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Sample records for crystal field

  1. Crystal field and magnetic properties

    NASA Technical Reports Server (NTRS)

    Flood, D. J.

    1977-01-01

    Magnetization and magnetic susceptibility measurements have been made in the temperature range 1.3 to 4.2 K on powdered samples of ErH3. The susceptibility exhibits Curie-Weiss behavior from 4.2 to 2 K, and intercepts the negative temperature axis at theta = 1.05 + or - 0.05 K, indicating that the material is antiferromagnetic. The low field effective moment is 6.77 + or - 0.27 Bohr magnetons per ion. The magnetization exhibits a temperature independent contribution, the slope of which is (5 + or - 1.2) x 10 to the -6th Weber m/kg Tesla. The saturation moment is 3.84 + or - 1 - 0.15 Bohr magnetons per ion. The results can be qualitatively explained by the effects of crystal fields on the magnetic ions. No definitive assignment of a crystal field ground state can be given, nor can a clear choice between cubically or hexagonally symmetric crystal fields be made. For hexagonal symmetry, the first excited state is estimated to be 86 to 100 K above the ground state. For cubic symmetry, the splitting is on the order of 160 to 180 K.

  2. Lectures on Crystal Field Theory

    DTIC Science & Technology

    1982-11-01

    used to calculate the electric dipole transition probabilities using the theory of Judd (1962) and Ofelt (1962)o As of 1970, all these objectives had...metry higher than C1 or C•. (4) The calculation of transltion probabilities, Zeeman splitting factors, Judd - Ofelt intensity parameters, branching ratios...INTERACTIONS ..................................... 37 4.1 Phenomenological Theory of Crystal Fields ................ 37 4.1.1 Matrix Elements of H in J States

  3. Crystal fields of porphyrins and phthalocyanines

    NASA Astrophysics Data System (ADS)

    Johnson, P. S.; Boukahil, I.; Himpsel, F. J.; Kennedy, C.; Jersett, N.; Cook, P. L.; Garcia-Lastra, J. M.

    2014-03-01

    Polarization-dependent X-ray absorption spectroscopy at the N 1s and metal 2p edges is combined with density functional and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal (Mn, Fe, Co, Ni) phthalocyanines and octaethylporphyrins. Octaethyl porphyrins are observed to lie flat on Si with native oxide, while phthalocyanines lie on edge. Strong polarization dependence is found at all edges, which facilitates a unique determination of the crystal field parameters. Crystal field values from PBE density functional calculations provide helpful starting values, which are refined by fitting atomic multiplet calculations to the data. Since the crystal field affects electron-hole separation in solar cells, the systematic set of crystal field parameters obtained here can be useful for optimizing dyes for solar cells.

  4. Crystal growth under external electric fields

    SciTech Connect

    Uda, Satoshi; Koizumi, Haruhiko; Nozawa, Jun; Fujiwara, Kozo

    2014-10-06

    This is a review article concerning the crystal growth under external electric fields that has been studied in our lab for the past 10 years. An external field is applied electrostatically either through an electrically insulating phase or a direct injection of an electric current to the solid-interface-liquid. The former changes the chemical potential of both solid and liquid and controls the phase relationship while the latter modifies the transport and partitioning of ionic solutes in the oxide melt during crystallization and changes the solute distribution in the crystal.

  5. Crystal-Field Engineering of Solid-State Laser Materials

    NASA Astrophysics Data System (ADS)

    Henderson, Brian; Bartram, Ralph H.

    2005-08-01

    Preface; 1. An introduction to lasers; 2. Symmetry considerations; 3. Optical crystals: their structures, colours and growth; 4. Energy levels of ions in crystals; 5. Spectra of ions in crystals; 6. Radiationless transitions; 7. Energy transfer and excited state absorption; 8. Covalency; 9. Engineering the crystal field; 10. The crystal field engineered.

  6. Phase-field-crystal model for ordered crystals

    NASA Astrophysics Data System (ADS)

    Alster, Eli; Elder, K. R.; Hoyt, Jeffrey J.; Voorhees, Peter W.

    2017-02-01

    We describe a general method to model multicomponent ordered crystals using the phase-field-crystal (PFC) formalism. As a test case, a generic B2 compound is investigated. We are able to produce a line of either first-order or second-order order-disorder phase transitions, features that have not been incorporated in existing PFC approaches. Further, it is found that the only elastic constant for B2 that depends on ordering is C11. This B2 model is then used to study antiphase boundaries (APBs). The APBs are shown to reproduce classical mean-field results. Dynamical simulations of ordering across small-angle grain boundaries predict that dislocation cores pin the evolution of APBs.

  7. Crystal fields in UO2 - revisited

    SciTech Connect

    Nakotte, Heinz; Rajatram, R; Mcqueeney, R J; Lander, G H; Robinson, R A

    2009-01-01

    We performed inelastic neutron scattering (INS) in order to re-investigate the crystal-field ground state and the level splitting in UO{sub 2}. Previous INS studies on UO{sub 2} by Amorelli et al. [Physical Review B 15, 1989, 1856] uncovered four excitations at low temperatures in the 150-180 meV range. Considering the dipole-allowed transitions, only three of these transitions could be explained by the published crystal-field model. Our INS results on a different UO{sub 2} sample revealed that the unaccounted peak at about 180 meV is a spurious one, and thus not intrinsic to UO{sub 2}. In good agreement with Amoretti's results, we corroborated that the ground-state of UO{sub 2} is the {Lambda}{sub 5} triplet, and we computed that the fourth- and six-order crystal field parameters are V{sub 4} = -116 meV and V{sub 6} = 26 meV, respectively. We also studied the INS response of the non-magnetic U{sub 0.4}Th{sub 0.6}O{sub 2}. The splitting for this thorium-doped compound is similar to the one of UO{sub 2}, which orders antiferromagnetically at low temperatures. Therefore, we can conclude that magnetic interactions only weakly perturb the energy level splitting, which is dominated by strong crystal fields.

  8. Unifying the crystallization behavior of hexagonal and square crystals with the phase-field-crystal model

    NASA Astrophysics Data System (ADS)

    Tao, Yang; Zheng, Chen; Jing, Zhang; Yongxin, Wang; Yanli, Lu

    2016-03-01

    By employing the phase-field-crystal models, the atomic crystallization process of hexagonal and square crystals is investigated with the emphasis on the growth mechanism and morphological change. A unified regime describing the crystallization behavior of both crystals is obtained with the thermodynamic driving force varying. By increasing the driving force, both crystals (in the steady-state) transform from a faceted polygon to an apex-bulged polygon, and then into a symmetric dendrite. For the faceted polygon, the interface advances by a layer-by-layer (LL) mode while for the apex-bulged polygonal and the dendritic crystals, it first adopts the LL mode and then transits into the multi-layer (ML) mode in the later stage. In particular, a shift of the nucleation sites from the face center to the area around the crystal tips is detected in the early growth stage of both crystals and is rationalized in terms of the relation between the crystal size and the driving force distribution. Finally, a parameter characterizing the complex shape change of square crystal is introduced. Project supported by the National Natural Science Foundation of China (Grant Nos. 54175378, 51474176, and 51274167), the Natural Science Foundation of Shaanxi Province, China (Grant No. 2014JM7261), and the Doctoral Foundation Program of Ministry of China (Grant No. 20136102120021).

  9. Study on Crystallization Properties of Mold Flux in Magnetic Field

    NASA Astrophysics Data System (ADS)

    Zhang, Congjing; Wang, Yu; Hu, Lang; Zhu, Mingmei; Wang, Hongpo

    Magnetic field has a great effect on the crystallization behavior of mold flux and properties of the flux film between mold and strand, on which the surface quality of strand was deeply depended in continuous casting process. Therefore, studying the change law of the crystallization properties of mold flux in magnetic field is of great significant. In the present work, based on intensity of the applied magnetic field with the range from 0mT to 60mT, the crystallization ratio, crystal size and mineralogical phases of the flux film were discussed. The results show that crystallization ratio increases with the increasing magnetic field intensity, and the crystal size becomes bigger at the same time. The magnetic field promotes the crystallization ratio and growth speed of the crystallized grains of mold flux. However, magnetic field doesn't change types of the mineralogical phases.

  10. Ab Initio Crystal Field for Lanthanides.

    PubMed

    Ungur, Liviu; Chibotaru, Liviu F

    2017-03-13

    An ab initio methodology for the first-principle derivation of crystal-field (CF) parameters for lanthanides is described. The methodology is applied to the analysis of CF parameters in [Tb(Pc)2 ](-) (Pc=phthalocyanine) and Dy4 K2 ([Dy(4) K(2) O(OtBu)(12) ]) complexes, and compared with often used approximate and model descriptions. It is found that the application of geometry symmetrization, and the use of electrostatic point-charge and phenomenological CF models, lead to unacceptably large deviations from predictions based on ab initio calculations for experimental geometry. It is shown how the predictions of standard CASSCF (Complete Active Space Self-Consistent Field) calculations (with 4f orbitals in the active space) can be systematically improved by including effects of dynamical electronic correlation (CASPT2 step) and by admixing electronic configurations of the 5d shell. This is exemplified for the well-studied Er-trensal complex (H3 trensal=2,2',2"-tris(salicylideneimido)trimethylamine). The electrostatic contributions to CF parameters in this complex, calculated with true charge distributions in the ligands, yield less than half of the total CF splitting, thus pointing to the dominant role of covalent effects. This analysis allows the conclusion that ab initio crystal field is an essential tool for the decent description of lanthanides.

  11. Oxidation and crystal field effects in uranium

    SciTech Connect

    Tobin, J. G.; Booth, C. H.; Shuh, D. K.; van der Laan, G.; Sokaras, D.; Weng, T. -C.; Yu, S. W.; Bagus, P. S.; Tyliszczak, T.; Nordlund, D.

    2015-07-06

    An extensive investigation of oxidation in uranium has been pursued. This includes the utilization of soft x-ray absorption spectroscopy, hard x-ray absorption near-edge structure, resonant (hard) x-ray emission spectroscopy, cluster calculations, and a branching ratio analysis founded on atomic theory. The samples utilized were uranium dioxide (UO2), uranium trioxide (UO3), and uranium tetrafluoride (UF4). As a result, a discussion of the role of non-spherical perturbations, i.e., crystal or ligand field effects, will be presented.

  12. Protein Crystal Growth in Gels and Stationary Magnetic Fields

    SciTech Connect

    Moreno,A.; Quiroz-Garcia, B.; Yokaichiya, F.; Stojanoff, V.; Rudolph, P.

    2007-01-01

    Thaumatin, lysozyme, and ferritin single crystals were grown in solutions and gels without and with surrounding strong stationary magnetic fields. The crystal size, number and alignment in dependence on the induction force were analyzed. The crystal quality, like mosaicity, as function of the magnetic force is discussed by using synchrotron X-ray diffraction analysis.

  13. Crystal field in TmCu 2 compound

    NASA Astrophysics Data System (ADS)

    Zajac, Š.; Diviš, M.; Šíma, V.; Smetana, Z.

    1988-12-01

    The crystal field splitting in orthorhombic TmCu 2 compound has been determined by the analysis of the specific heat and thermal expansion measurements. The crystal field parameters of second-order are in agreement and the parameters of fourth- and sixth-order are significantly different from that calculated in the point-charge model.

  14. Exchange charge model of crystal field for 3d ions

    NASA Astrophysics Data System (ADS)

    Brik, M. G.; Avram, N. M.; Avram, C. N.

    In the second chapter of the book the authors present the results of theoretical studies ofthe energy levels schemes of all 3dn (n=1, 9) ionsin various crystals at the substitutionalsites. Systematic calculations are described in all details; they include the overlap integrals between the impurityions' and ligands' wave functions; the crystal field parameters calculations, and diagonalization of the crystal field Hamiltonians for each considered case. The calculated results arediscussed and compared with experimental data and with similar results from literature. The chapter also contains a comprehensive literature review on the properties of 3d-ions doped crystals.

  15. Effects of magnetic fields on dissolution of arthritis causing crystals

    NASA Astrophysics Data System (ADS)

    Takeuchi, Y.; Iwasaka, M.

    2015-05-01

    The number of gout patients has rapidly increased because of excess alcohol and salt intake. The agent responsible for gout is the monosodium urate (MSU) crystal. MSU crystals are found in blood and consist of uric acid and sodium. As a substitute for drug dosing or excessive water intake, physical stimulation by magnetic fields represents a new medical treatment for gout. In this study, we investigated the effects of a magnetic field on the dissolution of a MSU crystal suspension. The white MSU crystal suspension was dissolved in an alkaline solution. We measured the light transmission of the MSU crystal suspension by a transmitted light measuring system. The magnetic field was generated by a horizontal electromagnet (maximum field strength was 500 mT). The MSU crystal suspension that dissolved during the application of a magnetic field of 500 mT clearly had a higher dissolution rate when compared with the control sample. We postulate that the alkali solution promoted penetration upon diamagnetic rotation and this magnetic field orienting is because of the pronounced diamagnetic susceptibility anisotropy of the MSU crystal. The results indicate that magnetic fields represent an effective gout treatment approach.

  16. Enhancement of crystal homogeneity of protein crystals under application of an external alternating current electric field

    SciTech Connect

    Koizumi, H.; Uda, S.; Fujiwara, K.; Nozawa, J.; Tachibana, M.; Kojima, K.

    2014-10-06

    X-ray diffraction rocking-curve measurements were performed on tetragonal hen egg white (HEW) lysozyme crystals grown with and without the application of an external alternating current (AC) electric field. The crystal quality was assessed by the full width at half maximum (FWHM) value for each rocking curve. For two-dimensional maps of the FWHMs measured on the 440 and the 12 12 0 reflection, the crystal homogeneity was improved under application of an external electric field at 1 MHz, compared with that without. In particular, the significant improvement of the crystal homogeneity was observed for the 12 12 0 reflection.

  17. Crystal-Field Engineering of Solid-State Laser Materials

    NASA Astrophysics Data System (ADS)

    Henderson, Brian; Bartram, Ralph H.

    2005-08-01

    This book examines the underlying science and design of laser materials. It emphasizes the principles of crystal-field engineering and discusses the basic physical concepts that determine laser gain and nonlinear frequency conversion in optical crystals. Henderson and Bartram develop the predictive capabilities of crystal-field engineering to show how modification of the symmetry and composition of optical centers can improve laser performance. They also discuss applications of the principles of crystal-field engineering to a variety of optical crystals in relation to the performances of laser devices. This book will be of considerable interest to physical, chemical and material scientists and to engineers involved in the science and technology of solid state lasers.

  18. Crystal-Field Engineering of Solid-State Laser Materials

    NASA Astrophysics Data System (ADS)

    Henderson, Brian; Bartram, Ralph H.

    2000-07-01

    This book examines the underlying science and design of laser materials. It emphasizes the principles of crystal-field engineering and discusses the basic physical concepts that determine laser gain and nonlinear frequency conversion in optical crystals. Henderson and Bartram develop the predictive capabilities of crystal-field engineering to show how modification of the symmetry and composition of optical centers can improve laser performance. They also discuss applications of the principles of crystal-field engineering to a variety of optical crystals in relation to the performances of laser devices. This book will be of considerable interest to physical, chemical and material scientists and to engineers involved in the science and technology of solid state lasers.

  19. Photonic crystal fiber sensor for magnetic field detection

    NASA Astrophysics Data System (ADS)

    Quintero, Sully M. M.; Martelli, Cicero; Kato, Carla C.; Valente, Luiz C. G.; Braga, Arthur M. B.

    2010-09-01

    A magnetic field sensor comprised of a high birefringence photonic crystal fiber coated by a Terfenol-D/Epoxy composite layer is proposed. Magnetic fields induce strains in the magnetostrictive composite that are transferred to the fiber interfering with light propagation. The sensitivity of the developed sensor with magnetic fields is measured to be 6 pm mT-1.

  20. Eighth-order phase-field-crystal model for two-dimensional crystallization

    NASA Astrophysics Data System (ADS)

    Jaatinen, A.; Ala-Nissila, T.

    2010-12-01

    We present a derivation of the recently proposed eighth-order phase-field crystal model [A. Jaatinen , Phys. Rev. E 80, 031602 (2009)10.1103/PhysRevE.80.031602] for the crystallization of a solid from an undercooled melt. The model is used to study the planar growth of a two-dimensional hexagonal crystal, and the results are compared against similar results from dynamical density functional theory of Marconi and Tarazona, as well as other phase-field crystal models. We find that among the phase-field crystal models studied, the eighth-order fitting scheme gives results in good agreement with the density functional theory for both static and dynamic properties, suggesting it is an accurate and computationally efficient approximation to the density functional theory.

  1. Crystal field effects in TmCu2 compound

    NASA Astrophysics Data System (ADS)

    Zajac, Š.; Šíma, V.; Smetana, Z.

    1987-01-01

    The splitting of the3H6 multiplet has been estimated for the Tm3+ ion in the crystal electric field of the orthorhombic TmCu2 compound. Using the energy levels and appropriate eigenfunctions the crystal field only susceptibility has been calculated along the principal orthorhombic axes at temperatures 10 to 300 K. The obtained results are compared with our measurements of specific heat and paramagnetic susceptibility on polycrystalline sample.

  2. Field induced heliconical structure of cholesteric liquid crystal

    DOEpatents

    Lavrentovich, Oleg D.; Shiyanovsii, Sergij V.; Xiang, Jie; Kim, Young-Ki

    2017-06-27

    A diffraction grating comprises a liquid crystal (LC) cell configured to apply an electric field through a cholesteric LC material that induces the cholesteric LC material into a heliconical state with an oblique helicoid director. The applied electric field produces diffracted light from the cholesteric LC material within the visible, infrared or ultraviolet. The axis of the heliconical state is in the plane of the liquid crystal cell or perpendicular to the plane, depending on the application. A color tuning device operates with a similar heliconical state liquid crystal material but with the heliconical director axis oriented perpendicular to the plane of the cell. A power generator varies the strength of the applied electric field to adjust the wavelength of light reflected from the cholesteric liquid crystal material within the visible, infrared or ultraviolet.

  3. Fiber field-effect device via in situ channel crystallization.

    PubMed

    Danto, Sylvain; Sorin, Fabien; Orf, Nicholas D; Wang, Zheng; Speakman, Scott A; Joannopoulos, John D; Fink, Yoel

    2010-10-01

    The in situ crystallization of the incorporated amorphous semiconductor within the multimaterial fiber device yields a large decrease in defect density and a concomitant five-order-of-magnitude decrease in resistivity of the novel metal-insulator-crystalline semiconductor structure. Using a post-drawing crystallization process, the first tens-of-meters-long single-fiber field-effect device is demonstrated. This work opens significant opportunities for incorporating higher functionality in functional fibers and fabrics.

  4. Magnetic Fields and the Crystallization of White Dwarfs

    NASA Astrophysics Data System (ADS)

    Isern, J.; García-Berro, E.; Külebi, B.; Lorén-Aguilar, P.

    2017-03-01

    The evolution of white dwarfs can be described as a cooling process. When the temperature is low enough, the interior experiences a phase transition and crystallizes. Crystallization introduces two new sources of energy, latent heat and chemical sedimentation, and induces the formation of a convective mantle around the solid core. This structure, which is analogous to that of the Earth, could induce the formation of a magnetic field via dynamo mechanism. In this work we discuss the viability of such mechanism, and its use as a diagnostic tool of crystallization.

  5. Crystal field spectra of lunar pyroxenes.

    NASA Technical Reports Server (NTRS)

    Burns, R. G.; Abu-Eid, R. M.; Huggins, F. E.

    1972-01-01

    Absorption spectra in the visible and near infrared regions have been obtained for pyroxene single crystals in rocks from the Apollo 11, 12, 14, and 15 missions. The polarized spectra are compared with those obtained from terrestrial calcic clinopyroxenes, subcalcic augites, pigeonites, and orthopyroxenes. The lunar pyroxenes contain several broad, intense absorption bands in the near infrared, the positions of which are related to bulk composition, Fe(2+) site occupancy and structure type of the pyroxene. The visible spectra contain several sharp, weak peaks mainly due to spin-forbidden transitions in Fe(2+). Additional weak bands in this region in Apollo 11 pyroxenes are attributed to Ti(3+) ions. Spectral features from Fe(3+), Mn(2+), Cr(3+), and Cr(2+) were not observed.

  6. Crystal field strength and spin-orbit interaction of Er3+- ground level in crystals

    NASA Astrophysics Data System (ADS)

    Petrov, Dimitar N.

    2017-08-01

    The matrix elements A of spin-orbit interaction (SOI) for the ground level 4I15/2 of Er3+ in crystals have been calculated and found compatible in magnitude with the difference to the first excited level 4I13/2. The dependence of the ratios A(cr)/A(fr), where cr and fr denote ion in crystal and free ion, respectively, on the scalar crystal field parameter Nv has revealed an effect similar to the nephelauxetic effect relative to the normalized limit of the free ion value. The variations with Nv of the maximum splitting of the ground level and the spin-orbit coupling constant for Er3+-containing crystals have been discussed. The examples comprise about 50 stoichiometric or doped halogenides, simple and complex oxides, oxohalides, complexes with organic ligands, and semiconductors, some of which include different site symmetries of Er3+ in a crystal.

  7. Magnetic Field Applications in Semiconductor Crystal Growth and Metallurgy

    NASA Technical Reports Server (NTRS)

    Mazuruk, Konstantin; Ramachandran, Narayanan; Grugel, Richard; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    The Traveling Magnetic Field (TMF) technique, recently proposed to control meridional flow in electrically conducting melts, is reviewed. In particular, the natural convection damping capability of this technique has been numerically demonstrated with the implication of significantly improving crystal quality. Advantages of the traveling magnetic field, in comparison to the more mature rotating magnetic field method, are discussed. Finally, results of experiments with mixing metallic alloys in long ampoules using TMF is presented

  8. Relaxation Dynamics of Ferroelectric Liquid Crystals in Pulsed Electric Field

    NASA Astrophysics Data System (ADS)

    Kudreyko, A. A.; Migranov, N. G.; Migranova, D. N.

    2016-11-01

    In this contribution we report a theoretical study of relaxation processes in surface-stabilized ferroelectric liquid crystals with spontaneous polarization. The influence of pulsed electric field on the behavior of ferroelectric liquid crystal in the SmC* phase, which is placed in a thin cell with strong anchoring of SmC* molecules with the boundary substrate, is studied. In the vicinity of the substrate interface, temporal dependence of the azimuthal motion of the director induced by electric field is obtained. The response to the external distortion of ferroelectric liquid crystal confined between two microstructured substrates is the occurrence of periodic temporal formation of solitons connected with the distortion of the director field n in the sample bulk. The interplay between microstructured substrates and director distribution of the ferroelectric SmC* phase is explained by the Frenkel-Kontorova model for a chain of atoms, but adapted for the continuum problem.

  9. Physical modelling of Czochralski crystal growth in horizontal magnetic field

    NASA Astrophysics Data System (ADS)

    Grants, Ilmārs; Pal, Josef; Gerbeth, Gunter

    2017-07-01

    This study addresses experimentally the heat transfer, the temperature azimuthal non-uniformity and the onset of oscillations in a low temperature physical model of a medium-sized Czochralski crystal growth process with a strong horizontal magnetic field (HMF). It is observed that under certain conditions the integral heat flux may decrease with increasing magnetic field strength at the same time as the flow velocity increases. The azimuthal non-uniformity of the temperature field in the melt near the crystal model rim is only little influenced by its rotation rate outside of a narrow range where the centrifugal force balances the buoyant one. The flow oscillation onset has been observed for two values of the HMF strength. Conditions of this onset are little influenced by the crystal rotation. The critical temperature difference of the oscillation onset considerably exceeds that of the Rayleigh-Bénard (RB) cell in a strong HMF.

  10. Temperature dependence of crystal field excitations in CuO.

    PubMed

    Huotari, S; Simonelli, L; Sahle, C J; Sala, M Moretti; Verbeni, R; Monaco, G

    2014-04-23

    We report a study on the temperature dependence of charge-neutral crystal field (dd) excitations in cupric oxide, using nonresonant inelastic x-ray scattering spectroscopy. Thanks to a very high-energy resolution (ΔE = 60 meV), we observe thermal effects on the dd excitation spectrum fine structure between temperatures of 10-320 K. The spectra broaden considerably with increasing temperature, consistently with an enhancement of the coupling between crystal field excitations and the temperature-dependent continuum of states above the band gap. We discuss this and other mechanisms that may explain this temperature dependence.

  11. Electromagnetic Field Effects in Semiconductor Crystal Growth

    NASA Technical Reports Server (NTRS)

    Dulikravich, George S.

    1996-01-01

    This proposed two-year research project was to involve development of an analytical model, a numerical algorithm for its integration, and a software for the analysis of a solidification process under the influence of electric and magnetic fields in microgravity. Due to the complexity of the analytical model that was developed and its boundary conditions, only a preliminary version of the numerical algorithm was developed while the development of the software package was not completed.

  12. Electromagnetic field patterning or crystal light

    NASA Astrophysics Data System (ADS)

    Słupski, Piotr; Wymysłowski, Artur; Czarczyński, Wojciech

    2016-12-01

    Using the orbital angular momentum of light for the development of a vortex interferometer, the underlying physics requires microwave/RF models,1 as well as quantum mechanics for light1, 2 and fluid flow for semiconductor devices.3, 4 The combination of the aforementioned physical models yields simulations and results such as optical lattices,1 or an Inverse Farday effect.5 The latter is explained as the absorption of optical angular momentum, generating extremely high instantenous magnetic fields due to radiation friction. An algorithmic reduction across the computational methods used in microwaves, lasers, quantum optics and holography is performed in order to explain electromagnetic field interactions in a single computational framework. This work presents a computational model for photon-electron interactions, being a simplified gauge theory described using differentials or disturbances (photons) instead of integrals or fields. The model is based on treating the Z-axis variables as a Laplace fluid with spatial harmonics, and the XY plane as Maxwell's equations on boundaries. The result is a unified, coherent, graphical computational method of describing the photon qualitatively, quantitatively and with proportion. The model relies on five variables and is described using two equations, which use emitted power, cavity wavelength, input frequency, phase and time. Phase is treated as a rotated physical dimension under gauge theory of Feynmann's QED. In essence, this model allows the electromagnetic field to be treated with it's specific crystallography. The model itself is described in Python programming language. PACS 42.50.Pq, 31.30.J-, 03.70.+k, 11.10.-z, 67.10.Hk

  13. Single crystal growth of organic semiconductors for field effect applications

    NASA Astrophysics Data System (ADS)

    Kloc, Christian

    2006-08-01

    Organic semiconductors attract considerable attention due to promising applications in organic light emitting diodes, field effect transistors, and organic solar cells. Moreover, solubility of some organic semiconductors in organic solvents favors them for printed large area OLED displays and inexpensive printed microelectronics. However, low mobility of carriers in organic semiconductors limits usability of organic semiconductors in integrated circuits and need to be overcome. For this reason, the knowledge of intrinsic properties achievable in very pure and perfect crystals is important. Therefore, we have carried out a program to grow high quality single crystals of organics. Solution growth, melt growth, solvothermal method and vapor transport crystal growth have been applied and will be reported. For research purpose, using a gas phase transport method, we have produced millimeter - sized crystals of numerous organic semiconductors with higher quality and purity. Structure quality has been evaluated by x-ray topography methods. Field effect transistors have been prepared on surfaces of single crystals. Some of organic semiconductors like rubrene, pentacene, copper phthalocyanine exhibit carrier mobilities comparable or even higher than amorphous silicon. However, characterization of starting materials, crystals, thin films and resulting devices remains the crucial issue. The relation between organic semiconductor properties, used device fabrication technologies and resulting device characteristics is the object of presented here studies.

  14. Crystallization of Calcium Carbonate in a Large Scale Field Study

    NASA Astrophysics Data System (ADS)

    Ueckert, Martina; Wismeth, Carina; Baumann, Thomas

    2017-04-01

    The long term efficiency of geothermal facilities and aquifer thermal energy storage in the carbonaceous Malm aquifer in the Bavarian Molasse Basin is seriously affected by precipitations of carbonates. This is mainly caused by pressure and temperature changes leading to oversaturation during production. Crystallization starts with polymorphic nuclei of calcium carbonate and is often described as diffusion-reaction controlled. Here, calcite crystallization is favoured by high concentration gradients while aragonite crystallization is occurring at high reaction rates. The factors affecting the crystallization processes have been described for simplified, well controlled laboratory experiments, the knowledge about the behaviour in more complex natural systems is still limited. The crystallization process of the polymorphic forms of calcium carbonate were investigated during a heat storage test at our test site in the eastern part of the Bavarian Molasse Basin. Complementary laboratory experiments in an autoclave were run. Both, field and laboratory experiments were conducted with carbonaceous tap water. Within the laboratory experiments additionally ultra pure water was used. To avoid precipitations of the tap water, a calculated amount of {CO_2} was added prior to heating the water from 45 - 110°C (laboratory) resp. 65 - 110°C (field). A total water volume of 0.5 L (laboratory) resp. 1 L (field) was immediately sampled and filtrated through 10 - 0.1

  15. Bridgman Growth of Germanium Crystals in a Rotating Magnetic Field

    NASA Technical Reports Server (NTRS)

    Volz, M. P.; Schweizer, M.; Cobb, S. D.; Walker, J. S.; Szofran, F. R.; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    A series of (100)-oriented gallium-doped germanium crystals have been grown by the Bridgman method and under the influence of a rotating magnetic field (RMF). The RMF has a marked affect on the interface shape, changing it from concave to nearly flat. The onset of time-dependent flow instabilities occurs when the critical magnetic Taylor number is exceeded, and this can be observed by noting the appearance of striations in the grown crystals. The critical magnetic Taylor number is a sensitive function of the aspect ratio and, as the crystal grows under a constant applied magnetic field, the induced striations change from nonperiodic to periodic, undergo a period-doubling transition, and then cease to exist. Also, by pulsing the RMF on and off, it is shown that intentional interface demarcations can be introduced.

  16. Tailor-made force fields for crystal-structure prediction.

    PubMed

    Neumann, Marcus A

    2008-08-14

    A general procedure is presented to derive a complete set of force-field parameters for flexible molecules in the crystalline state on a case-by-case basis. The force-field parameters are fitted to the electrostatic potential as well as to accurate energies and forces generated by means of a hybrid method that combines solid-state density functional theory (DFT) calculations with an empirical van der Waals correction. All DFT calculations are carried out with the VASP program. The mathematical structure of the force field, the generation of reference data, the choice of the figure of merit, the optimization algorithm, and the parameter-refinement strategy are discussed in detail. The approach is applied to cyclohexane-1,4-dione, a small flexible ring. The tailor-made force field obtained for cyclohexane-1,4-dione is used to search for low-energy crystal packings in all 230 space groups with one molecule per asymmetric unit, and the most stable crystal structures are reoptimized in a second step with the hybrid method. The experimental crystal structure is found as the most stable predicted crystal structure both with the tailor-made force field and the hybrid method. The same methodology has also been applied successfully to the four compounds of the fourth CCDC blind test on crystal-structure prediction. For the five aforementioned compounds, the root-mean-square deviations between lattice energies calculated with the tailor-made force fields and the hybrid method range from 0.024 to 0.053 kcal/mol per atom around an average value of 0.034 kcal/mol per atom.

  17. Density functional theory of the crystal field in dioxides

    NASA Astrophysics Data System (ADS)

    Diviš, M.; Kuriplach, J.; Richter, M.; Steinbeck, L.

    1996-04-01

    Presented are the results of ab-initio density functional calculations for PrO2 and UO2 using the general potential LAPW and optimized LCAO method in the local density approximation. The crystal field splitting of ionic Pr4+ and U4+ ground states was calculated and compared with predictions of a superposition model.

  18. Liquid crystal infiltrated photonic crystal fibers for electric field intensity measurements.

    PubMed

    Mathews, Sunish; Farrell, Gerald; Semenova, Yuliya

    2011-06-10

    The application of nematic liquid crystal infiltrated photonic crystal fiber as a sensor for electric field intensity measurement is demonstrated. The device is based on an intrinsic sensing mechanism for electric fields. The sensor probe, which consists of a 1  cm infiltrated section of photonic crystal fiber with a lateral size of ∼125  μm, is very compact with small size and weight. A simple all-fiber design for the sensor is employed in an intensity based measurement scheme. The transmitted and reflected power of the infiltrated photonic crystal fiber is shown to have a linear response with the applied electric field. The sensor is operated in the telecommunication window at 1550  nm. The temperature dependence of the device at this operating wavelength is also experimentally studied and discussed. These structures can be used to accurately measure electric field intensity and can be used for the fabrication of all-fiber sensors for high electric field environments as both an in-line and reflective type point sensor.

  19. Reflective liquid crystal light valve with hybrid field effect mode

    NASA Technical Reports Server (NTRS)

    Boswell, Donald D. (Inventor); Grinberg, Jan (Inventor); Jacobson, Alexander D. (Inventor); Myer, Gary D. (Inventor)

    1977-01-01

    There is disclosed a high performance reflective mode liquid crystal light valve suitable for general image processing and projection and particularly suited for application to real-time coherent optical data processing. A preferred example of the device uses a CdS photoconductor, a CdTe light absorbing layer, a dielectric mirror, and a liquid crystal layer sandwiched between indium-tin-oxide transparent electrodes deposited on optical quality glass flats. The non-coherent light image is directed onto the photoconductor; this reduces the impedance of the photoconductor, thereby switching the AC voltage that is impressed across the electrodes onto the liquid crystal to activate the device. The liquid crystal is operated in a hybrid field effect mode. It utilizes the twisted nematic effect to create a dark off-state (voltage off the liquid crystal) and the optical birefringence effect to create the bright on-state. The liquid crystal thus modulates the polarization of the coherent read-out or projection light responsively to the non-coherent image. An analyzer is used to create an intensity modulated output beam.

  20. Field stability of piezoelectric shear properties in PIN-PMN-PT crystals under large drive field.

    PubMed

    Zhang, Shujun; Li, Fei; Luo, Jun; Xia, Ru; Hackenberger, Wesley; Shrout, Thomas

    2011-02-01

    The coercive fields (E(C)) of Pb(In₀.₅Nb₀.₅)O₃-Pb(Mg(¹/₃)Nb(²/₃)O₃-PbTiO₃ (PIN-PMN-PT) ternary single crystals were found to be 5 kV/cm, double the value of binary Pb(Mg(¹/₃)Nb(²/₃)O₃-PbTiO₃ (PMNT) crystals, further increased to 6 to 9 kV/cm using Mn modifications. In addition to an increased EC, the acceptor modification resulted in the developed internal bias (E(int)), on the order of ~1 kV/cm. The piezoelectric shear properties of unmodified and Mn-modified PIN-PMN-PT crystals with various domain configurations were investigated. The shear piezoelectric coefficients and electromechanical coupling factors for different domain configurations were found to be >2000 pC/N and >0.85, respectively, with slightly reduced properties observed in Mn-modified tetragonal crystals. Fatigue/cycling tests performed on shearmode samples as a function of ac drive field level demonstrated that the allowable ac field levels (the maximum applied ac field before the occurrence of depolarization) were only ~2 kV/cm for unmodified crystals, less than half of their coercive field. Allowable ac drive levels were on the order of 4 to 6 kV/cm for Mn-modified crystals with rhombohedral/orthorhombic phase, further increased to 5 to 8 kV/cm in tetragonal crystals, because of their higher coercive fields. It is of particular interest that the allowable ac drive field level for Mn-modified crystals was found to be ≥ 60% of their coercive fields, because of the developed E(int), induced by the acceptor-oxygen vacancy defect dipoles.

  1. Field Stability of Piezoelectric Shear Properties in PIN-PMN-PT Crystals Under Large Drive Field

    PubMed Central

    Zhang, Shujun; Li, Fei; Luo, Jun; Xia, Ru; Hackenberger, Wesley; Shrout, Thomas R.

    2013-01-01

    The coercive fields (EC) of Pb(In0.5Nb0.5)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) ternary single crystals were found to be 5 kV/cm, double the value of binary Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMNT) crystals, further increased to 6 to 9 kV/cm using Mn modifications. In addition to an increased EC, the acceptor modification resulted in the developed internal bias (Eint), on the order of ~1 kV/cm. The piezoelectric shear properties of unmodified and Mn-modified PIN-PMN-PT crystals with various domain configurations were investigated. The shear piezoelectric coefficients and electromechanical coupling factors for different domain configurations were found to be >2000 pC/N and >0.85, respectively, with slightly reduced properties observed in Mn-modified tetragonal crystals. Fatigue/cycling tests performed on shear-mode samples as a function of ac drive field level demonstrated that the allowable ac field levels (the maximum applied ac field before the occurrence of depolarization) were only ~2 kV/cm for unmodified crystals, less than half of their coercive field. Allowable ac drive levels were on the order of 4 to 6 kV/cm for Mn-modified crystals with rhombohedral/orthorhombic phase, further increased to 5 to 8 kV/cm in tetragonal crystals, because of their higher coercive fields. It is of particular interest that the allowable ac drive field level for Mn-modified crystals was found to be ≥60% of their coercive fields, because of the developed Eint, induced by the acceptor-oxygen vacancy defect dipoles. PMID:21342812

  2. TOPICAL REVIEW: Organic field-effect transistors using single crystals

    NASA Astrophysics Data System (ADS)

    Hasegawa, Tatsuo; Takeya, Jun

    2009-04-01

    Organic field-effect transistors using small-molecule organic single crystals are developed to investigate fundamental aspects of organic thin-film transistors that have been widely studied for possible future markets for 'plastic electronics'. In reviewing the physics and chemistry of single-crystal organic field-effect transistors (SC-OFETs), the nature of intrinsic charge dynamics is elucidated for the carriers induced at the single crystal surfaces of molecular semiconductors. Materials for SC-OFETs are first reviewed with descriptions of the fabrication methods and the field-effect characteristics. In particular, a benchmark carrier mobility of 20-40 cm2 Vs-1, achieved with thin platelets of rubrene single crystals, demonstrates the significance of the SC-OFETs and clarifies material limitations for organic devices. In the latter part of this review, we discuss the physics of microscopic charge transport by using SC-OFETs at metal/semiconductor contacts and along semiconductor/insulator interfaces. Most importantly, Hall effect and electron spin resonance (ESR) measurements reveal that interface charge transport in molecular semiconductors is properly described in terms of band transport and localization by charge traps.

  3. Organic field-effect transistors using single crystals.

    PubMed

    Hasegawa, Tatsuo; Takeya, Jun

    2009-04-01

    Organic field-effect transistors using small-molecule organic single crystals are developed to investigate fundamental aspects of organic thin-film transistors that have been widely studied for possible future markets for 'plastic electronics'. In reviewing the physics and chemistry of single-crystal organic field-effect transistors (SC-OFETs), the nature of intrinsic charge dynamics is elucidated for the carriers induced at the single crystal surfaces of molecular semiconductors. Materials for SC-OFETs are first reviewed with descriptions of the fabrication methods and the field-effect characteristics. In particular, a benchmark carrier mobility of 20-40 cm(2) Vs(-1), achieved with thin platelets of rubrene single crystals, demonstrates the significance of the SC-OFETs and clarifies material limitations for organic devices. In the latter part of this review, we discuss the physics of microscopic charge transport by using SC-OFETs at metal/semiconductor contacts and along semiconductor/insulator interfaces. Most importantly, Hall effect and electron spin resonance (ESR) measurements reveal that interface charge transport in molecular semiconductors is properly described in terms of band transport and localization by charge traps.

  4. Organic field-effect transistors using single crystals

    PubMed Central

    Hasegawa, Tatsuo; Takeya, Jun

    2009-01-01

    Organic field-effect transistors using small-molecule organic single crystals are developed to investigate fundamental aspects of organic thin-film transistors that have been widely studied for possible future markets for ‘plastic electronics’. In reviewing the physics and chemistry of single-crystal organic field-effect transistors (SC-OFETs), the nature of intrinsic charge dynamics is elucidated for the carriers induced at the single crystal surfaces of molecular semiconductors. Materials for SC-OFETs are first reviewed with descriptions of the fabrication methods and the field-effect characteristics. In particular, a benchmark carrier mobility of 20–40 cm2 Vs−1, achieved with thin platelets of rubrene single crystals, demonstrates the significance of the SC-OFETs and clarifies material limitations for organic devices. In the latter part of this review, we discuss the physics of microscopic charge transport by using SC-OFETs at metal/semiconductor contacts and along semiconductor/insulator interfaces. Most importantly, Hall effect and electron spin resonance (ESR) measurements reveal that interface charge transport in molecular semiconductors is properly described in terms of band transport and localization by charge traps. PMID:27877287

  5. Far-field coupling in nanobeam photonic crystal cavities

    SciTech Connect

    Rousseau, Ian Sánchez-Arribas, Irene; Carlin, Jean-François; Butté, Raphaël; Grandjean, Nicolas

    2016-05-16

    We optimized the far-field emission pattern of one-dimensional photonic crystal nanobeams by modulating the nanobeam width, forming a sidewall Bragg cross-grating far-field coupler. By setting the period of the cross-grating to twice the photonic crystal period, we showed using three-dimensional finite-difference time-domain simulations that the intensity extracted to the far-field could be improved by more than three orders of magnitude compared to the unmodified ideal cavity geometry. We then experimentally studied the evolution of the quality factor and far-field intensity as a function of cross-grating coupler amplitude. High quality factor (>4000) blue (λ = 455 nm) nanobeam photonic crystals were fabricated out of GaN thin films on silicon incorporating a single InGaN quantum well gain medium. Micro-photoluminescence spectroscopy of sets of twelve identical nanobeams revealed a nine-fold average increase in integrated far-field emission intensity and no change in average quality factor for the optimized structure compared to the unmodulated reference. These results are useful for research environments and future nanophotonic light-emitting applications where vertical in- and out-coupling of light to nanocavities is required.

  6. Far-field coupling in nanobeam photonic crystal cavities

    NASA Astrophysics Data System (ADS)

    Rousseau, Ian; Sánchez-Arribas, Irene; Carlin, Jean-François; Butté, Raphaël; Grandjean, Nicolas

    2016-05-01

    We optimized the far-field emission pattern of one-dimensional photonic crystal nanobeams by modulating the nanobeam width, forming a sidewall Bragg cross-grating far-field coupler. By setting the period of the cross-grating to twice the photonic crystal period, we showed using three-dimensional finite-difference time-domain simulations that the intensity extracted to the far-field could be improved by more than three orders of magnitude compared to the unmodified ideal cavity geometry. We then experimentally studied the evolution of the quality factor and far-field intensity as a function of cross-grating coupler amplitude. High quality factor (>4000) blue (λ = 455 nm) nanobeam photonic crystals were fabricated out of GaN thin films on silicon incorporating a single InGaN quantum well gain medium. Micro-photoluminescence spectroscopy of sets of twelve identical nanobeams revealed a nine-fold average increase in integrated far-field emission intensity and no change in average quality factor for the optimized structure compared to the unmodulated reference. These results are useful for research environments and future nanophotonic light-emitting applications where vertical in- and out-coupling of light to nanocavities is required.

  7. Horizontal well taps bypassed Dundee oil in Crystal field, Mich.

    SciTech Connect

    Wood, J.R.; Allan, J.R.; Huntoon, J.E.; Pennington, W.D.; Harrison, W.B. III; Taylor, E.; Tester, C.J.

    1996-10-21

    The Dundee formation (Middle Devonian) has yielded more oil than any other producing interval in Michigan. The Dundee trend, which forms an east-west band across the central Michigan basin, consists of 137 fields which together have yielded more than 350 million bbl of oil. The first commercial Dundee production was established at Mt. Pleasant field in 1928, and most Dundee fields were discovered and brought on production during the 1930s--40s. Wells in many of the fields had very high initial production (IP) rates. IPs in excess of 1,000 b/d of oil were common, with values as high as 9,000 b/d reported. These high flow rates, combined with a thin (10--30 ft) oil column and a strong water drive, resulted in water coning that left significant volumes of oil unrecovered in some fields. One such field, Crystal field in Montcalm County, is the focus of a US Department of energy (DOE) Class 2 Reservoir Demonstration Project designed to demonstrate that horizontal drilling can recover significant volumes of this bypassed oil. The paper describes the demonstration project, regional setting, and the history of the Crystal field.

  8. Generation of Unprecedented high Electric Fields with Pyroelectric Crystals

    NASA Astrophysics Data System (ADS)

    Crimi, Sarah; Tornow, Werner; Corse, Zach

    2009-10-01

    Since a few years pyroelectric crystals in a deuterium gas environment have been used to produce neutrons via the ^2H(d,n)^3He reaction. The figure-of-merit for neutron production in the energy region of interest is about IE^3/2, where I is the deuterium ion current and E is the associated ion energy. Therefore, it is important to maximize E. Using single and double crystal arrangements with electric field enhancing nano-tips, the highest positive potentials reported in the literature were 115 keV [1] and 250 keV [2], respectively. Using longer LiTaO3 crystals than commonly employed (2.5 cm versus 1.0 cm) and without attaching a nano-tip, we have produced positive deuterium ion beams of energies up to 325 keV with a single crystal during the cooling phase from 130 ^oC to 0 ^oC. In a double crystal arrangement we have obtained positive ion energies of up to 390 keV. Details of our experimental approach will be presented.[4pt] [1] B. Naranjo et al., Nature 434, 1115 (2005).[0pt] [2] D. Gillich et al., Nucl. Instr. Meth. in Phys. Res. A 602, 306 (2009).

  9. Crystal field and magnetic properties of ErH3

    NASA Technical Reports Server (NTRS)

    Flood, D. J.

    1977-01-01

    Magnetization and magnetic susceptibility measurements have been made in the temperature range 1.3 to 4.2 K on powdered samples of ErH3. The susceptibility exhibits Curie-Weiss behavior from 4.2 to 2 K, and intercepts the negative temperature axis at 1.05 + or - 0.05 K, indicating that the material is antiferromagnetic. The low field effective moment is 6.77 + or - 0.27 Bohr magnetons per ion. The magnetization exhibits a temperature independent contribution, the slope of which is (5 + or - 1.2) times 10 to the minus 6 Weber m/kg Tesla. The saturation moment is 3.84 + or - 0.15 Bohr magnetons per ion. The results can be qualitatively explained by the effects of crystal fields on the magnetic ions. No definitive assignment of a crystal field ground state can be given, nor can a clear choice between cubically or hexagonally symmetric crystal fields be made. For hexagonal symmetry, the first excited state is estimated to be 86 to 100 K above the ground state. For cubic symmetry, the splitting is of the order of 160 to 180 K.

  10. Pulsed zero field NMR of solids and liquid crystals

    SciTech Connect

    Thayer, A.M.

    1987-02-01

    This work describes the development and applications to solids and liquid crystals of zero field nuclear magnetic resonance (NMR) experiments with pulsed dc magnetic fields. Zero field NMR experiments are one approach for obtaining high resolution spectra of amorphous and polycrystalline materials which normally (in high field) display broad featureless spectra. The behavior of the spin system can be coherently manipulated and probed in zero field with dc magnetic field pulses which are employed in a similar manner to radiofrequency pulses in high field NMR experiments. Nematic phases of liquid crystalline systems are studied in order to observe the effects of the removal of an applied magnetic field on sample alignment and molecular order parameters. In nematic phases with positive and negative magnetic susceptibility anisotropies, a comparison between the forms of the spin interactions in high and low fields is made. High resolution zero field NMR spectra of unaligned smectic samples are also obtained and reflect the symmetry of the liquid crystalline environment. These experiments are a sensitive measure of the motionally induced asymmetry in biaxial phases. Homonuclear and heteronuclear solute spin systems are compared in the nematic and smectic phases. Nonaxially symmetric dipolar couplings are reported for several systems. The effects of residual fields in the presence of a non-zero asymmetry parameter are discussed theoretically and presented experimentally. Computer programs for simulations of these and other experimental results are also reported. 179 refs., 75 figs.

  11. Magnetic field measurements based on Terfenol coated photonic crystal fibers.

    PubMed

    Quintero, Sully M M; Martelli, Cicero; Braga, Arthur M B; Valente, Luiz C G; Kato, Carla C

    2011-01-01

    A magnetic field sensor based on the integration of a high birefringence photonic crystal fiber and a composite material made of Terfenol particles and an epoxy resin is proposed. An in-fiber modal interferometer is assembled by evenly exciting both eigenemodes of the HiBi fiber. Changes in the cavity length as well as the effective refractive index are induced by exposing the sensor head to magnetic fields. The magnetic field sensor has a sensitivity of 0.006 (nm/mT) over a range from 0 to 300 mT with a resolution about ±1 mT. A fiber Bragg grating magnetic field sensor is also fabricated and employed to characterize the response of Terfenol composite to the magnetic field.

