties together with existing or developing transport models are sufficient and appropriate to describe real field conditions.

  2. Neural crest migration: trailblazing ahead

    PubMed Central

    McLennan, Rebecca

    2015-01-01

    Embryonic cell migration patterns are amazingly complex in the timing and spatial distribution of cells throughout the vertebrate landscape. However, advances in in vivo visualization, cell interrogation, and computational modeling are extracting critical features that underlie the mechanistic nature of these patterns. The focus of this review highlights recent advances in the study of the highly invasive neural crest cells and their migratory patterns during embryonic development. We discuss these advances within three major themes and include a description of computational models that have emerged to more rapidly integrate and test hypothetical mechanisms of neural crest migration. We conclude with technological advances that promise to reveal new insights and help translate results to human neural crest-related birth defects and metastatic cancer. PMID:25705385

  3. China: surplus labour and migration.

    PubMed

    Banister, J; Taylor, J R

    1989-12-01

    Surplus labor force and migration trends in China are examined, with emphasis on the impact of underemployment in rural areas. "Government policy encourages surplus labourers to transfer out of crop farming into agricultural sidelines or non-agricultural work. Peasants are urged to stay where they are, shifting jobs without shifting location; however, many rural areas are poorly endowed for providing alternative employment, so their surplus workers must also leave the village to find work. Many do not formally migrate, but rather move on a seasonal basis or set up 'temporary' residence in an urban place. This 'floating' population has been escalating rapidly in recent years....[The authors argue] that China's cities and towns can absorb millions of surplus labourers from rural areas each year, to the mutual benefit of sending and receiving areas." PMID:12342630

  4. Physicists' Forced Migrations under Hitler

    NASA Astrophysics Data System (ADS)

    Beyerchen, Alan

    2011-03-01

    When the Nazis came to power in early 1933 they initiated formal and informal measures that forced Jews and political opponents from public institutions such as universities. Some physicists retired and others went into industry, but most emigrated. International communication and contact made emigration a viable option despite the desperate economic times in the Great Depression. Another wave of emigrations followed the annexation of Austria in 1938. Individual cases as well as general patterns of migration and adaptation to new environments will be examined in this presentation. One important result of the forced migrations was that many of the physicists expelled under Hitler played important roles in strengthening physics elsewhere, often on the Allied side in World War II.

  5. Incentives and disincentives: international migration.

    PubMed

    Bhagwati, J N

    1984-01-01

    International migration is largely controlled by disincentives, or quotas, on immigration rather than checks on emigrations. Societies generally feel they have a right to exclude others from their boundaries, but they also usually feel that they do not have a right to control emigration. The single-planetary approach holds that people have the right to live wherever they like on the planet, and the cosmopolitan-utilitarian approach believes the same for reasons of world efficiency. The current feeling that societies have the right to exclude others may be explained best by territoriality in human animals. People also believe that their culture will be diluted if too many outsiders enter. In many cases, immigration systems cannot really control immigration, as in the cases of long landlocked borders between the US and Mexico and between Bangladesh and Assam. Immigration systems also contain legal loopholes. For example, in the US it is easier to get a student visa and convert to immigrant status than to gain immigrant status directly. Loopholes lead to plugs, which lead in turn to more loopholes. An upsurge in requests for political asylum followed increased restrictions on immigration in Western Europe. The US has investigated foreign aid and foreign investments to Mexico and Haiti to curb the flow of illegal migrants. The author suggests that foreign investments may lead to more migration because of the creation of a new proletariat used to the ways of developed countries. An estimate of what would happen if all immigration control were removed worldwide concludes that efficiency and income distribution would improve worldwide. Most migration from developing to developed countries currently consists of the migration of skilled professionals, the brain drain. The author proposes a tax on these professionals to be paid to the country of origin to compensate them for the loss in education and training. The author summarizes the differences between the West German

  6. Incentives and disincentives: international migration.

    PubMed

    Bhagwati, J N

    1984-01-01

    International migration is largely controlled by disincentives, or quotas, on immigration rather than checks on emigrations. Societies generally feel they have a right to exclude others from their boundaries, but they also usually feel that they do not have a right to control emigration. The single-planetary approach holds that people have the right to live wherever they like on the planet, and the cosmopolitan-utilitarian approach believes the same for reasons of world efficiency. The current feeling that societies have the right to exclude others may be explained best by territoriality in human animals. People also believe that their culture will be diluted if too many outsiders enter. In many cases, immigration systems cannot really control immigration, as in the cases of long landlocked borders between the US and Mexico and between Bangladesh and Assam. Immigration systems also contain legal loopholes. For example, in the US it is easier to get a student visa and convert to immigrant status than to gain immigrant status directly. Loopholes lead to plugs, which lead in turn to more loopholes. An upsurge in requests for political asylum followed increased restrictions on immigration in Western Europe. The US has investigated foreign aid and foreign investments to Mexico and Haiti to curb the flow of illegal migrants. The author suggests that foreign investments may lead to more migration because of the creation of a new proletariat used to the ways of developed countries. An estimate of what would happen if all immigration control were removed worldwide concludes that efficiency and income distribution would improve worldwide. Most migration from developing to developed countries currently consists of the migration of skilled professionals, the brain drain. The author proposes a tax on these professionals to be paid to the country of origin to compensate them for the loss in education and training. The author summarizes the differences between the West German