  12. Field emission properties of single crystal chromium disilicide nanowires

    SciTech Connect

    Valentin, L. A.; Carpena-Nunez, J.; Yang, D.; Fonseca, L. F.

    2013-01-07

    The composition, crystal structure, and field emission properties of high-crystallinity chromium disilicide (CrSi{sub 2}) nanowires synthesized by a vapor deposition method have been studied. High resolution transmission electron microscopy, energy dispersive spectroscopy, and selected area electron diffraction studies confirm the single-crystalline structure and composition of the CrSi{sub 2} nanowires. Field emission measurements show that an emission current density of 0.1 {mu}A/cm{sup 2} was obtained at a turn-on electric field intensity of 2.80 V/{mu}m. The maximum emission current measured was 1.86 mA/cm{sup 2} at 3.6 V/{mu}m. The relation between the emission current density and the electric field obtained follows the Fowler-Nordheim equation, with an enhancement coefficient of 1140. The electrical conductivity of single nanowires was measured by using four-point-probe specialized microdevices at different temperatures, and the calculated values are close to those reported in previous studies for highly conductive single crystal bulk CrSi{sub 2}. The thermal tolerance of the nanowires was studied up to a temperature of 1100 Degree-Sign C. The stability of the field emission current, the I-E values, their thermal tolerance, and high electrical conductivity make CrSi{sub 2} nanowires a promising material for field emission applications.

  13. Low-frequency electromagnetic field in a Wigner crystal

    SciTech Connect

    Stupka, Anton

    2013-03-15

    Long-wave low-frequency oscillations are described in a Wigner crystal by generalization of the reverse continuum model for the case of electronic lattice. The internal self-consistent long-wave electromagnetic field is used to describe the collective motions in the system. The eigenvectors and eigenvalues of the obtained system of equations are derived. The velocities of longitudinal and transversal sound waves are found.

  14. Semiconductor Crystal Growth in Static and Rotating Magnetic fields

    NASA Technical Reports Server (NTRS)

    Volz, Martin

    2004-01-01

    Magnetic fields have been applied during the growth of bulk semiconductor crystals to control the convective flow behavior of the melt. A static magnetic field established Lorentz forces which tend to reduce the convective intensity in the melt. At sufficiently high magnetic field strengths, a boundary layer is established ahead of the solid-liquid interface where mass transport is dominated by diffusion. This can have a significant effect on segregation behavior and can eliminate striations in grown crystals resulting from convective instabilities. Experiments on dilute (Ge:Ga) and solid solution (Ge-Si) semiconductor systems show a transition from a completely mixed convective state to a diffusion-controlled state between 0 and 5 Tesla. In HgCdTe, radial segregation approached the diffusion limited regime and the curvature of the solid-liquid interface was reduced by a factor of 3 during growth in magnetic fields in excess of 0.5 Tesla. Convection can also be controlled during growth at reduced gravitational levels. However, the direction of the residual steady-state acceleration vector can compromise this effect if it cannot be controlled. A magnetic field in reduced gravity can suppress disturbances caused by residual transverse accelerations and by random non-steady accelerations. Indeed, a joint program between NASA and the NHMFL resulted in the construction of a prototype spaceflight magnet for crystal growth applications. An alternative to the suppression of convection by static magnetic fields and reduced gravity is the imposition of controlled steady flow generated by rotating magnetic fields (RMF)'s. The potential benefits of an RMF include homogenization of the melt temperature and concentration distribution, and control of the solid-liquid interface shape. Adjusting the strength and frequency of the applied magnetic field allows tailoring of the resultant flow field. A limitation of RMF's is that they introduce deleterious instabilities above a

  15. Subsurface Stress Fields In Single Crystal (Anisotropic) Contacts

    NASA Technical Reports Server (NTRS)

    Arakere, Nagaraj K.; Knudsen, Erik C.; Duke, Greg; Battista, Gilda; Swanson, Greg

    2004-01-01

    Single crystal superalloy turbine blades used in high pressure turbomachinery are subject to conditions of high temperature, triaxial steady and alternating stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent HCF failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes, and noncrystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single crystal turbine blades. This paper presents analytical and numerical techniques for evaluating two and three dimensional subsurface stress fields in anisotropic contacts. The subsurface stress results are required for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single crystal nickel-base superalloy turbine blades. An analytical procedure is , presented, for evaluating the subsurface stresses in the elastic half-space, using a complex potential method outlined by Lekhnitskii. Numerical results are presented for cylindrical and spherical anisotropic contacts, using finite element analysis. Effects of crystal orientation on stress response and fatigue life are examined.

  16. Phase-field-crystal model for fcc ordering.

    PubMed

    Wu, Kuo-An; Adland, Ari; Karma, Alain

    2010-06-01

    We develop and analyze a two-mode phase-field-crystal model to describe fcc ordering. The model is formulated by coupling two different sets of crystal density waves corresponding to <111> and <200> reciprocal lattice vectors, which are chosen to form triads so as to produce a simple free-energy landscape with coexistence of crystal and liquid phases. The feasibility of the approach is demonstrated with numerical examples of polycrystalline and (111) twin growth. We use a two-mode amplitude expansion to characterize analytically the free-energy landscape of the model, identifying parameter ranges where fcc is stable or metastable with respect to bcc. In addition, we derive analytical expressions for the elastic constants for both fcc and bcc. Those expressions show that a nonvanishing amplitude of [200] density waves is essential to obtain mechanically stable fcc crystals with a nonvanishing tetragonal shear modulus (C11-C12)/2. We determine the model parameters for specific materials by fitting the peak liquid structure factor properties and solid-density wave amplitudes following the approach developed for bcc [K.-A. Wu and A. Karma, Phys. Rev. B 76, 184107 (2007)]. This procedure yields reasonable predictions of elastic constants for both bcc Fe and fcc Ni using input parameters from molecular dynamics simulations. The application of the model to two-dimensional square lattices is also briefly examined.

  17. Field-Effects in Large Axial Ratio Liquid Crystals

    NASA Astrophysics Data System (ADS)

    Lonberg, Franklin J.

    This paper consists of an introduction and four chapters, the abstracts of which are presented below. Chapter 2. The subject of this chapter is the dynamic periodic structures which are observed in the twist Frederiks transition. It is found that, for fields above a material dependent level, a transient periodic distortion is observed. The wave vector is parallel to the unperturbed director and increases with increasing field. A theoretical model and experimental data are presented. Chapter 3. The subject of this chapter is the discovery of a new equilibrium structure in the splay Frederiks transition. Experimental observation has shown that the imposition of a field, just above the critical strength, produces a periodic distortion in the polymer liquid crystal PBG. This periodic state is not dynamic in origin but it is a true ground state. An analysis of the energy of a liquid crystal, in the splay Frederiks transition geometry, shows that in materials with K(,1)/K(,3) > 3.3 the periodic distortion will have a lower critical field than the uniform distortion. Chapter 4. The subject of this chapter is the dynamics of the bend Frederiks transition in large axial ratio nematics. Experimental evidence is presented to show that there is a distortion mode which occurs at field greater than 2H(,c), which is very fast and does not grow exponentially. An analysis of the equations of motion shows that a mode with wave length half that of the static equilibrium mode will have these properties. Chapter 5. The bend Frederiks transition is use to show that the bend and splay elastic constants are linear in concentration in PBG. Interpretation of this result is made in connection with models of the elastic energy in liquid crystal made of semi-flexible partiles.

  18. Characterizing ice crystal growth behavior under electric field using phase field method.

    PubMed

    He, Zhi Zhu; Liu, Jing

    2009-07-01

    In this article, the microscale ice crystal growth behavior under electrostatic field is investigated via a phase field method, which also incorporates the effects of anisotropy and thermal noise. The multiple ice nuclei's competitive growth as disclosed in existing experiments is thus successfully predicted. The present approach suggests a highly efficient theoretical tool for probing into the freeze injury mechanisms of biological material due to ice formation during cryosurgery or cryopreservation process when external electric field was involved.

  19. Liquid Crystals for Organic Field-Effect Transistors

    NASA Astrophysics Data System (ADS)

    O'Neill, Mary; Kelly, Stephen M.

    Columnar, smectic and lamellar polymeric liquid crystals are widely recognized as very promising charge-transporting organic semiconductors due to their ability to spontaneously self-assemble into highly ordered domains in uniform thin films over large areas. The transport properties of smectic and columnar liquid crystals are discussed in Chaps. 2 (10.1007/978-90-481-2873-0_2) and 3 (10.1007/978-90-481-2873-0_3). Here we examine their application to organic field-effect transistors (OFETs): after a short introduction in Sect. 9.1 we introduce the OFET configuration and show how the mobility is measured in Sect. 9.2. Section 9.3 discusses polymeric liquid crystalline semiconductors in OFETs. We review research that shows that annealing of polymers in a fluid mesophase gives a more ordered microcrystalline morphology on cooling than that kinetically determined by solution processing of the thin film. We also demonstrate the benefits of monodomain alignment and show the application of liquid crystals in light-emitting field-effect transistors. Some columnar and smectic phases are highly ordered with short intermolecular separation to give large π-π coupling. We discuss their use in OFETs in Sects. 9.4, and 9.5 respectively. Section 9.6 summarises the conclusions of the chapter.

  20. Phase-field crystal model with a vapor phase

    NASA Astrophysics Data System (ADS)

    Schwalbach, Edwin J.; Warren, James A.; Wu, Kuo-An; Voorhees, Peter W.

    2013-08-01

    Phase-field crystal (PFC) models are able to resolve atomic length scale features of materials during temporal evolution over diffusive time scales. Traditional PFC models contain solid and liquid phases, however many important materials processing phenomena involve a vapor phase as well. In this work, we add a vapor phase to an existing PFC model and show realistic interfacial phenomena near the triple point temperature. For example, the PFC model exhibits density oscillations at liquid-vapor interfaces that compare favorably to data available for interfaces in metallic systems from both experiment and molecular-dynamics simulations. We also quantify the anisotropic solid-vapor surface energy for a two-dimensional PFC hexagonal crystal and find well-defined step energies from measurements on the faceted interfaces. Additionally, the strain field beneath a stepped interface is characterized and shown to qualitatively reproduce predictions from continuum models, simulations, and experimental data. Finally, we examine the dynamic case of step-flow growth of a crystal into a supersaturated vapor phase. The ability to model such a wide range of surface and bulk defects makes this PFC model a useful tool to study processing techniques such as chemical vapor deposition or vapor-liquid-solid growth of nanowires.

  1. Phase-field crystal model with a vapor phase.

    PubMed

    Schwalbach, Edwin J; Warren, James A; Wu, Kuo-An; Voorhees, Peter W

    2013-08-01

    Phase-field crystal (PFC) models are able to resolve atomic length scale features of materials during temporal evolution over diffusive time scales. Traditional PFC models contain solid and liquid phases, however many important materials processing phenomena involve a vapor phase as well. In this work, we add a vapor phase to an existing PFC model and show realistic interfacial phenomena near the triple point temperature. For example, the PFC model exhibits density oscillations at liquid-vapor interfaces that compare favorably to data available for interfaces in metallic systems from both experiment and molecular-dynamics simulations. We also quantify the anisotropic solid-vapor surface energy for a two-dimensional PFC hexagonal crystal and find well-defined step energies from measurements on the faceted interfaces. Additionally, the strain field beneath a stepped interface is characterized and shown to qualitatively reproduce predictions from continuum models, simulations, and experimental data. Finally, we examine the dynamic case of step-flow growth of a crystal into a supersaturated vapor phase. The ability to model such a wide range of surface and bulk defects makes this PFC model a useful tool to study processing techniques such as chemical vapor deposition or vapor-liquid-solid growth of nanowires.

  2. Subsurface Stress Fields in FCC Single Crystal Anisotropic Contacts

    NASA Technical Reports Server (NTRS)

    Arakere, Nagaraj K.; Knudsen, Erik; Swanson, Gregory R.; Duke, Gregory; Ham-Battista, Gilda

    2004-01-01

    Single crystal superalloy turbine blades used in high pressure turbomachinery are subject to conditions of high temperature, triaxial steady and alternating stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent high cycle fatigue (HCF) failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes, and non-crystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single crystal turbine blades. This paper presents analytical and numerical techniques for evaluating two and three dimensional subsurface stress fields in anisotropic contacts. The subsurface stress results are required for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single crystal nickel-base superalloy turbine blades. An analytical procedure is presented for evaluating the subsurface stresses in the elastic half-space, based on the adaptation of a stress function method outlined by Lekhnitskii. Numerical results are presented for cylindrical and spherical anisotropic contacts, using finite element analysis (FEA). Effects of crystal orientation on stress response and fatigue life are examined. Obtaining accurate subsurface stress results for anisotropic single crystal contact problems require extremely refined three-dimensional (3-D) finite element grids, especially in the edge of contact region. Obtaining resolved shear stresses (RSS) on the principal slip planes also involves

  3. Superconformal field theories from M-theory crystal lattices

    NASA Astrophysics Data System (ADS)

    Lee, Sangmin

    2007-05-01

    We propose a brane configuration for the (2+1)d, N=2 superconformal theories (CFT3) arising from M2 branes probing toric Calabi-Yau 4-fold cones, using a T-duality transformation of M theory. We obtain intersections of M5-branes on a three-torus which form a 3d bipartite crystal lattice in a way similar to the 2d dimer models for CFT4. The fundamental fields of the CFT3 are M2-brane discs localized around the intersections, and the superpotential terms are identified with the atoms of the crystal. The model correctly reproduces the Bogomol’nyi-Prasad-Sommerfield (BPS) spectrum of mesons.

  4. Bridgman Growth of Germanium Crystals in a Rotating Magnetic Field

    NASA Technical Reports Server (NTRS)

    Volz, M. P.; Szofran, F. R.; Cobb, S. D.; Schweizer, M.; Walker, J. S.

    2005-01-01

    A series of (100)-oriented gallium-doped germanium crystals has been grown by the vertical Bridgman method and under the influence of a rotating magnetic field (RMF). Time-dependent flow instabilities occur when the critical magnetic Taylor number (Tm(sup c)) is exceeded, and this can be observed by noting the appearance of striations in the grown crystals. Tm(sup c) decreases as the aspect ratio of the melt increases, and approaches the theoretical limit expected for an infinite cylinder. Intentional interface demarcations are introduced by pulsing the RMF on and off The RMF has a marked affect on the interface shape, changing it from concave to nearly flat as the RMF strength is increased.

  5. Bridgman Growth of Germanium Crystals in a Rotating Magnetic Field

    NASA Technical Reports Server (NTRS)

    Volz, M. P.; Walker, J. S.; Schweizer, M.; Cobb, S. D.; Szofran, F. R.

    2004-01-01

    A series of (100)-oriented gallium-doped germanium crystals have been grown by the Bridgman method and under the influence of a rotating magnetic field (RMF). Time-dependent flow instabilities occur when the critical magnetic Taylor number (Tm(sup c) is exceeded, and this can be observed by noting the appearance of striations in the grown crystals. The experimental data indicate that Tm(sup c) increases as the aspect ratio of the melt decreases. Modeling calculations predicting Tm(sup c) as a function of aspect ratio are in reasonable agreement with the experimental data. The RMF has a marked affect on the interface shape, changing it from concave to nearly flat as the RMF strength is increased. Also, by pulsing the RMF on and off, it is shown that intentional interface demarcations can be introduced.

  6. Magnetic measurements on single crystals of dysprosium trifluoromethanesulfonate nonahydrate; effects of crystal field perturbed energy levels

    NASA Astrophysics Data System (ADS)

    Neogy, D.; Paul, P.; Chattopadhyay, K. N.; Bisui, D.

    2002-07-01

    Magnetic susceptibility measurements on single crystals of dysprosium trifluoromethanesulfonate (DyTFMS) have been carried out from 300 K down to 13 K. The hexagonal crystal structure of DyTFMS renders the crystal uniaxial with the Kramers ion Dy 3+ occupying a site of C 3h symmetry. The principal magnetic susceptibilities, observed by us and the Friedberg group, over the wide range 300 to ˜1.0 K find an excellent theoretical simulation by the crystal field perturbed J-mixed eigenvectors with due consideration of the intermediate coupling effects. No ordering effects were noticed down to ˜13 K indicating the interionic interaction to be predominantly of the dipolar type which is consistent with the discovery of a ferromagnetic transition at T˜0.111 K by the Friedberg group. The g-values derived from other sources are reasonably accounted for. The thermal behavior of quadrupole splitting and that of electronic and nuclear heat capacities is also worked out.

  7. Intermittent dislocation density fluctuations in crystal plasticity from a phase-field crystal model.

    PubMed

    Tarp, Jens M; Angheluta, Luiza; Mathiesen, Joachim; Goldenfeld, Nigel

    2014-12-31

    Plastic deformation mediated by collective dislocation dynamics is investigated in the two-dimensional phase-field crystal model of sheared single crystals. We find that intermittent fluctuations in the dislocation population number accompany bursts in the plastic strain-rate fluctuations. Dislocation number fluctuations exhibit a power-law spectral density 1/f2 at high frequencies f. The probability distribution of number fluctuations becomes bimodal at low driving rates corresponding to a scenario where low density of defects alternates at irregular times with high populations of defects. We propose a simple stochastic model of dislocation reaction kinetics that is able to capture these statistical properties of the dislocation density fluctuations as a function of shear rate.

  8. Subsurface Stress Fields in Single Crystal (Anisotropic) Contacts

    NASA Technical Reports Server (NTRS)

    Arakere, Nagaraj K.

    2003-01-01

    Single crystal superalloy turbine blades used in high pressure turbomachinery are subject to conditions of high temperature, triaxial steady and fatigue stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent HCF failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes, and noncrystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single crystal turbine blades. Techniques for evaluating two and three dimensional subsurface stress fields in anisotropic contacts are presented in this report. Figure 1 shows typical damper contact locations in a turbine blade. The subsurface stress results are used for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single crystal nickel-base superalloy turbine blades.

  9. Phase-field-crystal methodology for modeling of structural transformations.

    PubMed

    Greenwood, Michael; Rottler, Jörg; Provatas, Nikolas

    2011-03-01

    We introduce and characterize free-energy functionals for modeling of solids with different crystallographic symmetries within the phase-field-crystal methodology. The excess free energy responsible for the emergence of periodic phases is inspired by classical density-functional theory, but uses only a minimal description for the modes of the direct correlation function to preserve computational efficiency. We provide a detailed prescription for controlling the crystal structure and introduce parameters for changing temperature and surface energies, so that phase transformations between body-centered-cubic (bcc), face-centered-cubic (fcc), hexagonal-close-packed (hcp), and simple-cubic (sc) lattices can be studied. To illustrate the versatility of our free-energy functional, we compute the phase diagram for fcc-bcc-liquid coexistence in the temperature-density plane. We also demonstrate that our model can be extended to include hcp symmetry by dynamically simulating hcp-liquid coexistence from a seeded crystal nucleus. We further quantify the dependence of the elastic constants on the model control parameters in two and three dimensions, showing how the degree of elastic anisotropy can be tuned from the shape of the direct correlation functions.

  10. Magnetic field sensor based on coupled photonic crystal nanobeam cavities

    NASA Astrophysics Data System (ADS)

    Du, Han; Zhou, Guangya; Zhao, Yunshan; Chen, Guoqiang; Chau, Fook Siong

    2017-02-01

    We report the design, fabrication, and characterization of a resonant Lorentz force magnetic field sensor based on dual-coupled photonic crystal nanobeam cavities. Compared with microelectromechanical systems (MEMS) Lorentz force magnetometers, the proposed magnetic field sensor has an ultra-small footprint (less than 70 μm × 40 μm) and a wider operation bandwidth (of 160 Hz). The sensing mechanism is based on the resonance wavelength shift of a selected supermode of the coupled cavities, which is caused by the Lorentz force-induced relative displacement of the cavity nanobeams, and thus the optical transmission variation. The sensitivity and resolution of the device demonstrated experimentally are 22.9 mV/T and 48.1 μT/Hz1/2, respectively. The results can be further improved by optimizing the initial offset of the two nanobeams.

  11. Phase-field-crystal models and mechanical equilibrium

    NASA Astrophysics Data System (ADS)

    Heinonen, V.; Achim, C. V.; Elder, K. R.; Buyukdagli, S.; Ala-Nissila, T.

    2014-03-01

    Phase-field-crystal (PFC) models constitute a field theoretical approach to solidification, melting, and related phenomena at atomic length and diffusive time scales. One of the advantages of these models is that they naturally contain elastic excitations associated with strain in crystalline bodies. However, instabilities that are diffusively driven towards equilibrium are often orders of magnitude slower than the dynamics of the elastic excitations, and are thus not included in the standard PFC model dynamics. We derive a method to isolate the time evolution of the elastic excitations from the diffusive dynamics in the PFC approach and set up a two-stage process, in which elastic excitations are equilibrated separately. This ensures mechanical equilibrium at all times. We show concrete examples demonstrating the necessity of the separation of the elastic and diffusive time scales. In the small-deformation limit this approach is shown to agree with the theory of linear elasticity.

  12. Heating behavior and crystal growth mechanism in microwave field.

    PubMed

    Yang, Gang; Kong, Yan; Hou, Wenhua; Yan, Qijie

    2005-02-03

    A simple microwave solid-state reactor was designed on the basis of a domestic microwave oven by using graphite powder as heating medium. The heating behavior of the reactor was studied by using an on-line computer to monitor the real-time temperature during irradiation. It was found that the temperature (T) was related to the time (t) and that microwave power depended on the duty cycle (x) of microwave irradiation. Two empirical equations were proposed and could be applied to the similar microwave solid-state reactors. Four inorganic layered materials, LiV(3)O(8), KNb(3)O(8), KTiNbO(5), and KSr(2)Nb(3)O(10), were successfully synthesized in the designed reactor at a suitable heating rate and temperature that were fully controlled by the empirical equations. Characterization results of X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy, and scanning (SEM) and transmission (TEM) electron microscopy indicated that the phases of samples prepared by traditional and microwave methods were in good agreement; nevertheless, the heating nature and the morphologies of products were quite different. The samples synthesized in the microwave field had crystallographic defects and showed an incompactly stacking structure of nanosheets. Due to the rapid formation of crystallites and different extended growth rate along the crystal axis of the products in microwave field, the crystal growth mechanism of layered metal oxides was not according to that of the traditional method and is briefly discussed.

  13. Hysteresis behaviors of the crystal field diluted general spin-S Ising model

    NASA Astrophysics Data System (ADS)

    Akıncı, Ümit

    2017-10-01

    Hysteresis characteristics of the crystal field diluted general Spin-S (S > 1) Blume-Capel model have been studied within the effective field approximation. Particular emphasis has been paid on the large negative valued crystal field and low temperature region and it has been demonstrated for this region that, rising dilution of the crystal field results in decreasing number of windowed hysteresis loops. The evolution of the multiple hysteresis loop with the dilution of the crystal field has been investigated and physical mechanism behind this evolution has been given.

  14. The Effects of a Magnetic Field on the Crystallization of a Fluorozirconate Glass

    NASA Technical Reports Server (NTRS)

    Tucker, Dennis S.; Lapointe, Michael R.; Jia, Zhiyong

    2006-01-01

    An axial magnetic field of 0.1T was applied to ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fibers during heating to the glass crystallization temperature. Scanning electron microscopy and x-ray diffraction were used to identify crystal phases. It was shown that fibers exposed to the magnetic field did not crystallize while fibers not exposed to the field did crystallize. A hypothesis based on magnetic work was proposed to explain the results and tested by measuring the magnetic susceptibilities of the glass and crystal.

  15. Crystal field effects on interionic distance in cubic MgO crystal doped with Fe2+ ions

    NASA Astrophysics Data System (ADS)

    Ivascu, S.; Gruia, A. S.; Avram, N. M.

    2014-10-01

    The exchange charge model of crystal field was applied to determine the dependence of the crystal field strength 10Dq on interionic distances R between the Fe2+ impurity ion and O2- ligands in cubic MgO:Fe2+. The obtained results were extrapolated by the power law and was shown that 10Dq depends on R as 1/R, with n=6.3486. The deviations of these values from the value n=5 (predicted by the simple point charge model of crystal field) is explained by the covalent and exchange effects between impurity ion and ligands; the contribution of these effects into the total crystal field strength was considered separately. The 10Dq functions obtained as a result of our calculations were used for estimations of the electron-vibrational constants, Huang-Rhys parameters, and Jahn-Teller stabilization energy, and compared with available literature data.

  16. Control of active liquid crystals with a magnetic field.

    PubMed

    Guillamat, Pau; Ignés-Mullol, Jordi; Sagués, Francesc

    2016-05-17

    Living cells sense the mechanical features of their environment and adapt to it by actively remodeling their peripheral network of filamentary proteins, known as cortical cytoskeleton. By mimicking this principle, we demonstrate an effective control strategy for a microtubule-based active nematic in contact with a hydrophobic thermotropic liquid crystal. By using well-established protocols for the orientation of liquid crystals with a uniform magnetic field, and through the mediation of anisotropic shear stresses, the active nematic reversibly self-assembles with aligned flows and textures that feature orientational order at the millimeter scale. The turbulent flow, characteristic of active nematics, is in this way regularized into a laminar flow with periodic velocity oscillations. Once patterned, the microtubule assembly reveals its intrinsic length and time scales, which we correlate with the activity of motor proteins, as predicted by existing theories of active nematics. The demonstrated commanding strategy should be compatible with other viable active biomaterials at interfaces, and we envision its use to probe the mechanics of the intracellular matrix.

  17. Control of active liquid crystals with a magnetic field

    PubMed Central

    Guillamat, Pau; Sagués, Francesc

    2016-01-01

    Living cells sense the mechanical features of their environment and adapt to it by actively remodeling their peripheral network of filamentary proteins, known as cortical cytoskeleton. By mimicking this principle, we demonstrate an effective control strategy for a microtubule-based active nematic in contact with a hydrophobic thermotropic liquid crystal. By using well-established protocols for the orientation of liquid crystals with a uniform magnetic field, and through the mediation of anisotropic shear stresses, the active nematic reversibly self-assembles with aligned flows and textures that feature orientational order at the millimeter scale. The turbulent flow, characteristic of active nematics, is in this way regularized into a laminar flow with periodic velocity oscillations. Once patterned, the microtubule assembly reveals its intrinsic length and time scales, which we correlate with the activity of motor proteins, as predicted by existing theories of active nematics. The demonstrated commanding strategy should be compatible with other viable active biomaterials at interfaces, and we envision its use to probe the mechanics of the intracellular matrix. PMID:27140604

  18. Raman spectrum of plutonium dioxide: Vibrational and crystal field modes

    NASA Astrophysics Data System (ADS)

    Naji, M.; Magnani, N.; Bonales, L. J.; Mastromarino, S.; Colle, J.-Y.; Cobos, J.; Manara, D.

    2017-03-01

    The Raman spectrum of plutonium dioxide is studied both experimentally and theoretically. Particular attention has been devoted to the identification of high-energy modes at 2110 and 2620 c m-1 , whose attribution has so far been controversial. The temperature dependence of both modes suggests an electronic origin for them. Original crystal field (CF) calculations reported in this work show that these two modes can be respectively assigned to the Γ1→Γ5 and Γ1→Γ3 CF transitions within the I54 manifold. These two modes, together with the only vibrational line foreseen by the group theory for the F m -3 m Pu O2 symmetry—the T2 gPu -O stretching mode observed at 478 c m-1 —can thus be used as a Raman fingerprint of fcc plutonium dioxide.

  19. Thermodynamics of bcc metals in phase-field-crystal models.

    PubMed

    Jaatinen, A; Achim, C V; Elder, K R; Ala-Nissila, T

    2009-09-01

    We examine the influence of different forms of the free-energy functionals used in the phase-field-crystal (PFC) model, and compare them with the second-order density-functional theory (DFT) of freezing, by using bcc iron as an example case. We show that there are large differences between the PFC and the DFT and it is difficult to obtain reasonable parameters for existing PFC models directly from the DFT. Therefore, we propose a way of expanding the correlation function in terms of gradients that allows us to incorporate the bulk modulus of the liquid as an additional parameter in the theory. We show that this functional reproduces reasonable values for both bulk and surface properties of bcc iron, and therefore it should be useful in modeling bcc materials. As a further demonstration, we also calculate the grain boundary energy as a function of misorientation for a symmetric tilt boundary close to the melting transition.

  20. Forms of crystal field Hamiltonians - A critical review

    NASA Astrophysics Data System (ADS)

    Rudowicz, C.; Gnutek, P.; Karbowiak, M.

    2011-08-01

    Our survey reveals that various disparate forms, both compact and expanded ones, of crystal field (CF) Hamiltonians, HCF, expressed in the Wybourne notation have been used in the literature. It turns out that the disparities in the symbolic explicit forms of HCF are especially important for monoclinic and triclinic site symmetry. The extent of the inconsistencies identified in selected papers has prompted us to embark on a systematic critical review of the HCF forms employed in optical spectroscopy and related areas. Most crucial results of this survey are presented here. Comparative analysis has been carried out to establish the interrelations between CF parameters (CFPs) expressed in disparate forms. The usage of inconsistent or confusing HCF forms has implications also for CFP conversions between the Stevens and Wybourne notations as well as for theoretical modeling of CFPs. This review reveals that comparison of CFP data taken from various sources should be carried out with special care, especially for low symmetry cases.

  1. Field-induced rectification in rutile single crystals.

    NASA Astrophysics Data System (ADS)

    Jameson, John R.; Fukuzumi, Yoshiaki; Tsunoda, Koji; Wang, Zheng; Griffin, Peter B.; Nishi, Yoshio

    2007-03-01

    A previously unknown resistive memory effect is reported in rutile titanium dioxide. Two Pt electrodes were placed on the surface of a rutile crystal, and a large voltage was applied between them. Afterwards, the device allowed current to pass in the direction of the voltage, but not in the other direction. The orientation of this rectification could then be switched by applying a large voltage of opposite sign. The effect was observed with electrodes on the (100) or (110) surfaces, but not the (001) surface. A plausible explanation is the field-induced motion of oxygen vacancies, which the large voltage might cause to pile up under the negative electrode, eliminating a Schottky barrier at that interface, but leaving a Schottky at the positive electrode intact. Parallels are drawn to other memory effects in titanium dioxide.

  2. Reductive renormalization of the phase-field crystal equation.

    PubMed

    Oono, Y; Shiwa, Y

    2012-12-01

    It has been known for some time that singular perturbation and reductive perturbation can be unified from the renormalization-group theoretical point of view: Reductive extraction of space-time global behavior is the essence of singular perturbation methods. Reductive renormalization was proposed to make this unification practically accessible; actually, this reductive perturbation is far simpler than most reduction methods, such as the rather standard scaling expansion. However, a rather cryptic exposition of the method seems to have been the cause of some trouble. Here, an explicit demonstration of the consistency of the reductive renormalization-group procedure is given for partial differentiation equations (of a certain type, including time-evolution semigroup type equations). Then, the procedure is applied to the reduction of a phase-field crystal equation to illustrate the streamlined reduction method. We conjecture that if the original system is structurally stable, the reductive renormalization-group result and that of the original equation are diffeomorphic.

  3. Interpretation of experimental results on Kondo systems with crystal field.

    PubMed

    Romero, M A; Aligia, A A; Sereni, J G; Nieva, G

    2014-01-15

    We present a simple approach to calculate the thermodynamic properties of single Kondo impurities including orbital degeneracy and crystal field effects (CFE) by extending a previous proposal by Schotte and Schotte (1975 Phys. Lett. 55A 38). Comparison with exact solutions for the specific heat of a quartet ground state split into two doublets shows deviations below 10% in the absence of CFE and a quantitative agreement for moderate or large CFE. As an application, we fit the measured specific heat of the compounds CeCu2Ge2, CePd3Si0.3, CePdAl, CePt, Yb2Pd2Sn and YbCo2Zn20. The agreement between theory and experiment is very good or excellent depending on the compound, except at very low temperatures due to the presence of magnetic correlations (not accounted for in the model).

  4. The Strength of PIN-PMN-PT Single Crystals under Bending with a Longitudinal Electric Field

    DTIC Science & Technology

    2011-04-06

    The strength of PIN– PMN – PT single crystals under bending with a longitudinal electric field This article has been downloaded from IOPscience. Please...COVERED 00-00-2011 to 00-00-2011 4. TITLE AND SUBTITLE The Strength Of PIN- PMN - PT Single Crystals Under Bending With A Longitudinal Electric Field... PMN ? PT ) single crystals was measured using a four point bending apparatus with a longitudinal electric field applied to the bar during bending. The

  5. Phase-field crystal modeling of heteroepitaxy and exotic modes of crystal nucleation

    NASA Astrophysics Data System (ADS)

    Podmaniczky, Frigyes; Tóth, Gyula I.; Tegze, György; Pusztai, Tamás; Gránásy, László

    2017-01-01

    We review recent advances made in modeling heteroepitaxy, two-step nucleation, and nucleation at the growth front within the framework of a simple dynamical density functional theory, the Phase-Field Crystal (PFC) model. The crystalline substrate is represented by spatially confined periodic potentials. We investigate the misfit dependence of the critical thickness in the StranskiKrastanov growth mode in isothermal studies. Apparently, the simulation results for stress release via the misfit dislocations fit better to the PeopleBean model than to the one by Matthews and Blakeslee. Next, we investigate structural aspects of two-step crystal nucleation at high undercoolings, where an amorphous precursor forms in the first stage. Finally, we present results for the formation of new grains at the solid-liquid interface at high supersaturations/supercoolings, a phenomenon termed Growth Front Nucleation (GFN). Results obtained with diffusive dynamics (applicable to colloids) and with a hydrodynamic extension of the PFC theory (HPFC, developed for simple liquids) will be compared. The HPFC simulations indicate two possible mechanisms for GFN.

  6. Modeling nuclear field shift isotope fractionation in crystals

    NASA Astrophysics Data System (ADS)

    Schauble, E. A.

    2013-12-01

    In this study nuclear field shift fractionations in solids (and chemically similar liquids) are estimated using calibrated density functional theory calculations. The nuclear field shift effect is a potential driver of mass independent isotope fractionation(1,2), especially for elements with high atomic number such as Hg, Tl and U. This effect is caused by the different shapes and volumes of isotopic nuclei, and their interactions with electronic structures and energies. Nuclear field shift isotope fractionations can be estimated with first principles methods, but the calculations are computationally difficult, limiting most theoretical studies so far to small gas-phase molecules and molecular clusters. Many natural materials of interest are more complex, and it is important to develop ways to estimate field shift effects that can be applied to minerals, solutions, in biomolecules, and at mineral-solution interfaces. Plane-wave density functional theory, in combination with the projector augmented wave method (DFT-PAW), is much more readily adapted to complex materials than the relativistic all-electron calculations that have been the focus of most previous studies. DFT-PAW is a particularly effective tool for studying crystals with periodic boundary conditions, and may also be incorporated into molecular dynamics simulations of solutions and other disordered phases. Initial calibrations of DFT-PAW calculations against high-level all-electron models of field shift fractionation suggest that there may be broad applicability of this method to a variety of elements and types of materials. In addition, the close relationship between the isomer shift of Mössbauer spectroscopy and the nuclear field shift isotope effect makes it possible, at least in principle, to estimate the volume component of field shift fractionations in some species that are too complex even for DFT-PAW models, so long as there is a Mössbauer isotope for the element of interest. Initial results

  7. Effect of an electric field on nucleation and growth of crystals

    NASA Astrophysics Data System (ADS)

    Yurov, V. M.; Guchenko, S. A.; Gyngazova, M. S.

    2016-02-01

    The effect of the electric field strength on nucleation and growth of the crystals of ammonium halides and alkali metal sulfates has been studied. The optimal electric field strength for NH4Cl and NH4Br crystals was found to be 15 kV/cm, and for NH4I, it equaled 10 kV/cm. No effect of the electric field strength on the crystal growth was found for alkali metal sulfates. This difference is analyzed in terms of the crystal growth thermodynamics. In case, when the electric field is small and the Gibbs energy is of a significant value, the influence of the electric field at the crystal growth is negligible. A method to estimate the critical radius of homogeneous nucleation of the crystal is suggested.

  8. An Overview of Hardware for Protein Crystallization in a Magnetic Field

    PubMed Central

    Yan, Er-Kai; Zhang, Chen-Yan; He, Jin; Yin, Da-Chuan

    2016-01-01

    Protein crystallization under a magnetic field is an interesting research topic because a magnetic field may provide a special environment to acquire improved quality protein crystals. Because high-quality protein crystals are very useful in high-resolution structure determination using diffraction techniques (X-ray, neutron, and electron diffraction), research using magnetic fields in protein crystallization has attracted substantial interest; some studies have been performed in the past two decades. In this research field, the hardware is especially essential for successful studies because the environment is special and the design and utilization of the research apparatus in such an environment requires special considerations related to the magnetic field. This paper reviews the hardware for protein crystallization (including the magnet systems and the apparatus designed for use in a magnetic field) and progress in this area. Future prospects in this field will also be discussed. PMID:27854318

  9. An Overview of Hardware for Protein Crystallization in a Magnetic Field.

    PubMed

    Yan, Er-Kai; Zhang, Chen-Yan; He, Jin; Yin, Da-Chuan

    2016-11-16

    Protein crystallization under a magnetic field is an interesting research topic because a magnetic field may provide a special environment to acquire improved quality protein crystals. Because high-quality protein crystals are very useful in high-resolution structure determination using diffraction techniques (X-ray, neutron, and electron diffraction), research using magnetic fields in protein crystallization has attracted substantial interest; some studies have been performed in the past two decades. In this research field, the hardware is especially essential for successful studies because the environment is special and the design and utilization of the research apparatus in such an environment requires special considerations related to the magnetic field. This paper reviews the hardware for protein crystallization (including the magnet systems and the apparatus designed for use in a magnetic field) and progress in this area. Future prospects in this field will also be discussed.

  10. Phase-field modeling on morphological landscape of isotactic polystyrene single crystals.

    PubMed

    Xu, Haijun; Matkar, Rushikesh; Kyu, Thein

    2005-07-01

    Spatio-temporal growth of isotactic polystyrene single crystals during isothermal crystallization has been investigated theoretically based on the phase field model by solving temporal evolution of a nonconserved phase order parameter coupled with a heat conduction equation. In the description of the total free energy, an asymmetric double-well local free energy density has been adopted to represent the metastable melt and the stable solid crystal. Unlike the small molecule systems, polymer crystallization rarely reaches thermodynamic equilibrium; most polymer crystals are kinetically stabilized in some metastable states. To capture various metastable polymer crystals, the phase field crystal order parameter at the solidification potential has been treated to be supercooling dependent such that it can assume an intermediate value between zero (melt) and unity (perfect crystal), reflecting imperfect polycrystalline nature of polymer crystals. Two-dimensional simulations exhibit various single crystal morphologies of isotactic polystyrene crystals such as faceted hexagonal patterns transforming to nonfaceted snowflakes with increasing supercooling. Of particular interest is that heat liberation from the crystallizing front influences the curvature of the crystal-melt interface, leading to directional growth of lamellar tips and side branches. The landscape of these morphological textures has been established as a function of anisotropy of surface energy and supercooling. With increasing supercooling and decreasing anisotropy, the hexagonal single crystal transforms to the dense lamellar branching morphology in conformity with the experimental findings.

  11. Rashba coupling amplification by a staggered crystal field

    NASA Astrophysics Data System (ADS)

    Santos-Cottin, David; Casula, Michele; Lantz, Gabriel; Klein, Yannick; Petaccia, Luca; Le Fèvre, Patrick; Bertran, François; Papalazarou, Evangelos; Marsi, Marino; Gauzzi, Andrea

    2016-04-01

    There has been increasing interest in materials where relativistic effects induce non-trivial electronic states with promise for spintronics applications. One example is the splitting of bands with opposite spin chirality produced by the Rashba spin-orbit coupling in asymmetric potentials. Sizable splittings have been hitherto obtained using either heavy elements, where this coupling is intrinsically strong, or large surface electric fields. Here by means of angular resolved photoemission spectroscopy and first-principles calculations, we give evidence of a large Rashba coupling of 0.25 eV Å, leading to a remarkable band splitting up to 0.15 eV with hidden spin-chiral polarization in centrosymmetric BaNiS2. This is explained by a huge staggered crystal field of 1.4 V Å-1, produced by a gliding plane symmetry, that breaks inversion symmetry at the Ni site. This unexpected result in the absence of heavy elements demonstrates an effective mechanism of Rashba coupling amplification that may foster spin-orbit band engineering.

  12. Rashba coupling amplification by a staggered crystal field

    PubMed Central

    Santos-Cottin, David; Casula, Michele; Lantz, Gabriel; Klein, Yannick; Petaccia, Luca; Le Fèvre, Patrick; Bertran, François; Papalazarou, Evangelos; Marsi, Marino; Gauzzi, Andrea

    2016-01-01

    There has been increasing interest in materials where relativistic effects induce non-trivial electronic states with promise for spintronics applications. One example is the splitting of bands with opposite spin chirality produced by the Rashba spin-orbit coupling in asymmetric potentials. Sizable splittings have been hitherto obtained using either heavy elements, where this coupling is intrinsically strong, or large surface electric fields. Here by means of angular resolved photoemission spectroscopy and first-principles calculations, we give evidence of a large Rashba coupling of 0.25 eV Å, leading to a remarkable band splitting up to 0.15 eV with hidden spin-chiral polarization in centrosymmetric BaNiS2. This is explained by a huge staggered crystal field of 1.4 V Å−1, produced by a gliding plane symmetry, that breaks inversion symmetry at the Ni site. This unexpected result in the absence of heavy elements demonstrates an effective mechanism of Rashba coupling amplification that may foster spin-orbit band engineering. PMID:27089869

  13. Description of hard-sphere crystals and crystal-fluid interfaces: a comparison between density functional approaches and a phase-field crystal model.

    PubMed

    Oettel, M; Dorosz, S; Berghoff, M; Nestler, B; Schilling, T

    2012-08-01

    In materials science the phase-field crystal approach has become popular to model crystallization processes. Phase-field crystal models are in essence Landau-Ginzburg-type models, which should be derivable from the underlying microscopic description of the system in question. We present a study on classical density functional theory in three stages of approximation leading to a specific phase-field crystal model, and we discuss the limits of applicability of the models that result from these approximations. As a test system we have chosen the three-dimensional suspension of monodisperse hard spheres. The levels of density functional theory that we discuss are fundamental measure theory, a second-order Taylor expansion thereof, and a minimal phase-field crystal model. We have computed coexistence densities, vacancy concentrations in the crystalline phase, interfacial tensions, and interfacial order parameter profiles, and we compare these quantities to simulation results. We also suggest a procedure to fit the free parameters of the phase-field crystal model. Thereby it turns out that the order parameter of the phase-field crystal model is more consistent with a smeared density field (shifted and rescaled) than with the shifted and rescaled density itself. In brief, we conclude that fundamental measure theory is very accurate and can serve as a benchmark for the other theories. Taylor expansion strongly affects free energies, surface tensions, and vacancy concentrations. Furthermore it is phenomenologically misleading to interpret the phase-field crystal model as stemming directly from Taylor-expanded density functional theory.

  14. Electric-field-assisted position and orientation control of organic single crystals.

    PubMed

    Kotsuki, Kenji; Obata, Seiji; Saiki, Koichiro

    2014-12-02

    We have investigated the motion of growing pentacene single crystals in solution under various electric fields. The pentacene single crystals in 1,2,4-trichlorobenzene responded to the electric field as if they were positively charged. By optimizing the strength and frequency of an alternating electric field, the pentacene crystals automatically bridged the electrodes on SiO2. The pentacene crystal with a large aspect ratio tended to direct the [1̅10] orientation parallel to the conduction direction, which will be suitable from a viewpoint of anisotropy in mobility. The present result shows a possibility of controlling the position and orientation of organic single crystals by the use of an electric field, which leads to high throughput and low cost industrial manufacturing of the single crystal array from solution.

  15. Holmium iron borate: high-resolution spectroscopy and crystal-field parameters

    NASA Astrophysics Data System (ADS)

    Erofeev, D. A.; Chukalina, E. P.; Popova, M. N.; Malkin, B. Z.; Bezmaternykh, L. N.; Gudim, I. A.

    2016-12-01

    High-resolution transmission spectra of HoFe3(BO3)4 single crystals were measured in broad spectral (5000-23000 cm-1) and temperature (1.7-300 K) ranges. Crystal-field energies of the Ho3+ ions were determined for a paramagnetic and easy-axis antiferromagnetic phases of the compound. On the basis of these data and of preliminary crystal-field calculations in the frame of the exchange-charge model, crystal-field parameters were found. A parameter of the isotropic Ho-Fe exchange interaction was estimated.

  16. Crystal-field calculations for transition-metal ions by application of an opposing potential

    DOE PAGES

    Zhou, Fei; Aberg, Daniel

    2016-02-16

    We propose a fully ab initio method, the opposing crystal potential (OCP), to calculate the crystal-field parameters of transition-metal impurities in insulator hosts. Through constrained density functional calculations, OCP obtains the constraining Lagrange multipliers, which act as a cancellation potential against the crystal field and lead to spherical d-electron distribution. Furthermore, the method is applied to several insulators doped with Mn4+ and Mn2+ ions and shown to be in good agreement with experiment.

  17. Using Magnetic Fields to Control Convection during Protein Crystallization: Analysis and Validation Studies

    NASA Technical Reports Server (NTRS)

    Ramachandran, N.; Leslie, F. W.

    2004-01-01

    The effect of convection during the crystallization of proteins is not very well understood. In a gravitational field, convection is caused by crystal sedimentation and by solutal buoyancy induced flow and these can lead to crystal imperfections. While crystallization in microgravity can approach diffusion limited growth conditions (no convection), terrestrially strong magnetic fields can be used to control fluid flow and sedimentation effects. In this work, we develop the analysis for magnetic flow control and test the predictions using analog experiments. Specifically, experiments on solutal convection in a paramagnetic fluid were conducted in a strong magnetic field gradient using a dilute solution of Manganese Chloride. The observed flows indicate that the magnetic field can completely counter the settling effects of gravity locally and are consistent with the theoretical predictions presented. This phenomenon suggests that magnetic fields may be useful in mimicking the microgravity environment of space for some crystal growth ana biological applications where fluid convection is undesirable.

  18. Using Magnetic Fields to Control Convection during Protein Crystallization: Analysis and Validation Studies

    NASA Technical Reports Server (NTRS)

    Ramachandran, N.; Leslie, F. W.

    2004-01-01

    The effect of convection during the crystallization of proteins is not very well understood. In a gravitational field, convection is caused by crystal sedimentation and by solutal buoyancy induced flow and these can lead to crystal imperfections. While crystallization in microgravity can approach diffusion limited growth conditions (no convection), terrestrially strong magnetic fields can be used to control fluid flow and sedimentation effects. In this work, we develop the analysis for magnetic flow control and test the predictions using analog experiments. Specifically, experiments on solutal convection in a paramagnetic fluid were conducted in a strong magnetic field gradient using a dilute solution of Manganese Chloride. The observed flows indicate that the magnetic field can completely counter the settling effects of gravity locally and are consistent with the theoretical predictions presented. This phenomenon suggests that magnetic fields may be useful in mimicking the microgravity environment of space for some crystal growth ana biological applications where fluid convection is undesirable.

  19. Effect of intense magnetic fields on the convection of biogenic guanine crystals in aqueous solution

    NASA Astrophysics Data System (ADS)

    Iwasaka, M.; Mizukawa, Y.

    2015-05-01

    In this study, the basic magneto-optic properties of biogenic microcrystals in aqueous media were investigated. Microcrystals, mica plates, silica, and microcrystals from a diatom cell and biogenic guanine crystals from goldfish showed light scattering inhibition when the crystals were observed in water under a 5 T magnetic field and dark-field illumination. In particular, in 50% ethanol/water medium, convection of the biogenic guanine particle aggregates was reversibly inhibited when the microcrystal suspension was exposed to a 5 T magnetic field. Microscopic observation comparing the biogenic guanine crystals in water with 95% ethanol or 99% acetone revealed that light flickering on the surface of the crystals was affected by the surface interaction of the crystal with the surrounding medium. By considering both the magnetic orientation of the microcrystals and the possible interactions of crystals with the surrounding medium, a magnetically controllable fluidic tracer was suggested.

  20. Crystallization of ethylene/alpha-olefin copolymers in shear fields

    NASA Astrophysics Data System (ADS)

    Shamsundar, R.

    2005-03-01

    Metallocene ethylene-co-α-olefins represent model materials to investigate the effect of ``non-crystallizable defects'' on crystallization of sheared copolymers. We present results on polymers having similar molecular weight and polydispersity (viz. same chain mobility), but varying comonomer percentage (viz. varying topological contraints on crystallization). Shear crystallization experiments are performed in a Linkam shear cell using optical techniques to monitor phase change. The polymer is melted at an elevated temperature, then sheared at a controlled rate for a fixed duration. After shearing, the polymer is cooled to a temperature chosen such that the quiescent crystallization time at that temperature is around 5000 s. Therefore, the crystallization temperature varies with comonomer content and serves only as a ``read-out'' to determine the effect of shearing. Shearing a copolymer (containing just 1 mole percent comonomer) has almost no effect on crystallization kinetics under conditions where shear greatly accelerates homopolymer crystallization. As chain relaxation dynamics are similar for all our polymers, shear enhanced crystallization kinetics in homopolymers is due to the formation of precursors during shear. These precursors are not formed for copolymers. Thus, while precursor formation happens via a rheological pathway, the ``crystallizability'' of the polymer chain determines the chance of precursor formation.

  1. Phase Field Modeling of Twinning in Indentation of Transparent Crystals

    DTIC Science & Technology

    2011-09-01

    functional of (32). Thus the mathematical problem of interest whose solution is sought numerically , as described later in sections 4 and 5, can be...of deformed and twinned crystals are attained numerically via direct energy minimization. Results are in qualitative agreement with experimental...deformed and twinned crystals are attained numerically via direct energy minimization. Results are in qualitative agreement with experimental

  2. Neutron study of crystal field excitations in single crystal CeCu2Ge2

    NASA Astrophysics Data System (ADS)

    Loewenhaupt, Michael; Faulhaber, Enrico; Schneidewind, Astrid; Deppe, Micha; Hradil, Klaudia

    2010-03-01

    CeCu2Ge2 is the counterpart of the heavy-fermion superconductor CeCu2Si2. CeCu2Ge2 is a magnetically ordering (TN= 4.1 K) Kondo lattice with a moderate Sommerfeld coefficient of 140 mJ/molK^2 [1]. Inelastic neutron measurements on a polycrystalline sample revealed a doublet ground state and a quasi-quartet at 16.5 meV [1] though a splitting of the 4f^1 (J=5/2) ground state multiplet into 3 doublets is expected from the point symmetry of the Ce^3+ ions. We performed detailed inelastic neutron scattering experiments on the thermal triple-axis spectrometer PUMA at FRM II at temperatures between 10 K and 300 K and for different crystallographic directions from low to high momentum transfers. In this way we obtained a reliable separation of magnetic and phonon contributions. From our results we infer that the quasi-quartet consists in fact of two doublets at 17 and 18 meV which exhibit a strong directional dependence of their transition matrix elements to the ground state doublet. Finally we will present a new set of crystal field parameters and their implications on other magnetic properties. [1] G. Knopp et al., Z. Physik B 77 (1989) 95

  3. Spatial Distribution of -Crystals in Metallocene-Made Isotactic Polypropylene Crystallized under Combined Thermal and Flow Fields

    SciTech Connect

    Wang, Y.; Pan, J; Mao, Y; Li, Z; Li, L; Hsiao, B

    2010-01-01

    The present Article reports the relationships between molecular orientation, formation, and spatial distribution of {gamma}-crystals in metallocene-made isotactic polypropylene (m-iPP) samples prepared by two industrial processes: conventional injection molding (CIM) and oscillatory shear injection molding (OSIM), in which combined thermal and flow fields typically exist. In particular, spatial distributions of crystallinity, fraction of {gamma}-crystal (f{gamma}) with respect to {alpha}-crystal, and lamella-branched shish-kebab structure in the shaped samples were characterized by synchrotron two-dimensional (2D) wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. The results showed that the crystallinity in any given region of OSIM samples was always higher than that of CIM samples. The value of f{gamma} increased monotonously from skin to core in CIM samples, whereas the corresponding f{gamma} increased nonmonotonically in OSIM samples. The spatial distribution of {gamma}-crystal in OSIM samples can be explained by the epitaxial arrangement between {gamma}- and {alpha}-crystal in a lamella-branched shish-kebab structure. In the proposed model, the parent lamellae of {alpha}-crystal provide secondary nucleation sites for daughter lamellae of {alpha}-crystal and {gamma}-crystal, and the different content of parent lamellae results in varying amounts of {gamma}-crystal. In OSIM samples, the smallest parent-daughter ratio ([R] = 1.38) in the core region led to the lowest fraction of {gamma}-crystal (0.57), but relatively higher {gamma}-crystal content (0.69) at 600 and 1200 {micro}m depth of the samples (corresponding to [R] of 4.5 and 5.8, respectively). This is consistent with the proposed model where more parent lamellae provide more nucleation sites for crystallization, thus resulting in higher content of {gamma}-crystal. The melting behavior of CIM and OSIM samples was studied by differential scanning calorimetery (DSC). The

  4. Magnetic field sensor based on selectively magnetic fluid infiltrated dual-core photonic crystal fiber

    NASA Astrophysics Data System (ADS)

    Gangwar, Rahul Kumar; Bhardwaj, Vanita; Singh, Vinod Kumar

    2016-02-01

    We reported the modeling result of selectively magnetic fluid infiltrated dual-core photonic crystal fiber based magnetic field sensor. Inside the cross-section of the designed photonic crystal fiber, the two fiber cores filled with magnetic fluid (Fe3O4) form two independent waveguides with mode coupling. The mode coupling under different magnetic field strengths is investigated theoretically. The sensitivity of the sensor as a function of the structural parameters of the photonic crystal fiber is calculated. The result shows that the proposed sensing device with 1 cm photonic crystal fiber length has a large sensitivity of 305.8 pm/Oe.

  5. Large area mode field photonic crystal fiber design

    NASA Astrophysics Data System (ADS)

    Guo, Shuqin; An, Wensheng; Wang, Kang; Zhu, Guangxin; Le, Zichun

    2005-11-01

    A novel design method about photonic crystal fiber (PCF) with large area model field (LAMF) is demonstrated. Different from ordinarily design that the core of PCF formed by missing one air holes in the center of section, many air holes distributed in heartland all together come into being the core region. Air holes are arranged regularly in core region and outer cladding regions according to different periodical character, respectively. The effective refractive index (n eff ) of core region should be higher than cladding region because of total internal reflection (TIR) requirement. In this paper, two kinds of typical scheme are offered to realize LAMF-PCF. First, Λ, the spacing of neighboring air holes in whole section is fixed, once the radius of air holes in the core region r c is smaller than the cladding air holes r cla, LAMF-PCF will be formed. The modal area only lessens a little as r c is reduced. Especially, optimal size of r c can nearly make MFA insensitive to wavelength. On the contrary, dispersion parameter of PCF will take place visible change along with r c reduced, and ultra-flattened dispersion character can be realized when r c is optimized. Another method of designing LAMF-PCF is keeping all air holes uniform in the whole section of PCF, but the space of neighboring air holes in the core region Λ c is longer than the cladding region Λ cla, so n eff of core region is higher than the cladding region and TIR can take place.

  6. Impedance of Polymer-Dispersed Liquid Crystals with Carbon Nanofibers in Weak Electric Fields

    NASA Astrophysics Data System (ADS)

    Zhdanov, K. R.; Romanenko, A. I.; Zharkova, G. M.; Podyacheva, O. Yu.

    2016-11-01

    Impedance of polymer-dispersed liquid crystals modified by carbon nanofibers is studied in fields lower than the threshold field of the director reorientation of a liquid crystal. It is shown that the real and imaginary parts of the impedance obey to the relationship (Zre - X0)2 + (Zim - Y0)2 = R 0 2 , where X0, Y0, and R0 are the fitting parameters depending on the frequency of the exciting electric field.

  7. Dynamic Behavior Analysis of Crystal with Magnetic Anisotropy under Imposition of Rotating Magnetic Field

    NASA Astrophysics Data System (ADS)

    Iwai, Kazuhiko

    2010-12-01

    The alignment behavior of a crystal with a magnetic anisotropy of χc < χa under the imposition of a rotating magnetic field has been investigated by numerical calculation. The promotion of the crystal alignment when the projection of the magnetically hard axis on the magnetic field rotating plane is parallel to the magnetic field direction and its suppression when the magnetically hard axis is perpendicular to the magnetic field direction can be explained by the fact that the direction of the driving torque acting on the crystal minimizes the magnetic energy. Non dimensional alignment time normalized by the alignment time under the imposition of a static field is constant in the out-of-step region where the crystal cannot follow the magnetic field rotation during its alignment. The initial phase difference between the projection of the magnetically hard axis on the magnetic field rotating plane and its direction hardly affects the alignment time in the out-of-step region but strongly affects that in the synchronous region where the crystal rotation synchronous with the magnetic field rotation. A crystal aligns quickly if the initial phase difference is between 0 and 90° in the synchronous region. The minimum alignment time is the same as that under the imposition of a static field.

  8. Phase-field simulations of crystal growth in a two-dimensional cavity flow

    NASA Astrophysics Data System (ADS)

    Lee, Seunggyu; Li, Yibao; Shin, Jaemin; Kim, Junseok

    2017-07-01

    In this paper, we consider a phase-field model for dendritic growth in a two-dimensional cavity flow and propose a computationally efficient numerical method for solving the model. The crystal is fixed in the space and cannot be convected in most of the previous studies, instead the supercooled melt flows around the crystal, which is hard to be realized in the real world experimental setting. Applying advection to the crystal equation, we have problems such as deformation of crystal shape and ambiguity of the crystal orientation for the anisotropy. To resolve these difficulties, we present a phase-field method by using a moving overset grid for the dendritic growth in a cavity flow. Numerical results show that the proposed method can predict the crystal growth under flow.

  9. SINGLE CRYSTAL CADMIUM SULFIDE AND CADMIUM SELENIDE INSULATED-GATE FIELD-EFFECT TRIODES.

    DTIC Science & Technology

    Insulated-gate field-effect triodes were fabricated on single crystal cadmium sulfide and cadmium selenide . Both bulk crystals and platelets were...used for single crystal samples. Chromium and aluminum were found to make low impedance contacts to cadmium sulfide and cadmium selenide . The...polycrystalline cadmium sulfide and cadmium selenide IGFET’s. The characteristics of the fabricated devices were unstable with respect to time and temperature

  10. Characterizing configurable transmission modes in plasma photonic crystals using scanning field mapping

    NASA Astrophysics Data System (ADS)

    Wang, Benjamin; Cappelli, Mark

    2016-10-01

    A fully tunable plasma photonic crystal is used to control the propagation of free space electromagnetic waves in the S to X band of the microwave spectrum. A structured array of discharge plasma tubes are arranged in a square crystal lattice with the individual plasma dielectric constant tuned through variation in the plasma density. Microwave field-mapping is used to characterize the transmitted electromagnetic fields of the tunable device operating in waveguiding and bending modes. These modes are obtained by introducing appropriate line defects in the photonic crystal structure by controlling the activity of individual plasma tubes. Comparisons are made of the measured fields to those simulated using commercially-available software.

  11. Static magnetic susceptibility, crystal field and exchange interactions in rare earth titanate pyrochlores.

    PubMed

    Malkin, B Z; Lummen, T T A; van Loosdrecht, P H M; Dhalenne, G; Zakirov, A R

    2010-07-14

    The experimental temperature dependence (T = 2-300 K) of single crystal bulk and site susceptibilities of rare earth titanate pyrochlores R(2)Ti(2)O(7) (R = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) is analyzed in the framework of crystal field theory and a mean field approximation. Analytical expressions for the site and bulk susceptibilities of the pyrochlore lattice are derived taking into account long range dipole-dipole interactions and anisotropic exchange interactions between the nearest neighbor rare earth ions. The sets of crystal field parameters and anisotropic exchange coupling constants have been determined and their variations along the lanthanide series are discussed.

  12. Anisotropic magnetic properties and crystal electric field studies on CePd2Ge2 single crystal.

    PubMed

    Maurya, Arvind; Kulkarni, R; Dhar, S K; Thamizhavel, A

    2013-10-30

    The anisotropic magnetic properties of the antiferromagnetic compound CePd2Ge2, crystallizing in the tetragonal crystal structure have been investigated in detail on a single crystal grown by the Czochralski method. From the electrical transport, magnetization and heat capacity data, the Néel temperature is confirmed to be 5.1 K. Anisotropic behaviour of the magnetization and resistivity is observed along the two principal crystallographic directions-namely, [100] and [001]. The isothermal magnetization measured in the magnetically ordered state at 2 K exhibits a spin reorientation at 13.5 T for the field applied along the [100] direction, whereas the magnetization is linear along the [001] direction attaining a value of 0.94 μ(B)/Ce at 14 T. The reduced value of the magnetization is attributed to the crystalline electric field (CEF) effects. A sharp jump in the specific heat at the magnetic ordering temperature is observed. After subtracting the phononic contribution, the jump in the heat capacity amounts to 12.5 J K(-1)mol(-1) which is the expected value for a spin ½ system. From the CEF analysis of the magnetization data the excited crystal field split energy levels were estimated to be at 120 K and 230 K respectively, which quantitatively explains the observed Schottky anomaly in the heat capacity. A magnetic phase diagram has been constructed based on the field dependence of magnetic susceptibility and the heat capacity data.

  13. Polymer crystallization in a temperature gradient field with controlled crystal growth rate

    NASA Technical Reports Server (NTRS)

    Hansen, D.; Taskar, A. N.; Casale, O.

    1971-01-01

    A method is described for studying the influence of a temperature gradient on the crystallization of quiescent polymer melts. The apparatus used consists of two brass plates with embedded electrical resistance heaters and cooling coils. The crystallizations experiments were conducted by placing polymer specimens between the paltes, and manually adjusting heaters and cooling fluids for temperature control. Linear polyethylene, isotactic polyprophylene, and a high density polyethylene were used. It is concluded that the role of a temperature gradient in producing oriented crystallization is in producing conditions which lead the spherulitic growth pattern to proceed primarily in one direction. Steep gradients diminish the penetration of supercooling and favors oriented growth.

  14. Growth, structure, spectral properties and crystal-field analysis of monoclinic Nd:YNbO4 single crystal

    NASA Astrophysics Data System (ADS)

    Ding, Shoujun; Zhang, Qingli; Gao, Jinyun; Liu, Wenpeng; Luo, Jianqiao; Sun, Dunlu; Sun, Guihua; Wang, Xiaofei

    2016-12-01

    A Nd:YNbO4 single crystal was successfully grown by Czochralski (Cz) method, its structural and spectroscopic properties were investigated. The X-ray rocking curve of the (010) diffraction face of Nd:YNbO4 crystal was measured, the full width at half maximum (FWHM) of this diffraction peak is 0.05°, which indicates a high crystalline quality of the as-grown crystal. Its lattice parameters, atomic coordinates and so on were obtained by Rietvield refinement to X-ray diffraction data. According to the Archimedes drainage method, the crystal density of Nd:YNbO4 is calculated to be 5.4 g/cm3. The Mohr‧s hardness value along (010) face was determined to be 6.0. The transmission spectrum along (010) face at room temperature was recorded and the excitation and emission spectra at 8 K were measured. Photoluminescence peaks of Nd:YNbO4 were assigned, and the crystal-field splitting of Nd3+ in YNbO4 was obtained. The fluorescence lifetime of the 4F3/2→4I11/2 transition of Nd3+ in YNbO4 is fitted to be 152 μs These spectroscopic and energy splitting data give an important reference for the research of laser property of Nd:YNbO4 crystal.

  15. Formation of temperature fields in doped anisotropic crystals under spatially inhomogeneous light beams passing through them

    SciTech Connect

    Zaitseva, E. V.; Markelov, A. S.; Trushin, V. N. Chuprunov, E. V.

    2013-12-15

    The features of formation of thermal fields in potassium dihydrophosphate crystal doped with potassium permanganate under a 532-nm laser beam passing through it have been investigated. Data on the influence of birefringence on the temperature distribution in an anisotropic crystal whose surface is illuminated by a spatially modulated light beam are presented.

  16. Imposed Orientation of Dye Molecules by Liquid Crystals and an Electric Field.

    ERIC Educational Resources Information Center

    Sadlej-Sosnowska, Nina

    1980-01-01

    Describes experiments using dye solutions in liquid crystals in which polar molecules are oriented in an electrical field and devices are constructed to change their color in response to an electric signal. (CS)

  17. Imposed Orientation of Dye Molecules by Liquid Crystals and an Electric Field.

    ERIC Educational Resources Information Center

    Sadlej-Sosnowska, Nina

    1980-01-01

    Describes experiments using dye solutions in liquid crystals in which polar molecules are oriented in an electrical field and devices are constructed to change their color in response to an electric signal. (CS)

  18. Transverse magnetic field impact on waveguide modes of photonic crystals.

    PubMed

    Sylgacheva, Daria; Khokhlov, Nikolai; Kalish, Andrey; Dagesyan, Sarkis; Prokopov, Anatoly; Shaposhnikov, Alexandr; Berzhansky, Vladimir; Nur-E-Alam, Mohammad; Vasiliev, Mikhail; Alameh, Kamal; Belotelov, Vladimir

    2016-08-15

    This Letter presents a theoretical and experimental study of waveguide modes of one-dimensional magneto-photonic crystals magnetized in the in-plane direction. It is shown that the propagation constants of the TM waveguide modes are sensitive to the transverse magnetization and the spectrum of the transverse magneto-optical Kerr effect has resonant features at mode excitation frequencies. Two types of structures are considered: a non-magnetic photonic crystal with an additional magnetic layer on top and a magneto-photonic crystal with a magnetic layer within each period. We found that the magneto-optical non-reciprocity effect is greater in the first case: it has a magnitude of δ∼10-4, while the second structure type demonstrates δ∼10-5 only, due to the higher asymmetry of the claddings of the magnetic layer. Experimental observations show resonant features in the optical and magneto-optical Kerr effect spectra. The measured dispersion properties are in good agreement with the theoretical predictions. An amplitude of light intensity modulation of up to 2.5% was observed for waveguide mode excitation within the magnetic top layer of the non-magnetic photonic crystal structure. The presented theoretical approach may be utilized for the design of magneto-optical sensors and modulators requiring pre-determined spectral features.

  19. Bragg diffraction of light from ultrasound in cubic centrosymmetric crystals in an external electric field

    SciTech Connect

    Kurilkina, S.N.

    1995-03-01

    Special features of noncollinear Bragg diffraction of light from ultrasound in centrosymmetric cubic crystals placed in an external electric field are considered. Particular cases of acousto-electro-optical interaction on (quasi-)longitudinal and (quasi-)transverse acoustic waves propagating the (001) and (110) planes are analyzed. The dependence of diffracted light energy characteristics on photoelastic and electro-optical parameters of a cubic centrosymmetric crystal, as well as external field strength and orientation, is determined. 11 refs., 2 figs.

  20. A study of the stress dependence of zero-field splitting for YGG: Mn 2+ crystal

    NASA Astrophysics Data System (ADS)

    Wen-Chen, Zheng

    1991-02-01

    The stress dependence of zero-field splitting for Mn 2+ ion in trigonal site of YGG crystal has been reasonably explained by calculating the spin-lattice coupling coefficients G44 within the framework of cubic symmetry approximation. On this basis, the zero-field splittings for Mn 2+ ion in the trigonal site of YGG and YAG crystals can also be interpreted in terms of the distinctive trigonal distortions.

  1. Study of Fluid Flow Control in Protein Crystallization using Strong Magnetic Fields

    NASA Technical Reports Server (NTRS)

    Ramachandran, Narayanan; Leslie, Fred; Ciszak, Ewa

    2002-01-01

    An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular crystals for diffraction analyses has been the central focus for biochemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of the container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and sedimentation, as is achieved in "microgravity", researchers have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, formation of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. Whether this limited convection in a magnetic field will provide the environment for the growth of high quality crystals is still a matter of conjecture that our research will address. The approach exploits the variation of fluid magnetic susceptibility with concentration for this purpose and the convective damping is realized by appropriately

  2. Method of bonding single crystal quartz by field-assisted bonding

    DOEpatents

    Curlee, R.M.; Tuthill, C.D.; Watkins, R.D.

    1991-04-23

    The method of producing a hermetic stable structural bond between quartz crystals includes providing first and second quartz crystals and depositing thin films of borosilicate glass and silicon on portions of the first and second crystals, respectively. The portions of the first and second crystals are then juxtaposed in a surface contact relationship and heated to a temperature for a period sufficient to cause the glass and silicon films to become electrically conductive. An electrical potential is then applied across the first and second crystals for creating an electrostatic field between the adjoining surfaces and causing the juxtaposed portions to be attracted into an intimate contact and form a bond for joining the adjoining surfaces of the crystals. 2 figures.

  3. Method of bonding single crystal quartz by field-assisted bonding

    DOEpatents

    Curlee, Richard M.; Tuthill, Clinton D.; Watkins, Randall D.

    1991-01-01

    The method of producing a hermetic stable structural bond between quartz crystals includes providing first and second quartz crystals and depositing thin films of borosilicate glass and silicon on portions of the first and second crystals, respectively. The portions of the first and second crystals are then juxtaposed in a surface contact relationship and heated to a temperature for a period sufficient to cause the glass and silicon films to become electrically conductive. An electrical potential is then applied across the first and second crystals for creating an electrostatic field between the adjoining surfaces and causing the juxtaposed portions to be attracted into an intimate contact and form a bond for joining the adjoining surfaces of the crystals.

  4. In-crystal and surface charge transport of electric-field-induced carriers in organic single-crystal semiconductors.

    PubMed

    Takeya, J; Kato, J; Hara, K; Yamagishi, M; Hirahara, R; Yamada, K; Nakazawa, Y; Ikehata, S; Tsukagoshi, K; Aoyagi, Y; Takenobu, T; Iwasa, Y

    2007-05-11

    Gate-voltage dependence of carrier mobility is measured in high-performance field-effect transistors of rubrene single crystals by simultaneous detection of the longitudinal conductivity sigma(square) and Hall coefficient R(H). The Hall mobility mu(H) (identical with sigma(square)R(H)) reaches nearly 10 cm(2)/V s when relatively low-density carriers (<10(11) cm(-2)) distribute into the crystal. mu(H) rapidly decreases with higher-density carriers as they are essentially confined to the surface and are subjected to randomness of the amorphous gate insulators. The mechanism to realize high carrier mobility in the organic transistor devices involves intrinsic-semiconductor character of the high-purity organic crystals and diffusive bandlike carrier transport in the bulk.

  5. A test of improved force field parameters for urea: molecular-dynamics simulations of urea crystals.

    PubMed

    Özpınar, Gül Altınbaş; Beierlein, Frank R; Peukert, Wolfgang; Zahn, Dirk; Clark, Timothy

    2012-08-01

    Molecular-dynamics (MD) simulations of urea crystals of different shapes (cubic, rectangular prismatic, and sheet) have been performed using our previously published force field for urea. This force field has been validated by calculating values for the cohesive energy, sublimation temperature, and melting point from the MD data. The cohesive energies computed from simulations of cubic and rectangular prismatic urea crystals in vacuo at 300 K agreed very well with the experimental sublimation enthalpies reported at 298 K. We also found very good agreement between the melting points as observed experimentally and from simulations. Annealing the crystals just below the melting point leads to reconstruction to form crystal faces that are consistent with experimental observations. The simulations reveal a melting mechanism that involves surface (corner/edge) melting well below the melting point, and rotational disordering of the urea molecules in the corner/edge regions of the crystal, which then facilitates the translational motion of these molecules.

  6. Crystal-field calculations for transition-metal ions by application of an opposing potential

    SciTech Connect

    Zhou, Fei; Aberg, Daniel

    2016-02-16

    We propose a fully ab initio method, the opposing crystal potential (OCP), to calculate the crystal-field parameters of transition-metal impurities in insulator hosts. Through constrained density functional calculations, OCP obtains the constraining Lagrange multipliers, which act as a cancellation potential against the crystal field and lead to spherical d-electron distribution. Furthermore, the method is applied to several insulators doped with Mn4+ and Mn2+ ions and shown to be in good agreement with experiment.

  7. Phase field crystal study of deformation and plasticity in nanocrystalline materials.

    PubMed

    Stefanovic, Peter; Haataja, Mikko; Provatas, Nikolas

    2009-10-01

    We introduce a modified phase field crystal (MPFC) technique that self-consistently incorporates rapid strain relaxation alongside the usual plastic deformation and multiple crystal orientations featured by the traditional phase field crystal (PFC) technique. Our MPFC formalism can be used to study a host of important phase transformation phenomena in material processing that require rapid strain relaxation. We apply the MPFC model to study elastic and plastic deformations in nanocrystalline materials, focusing on the "reverse" Hall-Petch effect. Finally, we introduce a multigrid algorithm for efficient numerical simulations of the MPFC model.

  8. Neutron study of crystal-field transitions in ErPO{sub 4}

    SciTech Connect

    Loong, C.-K.; Soderholm, L.; Hammonds, J.P.; Abraham, M.M.; Boatner, L.A.; Edelstein, N.M.

    1992-12-01

    The crystal-field splitting of the Er{sup 3+} ground multiplet, {sup 4}I{sub 15/2}, in ErPO{sub 4} is investigated by inelastic neutron scattering. Four excitations from the {Gamma}{sub 7} ground state to the excited states and several transitions between the excited states have been identified. The observed transition energies and intensities are used to refine the parameters of the crystal-field potential. The calculated magnetic susceptibility, {chi}(T), agrees well with experimental values from single-crystal measurements. A comparison of the neutron data with optical absorption and both nonresonance and resonance Raman scattering measurements has been made.

  9. Neutron study of crystal-field transitions in ErPO[sub 4

    SciTech Connect

    Loong, C.-K.; Soderholm, L.; Hammonds, J.P. ); Abraham, M.M.; Boatner, L.A. ); Edelstein, N.M. )

    1992-01-01

    The crystal-field splitting of the Er[sup 3+] ground multiplet, [sup 4]I[sub 15/2], in ErPO[sub 4] is investigated by inelastic neutron scattering. Four excitations from the [Gamma][sub 7] ground state to the excited states and several transitions between the excited states have been identified. The observed transition energies and intensities are used to refine the parameters of the crystal-field potential. The calculated magnetic susceptibility, [chi](T), agrees well with experimental values from single-crystal measurements. A comparison of the neutron data with optical absorption and both nonresonance and resonance Raman scattering measurements has been made.

  10. A Navier-Stokes phase-field crystal model for colloidal suspensions.

    PubMed

    Praetorius, Simon; Voigt, Axel

    2015-04-21

    We develop a fully continuous model for colloidal suspensions with hydrodynamic interactions. The Navier-Stokes Phase-Field Crystal model combines ideas of dynamic density functional theory with particulate flow approaches and is derived in detail and related to other dynamic density functional theory approaches with hydrodynamic interactions. The derived system is numerically solved using adaptive finite elements and is used to analyze colloidal crystallization in flowing environments demonstrating a strong coupling in both directions between the crystal shape and the flow field. We further validate the model against other computational approaches for particulate flow systems for various colloidal sedimentation problems.

  11. Comparison of the simulations of cellulosic crystals with three carbohydrate force fields.

    PubMed

    Miyamoto, Hitomi; Schnupf, Udo; Crowley, Michael F; Brady, John W

    2016-03-03

    Three independently developed molecular mechanics force fields for carbohydrates have been used to simulate a suite of small molecule analogs of cellulose for which crystal structures have been reported, as a test to determine which might be best for simulations of cellulose itself. Such evaluation is necessary since the reported cellulose crystal structure is not stable in molecular dynamics simulations with any available force field. The present simulations found that all three resulted in small deviations from the reported crystal structures, but that all were reasonably accurate and none was clearly superior to the others for the entire suite of structures examined. Copyright © 2016 Elsevier Ltd. All rights reserved.

  12. A Navier-Stokes phase-field crystal model for colloidal suspensions

    SciTech Connect

    Praetorius, Simon Voigt, Axel

    2015-04-21

    We develop a fully continuous model for colloidal suspensions with hydrodynamic interactions. The Navier-Stokes Phase-Field Crystal model combines ideas of dynamic density functional theory with particulate flow approaches and is derived in detail and related to other dynamic density functional theory approaches with hydrodynamic interactions. The derived system is numerically solved using adaptive finite elements and is used to analyze colloidal crystallization in flowing environments demonstrating a strong coupling in both directions between the crystal shape and the flow field. We further validate the model against other computational approaches for particulate flow systems for various colloidal sedimentation problems.

  13. Models of Mass Transport During Microgravity Crystal Growth of Alloyed Semiconductors in a Magnetic Field

    NASA Technical Reports Server (NTRS)

    Ma, Nancy

    2003-01-01

    Alloyed semiconductor crystals, such as germanium-silicon (GeSi) and various II-VI alloyed crystals, are extremely important for optoelectronic devices. Currently, high-quality crystals of GeSi and of II-VI alloys can be grown by epitaxial processes, but the time required to grow a certain amount of single crystal is roughly 1,000 times longer than the time required for Bridgman growth from a melt. Recent rapid advances in optoelectronics have led to a great demand for more and larger crystals with fewer dislocations and other microdefects and with more uniform and controllable compositions. Currently, alloyed crystals grown by bulk methods have unacceptable levels of segregation in the composition of the crystal. Alloyed crystals are being grown by the Bridgman process in space in order to develop successful bulk-growth methods, with the hope that the technology will be equally successful on earth. Unfortunately some crystals grown in space still have unacceptable segregation, for example, due to residual accelerations. The application of a weak magnetic field during crystal growth in space may eliminate the undesirable segregation. Understanding and improving the bulk growth of alloyed semiconductors in microgravity is critically important. The purpose of this grant to to develop models of the unsteady species transport during the bulk growth of alloyed semiconductor crystals in the presence of a magnetic field in microgravity. The research supports experiments being conducted in the High Magnetic Field Solidification Facility at Marshall Space Flight Center (MSFC) and future experiments on the International Space Station.

  14. Crystal field splitting and symmetry of Ce3 polyhedra in oxide crystals

    NASA Astrophysics Data System (ADS)

    Kodama, N.; Yamaga, M.; Kurahashi, T.

    Single crystals of Ce3+-doped oxides Ca2Al2SiO7 (CASM), CaYAl3O7 (CYAM) and CaYAlO4 (CYAP) have been grown in inert and reducing atmospheres, with intent of studying the effects of symmetry and ligand cordination on the dopant energy levels. The optical absorption spectra of Ce3+ in these crystals at most consist of five overlapping bands. The luminescence is also a broad band due to strong electron-phonon interaction in the 5d excited state. The luminescence bands for CASM, CYAM and CYAP are intermediate between GdAlO3 (GAP) and Y3Al5O12 (YAG). The energies of the lowest absorption band and the luminescence band decrease in order of GAP, CASM, CYAM, CYAP, and YAG. This trend may be explained by lowering symmetry and reducing size of the anion-coordinate polyhedra of Ce3+ in these crystals.

  15. Influence of Acoustic Field Structure on Polarization Characteristics of Acousto-optic Interaction in Crystals

    NASA Astrophysics Data System (ADS)

    Muromets, A. V.; Trushin, A. S.

    Influence of acoustic field structure on polarization characteristics of acousto-optic interaction is investigated. It is shown that inhomogeneity of acoustic field and mechanism of ultrasound excitation causes changes in values of acousto-optic figure of merit for ordinary and extraordinary light beams in comparison with theoretic values. The theoretic values were derived under assumption that acoustic wave is homogeneous. Experimental analysis was carried out in acousto-optic cell based on lithium niobate crystal where the acoustic wave propagates at the angle 13 degrees to Z axis of the crystal. We used three different methods of ultrasound generation in the crystal: by means of external piezotransducer, by interdigital transducer and by two sets of electrodes placed on top of the crystal surface. In the latter case, the first pair of the electrodes was directed along X crystal axis, while the second pair of the electrodes was directed orthogonally to X crystal axis and the direction of ultrasound. Obtained values for diffraction efficiencies for ordinary and extraordinary polarized optical beams were qualitatively different which may be caused by spatial inhomogeneity of the generated acoustic waves in the crystal. Structure of acoustic field generated by these sets of electrodes was examined by laser probing. We performed the analysis of the acoustic field intensity using acousto-optic method. A relation of diffraction efficiencies for ordinary and extraordinary light waves was measured during each iteration of the laser probing.

  16. Hidden local symmetry of Eu{sup 3+} in xenotime-like crystals revealed by high magnetic fields

    SciTech Connect

    Han, Yibo; Ma, Zongwei; Zhang, Junpei; Wang, Junfeng; Du, Guihuan; Xia, Zhengcai; Han, Junbo Li, Liang; Yu, Xuefeng

    2015-02-07

    The excellent optical properties of europium-doped crystals in visible and near infrared wavelength regions enable them to have broad applications in optoelectronics, laser crystals and sensing devices. The local site crystal fields can affect the intensities and peak positions of the photo-emission lines strongly, but they are usually difficult to be clarified due to magnetically degenerate 4f electronic levels coupling with the crystal fields. Here, we provide an effective way to explore the hidden local symmetry of the Eu{sup 3+} sites in different hosts by taking photoluminescence measurements under pulsed high magnetic fields up to 46 T. The zero-field photoluminescence peaks split further at high magnetic fields when the Zeeman splitting energy is comparable to or larger than that of the crystal field induced zero-field splitting. In particular, a magnetic field induced crossover of the local crystal fields has been observed in the GdVO{sub 4}:Eu{sup 3+} crystal, which resulted from the alignment of Gd{sup 3+} magnetic moment in high magnetic fields; and a hexagonally symmetric local crystal fields was observed in the YPO{sub 4} nanocrystals at the Eu{sup 3+} sites characterized by the special axial and rhombic crystal field terms. These distinct Zeeman splitting behaviors uncover the crystal fields-related local symmetry of luminescent Eu{sup 3+} centers in different hosts or magnetic environments, which are significant for their applications in optics and optoelectronics.

  17. Change in crystal structure and physical properties of the Multiferroics YMnO3 single crystals by Strong gravitational field

    NASA Astrophysics Data System (ADS)

    Tokuda, M.; Weijian, M.; Hayami, S.; Yoshiasa, A.; Mashimo, T.

    2017-04-01

    Many researchers have studied the multiferroicity of the hexagonal RMnO3 (R: rare-earth element) for both applications and fundamental studies. To investigate the relationship between the structure and physical properties of materials, some people apply the chemical pressure effect. The procedure of chemical pressure effect involves substituting rare-earth elements for ones which have a different ionic radius. Mashimo et al. have developed a high-temperature ultracentrifuge apparatus that can generate extended duration strong gravitational field in excess of 106 G under a wide range of temperatures (up to 500°C). Strong gravitational fields directly act on each atom as a different body force. This can cause the change in crystal structure. Thus, we subjected YMnO3 single crystal to strong gravity experiments (0.78×106 G, 400°C, 2 h) and investigated the resulting changes in the crystal structure and physical properties of the gravity sample. The single crystal four-circle X-ray diffraction measurements revealed the change in the nearest neighboring Mn-Mn and M-O bond distances. The temperature dependence of magnetic susceptibility by SQUID showed the change in the magnetic anisotropy of gravity sample.

  18. Controlled deposition or organic semiconductor single crystals and its application in field-effect transistors

    NASA Astrophysics Data System (ADS)

    Liu, Shuhong

    The search for low-cost, large area, flexible devices has led to a remarkable increase in the research and development of organic semiconductors. Single-crystal organic field-effect transistors (OFETs) are ideal device structures for studying fundamental science associated with charge transport in organic materials and have demonstrated high mobility and outstanding electrical characteristics. For example, an exceptionally high carrier mobility of 20 cm2/Vs has been demonstrated for rubrene single crystal field effect transistors. However, it remains a technical challenge to integrate single-crystal devices into practical electronic applications. A key difficulty is that organic single-crystal devices are usually fabricated one device at a time by handpicking a single crystal and placing it onto the device substrate. This makes it impossible to mass-produce at high density with reasonable throughput. Therefore, there is a great need for a high-throughput method for depositing large arrays of organic semiconductor single crystals directly onto device structures. In this dissertation, I develop several approaches towards realizing this goal. The first approach is a solution-processing technique, which relies on solvent wetting and de-wetting on substrates with patterned wettability to selectively direct the deposition or removal of organic crystals. The assembly of different organic crystals over centimeter-squared areas on Au, SiO 2 and flexible plastic substrates is demonstrated. By designing line features on the substrate, alignment of needle-like crystals is also achieved. As a demonstration of the potential application of this approach, arrays of organic single crystal FETs are fabricated by patterning organic single crystals directly onto and between transistor source and drain electrodes. Besides organic single crystals, this self-assembly strategy is also applicable for patterning other objects such as metallic nanowires. In the second technique, organic

  19. Propelling and spinning of microsheets in nematic liquid crystals driven by ac electric field

    NASA Astrophysics Data System (ADS)

    Rasna, M. V.; Ramudu, U. V.; Chandrasekar, R.; Dhara, Surajit

    2017-01-01

    Dynamics of microparticles in isotropic liquids by transducing the energy of an applied electric field have been studied for decades. Recently, such studies in anisotropic media like liquid crystals have opened up new perspectives in colloid science. Here, we report studies on ac-electric-field-driven dynamics of microsheets in nematic liquid crystals. In planar aligned liquid crystals, with negative dielectric anisotropy, the microsheets are propelled parallel to the director. A steady spinning of the microsheets is observed in homeotropic cells with positive dielectric anisotropy liquid crystals. The velocity of propelling and the angular frequency of spinning depends on the amplitude and the frequency of the applied electric field. The electrokinetic studies of anisotropic microparticles are important as they are potential for applications in microfluidics and in areas where the controlled transport or rotation is required.

  20. Composite Fermion Theory for the High Field Wigner Crystal State

    NASA Astrophysics Data System (ADS)

    Narevich, Romanas; Murthy, Ganpathy; Fertig, Herbert

    2001-03-01

    The low filling fraction Quantum Hall Effect is reexamined using the hamiltonian composite fermion theory developed by Shankar and Murthy(R. Shankar and G. Murthy, Phys. Rev. Lett. 79), 4437 (1997). We address the experiment by Jiang et. al.(H. W. Jiang et. al., Phys. Rev. B 44), 8107 (1991) where the insulating phase surrounding the ν=1/5 quantum liquid was observed and its activation energies (gaps) measured. Previous studies either found gaps that were off by few orders of magnitude (Hartree-Fock calculations of the electronic Wigner crystal(D. Yoshioka and H. Fukuyama, J. Phys. Soc. Japan 47), 394 (1979)) or were unable to calculate them because of the computational complexity (Monte-Carlo studies of the correlated crystal(H. Yi and H. A. Fertig, Phys. Rev. B 58), 4019 (1998)). We use the Hartree-Fock approximation for the periodic density state of composite fermions and find gaps that have a correct order of magnitude and reproduce the experimental dependence on the filling factor. We also report the results of the shear modulus calculation relevant for the collective pinning of the crystal.

  1. Phase diagrams of mixtures of a polymer and a cholesteric liquid crystal under an external field

    SciTech Connect

    Matsuyama, Akihiko

    2014-11-14

    We present a mean field theory to describe phase behaviors in mixtures of a polymer and a cholesteric liquid crystal under an external magnetic or electric field. Taking into account a chiral coupling between a polymer and a liquid crystal under the external field, we examine twist-untwist phase transitions and phase separations in the mixtures. It is found that a cholesteric-nematic phase transition can be induced by not only the external field but also concentration and temperature. Depending on the strength of the external field, we predict cholesteric-paranematic (Ch+pN), nematic-paranematic (N+pN), cholesteric-nematic (Ch+N) phase separations, etc., on the temperature-concentration plane. We also discuss mixtures of a non-chiral nematic liquid crystal and a chiral dopant.

  2. Magnetic field controlled single crystal growth and surface modification of titanium alloys exposed for biocompatibility

    NASA Astrophysics Data System (ADS)

    Hermann, Regina; Uhlemann, Margitta; Wendrock, Horst; Gerbeth, Gunter; Büchner, Bernd

    2011-03-01

    The aim of this work is growth and characterisation of Ti55Nb45 (wt%) single crystals by floating-zone single crystal growth of intermetallic compounds using two-phase radio-frequency (RF) electromagnetic heating. Thereby, the process and, in particular, the flow field in the molten zone is influenced by additional magnetic fields. The growth of massive intermetallic single crystals is very often unsuccessful due to an unfavourable solid-liquid interface geometry enclosing concave fringes. It is generally known that the crystallization process stability is enhanced if the crystallization interface is convex. For this, a tailored magnetic two-phase stirrer system has been developed, which enables a controlled influence on the melt ranging from intensive inwards to outwards flows. Since Ti is favourably light, strong and biocompatible, it is one of the few materials that naturally match the requirements for implantation in the human body. Therefore, the magnetic system was applied to crystal growth of Ti alloys. The grown crystals were oriented and cut to cubes with the desired crystallographic orientations [1 0 0] and [1 0 1] normally on a plane. The electron backscatter diffraction (EBSD) technique was applied to clearly determine crystal orientation and to localize grain boundaries. The formation of oxidic nanotubes on Ti surfaces in dependence of the grain orientation was investigated, performed electrochemically by anodic oxidation from fluoride containing electrolyte.

  3. Study of Fluid Flow Control In Protein Crystallization Using Strong Magnetic Fields

    NASA Technical Reports Server (NTRS)

    Ramachandran, N.; Leslie, F.; Ciszak, E.; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular crystals for diffraction analyses has been the central focus for biochemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of the container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and sedimentation, as is achieved in 'microgravity', researchers have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, formation of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. Whether this limited convection in a magnetic field will provide the environment for the growth of high quality crystals is still a matter of conjecture that our research will address. The approach exploits the variation of fluid magnetic susceptibility with concentration for this purpose and the convective damping is realized by appropriately

  4. Study of Fluid Flow Control In Protein Crystallization Using Strong Magnetic Fields

    NASA Technical Reports Server (NTRS)

    Ramachandran, N.; Leslie, F.; Ciszak, E.; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular crystals for diffraction analyses has been the central focus for biochemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of the container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and sedimentation, as is achieved in 'microgravity', researchers have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, formation of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. Whether this limited convection in a magnetic field will provide the environment for the growth of high quality crystals is still a matter of conjecture that our research will address. The approach exploits the variation of fluid magnetic susceptibility with concentration for this purpose and the convective damping is realized by appropriately

  5. Electric-field variations within a nematic-liquid-crystal layer.

    PubMed

    Cummings, L J; Mema, E; Cai, C; Kondic, L

    2014-07-01

    A thin layer of nematic liquid crystal (NLC) across which an electric field is applied is a setup of great industrial importance in liquid crystal display devices. There is thus a large literature modeling this situation and related scenarios. A commonly used assumption is that an electric field generated by electrodes at the two bounding surfaces of the layer will produce a field that is uniform: that is, the presence of NLC does not affect the electric field. In this paper, we use calculus of variations to derive the equations coupling the electric potential to the orientation of the NLC's director field, and use a simple one-dimensional model to investigate the limitations of the uniform field assumption in the case of a steady applied field. The extension of the model to the unsteady case is also briefly discussed.

  6. Electric-field variations within a nematic-liquid-crystal layer

    NASA Astrophysics Data System (ADS)

    Cummings, L. J.; Mema, E.; Cai, C.; Kondic, L.

    2014-07-01

    A thin layer of nematic liquid crystal (NLC) across which an electric field is applied is a setup of great industrial importance in liquid crystal display devices. There is thus a large literature modeling this situation and related scenarios. A commonly used assumption is that an electric field generated by electrodes at the two bounding surfaces of the layer will produce a field that is uniform: that is, the presence of NLC does not affect the electric field. In this paper, we use calculus of variations to derive the equations coupling the electric potential to the orientation of the NLC's director field, and use a simple one-dimensional model to investigate the limitations of the uniform field assumption in the case of a steady applied field. The extension of the model to the unsteady case is also briefly discussed.

  7. Monitoring of hydroxyapatite crystal formation using field-effect transistor

    NASA Astrophysics Data System (ADS)

    Kajisa, Taira; Sakata, Toshiya

    2016-04-01

    The biomineralization process of hydroxyapatite (HAp) in simulated body fluid (SBF) was monitored in realtime using extended-gate FETs whose gate electrode was modified with a variety of alkanethiol self-assembled monolayers (SAMs). It was found that the gate surface potential of the carboxyl- and amino-group-terminated SAM-coated gate FET was increased in SBF as HAp crystals grew on the gate surface. Moreover, in the carboxyl-group-terminated SAM-coated gate FET, the rate of increase and the shift of gate surface potential of the FET were found to depend on the concentration of calcium ions in the SBF. It was concluded that the process of HAp crystallization at a SAM-modified surface can be detected using FETs. Thus, a FET device that enables the easy detection of ionic charges in a real-time and label-free manner, will be useful for evaluating biomaterials based on biomineralization such as those in the bone regeneration process.

  8. Reorientation of single-wall carbon nanotubes in negative anisotropy liquid crystals by an electric field

    PubMed Central

    García-García, Amanda; Vergaz, Ricardo; Algorri, José F; Zito, Gianluigi; Cacace, Teresa; Marino, Antigone; Otón, José M

    2016-01-01

    Summary Single-wall carbon nanotubes (SWCNT) are anisotropic nanoparticles that can cause modifications in the electrical and electro-optical properties of liquid crystals. The control of the SWCNT concentration, distribution and reorientation in such self-organized fluids allows for the possibility of tuning the liquid crystal properties. The alignment and reorientation of CNTs are studied in a system where the liquid crystal orientation effect has been isolated. Complementary studies including Raman spectroscopy, microscopic inspection and impedance studies were carried out. The results reveal an ordered reorientation of the CNTs induced by an electric field, which does not alter the orientation of the liquid crystal molecules. Moreover, impedance spectroscopy suggests a nonnegligible anchoring force between the CNTs and the liquid crystal molecules. PMID:27547599

  9. Phase field modelling of strain induced crystal growth in an elastic matrix.

    PubMed

    Laghmach, Rabia; Candau, Nicolas; Chazeau, Laurent; Munch, Etienne; Biben, Thierry

    2015-06-28

    When a crystal phase grows in an amorphous matrix, such as a crystallisable elastomer, containing cross-links and/or entanglements, these "topological constraints" need to be pushed away from the crystal phase to allow further crystallization. The accumulation of these topological constraints in the vicinity of the crystal interface may store elastic energy and affect the phase transition. To evaluate the consequences of such mechanism, we introduce a phase field model based on the Flory theory of entropic elasticity. We show that the growth process is indeed sensibly affected, in particular, an exponential increase of the surface energy with the displacement of the interface is induced. This explains the formation of stable nano-crystallites as it is observed in the Strain Induced Crystallization (SIC) of natural rubber. Although simple, the model developed here is able to account for many interesting features of SIC, for instance, the crystallite shapes and their sizes which depend on the applied deformation.

  10. Extraction of crystal-field parameters for lanthanide ions from quantum-chemical calculations.

    PubMed

    Hu, Liusen; Reid, Michael F; Duan, Chang-Kui; Xia, Shangda; Yin, Min

    2011-02-02

    A simple method for constructing effective Hamiltonians for the 4f(N) and 4f(N - 1)5d energy levels of lanthanide ions in crystals from quantum-chemical calculations is presented. The method is demonstrated by deriving crystal-field and spin-orbit parameters for Ce(3 + ) ions doped in LiYF(4), Cs(2)NaYCl(6), CaF(2), KY(3)F(10) and YAG host crystals from quantum-chemical calculations based on the DV-Xα method. Good agreement between calculated and fitted values of the crystal-field parameters is obtained. The method can be used to calculate parameters even for low-symmetry sites where there are more parameters than energy levels.

  11. Extraction of crystal-field parameters for lanthanide ions from quantum-chemical calculations

    NASA Astrophysics Data System (ADS)

    Hu, Liusen; Reid, Michael F.; Duan, Chang-Kui; Xia, Shangda; Yin, Min

    2011-02-01

    A simple method for constructing effective Hamiltonians for the 4fN and 4fN - 15d energy levels of lanthanide ions in crystals from quantum-chemical calculations is presented. The method is demonstrated by deriving crystal-field and spin-orbit parameters for Ce3 + ions doped in LiYF4, Cs2NaYCl6, CaF2, KY3F10 and YAG host crystals from quantum-chemical calculations based on the DV-Xα method. Good agreement between calculated and fitted values of the crystal-field parameters is obtained. The method can be used to calculate parameters even for low-symmetry sites where there are more parameters than energy levels.