  7. Population forecasting with endogenous migration: an application to trans-Tasman migration.

    PubMed

    Gorbey, S; James, D; Poot, J

    1999-04-01

    "This article focuses on forecasting migration between Australia and New Zealand (trans-Tasman migration), which is largely visa-free and therefore resembles internal migration. Net trans-Tasman migration is a major component of New Zealand population change and is embedded in this article in a Bayesian or unrestricted vector autoregression (VAR) model, which includes foreign and domestic economic variables. When time series of net migration are available, this approach provides a useful input into forecasting population growth in the short run in the absence of major policy changes. This conclusion applies equally to interregional migration and to unrestricted international migration between economically integrated nations."

  8. Optimum swimming pathways of fish spawning migrations in rivers.

    PubMed

    McElroy, Brandon; DeLonay, Aaron; Jacobson, Robert

    2012-01-01

    Fishes that swim upstream in rivers to spawn must navigate complex fluvial velocity fields to arrive at their ultimate locations. One hypothesis with substantial implications is that fish traverse pathways that minimize their energy expenditure during migration. Here we present the methodological and theoretical developments necessary to test this and similar hypotheses. First, a cost function is derived for upstream migration that relates work done by a fish to swimming drag. The energetic cost scales with the cube of a fish's relative velocity integrated along its path. By normalizing to the energy requirements of holding a position in the slowest waters at the path's origin, a cost function is derived that depends only on the physical environment and not on specifics of individual fish. Then, as an example, we demonstrate the analysis of a migration pathway of a telemetrically tracked pallid sturgeon (Scaphirhynchus albus) in the Missouri River (USA). The actual pathway cost is lower than 10(5) random paths through the surveyed reach and is consistent with the optimization hypothesis. The implication--subject to more extensive validation--is that reproductive success in managed rivers could be increased through manipulation of reservoir releases or channel morphology to increase abundance of lower-cost migration pathways. PMID:22486084

  9. Optimum swimming pathways of fish spawning migrations in rivers

    USGS Publications Warehouse

    McElroy, Brandon; DeLonay, Aaron; Jacobson, Robert

    2012-01-01

    Fishes that swim upstream in rivers to spawn must navigate complex fluvial velocity fields to arrive at their ultimate locations. One hypothesis with substantial implications is that fish traverse pathways that minimize their energy expenditure during migration. Here we present the methodological and theoretical developments necessary to test this and similar hypotheses. First, a cost function is derived for upstream migration that relates work done by a fish to swimming drag. The energetic cost scales with the cube of a fish's relative velocity integrated along its path. By normalizing to the energy requirements of holding a position in the slowest waters at the path's origin, a cost function is derived that depends only on the physical environment and not on specifics of individual fish. Then, as an example, we demonstrate the analysis of a migration pathway of a telemetrically tracked pallid sturgeon (Scaphirhynchus albus) in the Missouri River (USA). The actual pathway cost is lower than 105 random paths through the surveyed reach and is consistent with the optimization hypothesis. The implication—subject to more extensive validation—is that reproductive success in managed rivers could be increased through manipulation of reservoir releases or channel morphology to increase abundance of lower-cost migration pathways.

  10. Lessons from the motorized migrations

    USGS Publications Warehouse

    Ellis, D.H.; Gee, G.F.; Clegg, K.R.; Duff, J.W.; Lishman, W.A.; Sladen, William J. L.

    2001-01-01

    Ten experiments have been conducted to determine if cranes can be led on migration and if those so trained will repeat migrations on their own. Results have been mixed as we have experienced the mishaps common to pilot studies. Nevertheless, we have learned many valuable lessons. Chief among these are that cranes can be led long distances behind motorized craft (air and ground), and those led over most or the entire route will return north come spring and south in fall to and from the general area of training. However, they will follow their own route. Groups transported south and flown at intervals along the route will migrate but often miss target termini. If certain protocol restrictions are followed, it is possible to make the trained cranes wild, however, the most practical way of so doing is to introduce them into a flock of wild cranes. We project that it is possible to create or restore wild migratory flocks of cranes by first leading small groups from chosen northern to southern termini.