  12. Self-Aligned Growth of Organic Semiconductor Single Crystals by Electric Field.

    PubMed

    Kotsuki, Kenji; Obata, Seiji; Saiki, Koichiro

    2016-01-19

    We proposed a novel but facile method for growing organic semiconductor single-crystals via solvent vapor annealing (SVA) under electric field. In the conventional SVA growth process, nuclei of crystals appeared anywhere on the substrate and their crystallographic axes were randomly distributed. We applied electric field during the SVA growth of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) on the SiO2/Si substrate on which a pair of electrodes had been deposited beforehand. Real-time observation of the SVA process revealed that rodlike single crystals grew with their long axes parallel to the electric field and bridged the prepatterned electrodes. As a result, C8-BTBT crystals automatically formed a field effect transistor (FET) structure and the mobility reached 1.9 cm(2)/(V s). Electric-field-assisted SVA proved a promising method for constructing high-mobility single-crystal FETs at the desired position by a low-cost solution process.

  13. X-ray and magnetic-field-enhanced change in physical characteristics of silicon crystals

    NASA Astrophysics Data System (ADS)

    Makara, V. A.; Steblenko, L. P.; Krit, A. N.; Kalinichenko, D. V.; Kurylyuk, A. N.; Naumenko, S. N.

    2012-07-01

    The effect of low-energy ( W = 8 keV) low-dose ((0.3-7.3) × 102 Gy) radiation and a dc magnetic field ( B = 0.17 T) on structural, micromechanical, and microplastic characteristics of silicon crystals has been studied. The features in the dynamic behavior of dislocations in silicon crystals, which manifest themselves upon only X-ray exposure and combined (X-ray and magnetic) exposure, have been revealed.

  14. Role of the crystal field stabilization energy in the formation of metal(II) formate mixed crystals

    NASA Astrophysics Data System (ADS)

    Balarew, Christo; Stoilova, Donka; Vassileva, Violeta

    A relationship between the distribution coefficient values and the factors determining the isomorphous substitution of some metal(II) formates (Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd) has been found, given by D=[exp⁡{aṡf[ΔR/R]+bṡϕ(Δɛ)+cṡψ(Δs)}/{RT}, where Δ R/R is the relative difference in the ionic radii of the intersubstituting ions, Δɛ is the difference in the Me sbnd O bond energy, Δ s is the difference in the crystal field stabilization energy. The pre-exponential term represents the balance in bonding factors between the ions in the crystal and in the aqueous solution, in the case of ideally mixing in the solid state. The exponential term takes into account the enthalpy of mixing in the solid state. For the isostructural formate salts in which the substitution of a given cation by another one occurs in equivalent octahedral positions, the difference in the crystal field stabilization energy exerts the most important influence on the enthalpy of mixing.

  15. Influence of lower frequency electromagnetic field on dendritic crystal growth in special alloys

    NASA Astrophysics Data System (ADS)

    Xu, Yu; Wang, Tao; Wang, Fei; Wang, Engang

    2017-06-01

    Based on a new developed chemical etching method to special alloys (e.g. Incoloy800H superalloy, GCr15 bearing steel and 1Cr13 stainless steel), the morphology of dendritic crystal growth and its transition to equiaxed crystal in the solidification structure with lower frequency electromagnetic fields were observed and investigated to understand the action mechanism of electromagnetic fields on the solidification of the special alloys. By applying a rotating electromagnetic field (REMF) in the casting of special alloys, the growth condition of dendritic crystals, such as the temperature gradient and concentration of elements in the front of dendritic crystals, have been changed by the forced convection of melt and bring the columnar dendrites to equiaxed dendrites. These phenomena are proved in the morphology observation of the Incoloy800H superalloy, GCr15 steel and 1Cr13 steel by the new developed chemical etching method, and the tips of dendritic crystals are changed from sharp to round. Meanwhile, with the application of REMF, the growing dendrites are broken in the front of dendritic crystals by the forced melt flow. Some of the dendrite fragments are partially remelted to become effective nuclei, and some of them are survived during the solidification process. Finally, a criterion for the dendrite fragmentation under REMF is derived based on the dendrite fragmentation theory of Campanella et al.

  16. A phase-field model coupled with lattice kinetics solver for modeling crystal growth in furnaces

    SciTech Connect

    Lin, Guang; Bao, Jie; Xu, Zhijie; Tartakovsky, Alexandre M.; Henager, Charles H.

    2014-02-02

    In this study, we present a new numerical model for crystal growth in a vertical solidification system. This model takes into account the buoyancy induced convective flow and its effect on the crystal growth process. The evolution of the crystal growth interface is simulated using the phase-field method. Two novel phase-field models are developed to model the crystal growth interface in vertical gradient furnaces with two temperature profile setups: 1) fixed wall temperature profile setup and 2) time-dependent temperature profile setup. A semi-implicit lattice kinetics solver based on the Boltzmann equation is employed to model the unsteady incompressible flow. This model is used to investigate the effect of furnace operational conditions on crystal growth interface profiles and growth velocities. For a simple case of macroscopic radial growth, the phase-field model is validated against an analytical solution. Crystal growth in vertical gradient furnaces with two temperature profile setups have been also investigated using the developed model. The numerical simulations reveal that for a certain set of temperature boundary conditions, the heat transport in the melt near the phase interface is diffusion dominant and advection is suppressed.

  17. Atomic density functional and diagram of structures in the phase field crystal model

    SciTech Connect

    Ankudinov, V. E.; Galenko, P. K.; Kropotin, N. V.; Krivilyov, M. D.

    2016-02-15

    The phase field crystal model provides a continual description of the atomic density over the diffusion time of reactions. We consider a homogeneous structure (liquid) and a perfect periodic crystal, which are constructed from the one-mode approximation of the phase field crystal model. A diagram of 2D structures is constructed from the analytic solutions of the model using atomic density functionals. The diagram predicts equilibrium atomic configurations for transitions from the metastable state and includes the domains of existence of homogeneous, triangular, and striped structures corresponding to a liquid, a body-centered cubic crystal, and a longitudinal cross section of cylindrical tubes. The method developed here is employed for constructing the diagram for the homogeneous liquid phase and the body-centered iron lattice. The expression for the free energy is derived analytically from density functional theory. The specific features of approximating the phase field crystal model are compared with the approximations and conclusions of the weak crystallization and 2D melting theories.

  18. Atomic density functional and diagram of structures in the phase field crystal model

    NASA Astrophysics Data System (ADS)

    Ankudinov, V. E.; Galenko, P. K.; Kropotin, N. V.; Krivilyov, M. D.

    2016-02-01

    The phase field crystal model provides a continual description of the atomic density over the diffusion time of reactions. We consider a homogeneous structure (liquid) and a perfect periodic crystal, which are constructed from the one-mode approximation of the phase field crystal model. A diagram of 2D structures is constructed from the analytic solutions of the model using atomic density functionals. The diagram predicts equilibrium atomic configurations for transitions from the metastable state and includes the domains of existence of homogeneous, triangular, and striped structures corresponding to a liquid, a body-centered cubic crystal, and a longitudinal cross section of cylindrical tubes. The method developed here is employed for constructing the diagram for the homogeneous liquid phase and the body-centered iron lattice. The expression for the free energy is derived analytically from density functional theory. The specific features of approximating the phase field crystal model are compared with the approximations and conclusions of the weak crystallization and 2D melting theories.

  19. Semiconductor crystal growth in crossed electric and magnetic fields: Center Director's Discretionary Fund

    NASA Technical Reports Server (NTRS)

    Mazuruk, K.; Volz, M. P.

    1996-01-01

    A unique growth cell was designed in which crossed electric and magnetic fields could be separately or simultaneously applied during semiconductor crystal growth. A thermocouple was inserted into an InSb melt inside the growth cell to examine the temperature response of the fluid to applied electromagnetic fields. A static magnetic field suppressed time-dependent convection when a destabilizing thermal field was applied. The simultaneous application of electric and magnetic fields resulted in forced convection in the melt. The InSb ingots grown in the cell were polycrystalline. An InGaSb crystal, 0.5 cm in diameter and 23-cm long, was grown without electromagnetic fields applied. The axial composition results indicated that complete mixing in the melt occurred for this large aspect ratio.

  20. Determination of optimal ionic liquid for organic single-crystal field-effect transistors

    NASA Astrophysics Data System (ADS)

    Ono, S.; Miwa, K.; Seki, S.

    2016-02-01

    We investigate organic single-crystal field-effect transistors with various ionic liquids as gate dielectric. We find that the mobility of the field-effect transistors for both p-type and n-type organic semiconductors increases with decreasing total capacitance of the ionic liquid. However, it does not depend on the ion species at the interface between the organic semiconductor and the ionic liquid. By choosing an appropriate ionic liquid, a high carrier mobility of 12.4 cm2/V s in rubrene single crystals (p-type) and 0.13 cm2/V s in 7.7.8.8-Tetracyanoquinodimethane single crystals (n-type) are achieved. This study clarifies the influence of ionic liquids on the device performance of organic field-effect transistors and shows a way to maximize carrier mobility at the solid/liquid interface.

  1. Random crystal field effect on the magnetic and hysteresis behaviors of a spin-1 cylindrical nanowire

    NASA Astrophysics Data System (ADS)

    Zaim, N.; Zaim, A.; Kerouad, M.

    2017-02-01

    In this work, the magnetic behavior of the cylindrical nanowire, consisting of a ferromagnetic core of spin-1 atoms surrounded by a ferromagnetic shell of spin-1 atoms is studied in the presence of a random crystal field interaction. Based on Metropolis algorithm, the Monte Carlo simulation has been used to investigate the effects of the concentration of the random crystal field p, the crystal field D and the shell exchange interaction Js on the phase diagrams and the hysteresis behavior of the system. Some characteristic behaviors have been found, such as the first and second-order phase transitions joined by tricritical point for appropriate values of the system parameters, triple and isolated critical points can be also found. Depending on the Hamiltonian parameters, single, double and para hysteresis regions are explicitly determined.

  2. Crystal-field interaction and oxygen stoichiometry effects in strontium-doped rare-earth cobaltates

    NASA Astrophysics Data System (ADS)

    Furrer, A.; Podlesnyak, A.; Frontzek, M.; Sashin, I.; Embs, J. P.; Mitberg, E.; Pomjakushina, E.

    2014-08-01

    Inelastic neutron scattering was employed to study the crystal-field interaction in the strontium-doped rare-earth compounds RxSr1-xCoO3-z (R=Pr, Nd, Ho, and Er). Particular emphasis is laid on the effect of oxygen deficiencies that naturally occur in the synthesis of these compounds. The observed energy spectra are found to be the result of a superposition of crystal fields with different nearest-neighbor oxygen coordination at the R sites. The experimental data are interpreted in terms of crystal-field parameters, which behave in a consistent manner through the rare-earth series, thereby allowing a reliable extrapolation for rare-earth ions not considered in the present work.

  3. Crystal field parameters and energy levels scheme of trivalent chromium doped BSO

    SciTech Connect

    Petkova, P.; Andreici, E.-L.; Avram, N. M.

    2014-11-24

    The aim of this paper is to give an analysis of crystal field parameters and energy levels schemes for the above doped material, in order to give a reliable explanation for experimental data. The crystal field parameters have been modeled in the frame of Exchange Charge Model (ECM) of the crystal field theory, taken into account the geometry of systems, with actually site symmetry of the impurity ions. The effect of the charges of the ligands and covalence bonding between chromium cation and oxygen anions, in the cluster approach, also were taken into account. With the obtained values of the crystal field parameters we simulated the scheme of energy levels of chromium ions by diagonalizing the matrix of the Hamiltonian of the doped crystal. The obtained energy levels and estimated Racah parameters B and C were compared with the experimental spectroscopic data and discussed. Comparison with experiment shows that the results are quite satisfactory which justify the model and simulation scheme used for the title system.

  4. An optimized intermolecular force field for hydrogen-bonded organic molecular crystals using atomic multipole electrostatics.

    PubMed

    Pyzer-Knapp, Edward O; Thompson, Hugh P G; Day, Graeme M

    2016-08-01

    We present a re-parameterization of a popular intermolecular force field for describing intermolecular interactions in the organic solid state. Specifically we optimize the performance of the exp-6 force field when used in conjunction with atomic multipole electrostatics. We also parameterize force fields that are optimized for use with multipoles derived from polarized molecular electron densities, to account for induction effects in molecular crystals. Parameterization is performed against a set of 186 experimentally determined, low-temperature crystal structures and 53 measured sublimation enthalpies of hydrogen-bonding organic molecules. The resulting force fields are tested on a validation set of 129 crystal structures and show improved reproduction of the structures and lattice energies of a range of organic molecular crystals compared with the original force field with atomic partial charge electrostatics. Unit-cell dimensions of the validation set are typically reproduced to within 3% with the re-parameterized force fields. Lattice energies, which were all included during parameterization, are systematically underestimated when compared with measured sublimation enthalpies, with mean absolute errors of between 7.4 and 9.0%.

  5. An optimized intermolecular force field for hydrogen-bonded organic molecular crystals using atomic multipole electrostatics

    PubMed Central

    Pyzer-Knapp, Edward O.; Thompson, Hugh P. G.; Day, Graeme M.

    2016-01-01

    We present a re-parameterization of a popular intermolecular force field for describing intermolecular interactions in the organic solid state. Specifically we optimize the performance of the exp-6 force field when used in conjunction with atomic multipole electrostatics. We also parameterize force fields that are optimized for use with multipoles derived from polarized molecular electron densities, to account for induction effects in molecular crystals. Parameterization is performed against a set of 186 experimentally determined, low-temperature crystal structures and 53 measured sublimation enthalpies of hydrogen-bonding organic molecules. The resulting force fields are tested on a validation set of 129 crystal structures and show improved reproduction of the structures and lattice energies of a range of organic molecular crystals compared with the original force field with atomic partial charge electrostatics. Unit-cell dimensions of the validation set are typically reproduced to within 3% with the re-parameterized force fields. Lattice energies, which were all included during parameterization, are systematically underestimated when compared with measured sublimation enthalpies, with mean absolute errors of between 7.4 and 9.0%. PMID:27484370

  6. Metal electrode dependent field effect transistors made of lanthanide ion-doped DNA crystals

    NASA Astrophysics Data System (ADS)

    Reddy Dugasani, Sreekantha; Hwang, Taehyun; Kim, Jang Ah; Gnapareddy, Bramaramba; Kim, Taesung; Park, Sung Ha

    2016-03-01

    We fabricated lanthanide ion (Ln3+, e.g. Dy3+, Er3+, Eu3+, and Gd3+)-doped self-assembled double-crossover (DX) DNA crystals grown on the surface of field effect transistors (FETs) containing either a Cr, Au, or Ni electrode. Here we demonstrate the metal electrode dependent FET characteristics as a function of various Ln3+. The drain-source current (I ds), controlled by the drain-source voltage (V ds) of Ln3+-doped DX DNA crystals with a Cr electrode on an FET, changed significantly under various gate voltages (V g) due to the relative closeness of the work function of Cr to the energy band gap of Ln3+-DNA crystals compared to those of Au and Ni. For Ln3+-DNA crystals on an FET with either a Cr or Ni electrode at a fixed V ds, I ds decreased with increasing V g ranging from  -2 to 0 V and from 0 to  +3 V in the positive and negative regions, respectively. By contrast, I ds for Ln3+-DNA crystals on an FET with Au decreased with increasing V g in only the positive region due to the greater electronegativity of Au. Furthermore, Ln3+-DNA crystals on an FET exhibited behaviour sensitive to V g due to the appreciable charge carriers generated from Ln3+. Finally, we address the resistivity and the mobility of Ln3+-DNA crystals on an FET with different metal electrodes obtained from I ds-V ds and I ds-V g curves. The resistivities of Ln3+-DNA crystals on FETs with Cr and Au electrodes were smaller than those of pristine DNA crystals on an FET, and the mobility of Ln3+-DNA crystals on an FET with Cr was relatively higher than that associated with other electrodes.

  7. Electric field-induced optical second harmonic generation in nematic liquid crystal 5CB

    NASA Astrophysics Data System (ADS)

    Torgova, S. I.; Shigorin, V. D.; Maslyanitsyn, I. A.; Todorova, L.; Marinov, Y. G.; Hadjichristov, G. B.; Petrov, A. G.

    2014-12-01

    Electric field-induced second harmonic generation (EFISH) was studied for the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) (a nematic phase material at room temperature). The intensity of coherent SHG from 5CB cells upon DC electric field was measured for various initial orientations of the liquid crystal. The dependence of the SHG intensity on the pump beam incidence angle was obtained in transmission geometry using sample rotation method. The experimental results (the registered light intensity in the output SHG interference patterns) were theoretically modelled and analyzed.

  8. Laboratory study of electrical discharges on vapor grown ice crystals subjected to strong electric fields

    NASA Astrophysics Data System (ADS)

    Petersen, Danyal A.

    Thundercloud electric-field observations have consistently yielded peak values that are an order of magnitude weaker than the dielectric strength of air at relevant altitudes. Various discharge processes have been proposed to explain how lightning can be initiated in such weak electric fields, including hydrometeor-initiated positive streamers and cosmic ray-initiated runaway breakdown. The historically favored positive streamer discharge process is problematic because it requires electric fields two to three times larger than the largest typically observed. The more recently favored runaway breakdown discharge process appear to be viable in electric fields comparable with those typically observed, but it is not clear how it may lead to creation of a hot lightning leader channel. It has been hypothesized previously by the author that a combination of these two discharge processes offers a more plausible solution, with each process solving a piece of the puzzle.162197 One of the important elements of the positive streamer system discharge process is the generation of initial "seed" positive streamers at the extremities of hydrometeors such as raindrops and ice crystals. The focus of this dissertation is an experimental study designed to investigate the generation of positive streamers and other corona discharges at the extremities of vapor-grown ice crystals. Of primary interest is the determination of the minimum electric field required to generate a positive streamer as a function of ambient air density, ice crystal length, and ice crystal tip geometry. The results of this study show a definite relationship between the minimum electric field required to generate a positive streamer, ambient air density, and ice crystal length. These results are useful insofar as they identify the electric fields required for seed positive streamer production from vapor-grown ice crystals such as are known to exist in the colder regions of thunderclouds. Another interesting result

  9. Field-induced phase transitions in chiral smectic liquid crystals studied by the constant current method

    NASA Astrophysics Data System (ADS)

    H, Dhaouadi; R, Zgueb; O, Riahi; F, Trabelsi; T, Othman

    2016-05-01

    In ferroelectric liquid crystals, phase transitions can be induced by an electric field. The current constant method allows these transition to be quickly localized and thus the (E,T) phase diagram of the studied product can be obtained. In this work, we make a slight modification to the measurement principles based on this method. This modification allows the characteristic parameters of ferroelectric liquid crystal to be quantitatively measured. The use of a current square signal highlights a phenomenon of ferroelectric hysteresis with remnant polarization at null field, which points out an effect of memory in this compound.

  10. Field-programmable rectification in rutile TiO2 crystals

    NASA Astrophysics Data System (ADS)

    Jameson, John R.; Fukuzumi, Yoshiaki; Wang, Zheng; Griffin, Peter; Tsunoda, Koji; Meijer, G. Ingmar; Nishi, Yoshio

    2007-09-01

    The authors report "field-programmable rectification" in crystals of rutile TiO2. A "programming" voltage is applied between two Pt electrodes on the surface of a crystal. Afterwards, current can pass in the direction of the programming voltage, but not in the reverse direction. The polarity of the rectification can be reversed by applying a programming voltage of opposite sign. The effect was observed on the (110) and (100) surfaces, but not the (001) surface. The proposed mechanism is field-induced motion of oxygen vacancies, which pile up under the negative terminal, eliminating a Schottky barrier, but leaving one at the positive terminal intact.

  11. Measurement of nonlinear coefficients of crystals at terahertz frequencies via High Field THzat the FELIX FEL

    DTIC Science & Technology

    2017-04-03

    plane. As the sample moves along the z-axis, the intensity of the incident radiation increases in a known fashion, and, at high intensities , a...AFRL-AFOSR-UK-TR-2017-0027 Measurement of nonlinear coefficients of crystals at terahertz frequencies via High - Field THz at the FELIX FEL Mira...coefficients of crystals at terahertz frequencies via High - Field THz at the FELIX FEL 5a.  CONTRACT NUMBER FA9550-15-C-0068 5b.  GRANT NUMBER 5c.  PROGRAM

  12. Ferromagnetic resonance and high field ESR in a TDAE-C60 single crystal

    NASA Astrophysics Data System (ADS)

    Arčon, D.; Cevc, P.; Omerzu, A.; Blinc, R.; Mehring, M.; Knorr, S.; Grupp, A.; Barra, A.-L.; Chouteau, G.

    1998-08-01

    Frequency variable ESR measurements have been performed on well annealed TDAE-C60 single crystals between 40 MHz and 245 GHz. A non-linear variation of the electron resonance frequency with the magnetic field has been observed below TC=16 K in the radio-frequency region. The observed ferromagnetic resonance data are characteristic for a three-dimensional Heisenberg ferromagnet with a small positive uniaxial anisotropy field. The easy axis coincides with the crystal c-direction which is the direction of closest approach of the C60- ions.

  13. Modelling of melt motion in a Czochralski crystal puller with an axial magnetic field

    NASA Astrophysics Data System (ADS)

    Hjellming, L. N.; Walker, J. S.

    1986-12-01

    The use of matched asymptotic expansions provide analytical solutions for the bulk flow in a Czochralski crystal puller in a strong axial magnetic field. Treating the crystal as a slight electrical conductor alters the radial and axial flows driven by centrifugal pumping. The motion due to buoyancy and thermocapillarity are found by considering the temperature as a known function and solving the non-linear heat equation numerically for different magnetic field strengths and melt depths. This note presents a summary of the analysis and results that are detailed in two papers.

  14. Fringing field suppression for liquid crystal gratings using equivalent capacitance configuration

    NASA Astrophysics Data System (ADS)

    Yang, Lei; Xia, Jun; Zhang, Xiaobing; Xie, Yi; Kang, Mingwu; Zhang, Qiuzhi

    2014-10-01

    A liquid crystal grating with high spatial frequency and equivalent capacitance configuration is proposed, where two layers of periodical ground electrodes are interlaced and aligned with the addressing electrodes. The equivalent capacitance configuration can reduce the fringing field effect efficiently owing to the generated electric field resisting the fringing field and redistributing the equivalent voltage exerting on the liquid crystal layer. The phase modulation depth and far-field diffraction patterns both for conventional and novel configurations were simulated. The results show that phase modulation is greatly enhanced and the maximum diffraction efficiency for a sinusoidal phase grating is 33.86%, which indicates that the equivalent capacitance configuration provides a good solution for suppressing the fringing field effect.

  15. Accurate force fields and methods for modelling organic molecular crystals at finite temperatures.

    PubMed

    Nyman, Jonas; Pundyke, Orla Sheehan; Day, Graeme M

    2016-06-21

    We present an assessment of the performance of several force fields for modelling intermolecular interactions in organic molecular crystals using the X23 benchmark set. The performance of the force fields is compared to several popular dispersion corrected density functional methods. In addition, we present our implementation of lattice vibrational free energy calculations in the quasi-harmonic approximation, using several methods to account for phonon dispersion. This allows us to also benchmark the force fields' reproduction of finite temperature crystal structures. The results demonstrate that anisotropic atom-atom multipole-based force fields can be as accurate as several popular DFT-D methods, but have errors 2-3 times larger than the current best DFT-D methods. The largest error in the examined force fields is a systematic underestimation of the (absolute) lattice energy.

  16. Striations in CZ silicon crystals grown under various axial magnetic field strengths

    NASA Astrophysics Data System (ADS)

    Kim, K. M.; Smetana, P.

    1985-10-01

    Inhibition of fluid flow instabilities in the melt by the axial magnetic field in Czochralski silicon crystal growth (AMCZ) is investigated precisely by a high-sensitivity striation etch in conjunction with temperature measurements. The magnetic field strength (B) was varied up to 4.0 kG, incremented mostly in 0.5-kG/2.5-cm crystal length. The convection flow was substantially suppressed at B greater than or equal to 1.0 kG. A low oxygen level of 2-3 ppm and a high resistivity of 400 ohm-cm is achieved in the AMCZ silicon crystals at B greater than or equal to 1.0 kG. Random striations at B = O, characteristic of turbulent convection, assumed progressively a periodicity, indicative of oscillatory convection at B from 0.35-4.0 kG. The striation contrast or 'intensity' decreased steadily with the increase in B. At B = 4 kG, most of the crystal was free of striations, although some weak, localized periodic striations persisted near the crystal periphery. Spreading-resistance measurement shows, however, a uniform dopant distribution in all crystal sections grown at B from 0.35-4.0 kG within a few percent.

  17. Study of Fluid Flow Control in Protein Crystallization using Strong Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Ramachandran, Narayanan; Leslie, Fred; Ciszak, Ewa

    2002-11-01

    An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular crystals for diffraction analyses has been the central focus for biochemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of the container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and sedimentation, as is achieved in "microgravity", researchers have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, formation of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. Whether this limited convection in a magnetic field will provide the environment for the growth of high quality crystals is still a matter of conjecture that our research will address. The approach exploits the variation of fluid magnetic susceptibility with concentration for this purpose and the convective damping is realized by appropriately

  18. Study of Fluid Flow Control in Protein Crystallization using Strong Magnetic Fields

    NASA Technical Reports Server (NTRS)

    Ramachandran, Narayanan; Leslie, Fred; Ciszak, Ewa

    2002-01-01

    An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular crystals for diffraction analyses has been the central focus for biochemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of the container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and sedimentation, as is achieved in "microgravity", researchers have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, formation of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. Whether this limited convection in a magnetic field will provide the environment for the growth of high quality crystals is still a matter of conjecture that our research will address. The approach exploits the variation of fluid magnetic susceptibility with concentration for this purpose and the convective damping is realized by appropriately

  19. Influence of computational domain size on the pattern formation of the phase field crystals

    NASA Astrophysics Data System (ADS)

    Starodumov, Ilya; Galenko, Peter; Alexandrov, Dmitri; Kropotin, Nikolai

    2017-04-01

    Modeling of crystallization process by the phase field crystal method (PFC) represents one of the important directions of modern computational materials science. This method makes it possible to research the formation of stable or metastable crystal structures. In this paper, we study the effect of computational domain size on the crystal pattern formation obtained as a result of computer simulation by the PFC method. In the current report, we show that if the size of a computational domain is changed, the result of modeling may be a structure in metastable phase instead of pure stable state. The authors present a possible theoretical justification for the observed effect and provide explanations on the possible modification of the PFC method to account for this phenomenon.

  20. Local strain fields in two-dimensional colloidal crystals with bond strength disorder

    NASA Astrophysics Data System (ADS)

    Gratale, Matthew; Xu, Ye; Still, Tim; Yodh, Arjun

    2013-03-01

    We study the local strain fields of two-dimensional colloidal crystals consisting of random distributions of hard polystyrene particles and soft microgel particles. Using standard video microscopy and particle tracking techniques, we analyze the variations of local configurations around each particle due to thermal motion. With this information we derive the best-fit affine strain tensor and the non-affinity for each particle in the sample, which allow us to study the mechanical properties of our colloidal crystals. We than observe the changes in these properties as we transition from a predominately hard-sphere crystal to predominately soft-sphere crystal, that is we explore how the mechanical properties are affected by replacing hard inter-particle bonds with soft inter-particle bonds. We gratefully acknowledge financial support from the National Science Foundation through DMR12-05463, the PENN MRSEC DMR11-20901, and NASA NNX08AO0G.

  1. An unforeseen polymorph of coronene by the application of magnetic fields during crystal growth

    PubMed Central

    Potticary, Jason; Terry, Lui R.; Bell, Christopher; Papanikolopoulos, Alexandros N.; Christianen, Peter C. M.; Engelkamp, Hans; Collins, Andrew M.; Fontanesi, Claudio; Kociok-Köhn, Gabriele; Crampin, Simon; Da Como, Enrico; Hall, Simon R.

    2016-01-01

    The continued development of novel drugs, proteins, and advanced materials strongly rely on our ability to self-assemble molecules in solids with the most suitable structure (polymorph) in order to exhibit desired functionalities. The search for new polymorphs remains a scientific challenge, that is at the core of crystal engineering and there has been a lack of effective solutions to this problem. Here we show that by crystallizing the polyaromatic hydrocarbon coronene in the presence of a magnetic field, a polymorph is formed in a β-herringbone structure instead of the ubiquitous γ-herringbone structure, with a decrease of 35° in the herringbone nearest neighbour angle. The β-herringbone polymorph is stable, preserves its structure under ambient conditions and as a result of the altered molecular packing of the crystals, exhibits significant changes to the optical and mechanical properties of the crystal. PMID:27161600

  2. An unforeseen polymorph of coronene by the application of magnetic fields during crystal growth

    NASA Astrophysics Data System (ADS)

    Potticary, Jason; Terry, Lui R.; Bell, Christopher; Papanikolopoulos, Alexandros N.; Christianen, Peter C. M.; Engelkamp, Hans; Collins, Andrew M.; Fontanesi, Claudio; Kociok-Köhn, Gabriele; Crampin, Simon; da Como, Enrico; Hall, Simon R.

    2016-05-01

    The continued development of novel drugs, proteins, and advanced materials strongly rely on our ability to self-assemble molecules in solids with the most suitable structure (polymorph) in order to exhibit desired functionalities. The search for new polymorphs remains a scientific challenge, that is at the core of crystal engineering and there has been a lack of effective solutions to this problem. Here we show that by crystallizing the polyaromatic hydrocarbon coronene in the presence of a magnetic field, a polymorph is formed in a β-herringbone structure instead of the ubiquitous γ-herringbone structure, with a decrease of 35° in the herringbone nearest neighbour angle. The β-herringbone polymorph is stable, preserves its structure under ambient conditions and as a result of the altered molecular packing of the crystals, exhibits significant changes to the optical and mechanical properties of the crystal.

  3. Pulsed field actuation of Ni-Mn-Ga ferromagnetic shape memory alloy single crystal

    NASA Astrophysics Data System (ADS)

    Marioni, M.; Bono, D.; Banful, A. B.; del Rosario, M.; Rodriguez, E.; Peterson, B. W.; Allen, S. M.; O'Handley, R. C.

    2003-10-01

    Ferromagnetic Shape Memory Alloy Ni-Mn-Ga has twin boundaries in the martensitic phase that move when a suitable magnetic field is applied. In this fashion strains of up to 6% have been observed for static fields in single crystals [1]. Recently 2.5% strain has been demonstrated [2] in Ni-Mn-Ga single crystals for oscillating fields up to frequencies of 75 Hz (150 Hz actuation). This work studies the actuation of single crystals when pulsed fields are applied. Fields in the 0.4-1.5MA/m-range were generated in an air coil with rise times of the order of 1ms and below. The elongation of the samples is measured with a light beam reflected off the tip of the crystal. Single twin boundaries have been observed to advance 0.16 mm during 600 μsec-ong pulses. Actuation has been shown to be possible at least up to frequencies of 1700 Hz.

  4. Effects of a Rotating Magnetic Field on Gas Transport During Detached Crystal Growth in Space

    NASA Technical Reports Server (NTRS)

    Walker, John S.; Volz, Martin P.; Szofran, Frank R.; Motakef, Shariar; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    During the detached Bridgman growth of semiconductor crystals, the melt has a short free surface which is detached from the ampoule wall near the crystal-melt interface, thus eliminating the crystal defects caused by contact with the ampoule wall. Recent modelling has indicated that initiation and continuation of detached growth depends on the rate of transport of dissolved gas from the crystal-melt interface, where gas is rejected into the melt, to the detached free surface, where evaporating gas maintains the pressure on the free surface. Here we use numerical modelling to investigate whether the application of a rotating magnetic field increases or decreases the transport of rejected gas to the detached free surface. Unfortunately the results show that a rotating magnetic field almost always decreases the evaporation rate at the detached free surface. The exception is an insignificant increase for a short period at the beginning of crystal growth for a few circumstances. The evaporation rate decreases as the strength of the rotating magnetic field is increased.

  5. A full field, 3-D velocimeter for microgravity crystallization experiments

    NASA Technical Reports Server (NTRS)

    Brodkey, Robert S.; Russ, Keith M.

    1991-01-01

    The programming and algorithms needed for implementing a full-field, 3-D velocimeter for laminar flow systems and the appropriate hardware to fully implement this ultimate system are discussed. It appears that imaging using a synched pair of video cameras and digitizer boards with synched rails for camera motion will provide a viable solution to the laminar tracking problem. The algorithms given here are simple, which should speed processing. On a heavily loaded VAXstation 3100 the particle identification can take 15 to 30 seconds, with the tracking taking less than one second. It seeems reasonable to assume that four image pairs can thus be acquired and analyzed in under one minute.

  6. Polymorphism, crystal nucleation and growth in the phase-field crystal model in 2D and 3D

    NASA Astrophysics Data System (ADS)

    Tóth, Gyula I.; Tegze, György; Pusztai, Tamás; Tóth, Gergely; Gránásy, László

    2010-09-01

    We apply a simple dynamical density functional theory, the phase-field crystal (PFC) model of overdamped conservative dynamics, to address polymorphism, crystal nucleation, and crystal growth in the diffusion-controlled limit. We refine the phase diagram for 3D, and determine the line free energy in 2D and the height of the nucleation barrier in 2D and 3D for homogeneous and heterogeneous nucleation by solving the respective Euler-Lagrange (EL) equations. We demonstrate that, in the PFC model, the body-centered cubic (bcc), the face-centered cubic (fcc), and the hexagonal close-packed structures (hcp) compete, while the simple cubic structure is unstable, and that phase preference can be tuned by changing the model parameters: close to the critical point the bcc structure is stable, while far from the critical point the fcc prevails, with an hcp stability domain in between. We note that with increasing distance from the critical point the equilibrium shapes vary from the sphere to specific faceted shapes: rhombic dodecahedron (bcc), truncated octahedron (fcc), and hexagonal prism (hcp). Solving the equation of motion of the PFC model supplied with conserved noise, solidification starts with the nucleation of an amorphous precursor phase, into which the stable crystalline phase nucleates. The growth rate is found to be time dependent and anisotropic; this anisotropy depends on the driving force. We show that due to the diffusion-controlled growth mechanism, which is especially relevant for crystal aggregation in colloidal systems, dendritic growth structures evolve in large-scale isothermal single-component PFC simulations. An oscillatory effective pair potential resembling those for model glass formers has been evaluated from structural data of the amorphous phase obtained by instantaneous quenching. Finally, we present results for eutectic solidification in a binary PFC model.

  7. Three dimensional simulation of melt flow in Czochralski crystal growth with steady magnetic fields

    NASA Astrophysics Data System (ADS)

    Cen, Xianrong; Li, Y. S.; Zhan, Jiemin

    2012-02-01

    Three-dimensional transient numerical simulations were carried out to investigate the melt convection and temperature fluctuations within an industrial Czochralski crucible. To study the magnetic damping effects on the growth process, a vertical magnetic field and a cusp magnetic field were considered. Due to our special interest in the melt convection, only local simulation was conducted. The melt flow was calculated by large-eddy simulation (LES) and the magnetic forces were implemented in the CFD code by solving a set of user-defined scalar (UDS) functions. In the absence of magnetic fields, the numerical results show that the buoyant plumes rise from the crucible to the free surface and the crystal-melt interface, which indicates that the heat and mass transfer phenomena in Si melt can be characterized by the turbulent flow patterns. In the presence of a vertical magnetic field, the temperature fluctuations in the melt are significantly damped, with the buoyant plumes forming regular cylindrical geometries. The cusp magnetic field could also markedly reduce the temperature fluctuations, but the buoyant plumes would break into smaller vortical structures, which gather around the crystal as well as in the center of the crucible bottom. With the present crucible configurations, it is found that the vertical magnetic field with an intensity of 128 mT can damp the temperature fluctuations more effectively than the 40 mT cusp magnetic field, especially in the region near the growing crystal.

  8. Thermoelectric Magnetohydrodynamic Flow During Crystal Growth with a Moderate or Weak Magnetic Field

    NASA Technical Reports Server (NTRS)

    Khine, Y. Y.; Walker, John S.; Szofran, Frank R.; Rose, M. Franklin (Technical Monitor)

    2000-01-01

    This paper treats a steady, axisymmetric melt motion in a cylindrical ampoule with a uniform, axial magnetic field and with an electric current due to a radial temperature variation along the crystal-melt interface, where the values of the absolute thermoelectric power for the crystal and melt are different. The radial component of the thermoelectric current in the melt produces an azimuthal body force, and the axial variation of the centrifugal force due to the azimuthal motion drives a meridional circulation with radial and axial velocities. For moderate magnetic field strengths, the azimuthal velocity and magnetic field produce a radial induced electric field which partially cancels the Seebeck electromotive force in the melt, so that the thermoelectric current and the melt motion are coupled. For weak magnetic fields, the thermoelectric current is decoupled from the melt motion, which is an ordinary hydrodynamic flow driven by a known azimuthal body force. The results show how the flow varies with the strength of the magnetic field and with the magnitude of the temperature variation along the crystal-melt interface. They also define the parameter ranges for which the simpler weak-field decoupled analysis gives accurate predictions.

  9. Waveguide modes of 1D photonic crystals in a transverse magnetic field

    SciTech Connect

    Sylgacheva, D. A. Khokhlov, N. E.; Kalish, A. N.; Belotelov, V. I.

    2016-11-15

    We analyze waveguide modes in 1D photonic crystals containing layers magnetized in the plane. It is shown that the magnetooptical nonreciprocity effect emerges in such structures during the propagation of waveguide modes along the layers and perpendicularly to the magnetization. This effect involves a change in the phase velocity of the mode upon reversal of the direction of magnetization. Comparison of the effects in a nonmagnetic photonic crystal with an additional magnetic layer and in a photonic crystal with magnetic layers shows that the magnitude of this effect is several times larger in the former case in spite of the fact that the electromagnetic field of the modes in the latter case is localized in magnetic regions more strongly. This is associated with asymmetry of the dielectric layers contacting with the magnetic layer in the former case. This effect is important for controlling waveguide structure modes with the help of an external magnetic field.

  10. Solid Separation from a Mixed Suspension through Electric-Field-Enhanced Crystallization.

    PubMed

    Li, Wei W; Radacsi, Norbert; Kramer, Herman J M; van der Heijden, Antoine E D M; Ter Horst, Joop H

    2016-12-23

    When applied to a pure component suspension in an apolar solvent, a strong inhomogeneous electric field induces particle movement, and the particles are collected at the surface of one of the two electrodes. This new phenomenon was used to separately isolate two organic crystalline compounds, phenazine and caffeine, from their suspension in 1,4-dioxane. First, crystals of both compounds were collected at different electrodes under the influence of an electric field. Subsequent cooling crystallization enabled the immobilization and growth of the particles on the electrodes, which were separately collected after the experiment with purities greater than 91 %. This method can be further developed into a technique for crystal separation and recovery in complex multicomponent suspensions of industrial processes. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

  11. Determination of odd-symmetry crystal-field parameters from optical spectra

    NASA Astrophysics Data System (ADS)

    Kornienko, A. A.; Dunina, E. B.; Fomicheva, L. A.

    2014-05-01

    We have obtained analytical expressions for effective parameters of the crystal field that acts on spin-orbit multiplets of 4 f N configurations taking into account admixture to them of 4 f N-15 d excited states and ligand-to-metal charge-transfer states. As an example, we analyze splittings of the ground and excited multiplets of Pr3+ and Tm2+ ions in some crystals without an inversion center. The effect of mixing of states of different configurations is most strongly pronounced for the 1 G 4 and 1 D 2 excited multiplets. The interconfigurational contribution to splittings is different for different multiplets. This circumstance makes it possible to estimate the values of the parameters of the odd-symmetry crystal field, which causes mixing of the 4 f N and 4 f N-15 d states, and the covalence parameters of rare-earth ion-ligand bonds.

  12. Electric Field-Controlled Crystallizing CaCO3 Nanostructures from Solution

    NASA Astrophysics Data System (ADS)

    Qi, Jian Quan; Guo, Rui; Wang, Yu; Liu, Xuan Wen; Chan, Helen Lai Wah

    2016-03-01

    The role of electric field is investigated in determining the structure, morphology, and crystallographic characteristics of CaCO3 nanostructures crystallized from solution. It is found that the lattice structure and crystalline morphology of CaCO3 can be tailed by the electric field applied to the solution during its crystallization. The calcite structure with cubic-like morphology can be obtained generally without electric field, and the vaterite structure with the morphology of nanorod is formed under the high electric field. The vaterite nanorods can be piled up to the petaliform layers. Both the nanorod and the petaliform layer can have mesocrystal structures which are piled up by much fine units of the rods with the size of several nanometers. Beautiful rose-like nanoflowers can be self-arranged by the petaliform layers. These structures can have potential application as carrier for medicine to involve into metabolism of living cell.

  13. Electric-field-induced domain intersection in BaTiO3 single crystal

    NASA Astrophysics Data System (ADS)

    He, Ming; Wang, Mengxia; Zhang, Zhihua

    2017-03-01

    Large-angle convergent beam electron diffraction was used to determine the directions of polarization vectors in a BaTiO3 single crystal. Domain intersections driven by an electric field were investigated by in situ transmission electron microscopy. The dark triangles observed in the domain intersection region can be accounted for by dislocations and the strain field. Domains nucleate at the domain tip depending on the dislocations and strain field to relieve the accumulated stress. Schematic representations of the intersecting domains and the microscopic structure are given, clarifying the special electric-field-induced domain structure.

  14. Crystal Field Theory and the Angular Overlap Model Applied to Hydrides of Main Group Elements.

    ERIC Educational Resources Information Center

    Moore, E. A.

    1990-01-01

    Described is how crystal field theory and the angular overlap model can be applied to very simple molecules which can then be used to introduce such concepts as bonding orbitals, MO diagrams, and Walsh diagrams. The main-group compounds are used as examples and a switch to the transition metal complexes. (KR)

  15. Crystal Field Theory and the Angular Overlap Model Applied to Hydrides of Main Group Elements.

    ERIC Educational Resources Information Center

    Moore, E. A.

    1990-01-01

    Described is how crystal field theory and the angular overlap model can be applied to very simple molecules which can then be used to introduce such concepts as bonding orbitals, MO diagrams, and Walsh diagrams. The main-group compounds are used as examples and a switch to the transition metal complexes. (KR)

  16. Assembly of colloidal molecules, polymers, and crystals in acoustic and magnetic fields.

    PubMed

    Yang, Ye; Pham, An T; Cruz, Daniela; Reyes, Christopher; Wiley, Benjamin J; Lopez, Gabriel P; Yellen, Benjamin B

    2015-08-26

    A dynamically adjustable colloidal assembly technique is presented, which combines magnetic and acoustic fields to produce a wide range of colloidal structures, ranging from discrete colloidal molecules, to polymer networks and crystals. The structures can be stabilized and dried, making them suitable for the fabrication of advanced materials.

  17. Study of crystal-field splitting in ultrathin CePt5 films by Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Halbig, B.; Bass, U.; Geurts, J.; Zinner, M.; Fauth, K.

    2017-04-01

    The low-temperature electronic properties of rare-earth intermetallics are substantially influenced by the symmetry and magnitude of the crystal electric field. The direct spectroscopic analysis of crystal-field splitting can be challenging, especially in low-dimensional systems, because it requires both high spectral resolution and pronounced sensitivity. We demonstrate the eligibility of electronic Raman spectroscopy for this purpose by the direct determination of the 4 f level splitting in ultrathin ordered CePt5 films down to ≈1.5 nm thickness on Pt(111). Crystal-field excitations of Ce 4 f electrons give rise to Raman peaks at energy losses up to ≈25 meV. Three distinct peaks occur which we attribute to inequivalent Ce sites, located (i) at the interface to the substrate, (ii) next to the Pt-terminated film surface, and (iii) in the CePt5 layers in between. The well-resolved Raman signatures allow us to identify a reduced crystal-field splitting at the interface and an enhancement at the surface, highlighting its strong dependence on the local atomic environment.

  18. Scaling of crystal field parameters between Pd 2REIn and Pd 2RESn

    NASA Astrophysics Data System (ADS)

    Babateen, M.; Neumann, K.-U.; Ziebeck, K. R. A.

    1995-02-01

    Experimentally it is found that crystal field (CF) parameters between the same rare earth compounds in the alloy series Pd 2REIn and Pd 2RESn (RE = rare earth element) exhibit scaling properties. A phenomenological model is put forward to explain this observation.

  19. Near-field observation of subwavelength confinement of photoluminescence by a photonic crystal microcavity.

    PubMed

    Louvion, Nicolas; Rahmani, Adel; Seassal, Christian; Callard, Ségolène; Gérard, Davy; de Fornel, Frédérique

    2006-07-15

    We present a direct, room-temperature near-field optical study of light confinement by a subwavelength defect microcavity in a photonic crystal slab containing quantum-well sources. The observations are compared with three-dimensional finite-difference time-domain calculations, and excellent agreement is found. Moreover, we use a subwavelength cavity to study the influence of a near-field probe on the imaging of localized optical modes.

  20. Crystal field and magnetoelastic interactions in Tb2Ti2O7

    NASA Astrophysics Data System (ADS)

    Klekovkina, V. V.; Malkin, B. Z.

    2014-06-01

    In terms of a semiphenomenological exchange charge model, we have obtained estimates of parameters of the crystal field and parameters of the electron-deformation interaction in terbium titanate Tb2Ti2O7 with a pyrochlore structure. The obtained set of parameters has been refined based on the analysis of spectra of neutron inelastic scattering and Raman light scattering, field dependences of the forced magnetostriction, and temperature dependences of elastic constants.

  1. High Field Magnetization measurements of uranium dioxide single crystals (P08358- E003-PF)

    SciTech Connect

    Gofryk, K.; Harrison, N.; Jaime, M.

    2014-12-01

    Our preliminary high field magnetic measurements of UO2 are consistent with a complex nature of the magnetic ordering in this material, compatible with the previously proposed non-collinear 3-k magnetic structure. Further extensive magnetic studies on well-oriented (<100 > and <111>) UO2 crystals are planned to address the puzzling behavior of UO2 in both antiferromagnetic and paramagnetic states at high fields.

  2. Measurement of temperature and velocity fields of freezing water using liquid crystal tracers

    NASA Astrophysics Data System (ADS)

    Kowalewski, Tomasz A.

    A new experimental technique based on a computational analysis of the colour and displacement of thermochromic liquid crystal tracers was applied to determine both the temperature and velocity fields of freezing water. The technique combines Digital Particle Image Thermometry and Digital Particle Image Velocimetry. Full 2-D temperature and velocity fields are determined from a pair or a longer sequence, of colour images taken for the selected cross-section of the flow.

  3. Bridging the terahertz near-field and far-field observations of liquid crystal based metamaterial absorbers

    NASA Astrophysics Data System (ADS)

    Wang, Lei; Ge, Shijun; Chen, Zhaoxian; Hu, Wei; Lu, Yanqing

    2016-09-01

    Metamaterial-based absorbers play a significant role in applications ranging from energy harvesting and thermal emitters to sensors and imaging devices. The middle dielectric layer of conventional metamaterial absorbers has always been solid. Researchers could not detect the near field distribution in this layer or utilize it effectively. Here, we use anisotropic liquid crystal as the dielectric layer to realize electrically fast tunable terahertz metamaterial absorbers. We demonstrate strong, position-dependent terahertz near-field enhancement with sub-wavelength resolution inside the metamaterial absorber. We measure the terahertz far-field absorption as the driving voltage increases. By combining experimental results with liquid crystal simulations, we verify the near-field distribution in the middle layer indirectly and bridge the near-field and far-field observations. Our work opens new opportunities for creating high-performance, fast, tunable, terahertz metamaterial devices that can be applied in biological imaging and sensing. Project supported by the National Basic Research Program of China (Grant No. 2012CB921803), the National Natural Science Foundation of China (Grants Nos. 61225026, 61490714, 11304151, and 61435008), the Natural Science Foundation of Jiangsu Province, China (Grant Nos. BK20150845 and 15KJB140004), the Open Foundation Project of National Laboratory of Solid State Microstructures, China (Grant No. M28003), and the Research Center of Optical Communications Engineering & Technology, Jiangsu Province, China.

  4. Multiscale approach to nematic liquid crystals via statistical field theory

    NASA Astrophysics Data System (ADS)

    Lu, Bing-Sui

    2017-08-01

    We propose an approach to a multiscale problem in the theory of thermotropic uniaxial nematics based on the method of statistical field theory. This approach enables us to relate the coefficients A , B , C , L1, and L2 of the Landau-de Gennes free energy for the isotropic-nematic phase transition to the parameters of a molecular model of uniaxial nematics, which we take to be a lattice gas model of nematogenic molecules interacting via a short-ranged potential. We obtain general constraints on the temperature and volume fraction of nematogens for the Landau-de Gennes theory to be stable against molecular orientation fluctuations at quartic order. In particular, for the case of a fully occupied lattice, we compute the values of the isotropic-nematic transition temperature and the order parameter discontinuity predicted by (i) a continuum approximation of the nearest-neighbor Lebwohl-Lasher model and (ii) a Lebwohl-Lasher-type model with a nematogenic interaction of finite range. We find that the predictions of (i) are in reasonably good agreement with known results of Monte Carlo simulation.

  5. Multiscale approach to nematic liquid crystals via statistical field theory.

    PubMed

    Lu, Bing-Sui

    2017-08-01

    We propose an approach to a multiscale problem in the theory of thermotropic uniaxial nematics based on the method of statistical field theory. This approach enables us to relate the coefficients A, B, C, L_{1}, and L_{2} of the Landau-de Gennes free energy for the isotropic-nematic phase transition to the parameters of a molecular model of uniaxial nematics, which we take to be a lattice gas model of nematogenic molecules interacting via a short-ranged potential. We obtain general constraints on the temperature and volume fraction of nematogens for the Landau-de Gennes theory to be stable against molecular orientation fluctuations at quartic order. In particular, for the case of a fully occupied lattice, we compute the values of the isotropic-nematic transition temperature and the order parameter discontinuity predicted by (i) a continuum approximation of the nearest-neighbor Lebwohl-Lasher model and (ii) a Lebwohl-Lasher-type model with a nematogenic interaction of finite range. We find that the predictions of (i) are in reasonably good agreement with known results of Monte Carlo simulation.

  6. Molecular field theory for biaxial smectic A liquid crystals

    NASA Astrophysics Data System (ADS)

    To, T. B. T.; Sluckin, T. J.; Luckhurst, G. R.

    2013-10-01

    Thermotropic biaxial nematic phases seem to be rare, but biaxial smectic A phases less so. Here we use molecular field theory to study a simple two-parameter model, with one parameter promoting a biaxial phase and the second promoting smecticity. The theory combines the biaxial Maier-Saupe and McMillan models. We use alternatively the Sonnet-Virga-Durand (SVD) and geometric mean approximations (GMA) to characterize molecular biaxiality by a single parameter. For non-zero smecticity and biaxiality, the model always predicts a ground state biaxial smectic A phase. For a low degree of smectic order, the phase diagram is very rich, predicting uniaxial and biaxial nematic and smectic phases, with the addition of a variety of tricritical and tetracritical points. For higher degrees of smecticity, the region of stability of the biaxial nematic phase is restricted and eventually disappears, yielding to the biaxial smectic phase. Phase diagrams from the two alternative approximations for molecular biaxiality are similar, except inasmuch that SVD allows for a first-order isotropic-biaxial nematic transition, whereas GMA predicts a Landau point separating isotropic and biaxial nematic phases. We speculate that the rarity of thermotropic biaxial nematic phases is partly a consequence of the presence of stabler analogous smectic phases.

  7. Electronic transitions, crystal field effects and phonons in UO 2

    NASA Astrophysics Data System (ADS)

    Schoenes, J.

    1980-08-01

    An extensive optical study of the 5f magnetic semiconductor UO 2 is presented. The experimental data include near normal incidence reflectivity measurements from 0.0025 to 13 eV, absorption and Faraday rotation measurements as function of temperature and of magnetic fields up to 100 kOe and photoemission results. From the data in the fundamental absorption region an energy level scheme is derived. This level scheme differs markedly from an earlier model but it is quantitatively supported by a calculation using the thermochemical Haber-Born process and also by cluster calculations. The localized nature of the 5f electrons is demonstrated. The absorption edge at 2 eV shows an abrupt shift to lower energies at the first order phase transition of UO 2 to the antiferromagnetic state. This shift is shown to be larger than expected from the lattice contraction indicating a magnetic order induced contribution to the total red shift. Below the absorption edge, intra-5f transitions and multiphonon excitations are reported, showing striking order induced effects at and below TN = 30.8 K. New results are presented for ε st, ε opt, ω TO and ω LO which fulfill the Lyddane-Sachs-Teller relation.

  8. Qubit addressing using hyperfine-interaction control by an electric field in a magnetic crystal

    SciTech Connect

    Song, Myeonghun; Lee, Soonchil; Lockwood, David J.

    2010-07-15

    We demonstrate experimentally the hyperfine-interaction control by an electric field, which is the operating principle of the addressable qubit operation in a silicon-based solid-state quantum computer in a new quantum computer system, a magnetic crystal. The transferred hyperfine field at a F{sup -} nucleus caused by neighboring Mn{sup 2+} electron spins in an antiferromagnetic MnF{sub 2} single crystal was measured by {sup 19}F nuclear magnetic resonance (NMR) with an external electric field applied along the [110] crystal direction. The electric field splits the {sup 19}F NMR peak into two resolved lines that come from the F nuclei located at geometrically equivalent sites. A line splitting of 56 kHz was achieved at an electric field of 3.4 V/{mu}m. One of the F{sup -} nuclear spins could be flipped selectively by a composite radio-frequency pulse while leaving the other unchanged, thereby demonstrating qubit addressing via electric field control of the hyperfine interaction.

  9. Anisotropic physical properties of single-crystal U2Rh2Sn in high magnetic fields

    NASA Astrophysics Data System (ADS)

    Prokeš, K.; Gorbunov, D. I.; Reehuis, M.; Klemke, B.; Gukasov, A.; Uhlířová, K.; Fabrèges, X.; Skourski, Y.; Yokaichiya, F.; Hartwig, S.; Andreev, A. V.

    2017-05-01

    We report on the crystal and magnetic structures, magnetic, transport, and thermal properties of U2Rh2Sn single crystals studied in part in high magnetic fields up to 58 T. The material adopts a U3Si2 -related tetragonal crystal structure and orders antiferromagnetically below TN=25 K. The antiferromagnetic structure is characterized by a propagation vector k =(00 1/2 ) . The magnetism in U2Rh2Sn is found to be associated mainly with 5 f states. However, both unpolarized and polarized neutron experiments reveal at low temperatures in zero field non-negligible magnetic moments also on Rh sites. U moments of 0.50(2) μB are directed along the tetragonal axis while Rh moments of 0.06(4) μB form a noncollinear arrangement confined to the basal plane. The response to applied magnetic field is highly anisotropic. Above ˜15 K the easy magnetization direction is along the tetragonal axis. At lower temperatures, however, a stronger response is found perpendicular to the c axis. While for the a axis no magnetic phase transition is observed up to 58 T, for the field applied at 1.8 K along the tetragonal axis we observe above 22.5 T a field-polarized state. A magnetic phase diagram for the field applied along the c axis is presented.

  10. Patterning technology for solution-processed organic crystal field-effect transistors

    PubMed Central

    Li, Yun; Sun, Huabin; Shi, Yi; Tsukagoshi, Kazuhito

    2014-01-01

    Organic field-effect transistors (OFETs) are fundamental building blocks for various state-of-the-art electronic devices. Solution-processed organic crystals are appreciable materials for these applications because they facilitate large-scale, low-cost fabrication of devices with high performance. Patterning organic crystal transistors into well-defined geometric features is necessary to develop these crystals into practical semiconductors. This review provides an update on recentdevelopment in patterning technology for solution-processed organic crystals and their applications in field-effect transistors. Typical demonstrations are discussed and examined. In particular, our latest research progress on the spin-coating technique from mixture solutions is presented as a promising method to efficiently produce large organic semiconducting crystals on various substrates for high-performance OFETs. This solution-based process also has other excellent advantages, such as phase separation for self-assembled interfaces via one-step spin-coating, self-flattening of rough interfaces, and in situ purification that eliminates the impurity influences. Furthermore, recommendations for future perspectives are presented, and key issues for further development are discussed. PMID:27877656

  11. Patterning technology for solution-processed organic crystal field-effect transistors

    NASA Astrophysics Data System (ADS)

    Li, Yun; Sun, Huabin; Shi, Yi; Tsukagoshi, Kazuhito

    2014-04-01

    Organic field-effect transistors (OFETs) are fundamental building blocks for various state-of-the-art electronic devices. Solution-processed organic crystals are appreciable materials for these applications because they facilitate large-scale, low-cost fabrication of devices with high performance. Patterning organic crystal transistors into well-defined geometric features is necessary to develop these crystals into practical semiconductors. This review provides an update on recent development in patterning technology for solution-processed organic crystals and their applications in field-effect transistors. Typical demonstrations are discussed and examined. In particular, our latest research progress on the spin-coating technique from mixture solutions is presented as a promising method to efficiently produce large organic semiconducting crystals on various substrates for high-performance OFETs. This solution-based process also has other excellent advantages, such as phase separation for self-assembled interfaces via one-step spin-coating, self-flattening of rough interfaces, and in situ purification that eliminates the impurity influences. Furthermore, recommendations for future perspectives are presented, and key issues for further development are discussed.

  12. Transition of vertically aligned liquid crystal driven by fan-shaped electric field

    NASA Astrophysics Data System (ADS)

    Tsung, J. W.; Ting, T. L.; Chen, C. Y.; Liang, W. L.; Lai, C. W.; Lin, T. H.; Hsu, W. H.

    2017-09-01

    Interdigital electrodes are implemented in many commercial and novel liquid crystal devices to align molecules. Although many empirical principles and patents apply to electrode design, only a few numerical simulations of alignment have been conducted. Why and how the molecules align in an ordered manner has never been adequately explained. Hence, this investigation addresses the Fréedericksz transition of vertically aligned liquid crystal that is driven by fishbone electrodes, and thereafter identifies the mechanism of liquid crystal alignment. Theoretical calculations suggest that the periodic deformation that is caused by the fan-shaped fringe field minimizes the free energy in the liquid crystal cell, and the optimal alignment can be obtained when the cell parameters satisfy the relation p /2 d =√{k11/k33 } , where p is the spatial period of the strips of the electrode; d denotes the cell gap; and k11 and k33 are the splay and bend elastic constants of the liquid crystal, respectively. Polymer-stabilized vertical alignment test cells with various p values and spacings between the electrodes were fabricated, and the process of liquid crystal alignment was observed under an optical microscope. The degree of alignment was evaluated by measuring the transmittance of the test cell. The experimental results were consistent with the theoretical predictions. The principle of design, p /2 d =√{k11/k33 } , greatly improves the uniformity and stability of the aligned liquid crystal. The methods that are presented here can be further applied to cholesteric liquid crystal and other self-assembled soft materials.

  13. Negative Refractive Bi-Crystal with Broken Symmetry Leading to Unusual Fields in Guided Wave Heterostructures

    NASA Astrophysics Data System (ADS)

    Krowne, Clifford

    2004-03-01

    A recent finding has shown that a unixial bi-crystal shows negative refraction (NR) [1], a property in common with recent left-handed metamaterials examined for physics of focusing behavior and of field distributions [2], and field contouring effects in electronic structures [3]. This is a very interesting property related to energy wave front motion and has an analog in electron ballistic motion in a semiconductor heterostructure too. The property which yields NR, breaks field symmetry, and allows asymmetric distributions of electromagnetic fields in the cross-section in which heterostructure layering occurs when propagation is normal to this cross-section in a longitudinal direction. What is all the more remarkable is that individual heterostructure layers are not field symmetry breaking and do not lead to asymmetric field distributions. In fact when a single crystal is inserted in a guiding structure, nothing special happens. When heterostructure layering is constructive, successive layers could enhance the effect. We demonstrate here for the first time, using a model stripline structure to guide the wave, that a bi-crystal will indeed produce asymmetric rf electric and magnetic distributions. Calculations were done with an ab initio approach using an anisotropic Greens function which allows the physical properties of the uniaxial crystals to be treated via their tensors. The results have important implications for microwave devices which rely on asymmetric field distributions. One could envision wide application in monolithic integrated circuits in terms of devices utilizing both microwave and millimeter transmission as well as optical transmission using dielectric waveguiding structures. [1] Y. Zhang, B. Fluegel and A. Mascarenhas, Phys. Rev. Lett. 91, 157404 (Oct. 2003). [2] C. M. Krowne, Phys. Rev. Lett. 92, to be publ. (2004). [3] C. M. Krowne, IEEE Trans. Microwave Theory & Tech. 51, (Dec. 2003).

  14. Wide-field-of-view narrow-band spectral filters based on photonic crystal nanocavities.

    PubMed

    Nakagawa, Wataru; Sun, Pang-Chen; Chen, Chyong-Hua; Fainman, Yeshaiahu

    2002-02-01

    We describe a novel approach to implementing wide-field-of-view narrow-band spectral filters, using an array of resonant nanocavities consisting of periodic defects in a two-dimensional three-material photonic-crystal nanostructure. We analyze the transmissivity of this type of filter for a range of wavelengths and in-plane incidence angles as a function of the defect's refractive index, the number of layers in the photonic-crystal reflectors, and the period of the defects and find that this structure diminishes the angular sensitivity of the resonance condition relative to that of a standard multilayer filter.

  15. Creating physically-based three-dimensional microstructures: Bridging phase-field and crystal plasticity models.

    SciTech Connect

    Lim, Hojun; Owen, Steven J.; Abdeljawad, Fadi F.; Hanks, Byron; Battaile, Corbett Chandler

    2015-09-01

    In order to better incorporate microstructures in continuum scale models, we use a novel finite element (FE) meshing technique to generate three-dimensional polycrystalline aggregates from a phase field grain growth model of grain microstructures. The proposed meshing technique creates hexahedral FE meshes that capture smooth interfaces between adjacent grains. Three dimensional realizations of grain microstructures from the phase field model are used in crystal plasticity-finite element (CP-FE) simulations of polycrystalline a -iron. We show that the interface conformal meshes significantly reduce artificial stress localizations in voxelated meshes that exhibit the so-called "wedding cake" interfaces. This framework provides a direct link between two mesoscale models - phase field and crystal plasticity - and for the first time allows mechanics simulations of polycrystalline materials using three-dimensional hexahedral finite element meshes with realistic topological features.

  16. Effect of electric field on reentrance transition in a binary mixture of liquid crystals

    NASA Astrophysics Data System (ADS)

    Kumari, Sunita; Singh, S.

    2015-12-01

    Employing a phenomenological mean field theory, we analyze the effect of an electric field on the N - SmA phase transition for pure liquid crystal and on the reentrant nematic phase in a binary mixture of liquid crystals exhibiting the phase sequence I - N - SmA - NR on cooling. The basic idea of the work is to explain the phase transition behavior of the system by assuming that certain Landau coefficients associated with the order parameters coupling terms of the free-energy density expansion are field dependent. These parameters play a crucial role and show a rapid variation at the SmA - NR transition as compared to the SmA - N transition.

  17. Drying dissipative patterns of the colloidal crystals of silica spheres in an dc-electric field.

    PubMed

    Okubo, Tsuneo; Kimura, Keisuke; Tsuchida, Akira

    2007-04-15

    Drying dissipative structural patterns of the colloidal crystals of silica spheres were studied under an dc-electric field. Platinum plate electrodes of anode and cathode were set on a cover glass. The broad hills accumulated with the spheres were observed at the outer edges of the dried film without and also with the electric fields. The column-like structures were formed by the electric flux, and movement of the spheres took place toward anode. The dried film kept colloidal crystal structure, where the nearest-neighbored spheres contact each other more compactly in the areas closer to the anode. Drying times needed for the complete dryness of the suspensions decreased as the strength of the electric field increased. Addition of sodium chloride to the suspensions retarded the movement of spheres toward the anode substantially.

  18. Effect of wake potential on Coulomb crystallization in the presence of magnetic field

    NASA Astrophysics Data System (ADS)

    Bhattacharjee, Saurav; Das, Nilakshi

    2012-10-01

    The formation of dust crystal in plasma under the influence of repulsive Yukawa (Debye-Hückel) potential is a well known phenomenon. The regular structure of dust particles is affected by anisotropic ion flow near the sheath region. The bombardment of the ions over dust grains distorts their Debye sphere by overshielding the dust cloud and gives rise to an attractive oscillatory wake potential. In this paper, we have obtained an expression for wake potential along with the Yukawa type of potential in a complex plasma in the presence of magnetic field, for subsonic ion flow towards the plasma sheath. In the presence of magnetic field, interaction potential gets modified and becomes anisotropic. We have studied the combined effect of the attractive wake potential as well as repulsive Yukawa potential on a 2D dust crystal, both in the presence and absence of magnetic field, using molecular dynamic simulation.

  19. Crystal growth and low coercive field 180{degree} domain switching characteristics of stoichiometric LiTaO{sub 3}

    SciTech Connect

    Kitamura, K.; Furukawa, Y.; Niwa, K.; Gopalan, V.; Mitchell, T.E.

    1998-11-01

    We grew LiTaO{sub 3} single crystals with a composition close to stoichiometry by using a double crucible Czochralski method. The switching field required for 180{degree} ferroelectric domain reversal and the internal fields originating from nonstoichiometric point defects were compared for the stoichiometric and conventional commercially available crystals. The switching fields for the domain reversal in the stoichiometric crystal with a Curie temperature of 685 {degree}C was 1.7 kV/mm. This is about one thirteenth of the switching field required for the conventional LiTaO{sub 3thinsp} crystals with a Curie temperature near 600 {degree}C. The internal field in the stoichiometric crystal drastically decreased to 0.1 kV/mm. {copyright} {ital 1998 American Institute of Physics.}

  20. Crystal field excitations in CeCu2Ge2: Revisited employing a single crystal and inelastic neutron scattering

    NASA Astrophysics Data System (ADS)

    Loewenhaupt, Michael; Faulhaber, Enrico; Schneidewind, Astrid; Deppe, Micha; Hradil, Klaudia

    2012-04-01

    The intermetallic compound, CeCu2Ge2, is the counterpart of the heavy-fermion superconductor CeCu2Si2. CeCu2Ge2 is a magnetically ordering (TN = 4.1K) Kondo lattice with a moderate Sommerfeld coefficient of 140 mJ/ molK2. Earlier inelastic neutron measurements on a polycrystalline sample revealed a doublet ground state and a quasi-quartet excited state at 16.5 meV, although a splitting of the 4f1 (J = 5/2) ground state multiplet into 3 doublets is expected from the point symmetry of the Ce3+ ions. We performed detailed inelastic neutron scattering experiments on a single crystal at the thermal triple-axis spectrometer PUMA at FRM II for different crystallographic directions. From our results we infer that the quasi-quartet, in fact, consists of two doublets at 17.0 and 18.3 meV which exhibit a strong directional dependence of their transition matrix elements to the ground state doublet. Finally, we will present a new set of crystal field parameters.

  1. Enhancement of local electromagnetic fields in plasmonic crystals of coaxial metallic nanostructures

    NASA Astrophysics Data System (ADS)

    Iwanaga, Masanobu; Ikeda, Naoki; Sugimoto, Yoshimasa

    2012-01-01

    We have experimentally and numerically examined resonant modes in plasmonic crystals (PlCs) of coaxial metallic nanostructures. Resonance enhancements of local electromagnetic (EM) fields were evaluated quantitatively. We clarified that a local mode induced in the coaxial metallic structure shows the most significant field enhancement. The enhancement factors are comprehensively discussed by comparison with other PlCs, indicating that the coaxial PlC provides a locally intense electric field and EM power flux in the annular slit of 50-nm metallic gaps.

  2. Optical near-field microscopy of light focusing through a photonic crystal flat lens.

    PubMed

    Fabre, Nathalie; Lalouat, Loïc; Cluzel, Benoit; Mélique, Xavier; Lippens, Didier; de Fornel, Frédérique; Vanbésien, Olivier

    2008-08-15

    We report here the direct observation by using a scanning near-field microscopy technique of the light focusing through a photonic crystal flat lens designed and fabricated to operate at optical frequencies. The lens is fabricated using a III-V semiconductor slab, and we directly visualize the propagation of the electromagnetic waves by using a scanning near-field optical microscope. We directly evidence spatially, as well as spectrally, the focusing operating regime of the lens. At last, in light of the experimental scanning near-field optical microscope pictures, we discuss the lens ability to focus light at a subwavelength scale.

  3. A coupled ductile fracture phase-field model for crystal plasticity

    NASA Astrophysics Data System (ADS)

    Hernandez Padilla, Carlos Alberto; Markert, Bernd

    2017-07-01

    Nowadays crack initiation and evolution play a key role in the design of mechanical components. In the past few decades, several numerical approaches have been developed with the objective to predict these phenomena. The objective of this work is to present a simplified, nonetheless representative phenomenological model to predict the crack evolution of ductile fracture in single crystals. The proposed numerical approach is carried out by merging a conventional elasto-plastic crystal plasticity model and a phase-field model modified to predict ductile fracture. A two-dimensional initial boundary value problem of ductile fracture is introduced considering a single-crystal setup and Nickel-base superalloy material properties. The model is implemented into the finite element context subjected to a quasi-static uniaxial tension test. The results are then qualitatively analyzed and briefly compared to current benchmark results in the literature.

  4. Fiber optic dynamic electric field sensor based on nematic liquid crystal Fabry-Perot etalon

    NASA Astrophysics Data System (ADS)

    Ko, Myeong Ock; Kim, Sung-Jo; Kim, Jong-Hyun; Jeon, Min Yong

    2014-05-01

    We propose a fiber-optic dynamic electric field sensor using a nematic liquid crystal (NLC) Fabry-Perot etalon and a wavelength-swept laser. The transmission wavelength of the NLC Fabry-Perot etalon depends on the applied electric field intensity. The change in the effective refractive index of the NLC is measured while changing the applied electric field intensity. It decreases from 1.67 to 1.51 as the applied the electric field intensity is increased. Additionally, we successfully measure the dynamic variation of the electric field using the high-speed wavelength-swept laser. By measuring the modulation frequency of the transmission peaks in the temporal domain, the frequency of the modulated electric field can be estimated.

  5. Role of internal demagnetizing field for the dynamics of a surface-modulated magnonic crystal

    NASA Astrophysics Data System (ADS)

    Langer, M.; Röder, F.; Gallardo, R. A.; Schneider, T.; Stienen, S.; Gatel, C.; Hübner, R.; Bischoff, L.; Lenz, K.; Lindner, J.; Landeros, P.; Fassbender, J.

    2017-05-01

    This work aims to demonstrate and understand the key role of local demagnetizing fields in hybrid structures consisting of a continuous thin film with a stripe modulation on top. To understand the complex spin dynamics of these structures, the magnonic crystal was reconstructed in two different ways—performing micromagnetic simulations based on the structural shape as well as based on the internal demagnetizing field, which both are mapped on the nanoscale using electron holography. The simulations yield the frequency-field dependence as well as the angular dependence revealing the governing role of the internal field landscape around the backward-volume geometry. Simple rules for the propagation vector and the mode localization are formulated in order to explain the calculated mode profiles. Treating internal demagnetizing fields equivalent to anisotropies, the complex angle-dependent spin-wave behavior is described for an in-plane rotation of the external field.

  6. Selective Precipitation and Concentrating of Perovskite Crystals from Titanium-Bearing Slag Melt in Supergravity Field

    NASA Astrophysics Data System (ADS)

    Gao, Jintao; Zhong, Yiwei; Guo, Zhancheng

    2016-08-01

    Selective precipitation and concentrating of perovskite crystals from titanium-bearing slag melt in the supergravity field was investigated in this study. Since perovskite was the first precipitated phase from the slag melt during the cooling process, and a greater precipitation quantity and larger crystal sizes of perovskite were obtained at 1593 K to 1563 K (1320 °C to 1290 °C), concentrating of perovskite crystals from the slag melt was carried out at this temperature range in the supergravity field, at which the perovskite transforms into solid particles while the other minerals remain in the liquid melt. The layered structures appeared significantly in the sample obtained by supergravity treatment, and all the perovskite crystals moved along the supergravity direction and concentrated as the perovskite-rich phase in the bottom area, whereas the molten slag concentrated in the upper area along the opposite direction, in which it was impossible to find any perovskite crystals. With the gravity coefficient of G = 750, the mass fraction of TiO2 in the perovskite-rich phase was up to 34.65 wt pct, whereas that of the slag phase was decreased to 12.23 wt pct, and the recovery ratio of Ti in the perovskite-rich phase was up to 75.28 pct. On this basis, an amplification experimental centrifugal apparatus was exploited and the continuous experiment with larger scale was further carried out, the results confirming that selective precipitation and concentrating of perovskite crystals from the titanium-bearing slag melt by supergravity was a feasible method.

  7. Influence of electromagnetic field intensity on the metastable zone width of CaCO3 crystallization in circulating water

    NASA Astrophysics Data System (ADS)

    Wang, Jianguo; Liang, Yandong; Chen, Si

    2016-09-01

    In this study, changes in the metastable zone width of CaCO3 crystallization was determined through conductivity titration by altering electromagnetic field parameters applied to the circulating water system. The critical conductivity value and metastable zone curves of CaCO3 crystallization were determined under different solution concentrations and electromagnetic field intensities. Experimental results indicate that the effect of the electromagnetic field intensity on the critical conductivity value intensifies with the increase of solution concentration. Moreover, the metastable zone width of CaCO3 crystallization increases with the increase of electromagnetic field intensity within 200 Gs, thereby prolonging the induction period of nucleation.

  8. Spectral and polarization structure of field-induced photonic bands in cholesteric liquid crystals

    NASA Astrophysics Data System (ADS)

    Palto, S. P.; Barnik, M. I.; Geivandov, A. R.; Kasyanova, I. V.; Palto, V. S.

    2015-09-01

    Transmission of planar layers of cholesteric liquid crystals is studied in pulsed electric fields perpendicular to the helix axis at normal incidence of both linearly polarized and unpolarized light. Spectral and light polarization properties of the primary photonic band and the field-induced bands up to fourth order of Bragg selective reflection are studied in detail. In our experiments we have achieved an electric field strength several times higher than the theoretical values corresponding to the critical field of full helix unwinding. However, the experiments show that despite the high strength of the electric field applied the helix does not unwind, but strongly deforms, keeping its initial spatial period. Strong helix deformation results in distinct spectral band splitting, as well as very high field-induced selective reflectance that can be applied in lasers and other optoelectronic devices. Peculiarities of inducing and splitting the bands are discussed in terms of the scattering coefficient approach. All observed effects are confirmed by numerical simulations. The simulations also show that liquid crystal surface anchoring is not the factor that prevents the helix unwinding. Thus, the currently acknowledged concept of continuous helix unwinding in the electric field should be reconsidered.

  9. Raman scattering investigation of crystal-field excitations in ErNi_2B_2C

    NASA Astrophysics Data System (ADS)

    Rho, H.; Klein, M. V.; Yang, In-Sang; Canfield, P. C.

    2002-03-01

    We present Raman scattering studies in a rare-earth magnetic superconductor ErNi_2B_2C to understand crystal-field excitations and interplay between magnetism and superconductivity. A recent Raman measurement on ErNi_2B_2C shows two crystal-field excitations at 46 and 146 cm-1 at 10 K above the Néel temperature (T_N) [H. Martinho et al., Journal of Magnetism and Magnetic Materials 226-230, 978 (2001)]. Our preliminary Raman scattering measurements show five excitations below T_N: double peak structures at 49, 56 cm-1 and 145, 153 cm-1, and a low-lying excitation at 6 cm-1. Our observations are in reasonably good agreement with inelastic neutron scattering, Mössbauer spectroscopy, and specific heat measurements. Influence of temperature and magnetic field on these crystal-field excitations will be discussed in detail and compared with the results from DyNi_2B_2C.

  10. Milestone in the History of Field-Effect Liquid Crystal Displays and Materials

    NASA Astrophysics Data System (ADS)

    Schadt, Martin

    2009-03-01

    The history of digital electronics would have been very different without the invention of field-effect liquid crystal displays (LCDs) in 1970 and their sophisticated development and implementation into numerous products. Transmissive and reflective LCDs have become a key interface between man and machine. After almost 40 years of interdisciplinary R+D and engineering, today's LCDs enable virtually all display applications, including high definition television. Field-effect LCDs are characterized by flat design, low weight, low driving voltage, design flexibility, compatibility with silicon-on-glass and very low power consumption, especially in reflection. Their polarization-sensitive layer concept is the basis for sandwiching and integration of optical and electronic thin-film functions. The liquid crystal technology has become a fast growing industry over the past 38 years, today surpassing 100 billion, with many spin-offs into new areas. Prerequisite for field-effect LCDs and their large diversification potential is the unique self-organization of liquid crystals. New applications beyond displays based on self-organisation, smart boundary alignment, dedicated liquid crystalline materials and the ability of LCs to respond to electromagnetic fields, including light, are being developed. Examples for new applications are LC polymer thin-film optics, or synergies between LCDs and solid state back-lighting, such as inorganic and organic light emitting diodes (LEDs/OLEDs).

  11. Phase field modeling of rapid crystallization in the phase-change material AIST

    NASA Astrophysics Data System (ADS)

    Tabatabaei, Fatemeh; Boussinot, Guillaume; Spatschek, Robert; Brener, Efim A.; Apel, Markus

    2017-07-01

    We carry out phase field modeling as a continuum simulation technique in order to study rapid crystallization processes in the phase-change material AIST (Ag4In3Sb67Te26). In particular, we simulate the spatio-temporal evolution of the crystallization of a molten area of the phase-change material embedded in a layer stack. The simulation model is adapted to the experimental conditions used for recent measurements of crystallization rates by a laser pulse technique. Simulations are performed for substrate temperatures close to the melting temperature of AIST down to low temperatures when an amorphous state is involved. The design of the phase field model using the thin interface limit allows us to retrieve the two limiting regimes of interface controlled (low temperatures) and thermal transport controlled (high temperatures) dynamics. Our simulations show that, generically, the crystallization velocity presents a maximum in the intermediate regime where both the interface mobility and the thermal transport, through the molten area as well as through the layer stack, are important. Simulations reveal the complex interplay of all different contributions. This suggests that the maximum switching velocity depends not only on material properties but also on the precise design of the thin film structure into which the phase-change material is embedded.

  12. Electric Field Induced Stable Micro Rotor in Nematic Liquid Crystal Drops Constrained on Thin Cellulosic Fibers

    NASA Astrophysics Data System (ADS)

    Godinho, Maria Helena; Geng, Yong; Almeida, Pedro; Figueirinhas, João; Terentjev, Eugene

    2012-02-01

    We directly visualize the response of nematic liquid crystal drops of toroidal topology constrained on thin fibers, suspended in air, to an AC applied electric field E. This new localized liquid crystal system can exhibit non-trivial point defects, which may become energetically unstable against expanding into ring disclinations depending on the fiber constraining geometries. The director anchoring tangential near the fiber surface and homeotropic at the air interface, making a hybrid shell distribution that in turn causes a ring of disclination line around the main axis of the fiber at the center of the droplet. Upon application of E, the disclination ring first expands and slightly moves along the fiber main axis, followed by the appearance of a stable ``spherical particle'' orbiting around the fiber at the center of the liquid crystal drop. The rotation speed of this particle was found to vary linearly with the applied voltage. This constrained liquid crystal geometry seems to meet the essential requirements in which soliton like particles can develop and exhibit stable orbiting in three dimensions upon application of an external electric field. This is another example of a soft energy transducer system which allows, at the micro scale, the transfer in a continuous way of electrical to mechanical energy.

  13. Temperature dependence of Fe/++/ crystal field spectra - Implications to mineralogical mapping of planetary surfaces

    NASA Technical Reports Server (NTRS)

    Sung, C.-M.; Singer, R. B.; Parkin, K. M.; Burns, R. G.; Osborne, M.

    1977-01-01

    Results are reported of Fe(++) crystal field spectral measurements for olivines and pyroxenes up to 400 C. The results are correlated with crystal structure data at elevated temperatures, and the validity of remote-sensed identifications of minerals on hot surfaces of the moon and Mercury is assessed. Two techniques were used to obtain spectra of minerals at elevated temperatures using a spectrophotometer. One employed a diamond cell assembly or a specially designed sample holder to measure polarized absorption spectra of heated single crystals. For the other technique, a sample holder was designed to attach to a diffuse reflectance accessory to produce reflectance spectra of heated powdered samples. Polarized absorption spectra of forsterite at 20-400 C are shown in a graph. Other graphs show the temperature dependence of Fe(++) crystal field bands in olivines, the diffuse reflectance spectra of olivine at 40-400 C, the polarization absorption spectra of orthopyroxene at 30-400 C, the diffuse reflectance spectra of pigeonite at 40-400 C, and unpolarized absorption spectra of lunar pyroxene from Apollo 15 rock 15058.

  14. Interfacial free energy adjustable phase field crystal model for homogeneous nucleation.

    PubMed

    Guo, Can; Wang, Jincheng; Wang, Zhijun; Li, Junjie; Guo, Yaolin; Huang, Yunhao

    2016-05-18

    To describe the homogeneous nucleation process, an interfacial free energy adjustable phase-field crystal model (IPFC) was proposed by reconstructing the energy functional of the original phase field crystal (PFC) methodology. Compared with the original PFC model, the additional interface term in the IPFC model effectively can adjust the magnitude of the interfacial free energy, but does not affect the equilibrium phase diagram and the interfacial energy anisotropy. The IPFC model overcame the limitation that the interfacial free energy of the original PFC model is much less than the theoretical results. Using the IPFC model, we investigated some basic issues in homogeneous nucleation. From the viewpoint of simulation, we proceeded with an in situ observation of the process of cluster fluctuation and obtained quite similar snapshots to colloidal crystallization experiments. We also counted the size distribution of crystal-like clusters and the nucleation rate. Our simulations show that the size distribution is independent of the evolution time, and the nucleation rate remains constant after a period of relaxation, which are consistent with experimental observations. The linear relation between logarithmic nucleation rate and reciprocal driving force also conforms to the steady state nucleation theory.

  15. Measurements of mode field diameter and effective area of photonic crystal fibers by far-field scanning technique

    NASA Astrophysics Data System (ADS)

    Miyagi, Kazuya; Namihira, Yoshinori; Razzak, S. M. Abdur; Kaijage, Shubi F.; Begum, Feroza

    2010-07-01

    We have demonstrated that the correction factor k n = A eff/( πw 2), where ω = MFD/2 (MFD: mode field diameter), is above 1.20 for photonic crystal fibers (PCFs) with structural parameters in the range of d/Λ ≅ 0.40 to 0.90 ( d/Λ ratio of hole diameter d and pitch Λ). By using the far-field scanning (FFS) technique and the finite difference method, the results of experimental measurements and numerical simulations differed by only 0.9 to 3.0% for two types of PCFs. The finding that k n ≠ 1.0 for PCFs indicates that their electrical field distribution is non-Gaussian and cannot be determined by assuming a conventional step-index distribution for PCFs. It was also found that the ITU-T Petermann II definition is the most suitable for MFD measurements of PCFs with non-Gaussian distribution.

  16. Self-consistent density functional calculations of the crystal field levels in lanthanide and actinide dioxides

    NASA Astrophysics Data System (ADS)

    Zhou, Fei; Ozoliņš, Vidvuds

    2012-02-01

    Using a recently developed method combining a nonspherical self-interaction corrected LDA + U scheme and an on-site multibody Hamiltonian [Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.83.085106 83, 085106 (2011)], we calculate the crystal field parameters and crystal field (CF) excitation levels of f-element dioxides in the fluorite structure with fn electronic configurations, including n=1 (PaO2, PrO2), n=2 (UO2), n=3 (NpO2), and n=4 (PuO2). It is shown that good agreement with experimental data (within approximately 10-20 meV) can be obtained in all cases. The properties of the multielectron CF ground states are analyzed.

  17. Glassy phases and driven response of the phase-field-crystal model with random pinning.

    PubMed

    Granato, E; Ramos, J A P; Achim, C V; Lehikoinen, J; Ying, S C; Ala-Nissila, T; Elder, K R

    2011-09-01

    We study the structural correlations and the nonlinear response to a driving force of a two-dimensional phase-field-crystal model with random pinning. The model provides an effective continuous description of lattice systems in the presence of disordered external pinning centers, allowing for both elastic and plastic deformations. We find that the phase-field crystal with disorder assumes an amorphous glassy ground state, with only short-ranged positional and orientational correlations, even in the limit of weak disorder. Under increasing driving force, the pinned amorphous-glass phase evolves into a moving plastic-flow phase and then, finally, a moving smectic phase. The transverse response of the moving smectic phase shows a vanishing transverse critical force for increasing system sizes.

  18. Rubrene crystal field-effect mobility modulation via conducting channel wrinkling.

    PubMed

    Reyes-Martinez, Marcos A; Crosby, Alfred J; Briseno, Alejandro L

    2015-05-05

    With the impending surge of flexible organic electronic technologies, it has become essential to understand how mechanical deformation affects the electrical performance of organic thin-film devices. Organic single crystals are ideal for the systematic study of strain effects on electrical properties without being concerned about grain boundaries and other defects. Here we investigate how the deformation affects the field-effect mobility of single crystals of the benchmark semiconductor rubrene. The wrinkling instability is used to apply local strains of different magnitudes along the conducting channel in field-effect transistors. We discover that the mobility changes as dictated by the net strain at the dielectric/semiconductor interface. We propose a model based on the plate bending theory to quantify the net strain in wrinkled transistors and predict the change in mobility. These contributions represent a significant step forward in structure-function relationships in organic semiconductors, critical for the development of the next generation of flexible electronic devices.

  19. Direct mapping of local director field of nematic liquid crystals at the nanoscale

    PubMed Central

    Xia, Yu; Serra, Francesca; Kamien, Randall D.; Stebe, Kathleen J.; Yang, Shu

    2015-01-01

    Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of wide interest in both the display industry and soft matter as a route to more sophisticated optical objects, to direct phase separation, and to facilitate colloidal assemblies. However, it remains challenging to directly probe the molecular-scale organization of nonglassy nematic LC molecules without altering the LC directors. We design and synthesize a new type of nematic liquid crystal monomer (LCM) system with strong dipole–dipole interactions, resulting in a stable nematic phase and strong homeotropic anchoring on silica surfaces. Upon photopolymerization, the director field can be faithfully “locked,” allowing for direct visualization of the LC director field and defect structures by scanning electron microscopy (SEM) in real space with 100-nm resolution. Using this technique, we study the nematic textures in more complex LC/colloidal systems and calculate the extrapolation length of the LCM. PMID:26621729

  20. Emergence of foams from the breakdown of the phase field crystal model

    NASA Astrophysics Data System (ADS)

    Guttenberg, Nicholas; Goldenfeld, Nigel; Dantzig, Jonathan

    2010-06-01

    The phase field crystal (PFC) model captures the elastic and topological properties of crystals with a single scalar field at small undercooling. At large undercooling, new foamlike behavior emerges. We characterize this foam phase of the PFC equation and propose a modified PFC equation that may be used for the simulation of foam dynamics. This minimal model reproduces von Neumann’s rule for two-dimensional dry foams and Lifshitz-Slyozov coarsening for wet foams. We also measure the coordination number distribution and find that its second moment is larger than previously reported experimental and theoretical studies of soap froths, a finding that we attribute to the wetness of the foam increasing with time.

  1. Flow and temperature field measurements of thermal convection in a small vertical gap using liquid crystals

    NASA Astrophysics Data System (ADS)

    Heiland, Hans Georg; Wozniak, Günter; Wozniak, Klaus

    2007-07-01

    Thermal convection in a small vertical gap is studied experimentally applying digital particle image velocimetry/thermometry. This optical method enables the simultaneous measurement of two-dimensional flow and temperature fields in a liquid. The principle is based on seeding the liquid flow medium with thermochromic liquid crystal particles. The temperature is measured by the crystal particles which change their reflected colour as function of temperature. The flow velocity is measured by using the same particles as flow tracers. The investigation shall contribute to the understanding of the fluid mechanical behaviour of biological liquids within micro reactor systems. However, the problem is also of fundamental interest as far as heat and mass transfer is concerned. Measured temperature and flow velocity fields are presented and discussed.

  2. Electric Field-Induced Skyrmion Crystals via Charged Monopoles in Insulating Helimagets

    NASA Astrophysics Data System (ADS)

    Watanabe, Haruki; Vishwanath, Ashvin

    2016-06-01

    Electrons propagating in a magnetically ordered medium experience an additional gauge field associated with the Berry phase of their spin following the local magnetic texture. In contrast to the usual electromagnetic field, this gauge field admits monopole excitations, corresponding to hedgehog defects of the magnetic order. In an insulator, these hedgehogs carry a well-defined electric charge allowing for them to be controlled by electric fields. One particularly robust mechanism that contributes to the charge is the orbital magnetoelectric effect, captured by a θ angle, which leads to a charge of eθ/2π on hedgehogs. This is a direct consequence of the Witten effect for magnetic monopoles in a θ medium. A physical consequence is that external electric fields can induce skyrmion crystal phases in insulating helimagnets.

  3. Intense THz radiation produced in organic salt crystals for high-field applications

    NASA Astrophysics Data System (ADS)

    Vicario, C.; Ruchert, C.; Hauri, C. P.

    2013-03-01

    Organic stilbazolium salt crystals pumped by intense, ultrashort mid-infrared laser have been investigated for efficient THz generation by optical rectification. In this paper we present our latest results in view of the generation of single-cycle and high-field THz transient in the THz gap (0.1-10 THz). The organic rectifiers like DAST, OH1 and DSTMS combine extremely large optical susceptibility with excellent velocity matching between the infrared pump and the THz radiation. Our simple collinear conversion scheme provides THz beams with excellent focusing properties and single cycle electric field larger than 1.5 MV/cm and magnetic field strength beyond 0.5 Tesla. The source can potentially cover the full THz gap at field strength which is barely provided by other THz sources. The THz pulse is carrier-envelope phase stable and the polarity of the field can be easily inverted.

  4. Threshold switching via electric field induced crystallization in phase-change memory devices

    NASA Astrophysics Data System (ADS)

    Vázquez Diosdado, Jorge A.; Ashwin, Peter; Kohary, Krisztian I.; Wright, C. David

    2012-06-01

    Phase-change devices exhibit characteristic threshold switching from the reset (off) to the set (on) state. Mainstream understanding of this electrical switching phenomenon is that it is initiated electronically via the influence of high electric fields on inter-band trap states in the amorphous phase. However, recent work has suggested that field induced (crystal) nucleation could instead be responsible. We compare and contrast these alternative switching "theories" via realistic simulations of device switching both with and without electric field dependent contributions to the system free energy. Results show that although threshold switching can indeed be obtained purely by electric field induced nucleation, the fields required are significantly larger than experimentally measured values.

  5. Enhanced magnetic-field-induced optical properties of nanostructured magnetic fluids by doping nematic liquid crystals

    NASA Astrophysics Data System (ADS)

    Wang, Xiang; Pu, Shengli; Ji, Hongzhu; Yu, Guojun

    2012-05-01

    Ferronematic materials composed of 4-cyano-4'-pentylbiphenyl nematic liquid crystal and oil-based Fe3O4 magnetic fluid were prepared using ultrasonic agitation. The birefringence (Δ n) and figure of merit of optical properties ( Q = Δ n/α, where α is the extinction coefficient) of pure magnetic fluids and the as-prepared ferronematic materials were examined and compared. The figure of merit of optical properties weighs the birefringence and extinction of the materials and is more appropriate to evaluate their optical properties. Similar magnetic-field- and magnetic-particle-concentration-dependent properties of birefringence and figure of merit of optical properties were obtained for the pure magnetic fluids and the ferronematic materials. For the ferronematic materials, the values of Q increase with the volume fractions of nematic liquid crystal under certain fixed field strength and are larger than those of their corresponding pure magnetic fluids at high field region. In addition, the enhancement of Q value increases monotonously with the magnetic field and becomes remarkable when the applied magnetic field is beyond 50 mT. The maximum relative enhanced value of Q R exceeds 6.8% in our experiments. The results of this work may conduce to extend the pragmatic applications of nanostructured magnetic fluids in optical field.

  6. Enhanced magnetic-field-induced optical properties of nanostructured magnetic fluids by doping nematic liquid crystals.

    PubMed

    Wang, Xiang; Pu, Shengli; Ji, Hongzhu; Yu, Guojun

    2012-05-15

    Ferronematic materials composed of 4-cyano-4'-pentylbiphenyl nematic liquid crystal and oil-based Fe3O4 magnetic fluid were prepared using ultrasonic agitation. The birefringence (Δn) and figure of merit of optical properties (Q = Δn/α, where α is the extinction coefficient) of pure magnetic fluids and the as-prepared ferronematic materials were examined and compared. The figure of merit of optical properties weighs the birefringence and extinction of the materials and is more appropriate to evaluate their optical properties. Similar magnetic-field- and magnetic-particle-concentration-dependent properties of birefringence and figure of merit of optical properties were obtained for the pure magnetic fluids and the ferronematic materials. For the ferronematic materials, the values of Q increase with the volume fractions of nematic liquid crystal under certain fixed field strength and are larger than those of their corresponding pure magnetic fluids at high field region. In addition, the enhancement of Q value increases monotonously with the magnetic field and becomes remarkable when the applied magnetic field is beyond 50 mT. The maximum relative enhanced value of QR exceeds 6.8% in our experiments. The results of this work may conduce to extend the pragmatic applications of nanostructured magnetic fluids in optical field.

  7. Recent Advances in the Understanding of the Influence of Electric and Magnetic Fields on Protein Crystal Growth

    DOE PAGES

    Pareja-Rivera, Carina; Cuéllar-Cruz, Mayra; Esturau-Escofet, Nuria; ...

    2016-12-05

    Here, in this contribution we use nonconventional methods that help to increase the success rate of a protein crystal growth, and consequently of structural projects using X-ray diffraction techniques. In order to achieve this purpose, this contribution presents new approaches involving more sophisticated techniques of protein crystallization, not just for growing protein crystals of different sizes by using electric fields, but also for controlling crystal size and orientation. Also, this latter was possible through the use of magnetic fields that allow to obtain protein crystals suitable for both high-resolution X-ray and neutron diffraction crystallography where big crystals are required. Thismore » contribution discusses some pros, cons and realities of the role of electromagnetic fields in protein crystallization research, and their effect on protein crystal contacts. Additionally, we discuss the importance of room and low temperatures during data collection. Finally, we also discuss the effect of applying a rather strong magnetic field of 16.5 T, for shorts and long periods of time, on protein crystal growth, and on the 3D structure of two model proteins.« less

  8. Recent Advances in the Understanding of the Influence of Electric and Magnetic Fields on Protein Crystal Growth

    SciTech Connect

    Pareja-Rivera, Carina; Cuéllar-Cruz, Mayra; Esturau-Escofet, Nuria; Demitri, Nicola; Polentarutti, Maurizio; Stojanoff, Vivian; Moreno, Abel

    2016-12-05

    Here, in this contribution we use nonconventional methods that help to increase the success rate of a protein crystal growth, and consequently of structural projects using X-ray diffraction techniques. In order to achieve this purpose, this contribution presents new approaches involving more sophisticated techniques of protein crystallization, not just for growing protein crystals of different sizes by using electric fields, but also for controlling crystal size and orientation. Also, this latter was possible through the use of magnetic fields that allow to obtain protein crystals suitable for both high-resolution X-ray and neutron diffraction crystallography where big crystals are required. This contribution discusses some pros, cons and realities of the role of electromagnetic fields in protein crystallization research, and their effect on protein crystal contacts. Additionally, we discuss the importance of room and low temperatures during data collection. Finally, we also discuss the effect of applying a rather strong magnetic field of 16.5 T, for shorts and long periods of time, on protein crystal growth, and on the 3D structure of two model proteins.

  9. Two Beam Energy Exchange in Hybrid Liquid Crystal Cells with Photorefractive Field Controlled Boundary Conditions (Postprint)

    DTIC Science & Technology

    2016-09-12

    energy gain when two light beams intersect in a hybrid nematic liquid crystal (LC) cell with photorefractive crystalline substrates. A periodic space...charge field induced by interfering light beams in the photorefractive substrates penetrates into the LC layer and reorients the director. We account...grating and the boundary-driven grating. Each light beam diffracts from the induced gratings leading to an energy exchange between beams. We

  10. Possibility of studying crystal-field levels and excitations by. mu. /sup +/SR spectroscopy

    SciTech Connect

    Yaouanc, A.

    1983-01-01

    We point out that ..mu../sup +/SR relaxation times T/sub 1/ and T/sub 2/ measured in metallic magnetic materials can sometimes be expressed in terms of the spin-spin correlation functions of the magnetic ions. We calculate these functions in a random phase approximation and notice they can strongly depend on the crystal field levels and excitations of the magnetic ions. The shortcomings of this approximation are discussed.

  11. Phonoritonic Crystals with a Synthetic Magnetic Field for an Acoustic Diode

    NASA Astrophysics Data System (ADS)

    Poshakinskiy, A. V.; Poddubny, A. N.

    2017-04-01

    We develop a rigorous theoretical framework to describe light-sound interaction in the laser-pumped periodic multiple-quantum-well structure accounting for hybrid phonon-polariton excitations, termed phonoritons. We show that phonoritons exhibit the pumping-induced synthetic magnetic field in the artificial "coordinate-energy" space that makes transmission of left- and right- going waves different. The sound transmission nonreciprocity allows one to use such phonoritonic crystals with realistic parameters as optically controlled nanoscale acoustic diodes.