  11. International migration statistics in Mexico.

    PubMed

    Garcia Y Griego, M

    1987-01-01

    During the past decade, Mexico has experienced both large-scale emigration directly, mostly to the US, and the mass immigration of Central American refugees. The implementation of the US Immigration and Control Act of 1986 and the possible escalation of armed conflicts in Central America may result in expanded inflows either of returning citizens or of new refugee waves. To develop appropriate policy responses, Mexico needs reliable information on international migration flows. This research note reviews available sources of that information--arrival and departure statistics, population censuses, refugee censuses, and survey data--and concludes that most of them are relatively weak. Currently, the published data on entries and departures provide little information on the demographic impact of legal migration, although they suggest that the inflow of foreigners is small. The census corroborates such findings, but it yields inadequate demographic detail. The movement of Mexican nationals, on the other hand, is poorly reflected by both sources. The void they leave has been palliated somewhat by surveys, but the only nationally representative survey on emigration was carried out in the late 1970s and might be a less than ideal basis for current policy formulation. It is hoped that as the relevance of international migration becomes more evident, steps towards the improvement of existing statistical systems may be undertaken. In the absence of such measures, policy-makers and researchers will have to continue relying on ad hoc surveys to answer the most pressing questions on the subject. PMID:12280914

  12. Transplantation stimulates interstitial cell migration in hydra

    SciTech Connect

    Fujisawa, T.; David, C.N.; Bosch, T.C. )

    1990-04-01

    Migration of interstitial cells and nerve cell precursors was analyzed in Hydra magnipapillata and Hydra vulgaris (formerly Hydra attenuata). Axial grafts were made between ({sup 3}H)thymidine-labeled donor and unlabeled host tissue. Migration of labeled cells into the unlabeled half was followed for 4 days. The results indicate that the rate of migration was initially high and then slowed on Days 2-4. Regrafting fresh donor tissue on Days 2-4 maintained high levels of migration. Thus, migration appears to be stimulated by the grafting procedure itself.

  13. Managing international migration: past, present, and future trends and issues.

    PubMed

    Hammar, T

    1991-01-01

    The author provides a brief overview of current international migration trends. Aspects considered include economic motives for migration, refugee migration, migration policies and human rights, and political factors. Comments by Jean-Yves Carlier are included (pp. 192-3).

  14. Trends in return migration to the South.

    PubMed

    Long, L H; Hansen, K A

    1975-11-01

    The rate of return migration to the South rose by nearly 19 percent between the late 1950's and the late 1960's and was an important factor in changing the South's overall migration pattern. But an increase in the rate of return migration was somewhat less important in changing Southern migration than (1) a decline in the rate of out-migration of native Southerners and (2) an increase in the rate at which non-Southern-born persons move to the South. The probability of former migrants returning to the South was over four times greater for whites than for blacks in the 1955-1960 period and three and one-fourth times greater in the 1965-1970 period. Since 1970 the rate of return migration has apparently continued to rise at a faster rate for blacks, but the black rate of return migration is still below the white rate.

  15. An economic analysis of migration in Mexico.

    PubMed

    Greenwood, M J; Ladman, J R

    1978-07-01

    This paper analyzes internal migration in Mexico over the 1960-70 period. A model of the determinants of migration is specified and estimated for aggregated interstate migration flows. Results show that distance serves as a significant deterrent to migration, that higher destination earning levels are attractive to migrants, and that regions with high unemployment rates experience lower rates of inmigration. An unanticipated finding is that regions with higher earning levels have greater rates of outmigration. The data are disaggregated to examine separate migration relationships for each state. The results are that distance is a lesser deterrent for those migrants with more accessible alternatives, that higher earning levels reduce the deterring effects of distance, and that regions with higher earning levels have lower associated elasticities of migration. It is concluded that economic factors have played a crucial role in internal migration and thus in the changing occupational and geographic structure of the Mexican labor force. PMID:12265626

  16. An economic analysis of migration in Mexico.

    PubMed

    Greenwood, M J; Ladman, J R

    1978-07-01

    This paper analyzes internal migration in Mexico over the 1960-70 period. A model of the determinants of migration is specified and estimated for aggregated interstate migration flows. Results show that distance serves as a significant deterrent to migration, that higher destination earning levels are attractive to migrants, and that regions with high unemployment rates experience lower rates of inmigration. An unanticipated finding is that regions with higher earning levels have greater rates of outmigration. The data are disaggregated to examine separate migration relationships for each state. The results are that distance is a lesser deterrent for those migrants with more accessible alternatives, that higher earning levels reduce the deterring effects of distance, and that regions with higher earning levels have lower associated elasticities of migration. It is concluded that economic factors have played a crucial role in internal migration and thus in the changing occupational and geographic structure of the Mexican labor force.