  12. Dust Lattice Waves in Two-Dimensional Hexagonal Dust Crystals with an External Magnetic Field

    SciTech Connect

    Farokhi, B.; Shahmansouri, M.

    2008-09-07

    The influence of a constant magnetic field on the propagation of dust-lattice (DL) modes in a two-dimensional hexagonal strongly coupled plasma crystal formed by paramagnetic particles is considered. The expression for the wave dispersion relation clearly shows that high-frequency and low-frequency branches exist as a result of the coupling of longitudinal and transverse modes due to the Lorentz force acting on the dust particles.

  13. Tunable extended depth of field using a liquid crystal annular spatial filter.

    PubMed

    Klapp, Iftach; Solodar, Asi; Abdulhalim, Ibrahim

    2014-03-15

    A tunable extended depth of field (EDOF) imaging is presented using temporal multiplexing and a low-cost eight-ring, annular liquid crystal spatial light modulator. By changing between different phase profiles in the pupil plane of a lens we perform several levels of EDOF. Using these levels as a "database" it is shown by temporal multiplexing how to decompose tunable levels of EDOF.

  14. Hyperspectral optical near-field imaging: Looking graded photonic crystals and photonic metamaterials in color

    NASA Astrophysics Data System (ADS)

    Dellinger, Jean; Van Do, K.; Le Roux, Xavier; de Fornel, Frédérique; Cassan, Eric; Cluzel, Benoît

    2012-10-01

    Using a scanning near-field optical microscope operating with a hyperspectral detection scheme, we report the direct observation of the mirage effect within an on-chip integrated artificial material made of a two dimensional graded photonic crystal. The light rainbow due to the material dispersion is quantified experimentally and quantitatively compared to three dimensional plane wave assisted Hamiltonian optics predictions of light propagation.

  15. Dual gauge field theory of quantum liquid crystals in two dimensions

    DOE PAGES

    Beekman, Aron J.; Nissinen, Jaakko; Wu, Kai; ...

    2017-04-18

    We present a self-contained review of the theory of dislocation-mediated quantum melting at zero temperature in two spatial dimensions. The theory describes the liquid-crystalline phases with spatial symmetries in between a quantum crystalline solid and an isotropic superfluid: quantum nematics and smectics. It is based on an Abelian-Higgs-type duality mapping of phonons onto gauge bosons (“stress photons”), which encode for the capacity of the crystal to propagate stresses. Dislocations and disclinations, the topological defects of the crystal, are sources for the gauge fields and the melting of the crystal can be understood as the proliferation (condensation) of these defects, givingmore » rise to the Anderson–Higgs mechanism on the dual side. For the liquid crystal phases, the shear sector of the gauge bosons becomes massive signaling that shear rigidity is lost. After providing the necessary background knowledge, including the order parameter theory of two-dimensional quantum liquid crystals and the dual theory of stress gauge bosons in bosonic crystals, the theory of melting is developed step-by-step via the disorder theory of dislocation-mediated melting. Resting on symmetry principles, we derive the phenomenological imaginary time actions of quantum nematics and smectics and analyze the full spectrum of collective modes. The quantum nematic is a superfluid having a true rotational Goldstone mode due to rotational symmetry breaking, and the origin of this ‘deconfined’ mode is traced back to the crystalline phase. The two-dimensional quantum smectic turns out to be a dizzyingly anisotropic phase with the collective modes interpolating between the solid and nematic in a non-trivial way. We also consider electrically charged bosonic crystals and liquid crystals, and carefully analyze the electromagnetic response of the quantum liquid crystal phases. In particular, the quantum nematic is a real superconductor and shows the Meissner effect. Furthermore

  16. Rheology of Pure Glasses and Crystal Bearing Melts: from the Newtonian Field to the Brittle Onset

    NASA Astrophysics Data System (ADS)

    Cordonnier, B.; Caricchi, L.; Pistone, M.; Castro, J. M.; Hess, K.; Dingwell, D. B.

    2010-12-01

    The brittle-ductile transition remains a central question of modern geology. If rocks can be perceived as a granular flow on geological time-scale, their behavior is brittle in dynamic areas. Understanding rock failure conditions is the main parameter in mitigating geological risks, more specifically the eruptive style transitions from effusive to explosive. If numerical simulations are the only way to fully understanding the physical processes involved, we are in a strong need of an experimental validation of the proposed models. here we present results obtained under torsion and uni-axial compression on both pure glasses and crystal bearing melts. We characterized the brittle onset of two phases magmas from 0 to 65% crystals. The strain-rates span a 5 orders magnitude range, from the Newtonian flow to the Brittle field (10-5 - 100 s-1). We particularly emphasize the time dependency of the measured rheology. The materials tested are a borosilicate glass from the National Bureau of Standards, a natural sample from Mt Unzen volcano and a synthetic sample. The lattest is an HPG8 melt with 7% sodium mole excess. The particles are quasi-isometric corundum crystalschosen for their shape and integrity under the stress range investigated. The crystal fraction ranges from 0 to 0.65. Concerning pure magmas, we recently demonstrated that the material passes from a Newtonian to a non-Nemtonian behavior with increasing strain-rate. This onset can mostly be explained by viscous-heating effects. However, for even greater strain-rates, the material cracks and finally fail. The brittle onset is here explained with the visco-elastic theory and corresponds to a Deborah number greater than 10-2. Concerning crystal bearing melts the departure from the Newtonian state is characterized by two effects: a shear-thinning and a time weakening effect. The first one is instantaneous and loading-unloading cyclic tests suggest an elastic contribution of the crystal network. The second one

  17. Crystal-Field and Covalency Effects in Uranates: An X-ray Spectroscopic Study.

    PubMed

    Butorin, Sergei M; Kvashnina, Kristina O; Smith, Anna L; Popa, Karin; Martin, Philippe M

    2016-07-04

    The electronic structure of U(V) - and U(VI) -containing uranates NaUO3 and Pb3 UO6 was studied by using an advanced technique, namely X-ray absorption spectroscopy (XAS) in high-energy-resolution fluorescence-detection (HERFD) mode. Due to a significant reduction in core-hole lifetime broadening, the crystal-field splittings of the 5f shell were probed directly in HERFD-XAS spectra collected at the U 3d edge, which is not possible by using conventional XAS. In addition, the charge-transfer satellites that result from U 5f-O 2p hybridization were clearly resolved. The crystal-field parameters, 5f occupancy, and degree of covalency of the chemical bonding in these uranates were estimated by using the Anderson impurity model by calculating the U 3d HERFD-XAS, conventional XAS, core-to-core (U 4f-3d transitions) resonant inelastic X-ray scattering (RIXS), and U 4f X-ray photoelectron spectra. The crystal field was found to be strong in these systems and the 5f occupancy was determined to be 1.32 and 0.84 electrons in the ground state for NaUO3 and Pb3 UO6 , respectively, which indicates a significant covalent character for these compounds. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  18. Evaluation of the exchange interaction and crystal fields in a prototype Dy2 SMM

    NASA Astrophysics Data System (ADS)

    Zhang, Qing; Sarachik, Myriam; Baker, Michael; Chen, Yizhang; Kent, Andrew; Pineda, Eufemio; McInnes, Eric

    In order to gain an understanding of the INS and magnetization data obtained for Dy2, the simplest member of a newly synthesized family of dysprosium-based molecular magnets, we report on calculations of the magnetic behavior of a Dy2 cluster with the formula [hqH2][Dy2(hq)4(NO3)3].MeOH. The molecular complex contains one high symmetry Dy(III) ion and one low symmetry Dy(III) ion. Our calculations suggest that exchange coupling between the two ions controls the behavior of the magnetization at low temperature, while the crystal field of the low symmetry Dy(III) ion controls the behavior at higher temperature. A point charge electrostatic model, based on crystallographic coordinates, provides a starting point for the determination of the crystal field. Parameters in these calculations are adjusted to provide best fits to inelastic neutron scattering data (INS) and low temperature magnetometry: the INS measurements access crystal field energies and low temperature magnetization probes the Dy-Dy exchange interaction. Work supported by ARO W911NF-13-1-1025 (CCNY) and NSF-DMR-1309202 (NYU).

  19. Crystal field parameters with Wannier functions: Application to rare-earth aluminates

    NASA Astrophysics Data System (ADS)

    Novák, P.; Knížek, K.; Kuneš, J.

    2013-05-01

    A method to calculate the crystal field parameters is proposed and applied to trivalent rare-earth impurities in yttrium aluminate and to Tb3+ ion in TbAlO3. To determine crystal field parameters local Hamiltonian expressed in the basis of Wannier functions is expanded in a series of spherical tensor operators. Wannier functions are obtained by transforming the Bloch functions calculated using the density functional theory based program. The results show that the crystal field is continuously decreasing as the number of 4f electrons increases and that the hybridization of 4f states with the states of oxygen ligands is important. The method contains a single adjustable parameter characterizing the 4f-ligand charge transfer. Theory is confronted with experiment for Nd3+ and Er3+ ions in the YAlO3 matrix and for the Tb3+ ion in TbAlO3, and a good agreement within a few meV is found.

  20. Crystal field and magnetism of Pr3+ and Nd3+ ions in orthorhombic perovskites

    NASA Astrophysics Data System (ADS)

    Novák, P.; Knížek, K.; Maryško, M.; Jirák, Z.; Kuneš, J.

    2013-11-01

    Fifteen parameters characterizing the crystal field of rare-earth ions in the RMO3 perovskites (R=Pr, Nd, M=Ga, Co) are calculated using a first-principles electronic structure and the Wannier projection. The method contains a single adjustable parameter that characterizes the hybridization of R(4f) states with the states of oxygen ligands. Subsequently the energy levels and magnetic moments of the trivalent R ion are determined by diagonalization of an effective Hamiltonian which, besides the crystal field, contains the 4f electron-electron repulsion, spin-orbit coupling and interaction with magnetic field. In the Ga compounds the energy levels of the ground multiplet agree within a few meV with those determined experimentally by other authors. For all four compounds in question the temperature dependence of magnetic susceptibility is measured on polycrystalline samples and compared with the results of calculation. For NdGaO3 the theory is also compared with the magnetic measurements on a single crystal presented by Luis et al (1998 Phys. Rev. B 58 798). Good agreement between the experiment and theory is found.

  1. Fabrication of Crystals and Devices for Studies of Field Induced Superconductivity

    NASA Astrophysics Data System (ADS)

    Kloc, Christian

    2002-03-01

    It was demonstrated that injection of electrons or holes into materials using field effect transistor can transform the surface layer into a metal or at low enough temperatures even into a superconductor. Which substances could exhibit electric field induced superconductivity is currently not well know. Superconductivity has been successfully induced in single crystals of arenes, (pentacene Tc = 2 K, tetracene Tc = 2.7K, anthracene Tc = 4 K) oligophenylenevinylenes (trimer Tc =4.2K , tetramer Tc=2.9 K and pentamer, Tc= 2 K) sexithiophene, polymer thin film (regioregular polythiophene, Tc = 2.35 K) and single crystals of pure and intercalated fullerenes (hole and electron doped C60 Tc = 52 and 11K, C70, Tc = 7K and substituted C60, Tc = up to 117 K). Additional, Schon et al. have reported the observation of field induced superconductivity in copper oxide compounds. Despite of relative simple device structures, they consist of semiconducting single crystals or well oriented thin films, metal source and drain electrodes, an aluminum oxide dielectric layer and a conducting gate electrode, the fabrication is onerous and the significance of particular technological steps to functionality of a whole device is not well explored. In this presentation I would like to discuss these technological procedures leading to superconducting devices and further developments in search for high temperature superconducting organics.

  2. Novel colloidal system: Magnetite-polymer particles/lyotropic liquid crystal under magnetic field

    NASA Astrophysics Data System (ADS)

    Mănăilă-Maximean, D.; Cîrtoaje, C.; Dănilă, O.; Donescu, D.

    2017-09-01

    We obtained a new highly ordered colloidal composite using specially manufactured magnetite-polymer nanoparticles and lyotropic liquid crystal. A good compatibility between the components was ensured by the functionalization of the particles during their synthesis. We studied the laser light transmission for the mixtures filled in sandwich-glass cells with homeotropic and planar treatment of the surfaces under external magnetic field. The Fréedericksz transition critical field was estimated, and its' behavior was compared to our new theoretical model based on the Brochard-de Gennes one.

  3. A photonic-crystal optical antenna for extremely large local-field enhancement.

    PubMed

    Chang, Hyun-Joo; Kim, Se-Heon; Lee, Yong-Hee; Kartalov, Emil P; Scherer, Axel

    2010-11-08

    We propose a novel design of an all-dielectric optical antenna based on photonic-band-gap confinement. Specifically, we have engineered the photonic-crystal dipole mode to have broad spectral response (Q~70) and well-directed vertical-radiation by introducing a plane mirror below the cavity. Considerably large local electric-field intensity enhancement~4,500 is expected from the proposed design for a normally incident planewave. Furthermore, an analytic model developed based on coupled-mode theory predicts that the electric-field intensity enhancement can easily be over 100,000 by employing reasonably high-Q (~10,000) resonators.

  4. Slave boson theory of orbital differentiation with crystal field effects: Application to UO2

    DOE PAGES

    Lanatà, Nicola; Yao, Yongxin; Deng, Xiaoyu; ...

    2017-03-23

    We derive an exact operatorial reformulation of the rotational invariant slave boson method, and we apply it to describe the orbital differentiation in strongly correlated electron systems starting from first principles. The approach enables us to treat strong electron correlations, spin-orbit coupling, and crystal field splittings on the same footing by exploiting the gauge invariance of the mean-field equations. Furthermore, we apply our theory to the archetypical nuclear fuel UO2 and show that the ground state of this system displays a pronounced orbital differentiation within the 5f manifold, with Mott-localized Γ8 and extended Γ7 electrons.

  5. Modeling Multiple Time Scales during Glass Formation with Phase-Field Crystals

    SciTech Connect

    Berry, Joel; Grant, Martin

    2011-04-29

    The dynamics of glass formation in monatomic and binary liquids are studied numerically using a microscopic field theory for the evolution of the time-averaged atomic number density. A stochastic framework combining phase-field crystal free energies and dynamic density functional theory is shown to successfully describe several aspects of glass formation over multiple time scales. Agreement with mode coupling theory is demonstrated for underdamped liquids at moderate supercoolings, and a rapidly growing dynamic correlation length is found to be associated with fragile behavior.

  6. Slave Boson Theory of Orbital Differentiation with Crystal Field Effects: Application to UO2

    NASA Astrophysics Data System (ADS)

    Lanatà, Nicola; Yao, Yongxin; Deng, Xiaoyu; Dobrosavljević, Vladimir; Kotliar, Gabriel

    2017-03-01

    We derive an exact operatorial reformulation of the rotational invariant slave boson method, and we apply it to describe the orbital differentiation in strongly correlated electron systems starting from first principles. The approach enables us to treat strong electron correlations, spin-orbit coupling, and crystal field splittings on the same footing by exploiting the gauge invariance of the mean-field equations. We apply our theory to the archetypical nuclear fuel UO2 and show that the ground state of this system displays a pronounced orbital differentiation within the 5 f manifold, with Mott-localized Γ8 and extended Γ7 electrons.

  7. Slave Boson Theory of Orbital Differentiation with Crystal Field Effects: Application to UO_{2}.

    PubMed

    Lanatà, Nicola; Yao, Yongxin; Deng, Xiaoyu; Dobrosavljević, Vladimir; Kotliar, Gabriel

    2017-03-24

    We derive an exact operatorial reformulation of the rotational invariant slave boson method, and we apply it to describe the orbital differentiation in strongly correlated electron systems starting from first principles. The approach enables us to treat strong electron correlations, spin-orbit coupling, and crystal field splittings on the same footing by exploiting the gauge invariance of the mean-field equations. We apply our theory to the archetypical nuclear fuel UO_{2} and show that the ground state of this system displays a pronounced orbital differentiation within the 5f manifold, with Mott-localized Γ_{8} and extended Γ_{7} electrons.

  8. Molecular-field-theory approach to the Landau theory of liquid crystals: uniaxial and biaxial nematics.

    PubMed

    Luckhurst, Geoffrey R; Naemura, Shohei; Sluckin, Timothy J; Thomas, Kenneth S; Turzi, Stefano S

    2012-03-01

    Nematic liquid crystal phase diagrams in temperature-biaxiality space are usually complex. We construct a Landau theory based on the analogous molecular-field theory for orthorhombic biaxial nematic fluids. A formal procedure yields coefficients (some of which, unusually, can be tensorial) in this Landau expansion, correctly predicts the complete set of invariants formed from the ordering tensors, and avoids ad hoc parametrization of the molecular biaxiality. By regularizing the Landau expansion to avoid unwanted order parameter divergences at low temperatures, we predict phase behavior over the whole range of biaxiality. The resulting phase diagrams have the same topology as those of molecular-field theory.

  9. A molecular-field approximation for quantum crystals. Ph.D. Thesis; [considering ground state properties

    NASA Technical Reports Server (NTRS)

    Danilowicz, R.

    1973-01-01

    Ground-state properties of quantum crystals have received considerable attention from both theorists and experimentalists. The theoretical results have varied widely with the Monte Carlo calculations being the most successful. The molecular field approximation yields ground-state properties which agree closely with the Monte Carlo results. This approach evaluates the dynamical behavior of each pair of molecules in the molecular field of the other N-2 molecules. In addition to predicting ground-state properties that agree well with experiment, this approach yields data on the relative importance of interactions of different nearest neighbor pairs.

  10. Influence of the magnetic field on isotropic wetting behavior of a nematic liquid crystal

    NASA Astrophysics Data System (ADS)

    Kadivar, Erfan

    2008-09-01

    I present a theoretical investigation of the temperature and magnetic field dependence of isotropic (paranematic) wetting layers close to an aligning substrate within a semi-infinite nematic liquid crystal with positive magnetic anisotropy under condition of weak homeotropic anchoring. Using the Landau-de Gennes model supplement by Nobili-Durand surface free energy, the existence and stability of paranematic wetting layers close to the substrate and below the nematic-isotropic temperature are discussed. Numerical results are presented showing the phase diagram for the isotropic (paranematic), nematic, and wetting layer states. In the present work, the dependence of the transition kind to the magnetic field is discussed.

  11. First visualization of temperature fields in liquids at high pressure using thermochromic liquid crystals

    NASA Astrophysics Data System (ADS)

    Pehl, M.; Werner, F.; Delgado, A.

    A first application of encapsulated thermochromic liquid crystals (TLCs) for visualizing temperature fields in pressurized liquids was studied experimentally. By means of a tempered high-pressure optical cell, investigations were performed in a wide temperature range and at pressures up to 7000bar. The measured calibration curves of isochromes in the pressure/temperature domain as well as photographically documented temperature fields at high pressure are presented and discussed. The results found illustrate that TLCs provide an efficient instrument for investigating thermofluiddynamical processes even at high pressure.

  12. Crystal fields of porphyrins and phthalocyanines from polarization-dependent 2p-to-3d multiplets.

    PubMed

    Johnson, Phillip S; García-Lastra, J M; Kennedy, Colton K; Jersett, Nathan J; Boukahil, Idris; Himpsel, F J; Cook, Peter L

    2014-03-21

    Polarization-dependent X-ray absorption spectroscopy is combined with density functional calculations and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal phthalocyanines and octaethylporphyrins (Mn, Fe, Co, Ni). The polarization dependence facilitates the assignment of the multiplets in terms of in-plane and out-of-plane orbitals and avoids ambiguities. Crystal field values from density functional calculations provide starting values close to the optimum fit of the data. The resulting systematics of the crystal field can be used for optimizing electron-hole separation in dye-sensitized solar cells.

  13. Crystal fields of porphyrins and phthalocyanines from polarization-dependent 2p-to-3d multiplets

    SciTech Connect

    Johnson, Phillip S.; Boukahil, Idris; Himpsel, F. J.; García-Lastra, J. M.; Kennedy, Colton K.; Jersett, Nathan J.; Cook, Peter L.

    2014-03-21

    Polarization-dependent X-ray absorption spectroscopy is combined with density functional calculations and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal phthalocyanines and octaethylporphyrins (Mn, Fe, Co, Ni). The polarization dependence facilitates the assignment of the multiplets in terms of in-plane and out-of-plane orbitals and avoids ambiguities. Crystal field values from density functional calculations provide starting values close to the optimum fit of the data. The resulting systematics of the crystal field can be used for optimizing electron-hole separation in dye-sensitized solar cells.

  14. Crystal fields of porphyrins and phthalocyanines from polarization-dependent 2p-to-3d multiplets

    NASA Astrophysics Data System (ADS)

    Johnson, Phillip S.; García-Lastra, J. M.; Kennedy, Colton K.; Jersett, Nathan J.; Boukahil, Idris; Himpsel, F. J.; Cook, Peter L.

    2014-03-01

    Polarization-dependent X-ray absorption spectroscopy is combined with density functional calculations and atomic multiplet calculations to determine the crystal field parameters 10Dq, Ds, and Dt of transition metal phthalocyanines and octaethylporphyrins (Mn, Fe, Co, Ni). The polarization dependence facilitates the assignment of the multiplets in terms of in-plane and out-of-plane orbitals and avoids ambiguities. Crystal field values from density functional calculations provide starting values close to the optimum fit of the data. The resulting systematics of the crystal field can be used for optimizing electron-hole separation in dye-sensitized solar cells.

  15. On the fringing-field effect in liquid-crystal beam-steering devices.

    PubMed

    Apter, Boris; Efron, Uzi; Bahat-Treidel, Eldad

    2004-01-01

    A detailed simulation of the fringing-field effect in liquid-crystal (LC)-based blazed-grating structures has been carried out. These studies are aimed at clarifying the relationship between the width of the fringing-field-broadened phase profile of the blazed grating and the LC cell thickness. This fringing-field broadening of the blazed grating's phase profile is shown to affect mostly the 2pi phase-step zone (fly-back zone) of the blazed grating. The results of the simulations carried out on the blazed-grating structure indicate two main effects of the fringing field: (1) reduction in the attainable diffraction efficiency and (2) limitation of the maximum deflection angle of the device. Both effects are shown to be directly linked to the broadening of the fly-back zone, which is shown to be proportional to the LC cell thickness.

  16. Wide-Viewing-Angle Hybrid Aligned Nematic Liquid Crystal Cell Controlled by Complex Electric Field

    NASA Astrophysics Data System (ADS)

    Hong, Seung Ho; Kim, Hyang Yul; Kim, Jae-Hyung; Nam, Sang-Hee; Lee, Myong-Hoon; Lee, Seung Hee

    2002-07-01

    We have developed a hybrid aligned nematic liquid crystal (LC) cell driven by a complex electric field. In the device, the pixel electrode exists on the bottom substrate and the counter electrodes exist on the top and bottom substrates such that with a bias voltage both vertical and horizontal fields are generated. The LC molecules are hybrid aligned with homogeneous alignment on the bottom substrate where the alignment direction is coincident with one of the transmission axes of the crossed polarizers. Therefore, the cell appears to be black in the absence of an electric field. When a voltage is applied to obtain a white state, both vertical and horizontal fields enable the LC molecules to rotate with lowered tilt angles than those in the dark state. The device shows a much wider viewing angle than that of the twisted nematic mode, high light efficiency and low driving voltage in electro-optic characteristics.

  17. Molecular dynamics simulations of dislocations in TlBr crystals under an electrical field

    SciTech Connect

    Zhou, X. W.; Foster, M. E.; Yang, P.; Doty, F. P.

    2016-07-13

    TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. As a result, this discovery can lead to new understanding of TlBr aging mechanisms under external fields.

  18. Molecular dynamics simulations of dislocations in TlBr crystals under an electrical field

    DOE PAGES

    Zhou, X. W.; Foster, M. E.; Yang, P.; ...

    2016-07-13

    TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. As a result, this discovery can lead to new understanding of TlBr agingmore » mechanisms under external fields.« less

  19. Nematic liquid crystals in a spatially step-wise magnetic field

    NASA Astrophysics Data System (ADS)

    Napoli, Gaetano; Scaraggi, Michele

    2016-01-01

    We study the molecular reorientation induced by a textured external field in a nematic liquid crystal (nLC). In particular, we consider an infinitely wide cell with strong planar anchoring boundary conditions, subjected to a spatially periodic piecewise magnetic field. In the framework of the Frank's continuum theory, we use the perturbation analysis to study in detail the field-induced splay-bend Fréedericksz transition. A numerical approach, based on the finite differences method, is instead employed to solve the fully nonlinear equations. At high field strengths, an analytic approach allows us to draw the bulk profile of the director in terms of elliptic integrals. Finally, through the application of the Bruggeman texture hydrodynamics theory, we qualitatively discuss on the LCs piecewise director configuration under sliding interfaces, which can be adopted to actively regulate friction. Our study opens the pathway for the application of highly controlled nLC texturing for tribotronics.

  20. Molecular dynamics simulations of dislocations in TlBr crystals under an electrical field

    SciTech Connect

    Zhou, X. W.; Foster, M. E.; Yang, P.; Doty, F. P.

    2016-07-13

    TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. As a result, this discovery can lead to new understanding of TlBr aging mechanisms under external fields.

  1. All-fiber magnetic field sensors based on magnetic fluid-filled photonic crystal fibers.

    PubMed

    Gao, R; Jiang, Y; Abdelaziz, S

    2013-05-01

    A method for measurement of a magnetic field by combining photonic crystal fibers (PCFs) and magnetic fluid is presented and experimentally demonstrated. The magnetic fluid is filled into the air holes of the cladding layer in the PCF. Due to the tunable refractive index property of the magnetic fluid, the refractive index difference between the fiber core and cladding layer is changed with the external magnetic field. The magnetic field can be directly detected by measuring the intensity of the transmission light. A series of magnetic fields with different strengths have been measured with the sensor. The experimental results show that a resolution of up to 0.09 Oe is achieved, and a good repetition is demonstrated experimentally. Compared with other expensive methods, the proposed method possesses high sensitivity and low cost.

  2. Theoretically informed Monte Carlo simulation of liquid crystals by sampling of alignment-tensor fields.

    PubMed

    Armas-Pérez, Julio C; Londono-Hurtado, Alejandro; Guzmán, Orlando; Hernández-Ortiz, Juan P; de Pablo, Juan J

    2015-07-28

    A theoretically informed coarse-grained Monte Carlo method is proposed for studying liquid crystals. The free energy functional of the system is described in the framework of the Landau-de Gennes formalism. The alignment field and its gradients are approximated by finite differences, and the free energy is minimized through a stochastic sampling technique. The validity of the proposed method is established by comparing the results of the proposed approach to those of traditional free energy minimization techniques. Its usefulness is illustrated in the context of three systems, namely, a nematic liquid crystal confined in a slit channel, a nematic liquid crystal droplet, and a chiral liquid crystal in the bulk. It is found that for systems that exhibit multiple metastable morphologies, the proposed Monte Carlo method is generally able to identify lower free energy states that are often missed by traditional approaches. Importantly, the Monte Carlo method identifies such states from random initial configurations, thereby obviating the need for educated initial guesses that can be difficult to formulate.

  3. Theoretically informed Monte Carlo simulation of liquid crystals by sampling of alignment-tensor fields

    SciTech Connect

    Armas-Pérez, Julio C.; Londono-Hurtado, Alejandro; Guzmán, Orlando; Hernández-Ortiz, Juan P.; Pablo, Juan J. de

    2015-07-28

    A theoretically informed coarse-grained Monte Carlo method is proposed for studying liquid crystals. The free energy functional of the system is described in the framework of the Landau-de Gennes formalism. The alignment field and its gradients are approximated by finite differences, and the free energy is minimized through a stochastic sampling technique. The validity of the proposed method is established by comparing the results of the proposed approach to those of traditional free energy minimization techniques. Its usefulness is illustrated in the context of three systems, namely, a nematic liquid crystal confined in a slit channel, a nematic liquid crystal droplet, and a chiral liquid crystal in the bulk. It is found that for systems that exhibit multiple metastable morphologies, the proposed Monte Carlo method is generally able to identify lower free energy states that are often missed by traditional approaches. Importantly, the Monte Carlo method identifies such states from random initial configurations, thereby obviating the need for educated initial guesses that can be difficult to formulate.

  4. Theoretically informed Monte Carlo simulation of liquid crystals by sampling of alignment-tensor fields.

    SciTech Connect

    Armas-Perez, Julio C.; Londono-Hurtado, Alejandro; Guzman, Orlando; Hernandez-Ortiz, Juan P.; de Pablo, Juan J.

    2015-07-27

    A theoretically informed coarse-grained Monte Carlo method is proposed for studying liquid crystals. The free energy functional of the system is described in the framework of the Landau-de Gennes formalism. The alignment field and its gradients are approximated by finite differences, and the free energy is minimized through a stochastic sampling technique. The validity of the proposed method is established by comparing the results of the proposed approach to those of traditional free energy minimization techniques. Its usefulness is illustrated in the context of three systems, namely, a nematic liquid crystal confined in a slit channel, a nematic liquid crystal droplet, and a chiral liquid crystal in the bulk. It is found that for systems that exhibit multiple metastable morphologies, the proposed Monte Carlo method is generally able to identify lower free energy states that are often missed by traditional approaches. Importantly, the Monte Carlo method identifies such states from random initial configurations, thereby obviating the need for educated initial guesses that can be difficult to formulate.

  5. High magnetic-field scales and critical currents in SmFeAs(O, F) crystals

    NASA Astrophysics Data System (ADS)

    Moll, Philip J. W.; Puzniak, Roman; Balakirev, Fedor; Rogacki, Krzysztof; Karpinski, Janusz; Zhigadlo, Nikolai D.; Batlogg, Bertram

    2010-08-01

    With the discovery of new superconducting materials, such as the iron pnictides, exploring their potential for applications is one of the foremost tasks. Even if the critical temperature Tc is high, intrinsic electronic properties might render applications difficult, particularly if extreme electronic anisotropy prevents effective pinning of vortices and thus severely limits the critical current density, a problem well known for cuprates. Although many questions concerning microscopic electronic properties of the iron pnictides have been successfully addressed and estimates point to a very high upper critical field, their application potential is less clear. Thus, we focus here on the critical currents, their anisotropy and the onset of electrical dissipation in high magnetic fields up to 65T. Our detailed study of the transport properties of SmFeAsO0.7F0.25 single crystals reveals a promising combination of high (>2×106Acm-2) and nearly isotropic critical current densities along all crystal directions. This favourable intragrain current transport in SmFeAs(O, F), which shows the highest Tc of 54K at ambient pressure, is a crucial requirement for possible applications. Essential in these experiments are four-probe measurements on focused-ion-beam-cut single crystals with a sub-square-micrometre cross-section, with current along and perpendicular to the crystallographic c axis.

  6. Magnetic-field-driven surface electromagnetic states in the graphene-antiferromagnetic photonic crystal system

    SciTech Connect

    Averkov, Yu. O. Tarapov, S. I.; Yakovenko, V. M.; Yampol’skii, V. A.

    2015-04-15

    The surface electromagnetic states (SEMSs) on graphene, which has a linear carrier dispersion law and is placed in an antiferromagnetic photonic crystal, are theoretically studied in the terahertz frequency range. The unit cell of such a crystal consists of layers of a nonmagnetic insulator and a uniaxial antiferromagnet, the easy axis of which is parallel to the crystal layers. A dc magnetic field is parallel to the easy axis of the antiferromagnet. An expression that relates the SEMS frequencies to the structure parameters is obtained. The problem of SEMS excitation by an external TE-polarized electromagnetic wave is solved, and the dependences of the transmission coefficient on the dc magnetic field and the carrier concentration are constructed. These dependences are shown to differ substantially from the case of a conventional two-dimensional electron gas with a quadratic electron dispersion law. Thus, the positions of the transmission coefficient peaks related to resonance SEMS excitation can be used to determine the character of carrier dispersion law in a two-dimensional electron gas.

  7. Effect of field driven phase transformations on the loss tangent of relaxor ferroelectric single crystals

    NASA Astrophysics Data System (ADS)

    Gallagher, John A.; Liu, Tieqi; Lynch, Christopher S.

    2013-02-01

    The effect of a bias stress induced phase transformation on the large field dielectric loss in [001] cut and poled single crystal stack actuators of (1 - x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT, x = 32) was experimentally characterized. Dielectric loss was observed to increase with compressive preload and electric field amplitude. The dielectric loss was determined by measuring the area within electric displacement vs. electric field hysteresis loops and the measured area was expressed in terms of an effective loss tangent. This approach matches the measured area within the hysteresis loop to an equivalent area ellipse in which the electric displacement lags the electric field by an amount, delta, under sinusoidal loading. The results collapse the measured loss as a function of bias stress and electric field amplitude reasonably close to a single curve. The measured dielectric loss behavior was attributed to the compressive stress preload driving a partial phase transformation from rhombohedral to orthorhombic and the electric field driving the reverse phase transformation from the stress induced orthorhombic phase to the zero stress rhombohedral phase. When the compressive bias stress partially or fully drives this phase transformation, the dielectric loss under unipolar electric field loading increases. This work is focused on quasi-static measurements. The large field dielectric loss is anticipated to be a function of frequency and temperature.

  8. Electromagnetic-field amplification in finite one-dimensional photonic crystals

    SciTech Connect

    Gorelik, V. S.; Kapaev, V. V.

    2016-09-15

    The electromagnetic-field distribution in a finite one-dimensional photonic crystal is studied using the numerical solution of Maxwell’s equations by the transfer-matrix method. The dependence of the transmission coefficient T on the period d (or the wavelength λ) has the characteristic form with M–1 (M is the number of periods in the structure) maxima with T = 1 in the allowed band of an infinite crystal and zero values in the forbidden band. The field-modulus distribution E(x) in the structure for parameters that correspond to the transmission maxima closest to the boundaries of forbidden bands has maxima at the center of the structure; the value at the maximum considerably exceeds the incident-field strength. For the number of periods M ~ 50, more than an order of magnitude increase in the field amplification is observed. The numerical results are interpreted with an analytic theory constructed by representing the solution in the form of a linear combination of counterpropagating Floquet modes in a periodic structure.

  9. Direct current electric field assembly of colloidal crystals displaying reversible structural color.

    PubMed

    Shah, Aayush A; Ganesan, Mahesh; Jocz, Jennifer; Solomon, Michael J

    2014-08-26

    We report the application of low-voltage direct current (dc) electric fields to self-assemble close-packed colloidal crystals in nonaqueous solvents from colloidal spheres that vary in size from as large as 1.2 μm to as small as 0.1 μm. The assemblies are created rapidly (∼2 min) from an initially low volume fraction colloidal particle suspension using a simple capacitor-like electric field device that applies a steady dc electric voltage. Confocal microscopy is used to observe the ordering that is produced by the assembly method. This spatial evidence for ordering is consistent with the 6-fold diffraction patterns identified by light scattering. Red, green, and blue structural color is observed for the ordered assemblies of colloids with diameters of 0.50, 0.40, and 0.29 μm, respectively, consistent with spectroscopic measurements of reflectance. The diffraction and spectrophotometry results were found to be consistent with the theoretical Bragg's scattering expected for closed-packed crystals. By switching the dc electric field from on to off, we demonstrate reversibility of the structural color response on times scales ∼60 s. The dc electric field assembly method therefore represents a simple method to produce reversible structural color in colloidal soft matter.

  10. Directed peptide amphiphile assembly using aqueous liquid crystal templates in magnetic fields.

    PubMed

    van der Asdonk, Pim; Keshavarz, Masoumeh; Christianen, Peter C M; Kouwer, Paul H J

    2016-08-21

    An alignment technique based on the combination of magnetic fields and a liquid crystal (LC) template uses the advantages of both approaches: the magnetic fields offer non-contact methods that apply to all sample sizes and shapes, whilst the LC templates offer high susceptibilities. The combination introduces a route to control the spatial organization of materials with low intrinsic susceptibilities. We demonstrate that we can unidirectionally align one such material, peptide amphiphiles in water, on a centimeter scale at a tenfold lower magnetic field by using a lyotropic chromonic liquid crystal as a template. We can transform the aligned supramolecular assemblies into optically active π-conjugated polymers after photopolymerization. Lastly, by reducing the magnetic field strength needed for addressing these assemblies, we are able to create more complex structures by initiating self-assembly of our supramolecular materials under competing alignment forces between the magnetically induced alignment of the assemblies (with a positive diamagnetic anisotropy) and the elastic force dominated alignment of the template (with a negative diamagnetic anisotropy), which is directed orthogonally. Although the approach is still in its infancy and many critical parameters need optimization, we believe that it is a very promising technique to create tailor-made complex structures of (aqueous) functional soft matter.

  11. Imaging the oblique propagation of electrons in germanium crystals at low temperature and low electric field

    SciTech Connect

    Moffatt, R. A. Cabrera, B.; Corcoran, B. M.; Kreikebaum, J. M.; Redl, P.; Shank, B.; Yen, J. J.; Young, B. A.; Brink, P. L.; Cherry, M.; Tomada, A.; Phipps, A.; Sadoulet, B.; Sundqvist, K. M.

    2016-01-11

    Excited electrons in the conduction band of germanium collect into four energy minima, or valleys, in momentum space. These local minima have highly anisotropic mass tensors which cause the electrons to travel in directions which are oblique to an applied electric field at sub-Kelvin temperatures and low electric fields, in contrast to the more isotropic behavior of the holes. This experiment produces a full two-dimensional image of the oblique electron and hole propagation and the quantum transitions of electrons between valleys for electric fields oriented along the [0,0,1] direction. Charge carriers are excited with a focused laser pulse on one face of a germanium crystal and then drifted through the crystal by a uniform electric field of strength between 0.5 and 6 V/cm. The pattern of charge density arriving on the opposite face is used to reconstruct the trajectories of the carriers. Measurements of the two-dimensional pattern of charge density are compared in detail with Monte Carlo simulations developed for the Cryogenic Dark Matter Search (SuperCDMS) to model the transport of charge carriers in high-purity germanium detectors.

  12. Effects of a high magnetic field on the primary zinc-rich crystals in hypoeutectic Zn-Sn alloy

    NASA Astrophysics Data System (ADS)

    Li, Lei; Ban, Chunyan; Shi, Xuchen; Zhang, Haitao; Zuo, Yubo; Zhu, Qingfeng; Wang, Xiangjie; Zhang, Hui; Cui, Jianzhong; Nagaumi, Hiromi

    2017-04-01

    The effects of a high magnetic field on the primary zinc-rich crystals during the solidification process of hypoeutectic Zn-Sn were investigated, along with the analysis of crystallographic orientations of them. The results show that the primary zinc-rich crystals in two dimensions show column-like or dendritic forms whether without and with the HMF. However, they are highly aligned and textured by the application of a high magnetic field: with the longer axis parallel to but the crystallographic c axis perpendicular to the magnetic field. The two-dimensional forms of the primary zinc-rich crystals are related to their orientations relative to the observation planes. The orientation and alignment modification by the magnetic field arises from the induced magnetic torques due to the magnetocrystalline anisotropy of the zinc-rich crystals, which makes them in energetically optimal orientations.

  13. Crystal field interaction in the pyrochlore magnet Ho{sub 2}Ti{sub 2}O{sub 7}.

    SciTech Connect

    Rosenkranz, S.; Ramirez, A. P.; Hayashi, A.; Cava, R. J.; Siddharthan, R.; Shastry, B. S.

    1999-09-30

    Neutron time-of-flight spectroscopy has been employed to study the crystal field interaction in the pyrochlore titanate Ho{sub 2}Ti{sub 2}O{sub 7}. The crystal field parameters and corresponding energy level scheme have been determined from a profile fit to the observed neutron spectra. The groundstate is a well separated E{sub g} doublet with a strong Ising like anisotropy, which can give rise to titration in the pyrochlore lattice. Using the crystal field parameters determined for the Ho compound as an estimate of the crystal field interaction in other pyrochlore magnets, we also find the Ising type behavior for Dy. In contrast, the almost planar like anisotropy found for Er and Yb prevents frustration, because of the continuous range of possible spin orientations in this case.

  14. Orientation of growing crystals of Co- or Gd-containing L-threonine dehydrogenase by magnetic fields

    NASA Astrophysics Data System (ADS)

    Maki, Syou; Ishikawa, Kazuhiko; Ataka, Mitsuo

    2009-12-01

    L-Threonine dehydrogenase from Pyrococcus horikoshii (TDH) is a water-soluble metalloenzyme, the molecular structure of which has been unknown until recently. The Zn 2+ ion in the native TDH, prepared as a recombinant protein, is replaced artificially with Co 2+, Ni 2+ or Gd 3+. These samples are crystallized in homogeneous magnetic fields of 2-10 T. Half of the Co- or Gd-substituted crystals show magnetic orientation in a field of 2 T at 278 K whereas the crystals of the native TDH require a 4 T magnetic field for half orientation. The sensitivity to magnetic orientation can thus be increased by metal substitution. On the other hand, we cannot assign clear changes in the size, number, and quality of the native and metal-substituted crystals with and without the presence of the magnetic field.

  15. Accelerated discovery of two crystal structure types in a complex inorganic phase field

    NASA Astrophysics Data System (ADS)

    Collins, C.; Dyer, M. S.; Pitcher, M. J.; Whitehead, G. F. S.; Zanella, M.; Mandal, P.; Claridge, J. B.; Darling, G. R.; Rosseinsky, M. J.

    2017-06-01

    The discovery of new materials is hampered by the lack of efficient approaches to the exploration of both the large number of possible elemental compositions for such materials, and of the candidate structures at each composition. For example, the discovery of inorganic extended solid structures has relied on knowledge of crystal chemistry coupled with time-consuming materials synthesis with systematically varied elemental ratios. Computational methods have been developed to guide synthesis by predicting structures at specific compositions and predicting compositions for known crystal structures, with notable successes. However, the challenge of finding qualitatively new, experimentally realizable compounds, with crystal structures where the unit cell and the atom positions within it differ from known structures, remains for compositionally complex systems. Many valuable properties arise from substitution into known crystal structures, but materials discovery using this approach alone risks both missing best-in-class performance and attempting design with incomplete knowledge. Here we report the experimental discovery of two structure types by computational identification of the region of a complex inorganic phase field that contains them. This is achieved by computing probe structures that capture the chemical and structural diversity of the system and whose energies can be ranked against combinations of currently known materials. Subsequent experimental exploration of the lowest-energy regions of the computed phase diagram affords two materials with previously unreported crystal structures featuring unusual structural motifs. This approach will accelerate the systematic discovery of new materials in complex compositional spaces by efficiently guiding synthesis and enhancing the predictive power of the computational tools through expansion of the knowledge base underpinning them.

  16. Crystal size of epidotes: A potentially exploitable geothermometer in geothermal fields

    SciTech Connect

    Patrier, P.; Beaufort, D.; Touchard, G. ); Fouillac, A.M. )

    1990-11-01

    Crystal size of epidotes crystallized in quartz + epidote veins is used as the basis for a new geothermometer from the fossil geothermal field of Saint Martin (Lesser Antilles). The epidote-bearing alteration paragenesis is developed as far as 3 km from a quartz diorite pluton at temperatures of 220-350C. The length/width ratio of the epidote grains is constant for all the analyzed samples and suggests isotropic growth environments. However, the length and width of the grains vary exponentially with temperature. The obtained results offer new perspectives for simple grain-size geothermomentry but must be extended to other geologic environments to clarify the influence of different rock types.

  17. Temperature and Field Induced Strain Measurements in Single Crystal Gd5Si2Ge2

    NASA Astrophysics Data System (ADS)

    McCall, S. K.; Nersessian, N.; Carman, G. P.; Pecharsky, V. K.; Schlagel, D. L.; Radousky, H. B.

    2016-06-01

    The first-order magneto-structural transformation that occurs in Gd5Si2Ge2 near room temperature makes it a strong candidate for many energy harvesting applications. Understanding the single crystal properties is crucial for allowing simulations of device performance. In this study, magnetically and thermally induced transformation strains were measured in a single crystal of Gd5Si2.05Ge1.95 as it transforms from a high-temperature monoclinic paramagnet to a lower-temperature orthorhombic ferromagnet. Thermally induced transformation strains of -8500 ppm, +960 ppm and +1800 ppm, and magnetically induced transformation strains of -8500 ppm, +900 ppm and +2300 ppm were measured along the a, b and c axes, respectively. Using experimental data coupled with general thermodynamic considerations, a universal phase diagram was constructed showing the transition from the monoclinic to the orthorhombic phase as a function of temperature and magnetic field.

  18. Phase-field modeling of two-dimensional crystal growth with anisotropic diffusion

    NASA Astrophysics Data System (ADS)

    Meca, Esteban; Shenoy, Vivek B.; Lowengrub, John

    2013-11-01

    In the present article, we introduce a phase-field model for thin-film growth with anisotropic step energy, attachment kinetics, and diffusion, with second-order (thin-interface) corrections. We are mainly interested in the limit in which kinetic anisotropy dominates, and hence we study how the expected shape of a crystallite, which in the long-time limit is the kinetic Wulff shape, is modified by anisotropic diffusion. We present results that prove that anisotropic diffusion plays an important, counterintuitive role in the evolving crystal shape, and we add second-order corrections to the model that provide a significant increase in accuracy for small supersaturations. We also study the effect of different crystal symmetries and discuss the influence of the deposition rate.

  19. General equations for the motions of ice crystals and water drops in gravitational and electric fields

    NASA Technical Reports Server (NTRS)

    Nisbet, John S.

    1988-01-01

    General equations for the Reynolds number of a variety of types of ice crystals and water drops are given in terms of the Davies, Bond, and Knudsen numbers. The equations are in terms of the basic physical parameters of the system and are valid for calculating velocities in gravitational and electric fields over a very wide range of sizes and atmospheric conditions. The equations are asymptotically matched at the bottom and top of the size spectrum, useful when checking large computer codes. A numerical system for specifying the dimensional properties of ice crystals is introduced. Within the limits imposed by such variables as particle density, which have large deviations, the accuracy of velocities appears to be within 10 percent over the entire range of sizes of interest.

  20. General equations for the motions of ice crystals and water drops in gravitational and electric fields

    NASA Technical Reports Server (NTRS)

    Nisbet, John S.

    1989-01-01

    General equations for the Reynolds number of a variety of types of ice crystals and water drops are given in terms of the Davies, Bond, and Knudsen numbers. The equations are in terms of the basic physical parameters of the system and are valid for calculating velocities in gravitational and electric fields over a very wide range of sizes and atmospheric conditions. The equations are asymptotically matched at the bottom and top of the size spectrum, useful when checking large computer codes. A numerical system for specifying the dimensional properties of ice crystals is introduced. Within the limits imposed by such variables as particle density, which have large deviations, the accuracy of velocities appears to be within 10 percent over the entire range of sizes of interest.

  1. Remanent magnetization of ceramic and single-crystal high-Tc superconductors in tilted magnetic fields

    NASA Astrophysics Data System (ADS)

    Bugoslavsky, Yu. V.; Minakov, A. A.; Vasyurin, S. I.

    1996-02-01

    Dependence of the remanent magnetization (Mr) anisotropy on the structure and shape of the superconductor and on the magnetizing procedure was studied for a number of high-Tc superconductor ceramics and single crystals. The experiments were done by means of a vibrating-sample magnetometer with a rotatable sample holder. It was found that the main contribution to the anisotropic behavior of Mr is due to the surface screening currents, and therefore the anisotropy is subject to variation when sample shape is changed. The question is resolved, why the effective demagnetization factors for decoupled ceramic samples are different from those calculated in the inscribed-ellipsoid approximation. Influence of inhomogeneous grain magnetization and global bulk currents on the angular dependencies of Mr in ceramic samples is investigated. The evolution of remanence in YBCO single crystals with an increase of the magnetizing field is described within an extended Bean model.

  2. Development of specialized modelling tools for crystal growth processes with magnetic fields

    NASA Astrophysics Data System (ADS)

    Rudevics, A.; Muiznieks, A.; Nacke, B.

    2007-06-01

    The present paper is devoted to some aspects of the development of specialized software for the modelling of crystal growth processes with magnetic fields. Due to the complexity of the mathematical models for such processes, the modern technology of software design and implementation has to be used. Our experience in numerical modelling of crystal growth processes has shown that such approach facilitates the development of complex software systems. It involves the so-called object-oriented design and programming as well as the use of powerful software libraries in order to benefit from its functionality. To illustrate the programs created by our group, some examples are briefly described in this paper. Figs 7, Refs 13.

  3. Electric-field gradient characterization at 181Ta impurities in sapphire single crystals

    NASA Astrophysics Data System (ADS)

    Rentería, M.; Darriba, G. N.; Errico, L. A.; Muñoz, E. L.; Eversheim, P. D.