  17. Social Physics and China's Population Migration

    NASA Astrophysics Data System (ADS)

    Wang, Yun-Lin; Li, Ding

    Based on the social physics theory, this paper analyzes the economic disparities between different regions in China, and contributes a conceptual model of population migration among eastern, central, western and north-eastern regions. The national 1% population sample investigation data is adopted to build a network of inter-provincial population migration, and the population migration network is analyzed with social network analysis. The results are shown that there is a very strong correlation between migrant population and economy disparity in China, and the migration with obviously geographical characteristics. The eastern region is the main areas for migration-inflow; the central region is the main areas of migration-outflow; the western region is relatively “locked-up”, with a little of population flow; and the migration of the northeast is mainly within its own regional territory.

  18. Migration mechanism of a GaN bicrystalline grain boundary as a model system

    NASA Astrophysics Data System (ADS)

    Lee, Sung Bo; Yoo, Seung Jo; Kim, Young-Min; Kim, Jin-Gyu; Han, Heung Nam

    2016-05-01

    Using in situ high-resolution transmission electron microscopy, we have explored migration mechanism of a grain boundary in a GaN bicrystal as a model system. During annealing at 500 °C, the grain-boundary region underwent a decrease in thickness, which occurred by decomposition or sublimation of GaN during annealing at 500 °C coupled with electron-beam sputtering. The decrease in thickness corresponds to an increase in the driving force for migration, because the migration of the grain boundary was driven by the surface energy difference. As the driving force increased with annealing time, the grain-boundary morphology turned from atomically smooth to rough, which is characterized by kinetic roughening. The observations indicate that a grain boundary exhibits a nonlinear relationship between driving force for migration and migration velocity, in discord with the general presumption that a grain boundary follows a linear relationship.

  19. A Kosloff/Basal method, 3D migration program implemented on the CYBER 205 supercomputer

    NASA Technical Reports Server (NTRS)

    Pyle, L. D.; Wheat, S. R.

    1984-01-01

    Conventional finite difference migration has relied on approximations to the acoustic wave equation which allow energy to propagate only downwards. Although generally reliable, such approaches usually do not yield an accurate migration for geological structures with strong lateral velocity variations or with steeply dipping reflectors. An earlier study by D. Kosloff and E. Baysal (Migration with the Full Acoustic Wave Equation) examined an alternative approach based on the full acoustic wave equation. The 2D, Fourier type algorithm which was developed was tested by Kosloff and Baysal against synthetic data and against physical model data. The results indicated that such a scheme gives accurate migration for complicated structures. This paper describes the development and testing of a vectorized, 3D migration program for the CYBER 205 using the Kosloff/Baysal method. The program can accept as many as 65,536 zero offset (stacked) traces.

  20. Migration mechanism of a GaN bicrystalline grain boundary as a model system

    PubMed Central

    Lee, Sung Bo; Yoo, Seung Jo; Kim, Young-Min; Kim, Jin-Gyu; Han, Heung Nam

    2016-01-01

    Using in situ high-resolution transmission electron microscopy, we have explored migration mechanism of a grain boundary in a GaN bicrystal as a model system. During annealing at 500 °C, the grain-boundary region underwent a decrease in thickness, which occurred by decomposition or sublimation of GaN during annealing at 500 °C coupled with electron-beam sputtering. The decrease in thickness corresponds to an increase in the driving force for migration, because the migration of the grain boundary was driven by the surface energy difference. As the driving force increased with annealing time, the grain-boundary morphology turned from atomically smooth to rough, which is characterized by kinetic roughening. The observations indicate that a grain boundary exhibits a nonlinear relationship between driving force for migration and migration velocity, in discord with the general presumption that a grain boundary follows a linear relationship. PMID:27210538

  1. Migration mechanism of a GaN bicrystalline grain boundary as a model system.

    PubMed

    Lee, Sung Bo; Yoo, Seung Jo; Kim, Young-Min; Kim, Jin-Gyu; Han, Heung Nam

    2016-01-01

    Using in situ high-resolution transmission electron microscopy, we have explored migration mechanism of a grain boundary in a GaN bicrystal as a model system. During annealing at 500 °C, the grain-boundary region underwent a decrease in thickness, which occurred by decomposition or sublimation of GaN during annealing at 500 °C coupled with electron-beam sputtering. The decrease in thickness corresponds to an increase in the driving force for migration, because the migration of the grain boundary was driven by the surface energy difference. As the driving force increased with annealing time, the grain-boundary morphology turned from atomically smooth to rough, which is characterized by kinetic roughening. The observations indicate that a grain boundary exhibits a nonlinear relationship between driving force for migration and migration velocity, in discord with the general presumption that a grain boundary follows a linear relationship. PMID:27210538

  2. Philippine migration policy: dilemmas of a crisis.

    PubMed

    Battistella, G

    1999-04-01

    Philippine migration policy is traced from the early 1970s to the present. The main migration trends in the 1990s are described. An assessment is made of the efficacy and appropriateness of present migration policy in light of the economic crisis. A regional approach to migration policy is necessary in order to encourage placing migration as a greater priority on national agendas and in bilateral agreements. In the Philippines, migrants are considered better paid workers, which diminishes their importance as a legislative or program priority. Santo Tomas (1998) conducted an empirical assessment of migration policies in the Philippines, but refinement is needed. Although migration is a transnational experience, there is little dialogue and cooperation among countries. Philippine migration policy defines its role as an information resource for migrants. Policy shifted from labor export to migrant management in the public and private sectors. Predeparture information program studies are recommending a multi-stage process that would involve all appropriate parties. There is talk of including migration information in the education curriculum. There are a variety of agendas, competing interests, and information resources between migration networks and officiating agencies. The Asian financial crisis may have a mild impact, but there are still issues of reintegration, protection, and employment conditions

  3. Clandestine labor migration to Taiwan.

    PubMed

    Tsay, C

    1992-01-01

    "Illegal migration to Taiwan is a recent phenomenon but with a rapid rate of increase. Most illegal foreign workers enter on visitor's visas and overstay. This paper's detailed analysis of official data reveals that Malaysia, Philippines, Indonesia and Thailand are the major sources, providing a stock of mostly male workers numbering around 40,000. Sociodemographic and attitudinal changes among Taiwanese workers coupled with labor shortages in low-skilled jobs are pressuring the Taiwanese government to formulate plans for a systematic importation of foreign labor." PMID:12285774

  4. Seismic Imaging Processing and Migration

    2000-06-26

    Salvo is a 3D, finite difference, prestack, depth migration code for parallel computers. It is also capable of processing 2D and poststack data. The code requires as input a seismic dataset, a velocity model and a file of parameters that allows the user to select various options. The code uses this information to produce a seismic image. Some of the options available to the user include the application of various filters and imaging conditions. Themore » code also incorporates phase encoding (patent applied for) to process multiple shots simultaneously.« less

  5. Savings, remittances, and return migration.

    PubMed

    Merkle, L; Zimmermann, K F

    1992-01-01

    "We use a data set of immigrants to West Germany to simultaneously study both savings and remittances which we relate to individual characteristics, economic variables, migration experiences and remigration plans. Section 2 discusses the basic hypotheses and explains the data. Section 3 presents the empirical study and Section 4 summarizes." The results suggest that "savings and remittances of migrants can be well explained by remigration plans and economic as well as demographic variables. However, the planned future duration of residence in Germany has a negative and significant effect only on remittances."

  6. Asian student migration to Australia.

    PubMed

    Shu, J; Hawthorne, L

    1996-01-01

    "This paper presents an overview of Asian student migration to Australia, together with an analysis of political and educational aspects of the overseas student programme. It focuses on some significant consequences of this flow for Australia. The characteristics of key student groups are contrasted to provide some perspective of the diversity of historical and cultural backgrounds, with the source countries of Malaysia, Indonesia and PRC [China] selected as case studies. Since the issue of PRC students in Australia has attracted considerable public attention and policy consideration, particular focus is placed on their experience." (SUMMARY IN FRE AND SPA)

  7. Clandestine labor migration to Taiwan.

    PubMed

    Tsay, C

    1992-01-01

    "Illegal migration to Taiwan is a recent phenomenon but with a rapid rate of increase. Most illegal foreign workers enter on visitor's visas and overstay. This paper's detailed analysis of official data reveals that Malaysia, Philippines, Indonesia and Thailand are the major sources, providing a stock of mostly male workers numbering around 40,000. Sociodemographic and attitudinal changes among Taiwanese workers coupled with labor shortages in low-skilled jobs are pressuring the Taiwanese government to formulate plans for a systematic importation of foreign labor."