    2005-07-01

    We report Perturbed-Angular-Correlation (PAC) experiments on corundum Al2O3 single crystals implanted with 181Hf/181Ta ions at the ISKP at Bonn and measured at La Plata with high efficiency and time-resolution. The magnitude, asymmetry, and orientation (with respect to the crystalline axes) of the electric-field gradient (EFG) tensor were determined measuring the spin-rotation curves as a function of different orientations of the single crystals relative to the detector system. These results are analyzed in the framework of point-charge model and ab initio Full-Potential Linearized-Augmented Plane Wave calculations, and compared with EFG results coming from PAC experiments with 111In/111Cd impurities. This combined study enables the determination of lattice relaxations induced by the presence of the impurity and the state of charge of a deep impurity donor level in the band gap of the semiconductor.

  4. Assessment of biomolecular force fields for molecular dynamics simulations in a protein crystal.

    PubMed

    Hu, Zhongqiao; Jiang, Jianwen

    2010-01-30

    Different biomolecular force fields (OPLS-AA, AMBER03, and GROMOS96) in conjunction with SPC, SPC/E and TIP3P water models are assessed for molecular dynamics simulations in a tetragonal lysozyme crystal. The root mean square deviations for the C(a) atoms of lysozymes are about 0.1 to 0.2 nm from OPLS-AA and AMBER03, smaller than 0.4 nm from GROMOS96. All force fields exhibit similar pattern in B-factors, whereas OPLS-AA and AMBER03 accurately reproduce experimental measurements. Despite slight variations, the primary secondary structures are well conserved using different force fields. Water diffusion in the crystal is approximately ten-fold slower than in bulk phase. The directional and average water diffusivities from OPLS-AA and AMBER03 along with SPC/E model match fairly well with experimental data. Compared to GROMOS96, OPLS-AA and AMBER03 predict larger hydrophilic solvent-accessible surface area of lysozyme, more hydrogen bonds between lysozyme and water, and higher percentage of water in hydration shell. SPC, SPC/E and TIP3P water models have similar performance in most energetic and structural properties, but SPC/E outperforms in water diffusion. While all force fields overestimate the mobility and electrical conductivity of NaCl, a combination of OPLS-AA for lysozyme and the Kirkwood-Buff model for ions is superior to others. As attributed to the steric restraints and surface interactions, the mobility and conductivity in the crystal are reduced by one to two orders of magnitude from aqueous solution.

  5. Measurement of spatio-temporal field distribution of THz pulses in electro-optic crystal by interferometry method

    SciTech Connect

    Chizhov, P A; Ushakov, A A; Bukin, V V; Garnov, S V

    2015-05-31

    We propose a scheme for measuring the spatial distribution of the THz pulse electric field strength in an electro-optic crystal using optical interferometry. The resulting images of the field distribution from a test source with a spherical wave front are presented. (extreme light fields and their applications)

  6. Three-Dimensional Stress Fields and Slip Systems for Single Crystal Superalloy Notched Specimens

    NASA Technical Reports Server (NTRS)

    Magnan, Shannon M.; Throckmorton, David (Technical Monitor)

    2002-01-01

    Single crystal superalloys have become increasingly popular for turbine blade and vane applications due to their high strength, and creep and fatigue resistance at elevated temperatures. The crystallographic orientation of a single crystal material greatly affects its material properties, including elastic modulus, shear modulus, and ductility. These directional properties, along with the type of loading and temperature, dictate an anisotropic response in the yield strength, creep resistance, creep rupture ductility, fatigue resistance, etc. A significant amount of research has been conducted to determine the material properties in the <001> orientation, yet the material properties deviating from the <001> orientation have not been assessed for all cases. Based on the desired application and design criteria, a crystal orientation is selected to yield the maximum properties. Currently, single crystal manufacturing is able to control the primary crystallographic orientation within 15 of the target orientation, which is an acceptable deviation to meet both performance and cost guidelines; the secondary orientation is rarely specified. A common experiment is the standard load-controlled tensile test, in which specimens with different orientations can be loaded to observe the material response. The deformation behavior of single-crystal materials under tension and compression is known to be a function of not only material orientation, but also of varying microdeformation (i.e. dislocation) mechanisms. The underlying dislocation motion causes deformation via slip, and affects the activation of specific slip systems based on load and orientation. The slip can be analyzed by observing the visible traces left on the surface of the specimen from the slip activity within the single crystal material. The goal of this thesis was to predict the slip systems activated in three-dimensional stress fields of a notched tensile specimen, as a function of crystal orientation, using

  7. Enhanced electrocaloric cooling in ferroelectric single crystals by electric field reversal

    NASA Astrophysics Data System (ADS)

    Ma, Yang-Bin; Novak, Nikola; Koruza, Jurij; Yang, Tongqing; Albe, Karsten; Xu, Bai-Xiang

    2016-09-01

    An improved thermodynamic cycle is validated in ferroelectric single crystals, where the cooling effect of an electrocaloric refrigerant is enhanced by applying a reversed electric field. In contrast to the conventional adiabatic heating or cooling by on-off cycles of the external electric field, applying a reversed field is significantly improving the cooling efficiency, since the variation in configurational entropy is increased. By comparing results from computer simulations using Monte Carlo algorithms and experiments using direct electrocaloric measurements, we show that the electrocaloric cooling efficiency can be enhanced by more than 20% in standard ferroelectrics and also relaxor ferroelectrics, like Pb (Mg1 /3 /Nb2 /3)0.71Ti0.29O3 .

  8. First-order transition and tricritical behavior of the transverse crystal field spin-1 Ising model

    NASA Astrophysics Data System (ADS)

    Costabile, Emanuel; Viana, J. Roberto; de Sousa, J. Ricardo; de Arruda, Alberto S.

    2015-06-01

    The phase diagram of the spin-1 Ising model in the presence of a transverse crystal-field anisotropy (Dx) is studied within the framework of an effective-field theory with correlation. The effect of the coordination number (z) on the phase diagram in the T -Dx plane is investigated. We observe only second-order transitions for coordination number z < 7, while that for z ≥ 7 we have first- and second-order transitions, with the presence of two tricritical points. The lower tricritical temperature (Tt) decreases monotonically with the increasing value of z, and in the limit of z → ∞ we found Tt = 0, corresponding to the mean-field solution [Ricardo de Sousa and Branco, Phys. Rev. E 77 (2008) 012104] with a single tricritical point in the phase diagram.

  9. Critical Fields, Thermally Activated Transport, and Critical Current Density of Beta-FeSe Single Crystals

    SciTech Connect

    Petrovic, C.; Lei, H.; Hu, R.

    2011-07-27

    We present critical fields, thermally activated flux flow (TAFF), and critical current density of tetragonal phase {beta}-FeSe single crystals. The upper critical fields H{sub c2}(T) for H {parallel} (101) and H {perpendicular} (101) are nearly isotropic and are likely governed by the Pauli limiting process. The large Ginzburg-Landau parameter {Kappa} {approx} 72.3(2) indicates that {beta}-FeSe is a type-II superconductor with a smaller penetration depth than in Fe(Te, Se). The resistivity below T{sub c} follows Arrhenius TAFF behavior. For both field directions below 30 kOe, single-vortex pinning is dominant, whereas collective creep becomes important above 30 kOe. The critical current density J{sub c} from M-H loops for H {parallel} (101) is about five times larger than for H {perpendicular} (101), yet much smaller than in other iron-based superconductors.

  10. Melt Motion Due to Peltier Marking During Bridgman Crystal Growth with an Axial Magnetic Field

    NASA Technical Reports Server (NTRS)

    Sellers, C. C.; Walker, John S.; Szofran, Frank R.; Motakef, Shariar

    2000-01-01

    This paper treats a liquid-metal flow inside an electrically insulating cylinder with electrically conducting solids above and below the liquid region. There is a uniform axial magnetic field, and there is an electric current through the liquid and both solids. Since the lower liquid-solid interface is concave into the solid and since the liquid is a better electrical conductor than the adjacent solid, the electric current is locally concentrated near the centerline. The return to a uniform current distribution involves a radial electric current which interacts with the axial magnetic field to drive an azimuthal flow. The axial variation of the centrifugal force due to the azimuthal velocity drives a meridional circulation with radial and axial velocities. This problem models the effects of Peltier marking during the vertical Bridgman growth of semiconductor crystals with an externally applied magnetic field, where the meridional circulation due to the Peltier Current may produce important mixing in the molten semiconductor.

  11. Electric field sensor based on cholesteric liquid crystal Fabry-Perot etalon

    NASA Astrophysics Data System (ADS)

    Ko, Myeong Ock; Kim, Sung-Jo; Kim, Jong-Hyun; Lee, Bong Wan; Jeon, Min Yong

    2015-09-01

    We propose an electric field sensor using a cholesteric liquid crystal (CLC) Fabry-Perot etalon and a broadband optical source. The CLC cell consists of glass substrates, polyimide layers, electrodes, and CLC layer. There is a threshold behavior for CLC cell and no change in the transmitted wavelength occurs until a threshold value. The threshold value is 0.8 V/μm for fabricated CLC cell in this experiment. The transmitted or reflected wavelength from the CLC Fabry-Perot etalon depends on the applied electric field. The valley wavelengths of the transmitted light from the CLC device are linearly increased from 1303 nm to 1317 nm as the applied electric field to the CLC device is increased from 0.8 V/μm to 1.9 V/μm.

  12. Melt Motion Due to Peltier Marking During Bridgman Crystal Growth with an Axial Magnetic Field

    NASA Technical Reports Server (NTRS)

    Sellers, C. C.; Walker, John S.; Szofran, Frank R.; Motakef, Shariar

    2000-01-01

    This paper treats a liquid-metal flow inside an electrically insulating cylinder with electrically conducting solids above and below the liquid region. There is a uniform axial magnetic field, and there is an electric current through the liquid and both solids. Since the lower liquid-solid interface is concave into the solid and since the liquid is a better electrical conductor than the adjacent solid, the electric current is locally concentrated near the centerline. The return to a uniform current distribution involves a radial electric current which interacts with the axial magnetic field to drive an azimuthal flow. The axial variation of the centrifugal force due to the azimuthal velocity drives a meridional circulation with radial and axial velocities. This problem models the effects of Peltier marking during the vertical Bridgman growth of semiconductor crystals with an externally applied magnetic field, where the meridional circulation due to the Peltier Current may produce important mixing in the molten semiconductor.

  13. Induced interaction and crystallization of self-localized impurity fields in a Bose-Einstein condensate

    SciTech Connect

    Rica, Sergio; Roberts, David C.

    2009-07-15

    We model the behavior of N classical impurity fields immersed in a larger Bose-Einstein condensate by N+1 coupled nonlinear Schroedinger equations in one, two, and three space dimensions. We discuss the stability of the uniform miscible system and show the importance of surface tension for self-localization of the impurity fields. We derive analytically the attractive tail of the impurity-impurity interaction due to mediation by the underlying condensate. Assuming all impurity fields interact with the same strength, we explore numerically the resulting phase diagram, which contains four phases: (I) all fields are miscible; (II) the impurity fields are miscible with each other but phase separate from the condensate as a single bubble; (III) the localized impurity fields stay miscible with the condensate, but not with each other; and (IV) the impurity fields phase separate from the condensate and each other, forming a crystalline structure within a bubble. Thus, we show that a crystal can be constructed solely from superfluid components. Finally, we argue that the crystalline phases maintain their superfluid behavior, i.e., they possess a nonclassical rotational inertia, which - combined with lattice order - is a characteristic of supersolidity.

  14. Electric field induced rotation of polymer cholesteric liquid crystal flakes: mechanisms and applications

    NASA Astrophysics Data System (ADS)

    Kosc, Tanya Z.; Marshall, Kenneth L.; Jacobs, Stephen D.; Lambropoulos, John C.

    2002-12-01

    Electric fields can induce motion of polymer cholesteric liquid crystal (pCLC) flakes suspended in a fluid medium. The platelet-shaped pCLC flakes with a Grandjean texture show strong selective reflection when lying flat in the plane of a conventional cell. As their orientation with respect to normally incident light changes, their selective reflection color shifts toward the blue and diminishes until the flakes are no longer easily visible beyond 7-12° of rotation. Reproducibility and control of motion has been observed in moderately conductive host fluid. Flakes in such hosts do not respond to a DC electric field, but they rotate 90° in an AC field within a given frequency band. The response times and frequency regions for motion depend partially on the field magnitude, the dielectric properties of the host fluid and the flake geometry. We observe flakes reorienting in less than 500 ms in an electric field of 0.17 Vrms/μm, while sub-second reorientation is seen in fields as low as 5x10-2 Vrms/μm. This response time is comparable with typical electronic-paper applications, but with a significantly lower electric field. Displays using pCLC flakes would not require backlighting, sheet polarizers, color filters or alignment layers. Numerous additional applications for pCLC flakes are envisioned, including filters, polarizers, and spatial light modulators.

  15. The crystallization of apo-form UMP kinase from Xanthomonas campestris is significantly improved in a strong magnetic field

    SciTech Connect

    Tu, Jhe-Le; Chin, Ko-Hsin; Wang, Andrew H.-J.; Chou, Shan-Ho

    2007-05-01

    A bacterial UMP kinase from the plant pathogen X. campestris pathovar campestris has been overexpressed in E. coli, purified and crystallized in a strong magnetic field. The crystals diffracted to 2.35 Å. Bacterial UMP kinases (UMPKs) are crucial enzymes that are responsible for microbial UTP biosynthesis. Interestingly, eukaryotic and prokaryotic cells use different enzymes for UMP-phosphorylation reactions. Prokaryotic UMPKs are thus believed to be potential targets for antimicrobial drug development. Here, the cloning, expression and crystallization of SeMet-substituted XC1936, a bacterial UMPK from Xanthomonas campestris pathovar campestris, are reported. The crystallization of the apo-form UMPK was found to be significantly improved in a strong magnetic field; the crystals diffracted to a resolution of 2.35 Å, a dramatic improvement over the original value of 3.6 Å. Preliminary structural analyses of apo-form XC1936 using crystals grown in a strong magnetic field clearly reveal well defined loop regions involved in substrate-analogue binding that were previously not visible. Crystallization in a strong magnetic field thus was found to be indispensable in determining the flexible region of the XC1936 UMPK structure.

  16. Geomagnetic field strength 3.2 billion years ago recorded by single silicate crystals.

    PubMed

    Tarduno, John A; Cottrell, Rory D; Watkeys, Michael K; Bauch, Dorothy

    2007-04-05

    The strength of the Earth's early geomagnetic field is of importance for understanding the evolution of the Earth's deep interior, surface environment and atmosphere. Palaeomagnetic and palaeointensity data from rocks formed near the boundary of the Proterozoic and Archaean eons, some 2.5 Gyr ago, show many hallmarks of the more recent geomagnetic field. Reversals are recorded, palaeosecular variation data indicate a dipole-dominated morphology and available palaeointensity values are similar to those from younger rocks. The picture before 2.8 Gyr ago is much less clear. Rocks of the Archaean Kaapvaal craton (South Africa) are among the best-preserved, but even they have experienced low-grade metamorphism. The variable acquisition of later magnetizations by these rocks is therefore expected, precluding use of conventional palaeointensity methods. Silicate crystals from igneous rocks, however, can contain minute magnetic inclusions capable of preserving Archaean-age magnetizations. Here we use a CO2 laser heating approach and direct-current SQUID magnetometer measurements to obtain palaeodirections and intensities from single silicate crystals that host magnetite inclusions. We find 3.2-Gyr-old field strengths that are within 50 per cent of the present-day value, indicating that a viable magnetosphere sheltered the early Earth's atmosphere from solar wind erosion.

  17. Crystal growth of pure substances: Phase-field simulations in comparison with analytical and experimental results

    NASA Astrophysics Data System (ADS)

    Nestler, B.; Danilov, D.; Galenko, P.

    2005-07-01

    A phase-field model for non-isothermal solidification in multicomponent systems [SIAM J. Appl. Math. 64 (3) (2004) 775-799] consistent with the formalism of classic irreversible thermodynamics is used for numerical simulations of crystal growth in a pure material. The relation of this approach to the phase-field model by Bragard et al. [Interface Science 10 (2-3) (2002) 121-136] is discussed. 2D and 3D simulations of dendritic structures are compared with the analytical predictions of the Brener theory [Journal of Crystal Growth 99 (1990) 165-170] and with recent experimental measurements of solidification in pure nickel [Proceedings of the TMS Annual Meeting, March 14-18, 2004, pp. 277-288; European Physical Journal B, submitted for publication]. 3D morphology transitions are obtained for variations in surface energy and kinetic anisotropies at different undercoolings. In computations, we investigate the convergence behaviour of a standard phase-field model and of its thin interface extension at different undercoolings and at different ratios between the diffuse interface thickness and the atomistic capillary length. The influence of the grid anisotropy is accurately analyzed for a finite difference method and for an adaptive finite element method in comparison.

  18. Large field-induced-strain at high temperature in ternary ferroelectric crystals.

    PubMed

    Wang, Yaojin; Chen, Lijun; Yuan, Guoliang; Luo, Haosu; Li, Jiefang; Viehland, D

    2016-10-13

    The new generation of ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ferroelectric single crystals have potential applications in high power devices due to their surperior operational stability relative to the binary system. In this work, a reversible, large electric field induced strain of over 0.9% at room temperature, and in particular over 0.6% above 380 K was obtained. The polarization rotation path and the phase transition sequence of different compositions in these ternary systems have been determined with increasing electric field applied along [001] direction based on x-ray diffraction data. Thereafter, composition dependence of field-temperature phase diagrams were constructed, which provide compositional and thermal prospectus for the electromechanical properties. It was found the structural origin of the large stain, especially at higher temperature is the lattice parameters modulated by dual independent variables in composition of these ternary solid solution crystals.

  19. Large field-induced-strain at high temperature in ternary ferroelectric crystals

    PubMed Central

    Wang, Yaojin; Chen, Lijun; Yuan, Guoliang; Luo, Haosu; Li, Jiefang; Viehland, D.

    2016-01-01

    The new generation of ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ferroelectric single crystals have potential applications in high power devices due to their surperior operational stability relative to the binary system. In this work, a reversible, large electric field induced strain of over 0.9% at room temperature, and in particular over 0.6% above 380 K was obtained. The polarization rotation path and the phase transition sequence of different compositions in these ternary systems have been determined with increasing electric field applied along [001] direction based on x-ray diffraction data. Thereafter, composition dependence of field-temperature phase diagrams were constructed, which provide compositional and thermal prospectus for the electromechanical properties. It was found the structural origin of the large stain, especially at higher temperature is the lattice parameters modulated by dual independent variables in composition of these ternary solid solution crystals. PMID:27734908

  20. Large field-induced-strain at high temperature in ternary ferroelectric crystals

    NASA Astrophysics Data System (ADS)

    Wang, Yaojin; Chen, Lijun; Yuan, Guoliang; Luo, Haosu; Li, Jiefang; Viehland, D.

    2016-10-01

    The new generation of ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ferroelectric single crystals have potential applications in high power devices due to their surperior operational stability relative to the binary system. In this work, a reversible, large electric field induced strain of over 0.9% at room temperature, and in particular over 0.6% above 380 K was obtained. The polarization rotation path and the phase transition sequence of different compositions in these ternary systems have been determined with increasing electric field applied along [001] direction based on x-ray diffraction data. Thereafter, composition dependence of field-temperature phase diagrams were constructed, which provide compositional and thermal prospectus for the electromechanical properties. It was found the structural origin of the large stain, especially at higher temperature is the lattice parameters modulated by dual independent variables in composition of these ternary solid solution crystals.

  1. Kovacs effect enhanced broadband large field of view electro-optic modulators in nanodisordered KTN crystals.

    PubMed

    Chang, Yun-Ching; Wang, Chao; Yin, Shizhuo; Hoffman, Robert C; Mott, Andrew G

    2013-07-29

    The unique physical effect-Kovacs effect is explored to enhance the performance of EO modulators by employing the non-thermal equilibrium state nanodisordered KTN crystals created by super-cooling process, which can have a significant 3.5 fold increase in quadratic electro-optic coefficient. This enables to reduce the switching half wave voltage (almost by half) so that a broadband (~GHz range) and large field of view (+/-30 deg) electro-optic modulator can be realized with much lowered driving power, which can be very useful for a variety of applications: laser Q-switches, laser pulse shaping, high speed optical shutters and modulating retro reflectors.

  2. Calculation of crystal-field parameters for rare-earth noble metal alloys

    NASA Astrophysics Data System (ADS)

    Steinbeck, L.; Richter, M.; Eschrig, H.; Nitzsche, U.

    1995-02-01

    The crystal-field (CF) parameters for 4f electrons of a series of rare earth impurities in Ag and Au have been evaluated from first-principles density functional calculations of the charge distribution which are based on an optimized LCAO scheme. The localized 4f states are treated as 'open core shell'. By including the self-interaction correction for the 4f states, artificial constraints on the 4f charge density employed in earlier density functional CF calculations are avoided. The calculated parameters are compared with recent neutron scattering data.

  3. Light quasiparticles dominate electronic transport in molecular crystal field-effect transistors

    SciTech Connect

    Li, Z. Q.; Podzorov, V.; Sai, N.; Martin, Michael C.; Gershenson, M. E.; Di Ventra, M.; Basov, D. N.

    2007-03-01

    We report on an infrared spectroscopy study of mobile holes in the accumulation layer of organic field-effect transistors based on rubrene single crystals. Our data indicate that both transport and infrared properties of these transistors at room temperature are governed by light quasiparticles in molecular orbital bands with the effective masses m[small star, filled]comparable to free electron mass. Furthermore, the m[small star, filled]values inferred from our experiments are in agreement with those determined from band structure calculations. These findings reveal no evidence for prominent polaronic effects, which is at variance with the common beliefs of polaron formation in molecular solids.

  4. Fluoride Binding and Crystal-Field Analysis of Lanthanide Complexes of Tetrapicolyl-Appended Cyclen.

    PubMed

    Blackburn, Octavia A; Kenwright, Alan M; Jupp, Andrew R; Goicoechea, Jose M; Beer, Paul D; Faulkner, Stephen

    2016-06-20

    Lanthanide complexes of tetrapicolyl cyclen displayed remarkably high affinities for fluoride (log K≈5) in water, and were shown to form 1:1 complexes. The behaviour of these systems can be rationalised by changes to the magnitude of the crystal-field parameter, B20 . However, such changes are not invariably accompanied by a change in sign of this parameter: for early lanthanides, the N8 donor set with a coordinated axial water molecule ensures that the magnetic anisotropy has the opposite sense to that observed in the analogous dehydrated lanthanide complexes. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  5. Linearly first- and second-order, unconditionally energy stable schemes for the phase field crystal model

    NASA Astrophysics Data System (ADS)

    Yang, Xiaofeng; Han, Daozhi

    2017-02-01

    In this paper, we develop a series of linear, unconditionally energy stable numerical schemes for solving the classical phase field crystal model. The temporal discretizations are based on the first order Euler method, the second order backward differentiation formulas (BDF2) and the second order Crank-Nicolson method, respectively. The schemes lead to linear elliptic equations to be solved at each time step, and the induced linear systems are symmetric positive definite. We prove that all three schemes are unconditionally energy stable rigorously. Various classical numerical experiments in 2D and 3D are performed to validate the accuracy and efficiency of the proposed schemes.

  6. Tunable far-field acoustic imaging by two-dimensional sonic crystal with concave incident surface

    NASA Astrophysics Data System (ADS)

    Shen, Feng-Fu; Lu, Dan-Feng; Zhu, Hong-Wei; Ji, Chang-Ying; Shi, Qing-Fan

    2017-01-01

    An additional concave incident surface comprised of two-dimensional (2D) sonic crystals (SCs) is employed to tune the acoustic image in the far-field region. The tunability is realized through changing the curvature of the concave surface. To explain the tuning mechanism, a simple ray-trace analysis is demonstrated based on the wave-beam negative refractive law. Then, a numerical confirmation is carried out. Results show that both the position and the intensity of the image can be tuned by the introduced concave surface.

  7. Crystal field analysis of Dy and Tm implanted silicon for photonic and quantum technologies.

    PubMed

    Hughes, Mark A; Lourenço, Manon A; Carey, J David; Murdin, Ben; Homewood, Kevin P

    2014-12-01

    We report the lattice site and symmetry of optically active Dy3+ and Tm3+ implanted Si. Local symmetry was determined by fitting crystal field parameters (CFPs), corresponding to various common symmetries, to the ground state splitting determined by photoluminescence measurements. These CFP values were then used to calculate the splitting of every J manifold. We find that both Dy and Tm ions are in a Si substitution site with local tetragonal symmetry. Knowledge of rare-earth ion symmetry is important in maximising the number of optically active centres and for quantum technology applications where local symmetry can be used to control decoherence.

  8. Free energy functionals for efficient phase field crystal modeling of structural phase transformations.

    PubMed

    Greenwood, Michael; Provatas, Nikolas; Rottler, Jörg

    2010-07-23

    The phase field crystal (PFC) method is a promising technique for modeling materials with atomic resolution on mesoscopic time scales. While numerically more efficient than classical density functional theory (CDFT), its single mode free energy limits the complexity of structural transformations that can be simulated. We introduce a new PFC model inspired by CDFT, which uses a systematic construction of two-particle correlation functions that allows for a broad class of structural transformations. Our approach considers planar spacings, lattice symmetries, planar atomic densities, and atomic vibrational amplitudes in the unit cell, and parameterizes temperature and anisotropic surface energies. The power of our approach is demonstrated by two examples of structural phase transformations.

  9. Renormalization-group theory for the phase-field crystal equation

    NASA Astrophysics Data System (ADS)

    Athreya, Badrinarayan P.; Goldenfeld, Nigel; Dantzig, Jonathan A.

    2006-07-01

    We derive a set of rotationally covariant amplitude equations for use in multiscale simulation of the two-dimensional phase-field crystal model by a variety of renormalization-group (RG) methods. We show that the presence of a conservation law introduces an ambiguity in operator ordering in the RG procedure, which we show how to resolve. We compare our analysis with standard multiple-scale techniques, where identical results can be obtained with greater labor, by going to sixth order in perturbation theory, and by assuming the correct scaling of space and time.

  10. Electrical Impact of SiC Structural Crystal Defects on High Electric Field Devices (Invited)

    NASA Technical Reports Server (NTRS)

    Neudeck, Philip G.

    1999-01-01

    As illustrated by the invited paper at this conference and other works, SiC wafers and epilayers contain a variety of crystallographic imperfections, including micropipes, closed-core screw dislocations, grain boundaries, basal plane dislocations, heteropolytypic inclusions, and surfaces that are often damaged and contain atomically rough features like step bunching and growth pits or hillocks. Present understanding of the operational impact of various crystal imperfections on SiC electrical devices is reviewed, with an emphasis placed on high-field SiC power devices and circuits.

  11. Rigorous analysis of an electric-field-driven liquid crystal lens for 3D displays

    NASA Astrophysics Data System (ADS)

    Kim, Bong-Sik; Lee, Seung-Chul; Park, Woo-Sang

    2014-08-01

    We numerically analyzed the optical performance of an electric field driven liquid crystal (ELC) lens adopted for 3-dimensional liquid crystal displays (3D-LCDs) through rigorous ray tracing. For the calculation, we first obtain the director distribution profile of the liquid crystals by using the Erickson-Leslie motional equation; then, we calculate the transmission of light through the ELC lens by using the extended Jones matrix method. The simulation was carried out for a 9view 3D-LCD with a diagonal of 17.1 inches, where the ELC lens was slanted to achieve natural stereoscopic images. The results show that each view exists separately according to the viewing position at an optimum viewing distance of 80 cm. In addition, our simulation results provide a quantitative explanation for the ghost or blurred images between views observed from a 3D-LCD with an ELC lens. The numerical simulations are also shown to be in good agreement with the experimental results. The present simulation method is expected to provide optimum design conditions for obtaining natural 3D images by rigorously analyzing the optical functionalities of an ELC lens.

  12. Simultaneously combined liquid crystal surface heat transfer and PIV flow-field measurements

    NASA Astrophysics Data System (ADS)

    Praisner, T. J.; Sabatino, D. R.; Smith, C. R.

    An experimental technique has been developed which combines simultaneous high-density particle image velocimetry (PIV) with a new thermochromic liquid-crystal based instantaneous convective heat transfer sensing system. The new heat transfer sensing system employs liquid crystals applied to the non-flow side of a constant heat flux foil, and isolated from the support plate by a thin insulating cavity. The resulting composite sensing sheet is capable of a 66Hz frequency response and, when utilized in a water flow, it can track instantaneous variations in convective heat transfer for turbulent flows. Placement of the liquid crystals on the non-flow side of the constant heat flux foil also provides optical separation from the flow field, allowing the simultaneous application of high image-density PIV measurements. A description of the experimental apparatus is provided along with a validation and uncertainty analysis of the combined technique. Selected quasi-three-dimensional constructs obtained using this new technique are presented for a variety of turbulent flow configurations.

  13. Direct mapping of local director field of nematic liquid crystals at the nano-scale

    NASA Astrophysics Data System (ADS)

    Xia, Yu; Serra, Francesca; Yang, Shu; Kamien, Randall

    2015-03-01

    The director field in liquid crystals (LCs) has been characterized mainly via polarized optical microscopy, fluorescence confocal microscopy, and Raman spectroscopy, all of which are limited by optical wavelengths - from hundreds of nanometers to several micrometers. Since LC orientation cannot be resolved directly by these methods, theory is needed to interpret the local director field of LC alignment. In this work, we introduce a new approach to directly visualize the local director field of a nematic LC (NLC) at the nano-scale using scanning electron microscopy (SEM). A new type of NLC monomer bearing crosslinkable groups was designed and synthesized. It can be well-oriented at particle surfaces and patterned polymer substrates, including micron-sized silica colloids, porous membranes, micropillar arrays, and 1D channels. After carefully crosslinking, the molecular orientation of NLCs around the particles or within the patterns could be directly visualized by SEM, showing oriented nanofibers representing LC director from the fractured samples. Here, we could precisely resolve not only the local director field by this approach, but the defect structures of NLCs, including hedgehogs and line defects. The direct mapping of LC directors at the nanoscale using this method will improve our understanding of NLC local director field, and thus their manipulation and applications. More importantly, a theoretical interpretation will no longer be a necessity to resolve a new material system in this field.

  14. Alignment Behavior of Crystal with Magnetic Anisotropy of χc < χa under Rotating Magnetic Field

    NASA Astrophysics Data System (ADS)

    Iwai, Kazuhiko; Niimi, Masahiro; Kohama, Takenori

    2009-10-01

    The alignment behavior of a crystal has been investigated by numerical calculation and an in situ observation experiment with a process combining magnetic field imposition and sample rotation to form unidirectionally aligned crystals with a magnetic anisotropy of χc < χa. The experimentally observed alignment behavior of a polymeric fiber and its alignment time agreed with the numerically calculated ones. Crystal alignment under the out-of-step condition alternately repeats the alignment duration and the keeping of a constant duration, and finally the crystal aligns in a specific direction. The alignment time under the synchronous condition is longer than that under the out-of-step condition if the magnetic field intensity is constant. To reduce the alignment time, a strong magnetic field under the out-of-step condition is desirable in this process.

  15. Electronic structure of ytterbium-implanted GaN at ambient and high pressure: experimental and crystal field studies.

    PubMed

    Kaminska, A; Ma, C-G; Brik, M G; Kozanecki, A; Boćkowski, M; Alves, E; Suchocki, A

    2012-03-07

    The results of high-pressure low-temperature optical measurements in a diamond-anvil cell of bulk gallium nitride crystals implanted with ytterbium are reported in combination with crystal field calculations of the Yb(3+) energy levels. Crystal field analysis of splitting of the (2)F(7/2) and (2)F(5/2) states has been performed, with the aim of assigning all features of the experimental luminescence spectra. A thorough analysis of the pressure behavior of the Yb(3+) luminescence lines in GaN allowed the determination of the ambient-pressure positions and pressure dependence of the Yb(3+) energy levels in the trigonal crystal field as well as the pressure-induced changes of the spin-orbit coupling coefficient.

  16. Study of photonic crystal structures and their application in the field of antennas

    NASA Astrophysics Data System (ADS)

    Gonzalo Garcia, Ramon

    2000-08-01

    "Photonic Crystals" defined as periodic structures in one, two or three dimensions, prevent the propagation of an incident electromagnetic wave inside them, over a determined frequency range (bandgap) and for any propagation direction. One of the main problems in the antenna field is the power transferred to the surface wave modes. This energy does not contribute to the main radiation of the antenna, considering this effect as a loss mechanism. Besides, this energy, depending on the dielectric constant of the substrate and its thickness, could be more than 70% of the total power. Even sometimes, this energy will be added in counter-phase to the main radiation pattern of the antenna making worse this pattern. The main idea is to use the "Photonic Crystal" structures as substrates for these antenna configurations leading to an increasing of the antenna efficiency and directivity, a more symmetric radiation pattern and a reduction of the back radiation. The thesis can be divided into two parts: (1) Firstly, the Maxwell's equations for dielectric periodic structures are presented in Chapter 2. An eigensystem has been formulated to solve for the eigenvalues and eigenmodes of any "Photonic Crystal" providing the information to determine a full or a partial bandgap. The gap behaviour of different structures as function of the geometry, dielectric constant contrast or fill factor of the unit cell has been analysed in Chapter 3. Besides, a new three-dimensional structure has been proposed. (2) Chapter 4 and 5 focus on using "Photonic Crystal" to avoid propagation of surface waves in microstrip patch antenna configurations, replacing conventional substrates for "Photonic Crystal" structures. A comparison between the conventional substrate features versus the "Photonic Crystal" is presented. The analysis reveals a decrease in coupled power to substrate modes and an improvement of efficiency, directivity, back radiation, pattern symmetry and mutual coupling. In Chapter 5 the

  17. The interface morphology of a spherical crystal in the undercooled melt affected by a far-field uniform flow

    NASA Astrophysics Data System (ADS)

    Chen, M. W.; Wang, Y. L.; Zhang, H.; Wu, L. Y.; Wang, Z. D.

    2011-05-01

    The effect of the convective flow caused by the far-field uniform flow on the interface morphology of a spherical crystal is studied by using the matched asymptotic expansion method. For the case that the far-field uniform flow is far less than the characteristic velocity of the interface, we obtain the uniformly valid asymptotic solution of the spherical crystal in the entire melt region. The analytical results show that the far-field uniform flow has significant effect on the interface morphology of the spherical crystal. The convection flow makes the interface of the growing spherical crystal enhance growth velocity in the upstream direction of the far-field uniform flow, inhibit growth in the downstream direction and decrease growth velocity on the two sides of the spherical crystal. The drag effect of the far-field uniform flow makes the interface morphology of the spherical crystal evolve into a pearlike oval shape. Our analytical result is consistent with the experimental and simulation results.

  18. The petrologic history of the Sanganguey volcanic field, Nayarit, Mexico: Comparisons in a suite of crystal-rich and crystal-poor lavas

    NASA Astrophysics Data System (ADS)

    Crabtree, Stephen M.; Waters, Laura E.

    2017-04-01

    To evaluate if intermediate magmas erupting from Volcán Sanganguey (Mexico) and the surrounding volcanic field are formed by mixing of basalts and rhyolites or if they initially exist as intermediate liquids, a detailed petrological study is presented for eight andesite and dacite magmas. Six of the samples erupted from the central edifice (four andesites and two dacites) are crystal-rich (≤ 50 vol%), whereas the remaining two samples (one andesite and one dacite) erupted from monogenetic vents in the peripheral volcanic field and are crystal poor (≤ 5 vol%). Despite the variation in crystallinity, all samples are multiply saturated in five to seven mineral phases (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± clinopyroxene ± hornblende). In all samples, plagioclase spans a 30-40 mol% An range in composition and orthopyroxene spans a range in Mg# of 5-10. Pre-eruptive temperatures and oxygen fugacites (relative to the NNO buffer) range from 853 (± 24) to 1085 (± 16) °C and - 0.1 (± 0.1) to 0.9 (± 0.1) ∆ NNO, on the basis of Fe-Ti two oxide thermometry. Application of the plagioclase-liquid hygrometer to the samples reveals maximum H2O contents that range from 1.7-6.2 wt%. Comparison with phase equilibrium experiments demonstrates that all plagioclase and orthopyroxene compositions in the crystal-poor samples could have grown from their respective whole rock compositions. Comparison of crystal rich samples with phase equilibrium experiments reveals the presence of sodic xenocrysts which reflect resorption textures and an estimated excess plagioclase crystal cargo of > 6 vol%. The excess plagioclase crystal cargo is not distinguishable from phenocrystic plagioclase based on composition or texture, suggesting that they were also grown in intermediate melts, and are therefore described as antecrystic. No calcic plagioclase xenocrysts (> An79) typical of hydrous arc basalts are observed, thus it is likely that the excess plagioclase

  19. Effects of Polymers on the Rotational Viscosities of Nematic Liquid Crystals and Dynamics of Field Alignment.

    NASA Astrophysics Data System (ADS)

    Kim, Du-Rim

    Many of the important physical phenomena exhibited by the nematic phase, such as its unusual flow properties and its responses to the electric and the magnetic fields, can be discussed regarding it as a continuous medium. The Leslie-Erickson dynamic theory has the six dissipative coefficients from continuum model of liquid crystal. Parodi showed that only five of them are independent, when Onsagar's reciprocal relations are used. One of these, which has no counterpart in the isotropic liquids, is the rotational viscosity coefficient, gamma_1. The main objective of this project is to study the rotational viscosities of selected micellar nematic systems and the effect of dissolved polymers in micellar and thermotropic liquid crystals. We used rotating magnetic field method which allows one to determine gamma _1 and the anisotropic magnetic susceptibility, chi_{a}. For the ionic surfactant liquid crystals of SDS and KL systems used in this study, the rotational viscosity exhibited an extraordinary drop after reaching the highest value gamma_1 as the temperature was lowered. This behavior is not observed in normal liquid crystals. But this phenomena can be attributed to the existence of nematic biaxial phase below the rod-like nematic N_{c} phase. The pretransitional increase in gamma _1 near the disk-like nematic to smectic -A phase transition of the pure CsPFO H_2O systems are better understood with the help of mean-field models of W. L. McMillan. He predicted a critical exponent nu = -{1over 2} for the divergence of gamma_1. The polymer (PEO, molecular weight = 10 ^5) dissolved in CsPFO H_2O system (which has 0.6% critical polymer concentration), suppressed the nematic to lamellar smectic phase transition in concentrated polymer solutions (0.75% and higher). In dilute polymer solutions with lower than 0.3% polyethylene-oxide, a linear increase of gamma_1 is observed, which agrees with Brochard theory. The polymer solutions in thermotropic liquid crystal solvents

  20. Effects of polymers on the rotational viscosities of nematic liquid crystals and dynamics of field alignment

    SciTech Connect

    Kim, D.

    1993-12-31

    Many of the important physical phenomena exhibited by the nematic phase, such as its unusual flow properties and its responses to the electric and the magnetic fields, can be discussed regarding it as a continous medium. The Leslie-Erickson dynamic theory has the six dissipative coefficients from continuum model of liquid crystal. Parodi showed that only five of them are independent, when Onsagar`s reciprocal relations are used. One of these, which has no counterpart in the isotropic liquids, is the rotational viscosity co-efficient, {gamma}{sub 1}. The main objective of this project is to study the rotational viscosities of selected micellar nematic systems and the effect of dissolved polymers in micellar and thermotropic liqud crystals. We used rotating magnetic field method which allows one to determine {gamma}{sub 1} and the anisotropic magnetic susceptibility, {chi}{sub a}. For the ionic surfactant liquid crystals of SDS and KL systems used in this study, the rotational viscosity exhibited an extraordinary drop after reaching the highest values {gamma}{sub 1} as the temperature was lowered. This behavior is not observed in normal liquid crystals. But this phenomena can be attributed to the existence of nematic biaxial phase below the rod-like nematic N{sub c} phase. The pretransitional increase in {gamma}{sub 1} near the disk-like nematic to smectic-A phase transition of the pure CsPFO/H{sub 2}O systems are better understood with the help of mean-field models of W.L. McMillan. He predicted a critical exponent {nu} = {1/2} for the divergence of {gamma}{sub 1}. The polymer (PEO, molecular weight = 10{sup 5}) dissolved in CsPFO/H{sub 2}O system (which has 0.6% critical polymer concentration), suppressed the nematic to lamellar smectic phase transition in concentrated polymer solutions (0.75% and higher). In dilute polymer solutions with lower than 0.3% polyethylene-oxide, a linear increase of {gamma}{sub 1} is observed, which agrees with Brochard theory.

  1. A phase field model for snow crystal growth in three dimensions

    NASA Astrophysics Data System (ADS)

    Demange, Gilles; Zapolsky, Helena; Patte, Renaud; Brunel, Marc

    2017-04-01

    Snowflake growth provides a fascinating example of spontaneous pattern formation in nature. Attempts to understand this phenomenon have led to important insights in non-equilibrium dynamics observed in various active scientific fields, ranging from pattern formation in physical and chemical systems, to self-assembly problems in biology. Yet, very few models currently succeed in reproducing the diversity of snowflake forms in three dimensions, and the link between model parameters and thermodynamic quantities is not established. Here, we report a modified phase field model that describes the subtlety of the ice vapour phase transition, through anisotropic water molecules attachment and condensation, surface diffusion, and strong anisotropic surface tension, that guarantee the anisotropy, faceting and dendritic growth of snowflakes. We demonstrate that this model reproduces the growth dynamics of the most challenging morphologies of snowflakes from the Nakaya diagram. We find that the growth dynamics of snow crystals matches the selection theory, consistently with previous experimental observations.

  2. Long-wavelength properties of phase-field-crystal models with second-order dynamics.

    PubMed

    Heinonen, V; Achim, C V; Ala-Nissila, T

    2016-05-01

    The phase-field-crystal (PFC) approach extends the notion of phase-field models by describing the topology of the microscopic structure of a crystalline material. One of the consequences is that local variation of the interatomic distance creates an elastic excitation. The dynamics of these excitations poses a challenge: pure diffusive dynamics cannot describe relaxation of elastic stresses that happen through phonon emission. To this end, several different models with fast dynamics have been proposed. In this article we use the amplitude expansion of the PFC model to compare the recently proposed hydrodynamic PFC amplitude model with two simpler models with fast dynamics. We compare these different models analytically and numerically. The results suggest that in order to have proper relaxation of elastic excitations, the full hydrodynamical description of the PFC amplitudes is required.

  3. Long-wavelength properties of phase-field-crystal models with second-order dynamics

    NASA Astrophysics Data System (ADS)

    Heinonen, V.; Achim, C. V.; Ala-Nissila, T.

    2016-05-01

    The phase-field-crystal (PFC) approach extends the notion of phase-field models by describing the topology of the microscopic structure of a crystalline material. One of the consequences is that local variation of the interatomic distance creates an elastic excitation. The dynamics of these excitations poses a challenge: pure diffusive dynamics cannot describe relaxation of elastic stresses that happen through phonon emission. To this end, several different models with fast dynamics have been proposed. In this article we use the amplitude expansion of the PFC model to compare the recently proposed hydrodynamic PFC amplitude model with two simpler models with fast dynamics. We compare these different models analytically and numerically. The results suggest that in order to have proper relaxation of elastic excitations, the full hydrodynamical description of the PFC amplitudes is required.

  4. Two beam energy exchange in hybrid liquid crystal cells with photorefractive field controlled boundary conditions

    NASA Astrophysics Data System (ADS)

    Reshetnyak, V. Yu.; Pinkevych, I. P.; Subota, S. I.; Evans, D. R.

    2016-09-01

    We develop a theory describing energy gain when two light beams intersect in a hybrid nematic liquid crystal (LC) cell with photorefractive crystalline substrates. A periodic space-charge field induced by interfering light beams in the photorefractive substrates penetrates into the LC layer and reorients the director. We account for two main mechanisms of the LC director reorientation: the interaction of the photorefractive field with the LC flexopolarization and the director easy axis at the cell boundaries. It is shown that the resulting director grating is a sum of two in-phase gratings: the flexoelectric effect driven grating and the boundary-driven grating. Each light beam diffracts from the induced gratings leading to an energy exchange between beams. We evaluate the signal beam gain coefficient and analyze its dependence on the director anchoring energy and the magnitude of the director easy axis modulation.

  5. Mean-Field Limit and Phase Transitions for Nematic Liquid Crystals in the Continuum

    NASA Astrophysics Data System (ADS)

    Bachmann, Sven; Genoud, François

    2017-08-01

    We discuss thermotropic nematic liquid crystals in the mean-field regime. In the first part of this article, we rigorously carry out the mean-field limit of a system of N rod-like particles as N→ ∞, which yields an effective `one-body' free energy functional. In the second part, we focus on spatially homogeneous systems, for which we study the associated Euler-Lagrange equation, with a focus on phase transitions for general axisymmetric potentials. We prove that the system is isotropic at high temperature, while anisotropic distributions appear through a transcritical bifurcation as the temperature is lowered. Finally, as the temperature goes to zero we also prove, in the concrete case of the Maier-Saupe potential, that the system converges to perfect nematic order.

  6. Interdigitated pixel electrodes with alternating tilts for fast fringe-field switching of liquid crystals.

    PubMed

    Choi, Tae-Hoon; Woo, Jae-Hyeon; Choi, Yeongyu; Yoon, Tae-Hoon

    2016-11-28

    We propose an interdigitated pixel electrode structure with alternating tilts for fast fringe-field switching of liquid crystals (LCs). In contrast to an LC cell, where the pixel electrodes are parallel to the LC alignment direction, this device does not require a non-zero pretilt angle, owing to an obliquely applied electric field; thus, it can retain a much wider viewing angle by aligning the LCs without a pretilt. In addition to a short response time and wide viewing angle, the proposed device allows a much larger deviation of the LC alignment direction, which is essential for mass production. Moreover, LCs with negative dielectric anisotropy can be used to minimize the transmittance decrease.

  7. Nanoscale mechanical actuation and near-field read-out of photonic crystal molecules

    NASA Astrophysics Data System (ADS)

    Petruzzella, M.; La China, F.; Intonti, F.; Caselli, N.; De Pas, M.; van Otten, F. W. M.; Gurioli, M.; Fiore, A.

    2016-09-01

    We employed the contact forces induced by a near-field tip to tune and probe the optical resonances of a mechanically compliant photonic crystal molecule. Here, the pressure induced by the near-field tip is exploited to control the spectral proprieties of the coupled cavities in an ultrawide spectral range, demonstrating a reversible mode shift of 37.5 nm . Besides, by monitoring the coupling strength variation due to the vertical nanodeformation of the dielectric structure, distinct tip-sample interaction regimes have been unambiguously reconstructed with a nano-Newton sensitivity. These results demonstrate an optical method for mapping mechanical forces at the nanoscale with a lateral spatial resolution below 100 nm.

  8. Bicritical universality of the anisotropic Heisenberg model in a crystal field.

    PubMed

    Freire, R T S; Plascak, J A

    2015-03-01

    The bicritical properties of the three-dimensional classical anisotropic Heisenberg model in a crystal field are investigated through extensive Monte Carlo simulations on a simple cubic lattice, using Metropolis and Wolff algorithms. Field-mixing and multidimensional histogram techniques were employed in order to compute the probability distribution function of the extensive conjugate variables of interest and, using finite-size scaling analysis, the first-order transition line of the model was precisely located. The fourth-order cumulant of the order parameter was then calculated along this line and the bicritical point located with good precision from the cumulant crossings. The bicritical properties of this point were further investigated through the measurement of the universal probability distribution function of the order parameter. The results lead us to conclude that the studied bicritical point belongs in fact to the three-dimensional Heisenberg universality class.

  9. Phase-field crystal model for a diamond-cubic structure.

    PubMed

    Chan, V W L; Pisutha-Arnond, N; Thornton, K

    2015-05-01

    We present a structural phase-field crystal model [M. Greenwood et al., Phys. Rev. Lett. 105, 045702 (2010)] that yields a stable dc structure. The stabilization of a dc structure is accomplished by constructing a two-body direct correlation function (DCF) approximated by a combination of two Gaussian functions in Fourier space. A phase diagram containing a dc-liquid phase coexistence region is calculated for this model. We examine the energies of solid-liquid interfaces with normals along the [100], [110], and [111] directions. The dependence of the interfacial energy on a temperature parameter, which controls the heights of the peaks in the two-body DCF, is described by a Gaussian function. Furthermore, the dependence of the interfacial energy on the peak widths of the two-body DCF, which controls the excess energy associated with interfaces, defects, and strain, is described by an inverse power law. These relationships can be used to parametrize the phase-field crystal model for the dc structure to match solid-liquid interfacial energies to those measured experimentally or calculated from atomistic simulations.