  8. Migration timing and its determinants for nocturnal migratory birds during autumn migration.

    PubMed

    La Sorte, Frank A; Hochachka, Wesley M; Farnsworth, Andrew; Sheldon, Daniel; Fink, Daniel; Geevarghese, Jeffrey; Winner, Kevin; Van Doren, Benjamin M; Kelling, Steve

    2015-09-01

    1. Migration is a common strategy used by birds that breed in seasonal environments, and multiple environmental and biological factors determine the timing of migration. How these factors operate in combination during autumn migration, which is considered to be under weaker time constraints relative to spring migration, is not clear. 2. Here, we examine the patterns and determinants of migration timing for nocturnal migrants during autumn migration in the north-eastern USA using nocturnal reflectivity data from 12 weather surveillance radar stations and modelled diurnal probability of occurrence for 142 species of nocturnal migrants. We first model the capacity of seasonal atmospheric conditions (wind and precipitation) and ecological productivity (vegetation greenness) to predict autumn migration intensity. We then test predictions, formulated under optimal migration theory, on how migration timing should be related to assemblage-level estimates of body size and total migration distance within the context of dietary guild (insectivore and omnivore) and level of dietary plasticity during autumn migration. 3. Our results indicate seasonal declines in ecological productivity delineate the beginning and end of peak migration, whose intensity is best predicted by the velocity of winds at migration altitudes. Insectivorous migrants departed earlier in the season and, consistent with our predictions, large-bodied and long-distance insectivorous migrants departed the earliest. Contrary to our predictions, large-bodied and some long-distance omnivorous migrants departed later in the season, patterns that were replicated in part by insectivorous migrants that displayed dietary plasticity during autumn migration. 4. Our findings indicate migration timing in the region is dictated by optimality strategies, modified based on the breadth and flexibility of migrant's foraging diets, with declining ecological productivity defining possible resource thresholds during which

  9. A model to resolve organochlorine pharmacokinetics in migrating humpback whales.

    PubMed

    Cropp, Roger; Nash, Susan Bengtson; Hawker, Darryl

    2014-07-01

    Humpback whales are iconic mammals at the top of the Antarctic food chain. Their large reserves of lipid-rich tissues such as blubber predispose them to accumulation of lipophilic contaminants throughout their lifetime. Changes in the volume and distribution of lipids in humpback whales, particularly during migration, could play an important role in the pharmacokinetics of lipophilic contaminants such as the organochlorine pesticide hexachlorobenzene (HCB). Previous models have examined constant feeding and nonmigratory scenarios. In the present study, the authors develop a novel heuristic model to investigate HCB dynamics in a humpback whale and its environment by coupling an ecosystem nutrient-phytoplankton-zooplankton-detritus (NPZD) model, a dynamic energy budget (DEB) model, and a physiologically based pharmacokinetic (PBPK) model. The model takes into account the seasonal feeding pattern of whales, their energy requirements, and fluctuating contaminant burdens in the supporting plankton food chain. It is applied to a male whale from weaning to maturity, spanning 20 migration and feeding cycles. The model is initialized with environmental HCB burdens similar to those measured in the Southern Ocean and predicts blubber HCB concentrations consistent with empirical concentrations observed in a southern hemisphere population of male, migrating humpback whales. Results show for the first time some important details of the relationship between energy budgets and organochlorine pharmacokinetics. PMID:24733631

  10. A model to resolve organochlorine pharmacokinetics in migrating humpback whales.

    PubMed

    Cropp, Roger; Nash, Susan Bengtson; Hawker, Darryl

    2014-07-01

    Humpback whales are iconic mammals at the top of the Antarctic food chain. Their large reserves of lipid-rich tissues such as blubber predispose them to accumulation of lipophilic contaminants throughout their lifetime. Changes in the volume and distribution of lipids in humpback whales, particularly during migration, could play an important role in the pharmacokinetics of lipophilic contaminants such as the organochlorine pesticide hexachlorobenzene (HCB). Previous models have examined constant feeding and nonmigratory scenarios. In the present study, the authors develop a novel heuristic model to investigate HCB dynamics in a humpback whale and its environment by coupling an ecosystem nutrient-phytoplankton-zooplankton-detritus (NPZD) model, a dynamic energy budget (DEB) model, and a physiologically based pharmacokinetic (PBPK) model. The model takes into account the seasonal feeding pattern of whales, their energy requirements, and fluctuating contaminant burdens in the supporting plankton food chain. It is applied to a male whale from weaning to maturity, spanning 20 migration and feeding cycles. The model is initialized with environmental HCB burdens similar to those measured in the Southern Ocean and predicts blubber HCB concentrations consistent with empirical concentrations observed in a southern hemisphere population of male, migrating humpback whales. Results show for the first time some important details of the relationship between energy budgets and organochlorine pharmacokinetics.

  11. WLCI researchers employ new approaches to help managers conserve deer migrations

    USGS Publications Warehouse

    Allen, Leslie A.; Kauffman, Matthew J.

    2012-01-01

    Elk, mule deer, pronghorn antelope, moose, and bighorn sheep are iconic animals of the American West. These hooved animals, known as ungulates, commonly travel 30–60 miles between seasonal ranges. These migrations between winter and summer ranges are vital for survival and reproduction. As habitat fragmentation continues, the conservation of ungulate migration routes has received considerable attention in the West and across the globe. For example, it is estimated that many ungulate migration routes in the Greater Yellowstone Ecosystem have already been lost. The traditional migration routes of Wyoming ungulates are threatened by unprecedented levels of energy development and by increasing levels of rural ranchette development (including fences, structures, and roads). In the past, migration corridors have been mapped based primarily on the expert opinions of state game managers, but long-term conservation of Wyoming's ungulate migration routes requires a better understanding of migration ecology and more sophisticated management tools. Wyoming Landscape Conservation Initiative (WLCI) researchers investigated the migration of a large mule deer herd across the Dad and Wild Horse winter ranges in southwest Wyoming, where 2,000 gas wells and 1,609 kilometers of pipelines and roads have been proposed for development.

  12. Bird species migration ratio in East Asia, Australia, and surrounding islands

    NASA Astrophysics Data System (ADS)

    Kuo, Yiliang; Lin, Da-Li; Chuang, Fu-Man; Lee, Pei-Fen; Ding, Tzung-Su

    2013-08-01

    Bird migration and its relationship with the contemporary environment have attracted long-term discussion. We calculated the avian migration ratio (the proportion of breeding species that migrate) in the areas from 70°E to 180°E and examined its relationship with the annual ranges of ambient temperature, primary productivity (estimated by the Enhanced Vegetation Index), and precipitation, along with island isolation and elevational range. The avian migration ratio increased with increasing latitude in general but varied greatly between the two hemispheres. Additionally, it showed minimal differences between continents and islands. Our analyses revealed that the seasonality of ambient temperature, which represents the energy expenditure of birds, is the dominant factor in determining bird species migration. Seasonality in primary productivity and other environmental factors play an indirect or limited role in bird species migration. The lower avian migration ratio in the Southern Hemisphere can be attributed to its paleogeographical isolation, stable paleoclimate, and warm contemporary environment. Under current trends of global warming, our findings should lead to further studies of the impact of warming on bird migration.

  13. Thermocapillary Bubble Migration: Thermal Boundary Layers for Large Marangoni Numbers

    NASA Technical Reports Server (NTRS)

    Balasubramaniam, R.; Subramanian, R. S.

    1996-01-01

    The migration of an isolated gas bubble in an immiscible liquid possessing a temperature gradient is analyzed in the absence of gravity. The driving force for the bubble motion is the shear stress at the interface which is a consequence of the temperature dependence of the surface tension. The analysis is performed under conditions for which the Marangoni number is large, i.e. energy is transferred predominantly by convection. Velocity fields in the limit of both small and large Reynolds numbers are used. The thermal problem is treated by standard boundary layer theory. The outer temperature field is obtained in the vicinity of the bubble. A similarity solution is obtained for the inner temperature field. For both small and large Reynolds numbers, the asymptotic values of the scaled migration velocity of the bubble in the limit of large Marangoni numbers are calculated. The results show that the migration velocity has the same scaling for both low and large Reynolds numbers, but with a different coefficient. Higher order thermal boundary layers are analyzed for the large Reynolds number flow field and the higher order corrections to the migration velocity are obtained. Results are also presented for the momentum boundary layer and the thermal wake behind the bubble, for large Reynolds number conditions.

  14. Direct imaging Au nanoparticle migration inside mesoporous silica channels.

    PubMed

    Liu, Zhengwang; Che, Renchao; Elzatahry, Ahmed A; Zhao, Dongyuan

    2014-10-28

    Supported metal nanoparticle (NP) catalysts have been widely used in many industry processes and catalytic reactions. Catalyst deactivation is mainly caused by the sintering of supported metal NPs. Hence, understanding the metal NPs' sintering behaviors has great significance in preventing catalyst deactivation. Here we report the metal particle migration inside/between mesochannels by scanning transmission electron microscopy and electron energy loss spectroscopy via an in situ TEM heating technique. A sintering process is proposed that particle migration predominates, driven by the difference of gravitational potential from the height of the uneven internal surface of the mesopores; when the distance of the gold nanoparticles with a size of about 3 and 5 nm becomes short after migration, the coalescence process is completed, which is driven by an "octopus-claw-like" expansion of a conduction electron cloud outside the Au NPs. The supports containing an abundance of micropores help to suppress particle migration and coalescence. Our findings provide the understanding toward the rational design of supported industrial catalysts and other nanocomposites with enhanced activity and stability for applications such as batteries, catalysis, drug delivery, gas sensors, and solar cells.

  15. Migrational Instabilities in Particle Suspensions

    NASA Technical Reports Server (NTRS)

    Goddard, Joe D.

    1996-01-01

    This work deals with an instability arising from the shear-induced migration of particles in dense suspensions coupled with a dependence of viscosity on particle concentration. The analysis summarized here treats the inertialess (Re = O) linear stability of homogeneous simple shear flows for a Stokesian suspension model of the type proposed by Leighton and Acrivos (1987). Depending on the importance of shear-induced migration relative to concentration-driven diffusion, this model admits short-wave instability arising from wave-vector stretching by the base flow and evolving into particle-depleted shear bands. Moreover, this instability in the time-dependent problem corresponds to loss of ellipticity in the associated static problem (Re = O, Pe = O). While the isotropic version of the Leighton-Acrivos model is found to be stable with their experimentally determined parameters for simple shear, it is known that the stable model does not give a good quantitative description of particle clustering in the core of pipe flow (Nott and Brady 1994). This leads to the conjecture that an appropriate variant on the above model could explain such clustering as a two-phase bifurcation in the base flow.

  16. Endocannabinoid signalling in neuronal migration.

    PubMed

    Zhou, Ya; Falenta, Katarzyna; Lalli, Giovanna

    2014-02-01

    The endocannabinoid (eCB) system consists of several endogenous lipids, their target CB1 and CB2 receptors and enzymes responsible for their synthesis and degradation. The most abundant eCB in the central nervous system (CNS), 2-arachidonoyl glycerol (2-AG), triggers a broad range of signalling events by acting on CB1, the most abundant G protein-coupled receptor in the CNS. The eCB system regulates many physiological processes including neurogenesis, axon guidance and synaptic plasticity. Recent studies have highlighted an additional important role for eCB signalling in neuronal migration, which is crucial to achieve the complex architecture and efficient wiring of the CNS. Indeed, eCB signalling controls migration both pre- and post-natally, regulating interneuron positioning in the developing cortex and hippocampus and the polarised motility of stem cell-derived neuroblasts. While these effects may contribute to cognitive deficits associated with cannabis consumption, they also provide potential opportunities for endogenous stem cell-based neuroregenerative strategies.

  17. Energy.

    ERIC Educational Resources Information Center

    Online-Offline, 1998

    1998-01-01

    This issue focuses on the theme of "Energy," and describes several educational resources (Web sites, CD-ROMs and software, videos, books, activities, and other resources). Sidebars offer features on alternative energy, animal energy, internal combustion engines, and energy from food. Subthemes include harnessing energy, human energy, and natural…

  18. Globalisation and international migration in Asia.

    PubMed

    Hugo, G

    1998-03-01

    "The present paper attempts to summarise the major contemporary trends in international migration in Asia and how this is related to the rapid demographic transition occurring across most of the region. The diversity of types of mobility is one of the striking features of Asian international migration and developments in each major type [are] discussed in the paper. It is then argued that there are elements within the Asian migration system that give it a momentum which to some extent operates outside of the influence of economic and political trends. Finally some emerging issues in Asian international migration are briefly addressed. These include the scale and effects of remittances, the feminisation of migration, undocumented migration, increasing government involvement and brain drain issues." (EXCERPT) PMID:12293967

  19. Measuring spatial focusing in a migration system.

    PubMed

    Plane, D A; Mulligan, G F

    1997-05-01

    Equality indexes used in other geographical contexts may be used to gauge the degree of spatial focusing in an entire migration system or within the gross in- and out-migration fields of specific regions. They provide useful indicators of overall shifts in the patterns of interregional migration and can help give insight into the population redistributive roles played by specific regions. Perhaps the most common equality index used to measure income distribution is the Gini coefficient, yet it appears almost never to have been applied in migration research. In this paper we set forth a variety of Gini indexes to be used for different migration analyses and illustrate their application with recent data on U.S. interstate movements. We argue that the Gini index provides some singularly useful insights that differ from those afforded by other measures more commonly found to date in the migration analyst's tool kit.

  20. Regional interdependence and migration in Asia.

    PubMed

    Kim, W B

    1995-01-01

    "The 1980s witnessed increasing regional interdependence in Asia through trade and investment. Increasing flows of labor within the region, however, raise questions about three important issues: (1) the assumption that trade, investment and aid will eventually mitigate migration pressure in source countries and the effectiveness of migration policies based on that assumption; (2) whether increasing regional interdependence stimulates or deters migration; [and] (3) the effect of rising interdependence on the political and international relations aspects of migration. As a partial attempt to address these questions, this article examines the regional pattern of economic interdependence by utilizing information concerning trade, investment and migration flows. The concept of interdependence/dependence is discussed within a political context, focusing on migration and policy responses to it. Observations are made on the implications for regional stability and development."