  10. Phase-field crystal model for a diamond-cubic structure

    NASA Astrophysics Data System (ADS)

    Chan, V. W. L.; Pisutha-Arnond, N.; Thornton, K.

    2015-05-01

    We present a structural phase-field crystal model [M. Greenwood et al., Phys. Rev. Lett. 105, 045702 (2010), 10.1103/PhysRevLett.105.045702] that yields a stable d c structure. The stabilization of a d c structure is accomplished by constructing a two-body direct correlation function (DCF) approximated by a combination of two Gaussian functions in Fourier space. A phase diagram containing a d c -liquid phase coexistence region is calculated for this model. We examine the energies of solid-liquid interfaces with normals along the [100], [110], and [111] directions. The dependence of the interfacial energy on a temperature parameter, which controls the heights of the peaks in the two-body DCF, is described by a Gaussian function. Furthermore, the dependence of the interfacial energy on the peak widths of the two-body DCF, which controls the excess energy associated with interfaces, defects, and strain, is described by an inverse power law. These relationships can be used to parametrize the phase-field crystal model for the d c structure to match solid-liquid interfacial energies to those measured experimentally or calculated from atomistic simulations.

  11. Modified phase-field-crystal model for solid-liquid phase transitions

    NASA Astrophysics Data System (ADS)

    Guo, Can; Wang, Jincheng; Wang, Zhijun; Li, Junjie; Guo, Yaolin; Tang, Sai

    2015-07-01

    A modified phase-field-crystal (PFC) model is proposed to describe solid-liquid phase transitions by reconstructing the correlation function. The effects of fitting parameters of our modified PFC model on the bcc-liquid phase diagram, numerical stability, and solid-liquid interface properties during planar interface growth are examined carefully. The results indicate that the increase of the correlation function peak width at k =km will enhance the stability of the ordered phase, while the increase of peak height at k =0 will narrow the two-phase coexistence region. The third-order term in the free-energy function and the short wave-length of the correlation function have significant influences on the numerical stability of the PFC model. During planar interface growth, the increase of peak width at k =km will decrease the interface width and the velocity coefficient C , but increase the anisotropy of C and the interface free energy. Finally, the feasibility of the modified phase-field-crystal model is demonstrated with a numerical example of three-dimensional dendritic growth of a body-centered-cubic structure.

  12. Rubrene crystal field-effect mobility modulation via conducting channel wrinkling

    PubMed Central

    Reyes-Martinez, Marcos A.; Crosby, Alfred J.; Briseno, Alejandro L.

    2015-01-01

    With the impending surge of flexible organic electronic technologies, it has become essential to understand how mechanical deformation affects the electrical performance of organic thin-film devices. Organic single crystals are ideal for the systematic study of strain effects on electrical properties without being concerned about grain boundaries and other defects. Here we investigate how the deformation affects the field-effect mobility of single crystals of the benchmark semiconductor rubrene. The wrinkling instability is used to apply local strains of different magnitudes along the conducting channel in field-effect transistors. We discover that the mobility changes as dictated by the net strain at the dielectric/semiconductor interface. We propose a model based on the plate bending theory to quantify the net strain in wrinkled transistors and predict the change in mobility. These contributions represent a significant step forward in structure–function relationships in organic semiconductors, critical for the development of the next generation of flexible electronic devices. PMID:25939864

  13. Magnetic properties and crystal field in Pr2Zr2O7

    NASA Astrophysics Data System (ADS)

    Bonville, P.; Guitteny, S.; Gukasov, A.; Mirebeau, I.; Petit, S.; Decorse, C.; Hatnean, M. Ciomaga; Balakrishnan, G.

    2016-10-01

    In this work, we revisit the crystal field acting on the non-Kramers Pr3 + ion (4 f2 ) in the quantum spin-ice candidate Pr2Zr2O7 using both a standard calculation restricted to the ground spin-orbit multiplet and intermediate coupling states in the full basis of the f2 configuration. Analysis of the thermal variation of the polycrystal magnetic susceptibility and of the local susceptibilities χ⊥ and χ∥ determined by means of polarized neutron diffraction experiments reveals that the effective antiferromagnetic exchange is strongly depleted at low temperature with respect to its high-temperature value. We then discuss the influence of crystal field imperfections arising from residual strains, which are especially important for a non-Kramers ion. We find that they are an essential ingredient to account for the very low temperature M (H ) magnetization curves, showing that the saturation is not achieved even at 8 T. Furthermore, as possible candidates to qualitatively understand the Curie-like behavior observed below 0.5 K, we discuss the influence of the magnetic hyperfine interaction.

  14. Crystal field analysis of rare-earth ions energy levels in GaN

    NASA Astrophysics Data System (ADS)

    Stachowicz, M.; Kozanecki, A.; Ma, C.-G.; Brik, M. G.; Lin, J. Y.; Jiang, Hx; Zavada, J. M.

    2014-11-01

    Much effort has been put to achieve optoelectronic devices based on Er doped GaN, operating on the intra-4f-shell transitions of erbium. The key issue for good understanding of energy transfer mechanisms to Er and its luminescence properties is the position of Er3+ ions in the crystalline lattice of GaN. After doping, Er3+ ions are assumed to be placed in substitutional position for Ga3+ in GaN. Although Ga is positioned in high symmetry, tetrahedral [ErN4]9 - cluster, deviations from this after doping are impossible to avoid because of a large difference in ionic radii of Ga3+ (47 pm) and Er3+ (89 pm). In this work we report on crystal field analysis of Er ion energy levels in cubic and hexagonal GaN. It is shown that local symmetry of Er in cubic GaN is D2, whereas calculations reveal that in hexagonal GaN local symmetry is C3V. Some trends in crystal field parameters of trivalent lanthanides in hexagonal GaN are discussed.

  15. Precise Patterning of Organic Single Crystals via Capillary-Assisted Alternating-Electric Field.

    PubMed

    Zhang, Yedong; Jie, Jiansheng; Sun, Yuning; Jeon, Seok-Gy; Zhang, Xiujuan; Dai, Gaole; Lee, Cheol Jin; Zhang, Xiaohong

    2017-07-01

    Owing to the extraordinary properties, organic micro/nanocrystals are important building blocks for future low-cost and high-performance organic electronic devices. However, integrated device application of the organic micro/nanocrystals is hampered by the difficulty in high-throughput, high-precision patterning of the micro/nanocrystals. In this study, the authors demonstrate, for the first time, a facile capillary-assisted alternating-electric field method for the large-scale assembling and patterning of both 0D and 1D organic crystals. These crystals can be precisely patterned at the photolithography defined holes/channels at the substrate with the yield up to 95% in 1 mm(2) . The mechanism of assembly kinetics is systematically studied by the electric field distribution simulation and experimental investigations. By using the strategy, various organic micro/nanocrystal patterns are obtained by simply altering the geometries of the photoresist patterns on substrates. Moreover, ultraviolet photodetectors based on the patterned Alq3 micro/nanocrystals exhibit visible-blind photoresponse with high sensitivity as well as excellent stability and reproducibility. This work paves the way toward high-integration, high-performance organic electronic, and optoelectronic devices from the organic micro/nanocrystals. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  16. Large piezoelectricity in electric-field modified single crystals of SrTiO3

    NASA Astrophysics Data System (ADS)

    Khanbabaee, B.; Mehner, E.; Richter, C.; Hanzig, J.; Zschornak, M.; Pietsch, U.; Stöcker, H.; Leisegang, T.; Meyer, D. C.; Gorfman, S.

    2016-11-01

    Defect engineering is an effective and powerful tool to control the existing material properties and produce completely new ones, which are symmetry-forbidden in a defect-free crystal. For example, the application of a static electric field to a single crystal of SrTiO3 forms a strained near-surface layer through the migration of oxygen vacancies out of the area beneath the positively charged electrode. While it was previously shown that this near-surface phase holds pyroelectric properties, which are symmetry-forbidden in centrosymmetric bulk SrTiO3, this paper reports that the same phase is strongly piezoelectric. We demonstrate the piezoelectricity of this phase through stroboscopic time-resolved X-ray diffraction under alternating electric field and show that the effective piezoelectric coefficient d33 ranges between 60 and 100 pC/N. The possible atomistic origins of the piezoelectric activity are discussed as a coupling between the electrostrictive effect and spontaneous polarization of this near-surface phase.

  17. Crystal field effect induced topological crystalline insulators in monolayer IV-VI semiconductors.

    PubMed

    Liu, Junwei; Qian, Xiaofeng; Fu, Liang

    2015-04-08

    Two-dimensional (2D) topological crystalline insulators (TCIs) were recently predicted in thin films of the SnTe class of IV-VI semiconductors, which can host metallic edge states protected by mirror symmetry. As thickness decreases, quantum confinement effect will increase and surpass the inverted gap below a critical thickness, turning TCIs into normal insulators. Surprisingly, based on first-principles calculations, here we demonstrate that (001) monolayers of rocksalt IV-VI semiconductors XY (X = Ge, Sn, Pb and Y = S, Se, Te) are 2D TCIs with the fundamental band gap as large as 260 meV in monolayer PbTe. This unexpected nontrivial topological phase stems from the strong crystal field effect in the monolayer, which lifts the degeneracy between p(x,y) and p(z) orbitals and leads to band inversion between cation pz and anion px,y orbitals. This crystal field effect induced topological phase offers a new strategy to find and design other atomically thin 2D topological materials.

  18. Modified phase-field-crystal model for solid-liquid phase transitions.

    PubMed

    Guo, Can; Wang, Jincheng; Wang, Zhijun; Li, Junjie; Guo, Yaolin; Tang, Sai

    2015-07-01

    A modified phase-field-crystal (PFC) model is proposed to describe solid-liquid phase transitions by reconstructing the correlation function. The effects of fitting parameters of our modified PFC model on the bcc-liquid phase diagram, numerical stability, and solid-liquid interface properties during planar interface growth are examined carefully. The results indicate that the increase of the correlation function peak width at k=k(m) will enhance the stability of the ordered phase, while the increase of peak height at k=0 will narrow the two-phase coexistence region. The third-order term in the free-energy function and the short wave-length of the correlation function have significant influences on the numerical stability of the PFC model. During planar interface growth, the increase of peak width at k=k(m) will decrease the interface width and the velocity coefficient C, but increase the anisotropy of C and the interface free energy. Finally, the feasibility of the modified phase-field-crystal model is demonstrated with a numerical example of three-dimensional dendritic growth of a body-centered-cubic structure.

  19. Field-driven dynamics of microcapillaries filled with nematic liquid crystal

    NASA Astrophysics Data System (ADS)

    Fu, Fred; Khayyatzadeh, Pouya; Abukhdeir, Nasser M.

    Polymer-dispersed liquid crystal (PDLC) composites have long been a focus of study for their unique electro-optical properties and the feasibility of manufacturing them on a large scale, resulting in applications such as switchable windows. LC domains within PDLCs are typically spheroidal, as opposed to rectangular in LCD technology, and thus exhibit substantially different behaviour in the presence of an external field. In this work, continuum simulations were performed in order to capture the complex formation and electric field-driven switching dynamics of approximations of PDLC domains. A simplified elliptic cylinder (microcapillary) geometry is used and the effects of varying aspect ratio, surface anchoring, and external field strength were studied using the Landau-de Gennes model. The observed nematic formation and reorientation dynamics were found to be governed by the presence and motion of defects within the domain. Aspect ratio was found to strongly influence domain texture by providing regions of high curvature to which defects are attracted. Simulations also predict the presence of a geometry-controlled transition from nematic order enhanced by an external field (low aspect ratio) to nematic order frustrated by an external field (high aspect ratio). This work was made possible by the Natural Sciences and Engineering Research Council of Canada and Compute Ontario.

  20. Exchange and crystal field in Sm-based magnets. II. Phenomenological analysis and density functional calculations

    NASA Astrophysics Data System (ADS)

    Kuz'Min, Michael D.; Steinbeck, Lutz; Richter, Manuel

    2002-02-01

    A technique of determining the exchange field Bex on the 4f shell of Sm atoms in Sm-based magnets is proposed. It makes use of the 4f intermultiplet transition in Sm, observed in inelastic neutron scattering (INS) experiments. The method is used to analyze previously published data for a number of Sm-Fe and Sm-Co intermetallics, for all of which Bex is determined. Additional information on intramultiplet transitions in SmCo5 and Sm2Co17 makes it possible to obtain more accurate Bex values as well as to estimate the leading crystal field parameter (CFP) A02 for these compounds. For the same systems an independent determination of A02 is carried out using published magnetization curves and the Bex values found from the INS spectra. The two ``experimental'' values of A02 (INS and magnetization) agree well. For comparison, theoretical Sm-Co exchange fields and CFP for SmCo5 and Sm2Co17 are obtained from full-potential density-functional calculations. The theoretical A02 are shifted toward more negative values with respect to their experimental counterparts by a few millielectronvolts. The calculated Sm-Co exchange fields are in fair agreement with the experimentally determined values of the total exchange field on Sm, Bex, the weak Sm-Sm exchange interaction being accountable for the remaining small discrepancies.

  1. Effects of in-plane electric fields on the optical properties of cholesteric liquid crystals

    NASA Astrophysics Data System (ADS)

    Rumi, Mariacristina; Tondiglia, Vincent P.; Natarajan, Lalgudi V.; White, Timothy J.; Bunning, Timothy J.

    2013-09-01

    A considerable body of knowledge has been developed on the general behavior of cholesteric liquid crystal (CLC) materials in electric fields. One approach that has been reported to achieve tunability in optical filters based on CLCs with a positive dielectric anisotropy and in the planar homogeneous state involves the application of electric fields perpendicular to the axis of the CLC helix. The field leads to a progressive unwinding of the helix and a corresponding red-shift in the position of the reflection band of the CLC. In this work, a microspectrophotometer was employed to probe the spatial heterogeneity of the optical spectra of the CLC in cells with interdigitated electrodes. We will show that a complex behavior of the Bragg reflection band is obtained in the gap between electrodes for certain parameters of cells with interdigitated electrodes as a function of the applied field. This is ascribed to variations in the field magnitude and direction in the cell, which lead to a spatial variation of helix unwinding.

  2. Double hydrogen bonded ferroelectric liquid crystals: A study of field induced transition (FiT)

    NASA Astrophysics Data System (ADS)

    Vijayakumar, V. N.; Madhu Mohan, M. L. N.

    2009-12-01

    A novel series of chiral hydrogen bonded liquid crystals have been isolated. Hydrogen bond was formed between chiral nonmesogen ingredient levo tartaric acid and mesogenic p-n-alkoxybenzoic acids. Phase diagram was constructed from the transition temperatures obtained by DSC and polarizing optical microscopic (POM) studies. Thermal and electrical properties exhibited by three complexes namely LTA+8BA, LTA+7BA and LTA+5BA were discussed. Salient feature of the present work was the observation of a reentrant smectic ordering in LTA+8BA complex designated as C r∗ phase. This reentrant phenomenon was confirmed by DSC thermograms, optical textures of POM and temperature variation of capacitance and dielectric loss studies. Tilt angle was measured in smectic C ∗ and reentrant smectic C r∗ phases. Another interesting feature of the present investigation was the observation of a field induced transition (FiT) in the LTA+ nBA homologous series. Three threshold field values were noticed which give rise to two new phases (E 1 and E 2) induced by electric field and on further enhancement of the applied field the mesogen behaves like an optical shutter. FiT is reversible in the sense that when applied field is removed the original texture was restored.

  3. Crystal-oriented tungsten-bronze type ceramics prepared by a rotating magnetic field

    NASA Astrophysics Data System (ADS)

    Tanaka, S.; Doshida, Y.; Shimizu, H.; Furushima, R.; Uematsu, K.

    2011-03-01

    Forming and sintering of c-axis-oriented Sr2NaNb5O15 (SNN) ceramics were examined. Particle-oriented SNN was fabricated by using a rotating high magnetic field and subsequent sintering without magnetic field. SNN ceramics are tungsten-bronze-type ferroelectric materials with a tetragonal crystal system. The diamagnetic susceptibilities of the c-axis are smaller than that of the a- and b-axis (χc < χa,b < 0). SNN powder was prepared by conventional solid-state reaction. The synthesized powder was mixed with distilled water and a dispersant by using ball milling to give a slurry with solid loading of 30 vol%. The slurry was poured into a plastic mold and this was placed in a 10Tesla magnetic field in a superconducting magnet. The mold was rotated at 30 rpm while the slurry dried at room temperature. The resulting powder compact with a columnar shape was heated at 5 K/min to 1473 K, held for 6 h, and then heated at 1525 K for 2 h to prevent exaggerated grain growth. XRD patterns showed that c-axis-oriented SNN polycrystalline ceramics were produced in the presence of the rotating magnetic field. In XRD patterns viewed from the top surface of the sintered specimens, peaks from the c-planes of the crystal, such as 001 and 002, were very strong. Diffraction peaks which were very strong in the ceramics, such as 320 and 410, were absent in the specimen. Oriented microstructure was developed well by sintering. Grain-growth along to c-axis was observed in the SNN ceramics heated at 1525 K.

  4. MAS Level-1B HDF Data and Derived Products during the CRYSTAL-FACE Field Campaign

    NASA Astrophysics Data System (ADS)

    Fitzgerald, M.; Dominguez, R.; Arnold, T.; Moody, E.

    2002-12-01

    During July 2002, the Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE) field campaign was conducted to study the radiative properties of tropical cirrus clouds. The field experiment was a cooperative effort of several government and university organizations, and involved coordination of 6 research aircraft and various ground-based instrumentation. During CRYSTAL-FACE, the MODIS Airborne Simulator (MAS), a 50-channel scanning radiometer was flown onboard the NASA ER-2 aircraft. The MAS was the only visible and infrared imaging instrument flown during the campaign. The purpose of this presentation is to demonstrate how collected raw MAS data is processed "in the field" to Level-1B HDF format and subsequent "science" products were then generated and used. MAS data was collected on all 13 ER-2 missions. All straight and level flight data were processed from raw data to level-1b (geo-located and calibrated to radiance at sensor) and written into a HDF file format. These preliminary data products were generated within 48-hours of data collection. In total 84 gigabytes of MAS data were processed to a Level-1B, HDF format. The MODIS science team generated several derived post processed and level-2 products, which utilized this MAS Level-1b HDF data. These field-derived products included a ecosystem map, several cloud phase determination models and brightness temperature. Later processing with this MAS HDF data product will be used for determination of cloud optical thickness and cloud particle radius. The MAS level-1b HDF data results will ultimately be compared with MODIS HDF data products for validation and comparison.

  5. A three-dimensional phase field model coupled with lattice kinetics solver for modeling crystal growth in furnaces with accelerated crucible rotation and traveling magnetic field

    SciTech Connect

    Lin, Guang; Bao, Jie; Xu, Zhijie

    2014-11-01

    In this study, which builds on other related work, we present a new three-dimensional numerical model for crystal growth in a vertical solidification system. This model accounts for buoyancy, accelerated crucible rotation technique (ACRT), and traveling magnetic field (TMF) induced convective flow and their effect on crystal growth and the chemical component's transport process. The evolution of the crystal growth interface is simulated using the phase field method. A semi-implicit lattice kinetics solver based on the Boltzmann equation is employed to model the unsteady incompressible flow. A one-way coupled concentration transport model is used to simulate the component fraction variation in both the liquid and solid phases, which can be used to check the quality of the crystal growth.

  6. A Photonic Crystal Magnetic Field Sensor Using a Shoulder-Coupled Resonant Cavity Infiltrated with Magnetic Fluid.

    PubMed

    Su, Delong; Pu, Shengli; Mao, Lianmin; Wang, Zhaofang; Qian, Kai

    2016-12-16

    A kind of photonic crystal magnetic field sensor is proposed and investigated numerically. The shoulder-coupled resonant cavity is introduced in the photonic crystal, which is infiltrated with magnetic fluid. Through monitoring the shift of resonant wavelength, the magnetic field sensing is realized. According to the designed infiltration schemes, both the magnetic field sensitivity and full width at half maximum increase with the number of infiltrated air holes. The figure of merit of the structure is defined to evaluate the sensing performance comprehensively. The best structure corresponding to the optimal infiltration scheme with eight air holes infiltrated with magnetic fluid is obtained.

  7. Photonic crystal fiber injected with Fe{sub 3}O{sub 4} nanofluid for magnetic field detection

    SciTech Connect

    Thakur, Harneet V.; Nalawade, Sandipan M.; Gupta, Swati; Kitture, Rohini; Kale, S. N.

    2011-10-17

    We report a magnetic field sensor having advantages of both photonic crystal fiber and optofluidics, combining them on a single platform by infiltrating small amount of Fe{sub 3}O{sub 4} magnetic optofluid/nanofluid in cladding holes of polarization-maintaining photonic crystal fiber. We demonstrated that magnetic field of few mT can be easily and very well detected with higher sensitivity of 242 pm/mT. The change in the birefringence values has been correlated to the response of nanofluid to applied field.

  8. Effects of the biaxial transverse crystal-field on the phase diagrams of a spin-1 nanowire

    NASA Astrophysics Data System (ADS)

    Magoussi, H.; Zaim, A.; Boughrara, M.; Kerouad, M.

    2016-09-01

    By using the effective field theory based on a probability distribution method, the phase diagrams and the magnetic properties of an Ising nanowire in the presence of the biaxial transverse crystal-field are investigated. The effects of the biaxial transverse crystal field, the interfacial coupling and the exchange interaction in the surface on the phase diagram, the magnetization and the internal energy are examined. Some characteristic phenomena are found such as the tricritical behavior, the critical end point and the re-entrant phenomenon.

  9. A Photonic Crystal Magnetic Field Sensor Using a Shoulder-Coupled Resonant Cavity Infiltrated with Magnetic Fluid

    PubMed Central

    Su, Delong; Pu, Shengli; Mao, Lianmin; Wang, Zhaofang; Qian, Kai

    2016-01-01

    A kind of photonic crystal magnetic field sensor is proposed and investigated numerically. The shoulder-coupled resonant cavity is introduced in the photonic crystal, which is infiltrated with magnetic fluid. Through monitoring the shift of resonant wavelength, the magnetic field sensing is realized. According to the designed infiltration schemes, both the magnetic field sensitivity and full width at half maximum increase with the number of infiltrated air holes. The figure of merit of the structure is defined to evaluate the sensing performance comprehensively. The best structure corresponding to the optimal infiltration scheme with eight air holes infiltrated with magnetic fluid is obtained. PMID:27999254

  10. Synthesis and superconducting properties of FeTe1-xSe Single Crystals under high magnetic fields

    NASA Astrophysics Data System (ADS)

    Gebre, Tesfaye; Li, G.; Whalen, J.; Conner, B.; Kostov, M.; Siegrist, T.; Balicas, L.

    2011-03-01

    Single crystals of superconductor FeTe 1-x Se x (0.1 <= x <= 0.5) were synthesized using optical floating zone, Bridgeman technique, and solid stat reaction. The samples were synthesized under various temperature gradients and cooling rates. Crystals were characterized via EDX, X-ray scattering, magnetization and transport measurements. Upper critical fields Hc 2 as estimated through the Werthamer-Hohenberg-Helfand (WHH) formalism indicate that these materials strongly surpass the weak coupling Pauli limiting field indicating that the shape of their phase diagram under field is essentially controlled by the Pauli effect. Annealing, leads to a metallic temperature dependence of the resistivity, and to sharper superconducting transitions. Despite the relatively small increase in single crystallinity, as quantified by single crystal x-ray diffraction measurements, we observe a different phase diagram under high magnetic fields when compared to non-annealed samples.

  11. Tunable Lamb wave band gaps in two-dimensional magnetoelastic phononic crystal slabs by an applied external magnetostatic field.

    PubMed

    Zhou, Changjiang; Sai, Yi; Chen, Jiujiu

    2016-09-01

    This paper theoretically investigates the band gaps of Lamb mode waves in two-dimensional magnetoelastic phononic crystal slabs by an applied external magnetostatic field. With the assumption of uniformly oriented magnetization, an equivalent piezomagnetic material model is used. The effects of magnetostatic field on phononic crystals are considered carefully in this model. The numerical results indicate that the width of the first band gap is significantly changed by applying the external magnetic field with different amplitude, and the ratio between the maximum and minimum gap widths reaches 228%. Further calculations demonstrate that the orientation of the magnetic field obviously affects the width and location of the first band gap. The contactless tunability of the proposed phononic crystal slabs shows many potential applications of vibration isolation in engineering. Copyright © 2016 Elsevier B.V. All rights reserved.

  12. Investigation of the linear and second-order nonlinear optical properties of molecular crystals within the local field theory.

    PubMed

    Seidler, Tomasz; Stadnicka, Katarzyna; Champagne, Benoît

    2013-09-21

    In this paper it is shown that modest calculations combining first principles evaluations of the molecular properties with electrostatic interaction schemes to account for the crystal environment effects are reliable for predicting and interpreting the experimentally measured electric linear and second-order nonlinear optical susceptibilities of molecular crystals within the experimental error bars. This is illustrated by considering two molecular crystals, namely: 2-methyl-4-nitroaniline and 4-(N,N-dimethylamino)-3-acetamidonitrobenzene. Three types of surrounding effects should be accounted for (i) the polarization due to the surrounding molecules, described here by static electric fields originating from their electric dipoles or charge distributions, (ii) the intermolecular interactions, which affect the geometry and particularly the molecular conformation, and (iii) the screening of the external electric field by the constitutive molecules. This study further highlights the role of electron correlation on the linear and nonlinear responses of molecular crystals and the challenge of describing frequency dispersion.

  13. Ferroelectric Single-Crystal Gated Graphene/Hexagonal-BN/Ferroelectric Field-Effect Transistor.

    PubMed

    Park, Nahee; Kang, Haeyong; Park, Jeongmin; Lee, Yourack; Yun, Yoojoo; Lee, Jeong-Ho; Lee, Sang-Goo; Lee, Young Hee; Suh, Dongseok

    2015-11-24

    The effect of a ferroelectric polarization field on the charge transport in a two-dimensional (2D) material was examined using a graphene monolayer on a hexagonal boron nitride (hBN) field-effect transistor (FET) fabricated using a ferroelectric single-crystal substrate, (1-x)[Pb(Mg1/3Nb2/3)O3]-x[PbTiO3] (PMN-PT). In this configuration, the intrinsic properties of graphene were preserved with the use of an hBN flake, and the influence of the polarization field from PMN-PT could be distinguished. During a wide-range gate-voltage (VG) sweep, a sharp inversion of the spontaneous polarization affected the graphene channel conductance asymmetrically as well as an antihysteretic behavior. Additionally, a transition from antihysteresis to normal ferroelectric hysteresis occurred, depending on the V(G) sweep range relative to the ferroelectric coercive field. We developed a model to interpret the complex coupling among antihysteresis, current saturation, and sudden conductance variation in relation with the ferroelectric switching and the polarization-assisted charge trapping, which can be generalized to explain the combination of 2D structured materials with ferroelectrics.

  14. Terahertz probes of magnetic field induced spin reorientation in YFeO{sub 3} single crystal

    SciTech Connect

    Lin, Xian; Jiang, Junjie; Ma, Guohong; Jin, Zuanming; Wang, Dongyang; Tian, Zhen; Han, Jiaguang; Cheng, Zhenxiang

    2015-03-02

    Using the terahertz time-domain spectroscopy, we demonstrate the spin reorientation of a canted antiferromagnetic YFeO{sub 3} single crystal, by evaluating the temperature and magnetic field dependence of resonant frequency and amplitude for the quasi-ferromagnetic (FM) and quasi-antiferromagnetic modes (AFM), a deeper insight into the dynamics of spin reorientation in rare-earth orthoferrites is established. Due to the absence of 4f-electrons in Y ion, the spin reorientation of Fe sublattices can only be induced by the applied magnetic field, rather than temperature. In agreement with the theoretical predication, the frequency of FM mode decreases with magnetic field. In addition, an obvious step of spin reorientation phase transition occurs with a relatively large applied magnetic field of 4 T. By comparison with the family members of RFeO{sub 3} (R = Y{sup 3+} or rare-earth ions), our results suggest that the chosen of R would tailor the dynamical rotation properties of Fe ions, leading to the designable spin switching in the orthoferrite antiferromagnetic systems.

  15. In Field Detection of Biologicals in Human Blood Serum, Saliva and Urine Using Pan Coated Quartz Crystals

    DTIC Science & Technology

    1996-10-01

    removed and stored at 40 C for use in assays. 11 inhibit these proteins. Other enzymes, such as a - amylase , are also found in abundance in saliva . This...Detection of Biologicals in Human Blood Serum, Saliva and Urine Using Pan Coated Quartz Crystals PRINCIPAL INVESTIGATOR: Robert Carter CONTRACTING...TITLE AND SUBTITLE In Field Detection of Biologicals in 5. FUNDING NUMBERS Human Blood Serum, Saliva and Urine Using Pan Coated Quartz Crystals

  16. Beam Collapse and Polarization Self-Modulation in an Optically Active Photorefractive Crystal in an Alternating Electric Field

    NASA Astrophysics Data System (ADS)

    Fuentes-Hernández, C. A.; Khomenko, A. V.

    1999-08-01

    Consistent experimental and numerical simulation studies of the propagation of a one-dimensional Gaussian beam in optically active BSO crystal in the presence of an alternating external field are presented. We have observed three forms of the beam evolution: (i) polarization-dependent self-bending, in which the crystal acts as a nonlinear polarization beam splitter; (ii) spatially nonuniform self-modulation of the state of polarization; (iii) formation of narrow inclined waveguides within the Gaussian beam. The effects of optical activity and the crystal orientation are discussed.

  17. Phase-field crystal approach for modeling the role of microstructure in multiferroic composite materials

    NASA Astrophysics Data System (ADS)

    Seymour, Matthew; Sanches, F.; Elder, Ken; Provatas, Nikolas

    2015-11-01

    This paper introduces a phase-field crystal (PFC) approach that couples the atomic-scale PFC density field to order parameters describing ferromagnetic and ferroelectric ordering, as well to a solute impurity field. This model extends the magnetic PFC model introduced by Faghihi et al. [N. Faghihi, Ph.D. Thesis, The University of Western Ontario, 2012; N. Faghihi, N. Provatas, K. R. Elder, M. Grant, and M. Karttunen, Phys. Rev. E 88, 032407 (2013), 10.1103/PhysRevE.88.032407] to incorporate polarization and concentration fields, as well as anisotropic ordering of the magnetization and polarization fields as determined by the local crystalline orientation. Magnetoelectric coupling is incorporated through the elastic coupling. Analytic calculations for a body centered-cubic (BCC) system are presented to illustrate that the model reduces to the standard multiferroic phase-field models when only a single crystal is considered. Two special cases of the model are then studied, the first focusing on magnetocrystalline interactions in a system described by the two-point correlation function of the XPFC model developed by Greenwood et al. [M. Greenwood, N. Provatas, and J. Rottler, Phys. Rev. Lett. 105, 045702 (2010), 10.1103/PhysRevLett.105.045702; M. Greenwood, J. Rottler, and N. Provatas, Phys. Rev. E 83, 031601 (2011), 10.1103/PhysRevE.83.031601], and the second focusing on electrocrystalline interactions in a system described by the original PFC kernel developed by Elder et al. K. R. Elder, M. Katakowski, M. Haataja, and M. Grant, Phys. Rev. Lett. 88, 245701 (2002), 10.1103/PhysRevLett.88.245701; K. R. Elder and M. Grant, Phys. Rev. E 70, 051605 (2004), 10.1103/PhysRevE.70.051605]. We examine the small deformation properties of these two realizations of the model . Numerical simulations are performed to illustrate how magnetocrystalline coupling can be exploited to design a preferential grain texture and how defects and grain boundaries influence the ferroelectric

  18. Wide-field imaging of birefringent synovial fluid crystals using lens-free polarized microscopy for gout diagnosis

    PubMed Central

    Zhang, Yibo; Lee, Seung Yoon Celine; Zhang, Yun; Furst, Daniel; Fitzgerald, John; Ozcan, Aydogan

    2016-01-01

    Gout is a form of crystal arthropathy where monosodium urate (MSU) crystals deposit and elicit inflammation in a joint. Diagnosis of gout relies on identification of MSU crystals under a compensated polarized light microscope (CPLM) in synovial fluid aspirated from the patient’s joint. The detection of MSU crystals by optical microscopy is enhanced by their birefringent properties. However, CPLM partially suffers from the high-cost and bulkiness of conventional lens-based microscopy, and its relatively small field-of-view (FOV) limits the efficiency and accuracy of gout diagnosis. Here we present a lens-free polarized microscope which adopts a novel differential and angle-mismatched polarizing optical design achieving wide-field and high-resolution holographic imaging of birefringent objects with a color contrast similar to that of a standard CPLM. The performance of this computational polarization microscope is validated by imaging MSU crystals made from a gout patient’s tophus and steroid crystals used as negative control. This lens-free polarized microscope, with its wide FOV (>20 mm2), cost-effectiveness and field-portability, can significantly improve the efficiency and accuracy of gout diagnosis, reduce costs, and can be deployed even at the point-of-care and in resource-limited clinical settings. PMID:27356625

  19. Wide-field imaging of birefringent synovial fluid crystals using lens-free polarized microscopy for gout diagnosis

    NASA Astrophysics Data System (ADS)

    Zhang, Yibo; Lee, Seung Yoon Celine; Zhang, Yun; Furst, Daniel; Fitzgerald, John; Ozcan, Aydogan

    2016-06-01

    Gout is a form of crystal arthropathy where monosodium urate (MSU) crystals deposit and elicit inflammation in a joint. Diagnosis of gout relies on identification of MSU crystals under a compensated polarized light microscope (CPLM) in synovial fluid aspirated from the patient’s joint. The detection of MSU crystals by optical microscopy is enhanced by their birefringent properties. However, CPLM partially suffers from the high-cost and bulkiness of conventional lens-based microscopy, and its relatively small field-of-view (FOV) limits the efficiency and accuracy of gout diagnosis. Here we present a lens-free polarized microscope which adopts a novel differential and angle-mismatched polarizing optical design achieving wide-field and high-resolution holographic imaging of birefringent objects with a color contrast similar to that of a standard CPLM. The performance of this computational polarization microscope is validated by imaging MSU crystals made from a gout patient’s tophus and steroid crystals used as negative control. This lens-free polarized microscope, with its wide FOV (>20 mm2), cost-effectiveness and field-portability, can significantly improve the efficiency and accuracy of gout diagnosis, reduce costs, and can be deployed even at the point-of-care and in resource-limited clinical settings.

  20. Controlling electromagnetic fields using periodic structures: Gratings, metamaterials, and photonic crystals

    NASA Astrophysics Data System (ADS)

    Memarian, Mohammad

    This thesis presents novel devices and techniques that enable new methods for enhancement, concentration, refraction, shaping, collimation, and directive beaming of electromagnetic fields. These unprecedented methods to control electromagnetic fields are achieved by exploring and harnessing the unique wave-interactions in periodic gratings, metamaterials, and photonic crystals, with emphasis on Epsilon-Near-Zero (ENZ) metamaterials and zero-index media. The presented solutions impact a wide variety of applications ranging from microwave to optical frequencies. A discovery of dramatic radiation enhancement of an invisible array of sources next to a sub-wavelength periodic metal strip grating is reported, both theoretically and experimentally. The phenomenon is first systematically theorized by introducing the 'spectral impulse response' approach for the aperiodic excitation problem, followed by the 'spectral array factor' approach for designing the near-field of array of sources. Such radiation enhancement has applications in sensing, detection, and accurate measurement of distance. The shaping and collimation of radiation of a simple dipole source near or buried inside a general anisotropic ENZ half-space is then systematically studied using the Lorentz reciprocity method. Various elliptic and hyperbolic anisotropic ENZ media are considered, showing how the air-side radiation can be enhanced and shaped using certain ENZs. A novel device and technique is proposed for collecting, refracting and concentrating incident waves into an area of high power concentration, at extremely short distances. This flat low-profile light-concentrator comprises a hetero-junction of anisotropic ENZ metamaterials (hyperbolic or elliptic), and is realized with plasmonic layered media at optical frequencies. By harnessing an extremely oblique refraction process in ENZs, the light-concentrator significantly outperforms the size requirements of existing thick high curvature lenses, useful

  1. A statistical analysis of the elastic distortion and dislocation density fields in deformed crystals

    NASA Astrophysics Data System (ADS)

    Mohamed, Mamdouh S.; Larson, Bennett C.; Tischler, Jonathan Z.; El-Azab, Anter

    2015-09-01

    The statistical properties of the elastic distortion fields of dislocations in deforming crystals are investigated using the method of discrete dislocation dynamics to simulate dislocation structures and dislocation density evolution under tensile loading. Probability distribution functions (PDF) and pair correlation functions (PCF) of the simulated internal elastic strains and lattice rotations are generated for tensile strain levels up to 0.85%. The PDFs of simulated lattice rotation are compared with sub-micrometer resolution three-dimensional X-ray microscopy measurements of rotation magnitudes and deformation length scales in 1.0% and 2.3% compression strained Cu single crystals to explore the linkage between experiment and the theoretical analysis. The statistical properties of the deformation simulations are analyzed through determinations of the Nye and Kröner dislocation density tensors. The significance of the magnitudes and the length scales of the elastic strain and the rotation parts of dislocation density tensors are demonstrated, and their relevance to understanding the fundamental aspects of deformation is discussed.

  2. A statistical analysis of the elastic distortion and dislocation density fields in deformed crystals

    DOE PAGES

    Mohamed, Mamdouh S.; Larson, Bennett C.; Tischler, Jonathan Z.; ...

    2015-05-18

    The statistical properties of the elastic distortion fields of dislocations in deforming crystals are investigated using the method of discrete dislocation dynamics to simulate dislocation structures and dislocation density evolution under tensile loading. Probability distribution functions (PDF) and pair correlation functions (PCF) of the simulated internal elastic strains and lattice rotations are generated for tensile strain levels up to 0.85%. The PDFs of simulated lattice rotation are compared with sub-micrometer resolution three-dimensional X-ray microscopy measurements of rotation magnitudes and deformation length scales in 1.0% and 2.3% compression strained Cu single crystals to explore the linkage between experiment and the theoreticalmore » analysis. The statistical properties of the deformation simulations are analyzed through determinations of the Nye and Kr ner dislocation density tensors. The significance of the magnitudes and the length scales of the elastic strain and the rotation parts of dislocation density tensors are demonstrated, and their relevance to understanding the fundamental aspects of deformation is discussed.« less

  3. A statistical analysis of the elastic distortion and dislocation density fields in deformed crystals

    SciTech Connect

    Mohamed, Mamdouh S.; Larson, Bennett C.; Tischler, Jonathan Z.; El-Azab, Anter

    2015-05-18

    The statistical properties of the elastic distortion fields of dislocations in deforming crystals are investigated using the method of discrete dislocation dynamics to simulate dislocation structures and dislocation density evolution under tensile loading. Probability distribution functions (PDF) and pair correlation functions (PCF) of the simulated internal elastic strains and lattice rotations are generated for tensile strain levels up to 0.85%. The PDFs of simulated lattice rotation are compared with sub-micrometer resolution three-dimensional X-ray microscopy measurements of rotation magnitudes and deformation length scales in 1.0% and 2.3% compression strained Cu single crystals to explore the linkage between experiment and the theoretical analysis. The statistical properties of the deformation simulations are analyzed through determinations of the Nye and Kr ner dislocation density tensors. The significance of the magnitudes and the length scales of the elastic strain and the rotation parts of dislocation density tensors are demonstrated, and their relevance to understanding the fundamental aspects of deformation is discussed.

  4. Temperature and field induced strain measurements in single crystal Gd5Si2Ge2

    DOE PAGES

    McCall, S. K.; Nersessian, N.; Carman, G. P.; ...

    2016-03-29

    The first-order magneto-structural transformation that occurs in Gd5Si2Ge2 near room temperature makes it a strong candidate for many energy harvesting applications. Understanding the single crystal properties is crucial for allowing simulations of device performance. In this study, magnetically and thermally induced transformation strains were measured in a single crystal of Gd5Si2.05Ge1.95 as it transforms from a high-temperature monoclinic paramagnet to a lower-temperature orthorhombic ferromagnet. Thermally induced transformation strains of –8500 ppm, +960 ppm and +1800 ppm, and magnetically induced transformation strains of –8500 ppm, +900 ppm and +2300 ppm were measured along the a, b and c axes, respectively. Furthermore,more » using experimental data coupled with general thermodynamic considerations, a universal phase diagram was constructed showing the transition from the monoclinic to the orthorhombic phase as a function of temperature and magnetic field.« less

  5. Phase field crystal modeling as a unified atomistic approach to defect dynamics

    NASA Astrophysics Data System (ADS)

    Berry, Joel; Provatas, Nikolas; Rottler, Jörg; Sinclair, Chad W.

    2014-06-01

    Material properties controlled by evolving defect structures, such as mechanical response, often involve processes spanning many length and time scales which can not be modeled using a single approach. We present a variety of results that demonstrate the ability of phase field crystal (PFC) models to describe complex defect evolution phenomena on atomistic length scales and over long, diffusive time scales. Primary emphasis is given to the unification of conservative and nonconservative dislocation creation mechanisms in three-dimensional fcc and bcc materials. These include Frank-Read-type glide mechanisms involving closed dislocation loops or grain boundaries as well as Bardeen-Herring-type climb mechanisms involving precipitates, inclusions, and/or voids. Both source classes are naturally and simultaneously captured at the atomistic level by PFC descriptions, with arbitrarily complex defect configurations, types, and environments. An unexpected dipole-to-quadrupole source transformation is identified, as well as various complex geometrical features of loop nucleation via climb from spherical particles. Results for the strain required to nucleate a dislocation loop from such a particle are in agreement with analytic continuum theories. Other basic features of fcc and bcc dislocation structure and dynamics are also outlined, and initial results for dislocation-stacking fault tetrahedron interactions are presented. These findings together highlight various capabilities of the PFC approach as a coarse-grained atomistic tool for the study of three-dimensional crystal plasticity.

  6. Field and temperature dependences of anisotropic magnetic susceptibility of CaNdAlO{sub 4} crystal

    SciTech Connect

    Fink-Finowicki, J.; Puzniak, R.; Baran, M.; Byszewski, P.; Gutowski, M.; Szymczak, H.; Pajaczkowska, A. |

    1994-03-01

    The measurements of temperature dependences of magnetic susceptibility both in (a-b) plane, and along c-axis of CaNdAlO{sub 4} single crystal have been performed. The strong uniaxial anisotropy of magnetic properties has been found. The low field susceptibility along c-axis is well described by the Curie law, while the susceptibility in (a-b) plane can be described by Curie-Weiss law suggesting the presence of antiferromagnetic interactions between neodymium ions. However, the anisotropy of magnetic susceptibility can also appear to be due to crystal field acting on magnetic ions in a system without any exchange interaction. The successful description of experimental data was done in frames of the crystal field approximation.

  7. Field induced phase transitions and energy harvesting performance of (Pb,La)(Zr,Sn,Ti)O3 single crystal

    NASA Astrophysics Data System (ADS)

    Zhuo, Fangping; Li, Qiang; Li, Yuanyuan; Gao, Jinghan; Yan, Qingfeng; Zhang, Yiling; Xi, Xiaoqing; Chu, Xiangcheng; Cao, Wenwu

    2017-02-01

    (Pb,La)(Zr,Sn,Ti)O3 (PLZST) single crystals with composition close to the morphotropic phase boundary had been grown by the flux method. The antiferroelectric-ferroelectric phase switching electric field was 0.8 kV/mm. Temperature-dependent dielectric and polarization versus electric field hysteresis loops revealed that the electric field induced ferroelectric phase could transform back into the antiferroelectric phase at depolarization temperature (145 °C). An enhanced pyroelectric coefficient value of 1.46 μC/cm2/K was obtained at 145 °C, which is several times larger than that of conventional pyroelectric materials. Furthermore, multiple peak pyroelectric responses and an enhanced harvested energy density value of 0.4 J/cm3 were achieved in the PLZST crystal. The enhanced harvested energy density and multiple peak pyroelectric responses make the PLZST crystal a promising candidate for high sensitive temperature sensors and energy conversion technologies.

  8. First and second order numerical methods based on a new convex splitting for phase-field crystal equation

    NASA Astrophysics Data System (ADS)

    Shin, Jaemin; Lee, Hyun Geun; Lee, June-Yub

    2016-12-01

    The phase-field crystal equation derived from the Swift-Hohenberg energy functional is a sixth order nonlinear equation. We propose numerical methods based on a new convex splitting for the phase-field crystal equation. The first order convex splitting method based on the proposed splitting is unconditionally gradient stable, which means that the discrete energy is non-increasing for any time step. The second order scheme is unconditionally weakly energy stable, which means that the discrete energy is bounded by its initial value for any time step. We prove mass conservation, unique solvability, energy stability, and the order of truncation error for the proposed methods. Numerical experiments are presented to show the accuracy and stability of the proposed splitting methods compared to the existing other splitting methods. Numerical tests indicate that the proposed convex splitting is a good choice for numerical methods of the phase-field crystal equation.

  9. Large field enhancement obtained by combining Fabry–Perot resonance and Rayleigh anomaly in photonic crystal slabs

    NASA Astrophysics Data System (ADS)

    Dossou, Kokou B.

    2017-04-01

    By applying the properties of Fabry–Perot resonance and Rayleigh anomaly, we have shown that a photonic crystal slab can scatter the light from an incident plane wave into a diffracted light with a very large reflection or transmission coefficient. The enhanced field is either a propagating diffracted wave (with a grazing angle of diffraction) or a weakly evanescent diffracted wave, so it can be particularly useful for applications requiring an enhanced propagating field (or an enhanced field with a low attenuation). An efficient effective medium technique is developed for the design of the resonant photonic crystal slabs. Numerical simulations have shown that photonic crystal slabs with low index contrast, such as the ones found in the cell wall of diatoms, can enhance the intensity of the incident light by four orders of magnitude.

  10. Inductive crystal field control in layered metal oxides with correlated electrons

    DOE PAGES

    Balachandran, P. V.; Cammarata, A.; Nelson-Cheeseman, B. B.; ...

    2014-07-25

    Here, we show that the NiO6 crystal field energies can be tailored indirectly via heterovalent A cation ordering in layered (La,A) NiO4 Ruddlesden-Popper (RP) oxides, where A = Sr, Ca, or Ba, using density functional calculations. We leverage as a driving force the electrostatic interactions between charged [LaO]1+ and neutral [AO]0 planes to inductively tune the Ni-O bond distortions, without intentional doping or epitaxial strain, altering the correlated d-orbital energies. We use this strategy to design cation ordered LaCaNiO4 and LaBaNiO4 with distortions favoring enhanced Ni eg orbital polarization, and find local electronic structure signatures analogous to those in RPmore » La-cuprates, i.e., parent phases of the high-temperature superconducting oxides.« less

  11. Photonic crystal membrane reflectors by magnetic field-guided metal-assisted chemical etching

    NASA Astrophysics Data System (ADS)

    Balasundaram, Karthik; Mohseni, Parsian K.; Shuai, Yi-Chen; Zhao, Deyin; Zhou, Weidong; Li, Xiuling

    2013-11-01

    Metal-assisted chemical etching (MacEtch) is a simple etching method that uses metal as the catalyst for anisotropic etching of semiconductors. However, producing nano-structures using MacEtch from discrete metal patterns, in contrast to interconnected ones, has been challenging because of the difficulties in keeping the discrete metal features in close contact with the semiconductor. We report the use of magnetic field-guided MacEtch (h-MacEtch) to fabricate periodic nanohole arrays in silicon-on-insulator (SOI) wafers for high reflectance photonic crystal membrane reflectors. This study demonstrates that h-MacEtch can be used in place of conventional dry etching to produce ordered nanohole arrays for photonic devices.

  12. First and second order operator splitting methods for the phase field crystal equation

    NASA Astrophysics Data System (ADS)

    Lee, Hyun Geun; Shin, Jaemin; Lee, June-Yub

    2015-10-01

    In this paper, we present operator splitting methods for solving the phase field crystal equation which is a model for the microstructural evolution of two-phase systems on atomic length and diffusive time scales. A core idea of the methods is to decompose the original equation into linear and nonlinear subequations, in which the linear subequation has a closed-form solution in the Fourier space. We apply a nonlinear Newton-type iterative method to solve the nonlinear subequation at the implicit time level and thus a considerably large time step can be used. By combining these subequations, we achieve the first- and second-order accuracy in time. We present numerical experiments to show the accuracy and efficiency of the proposed methods.

  13. Vectorial near-field imaging of a GaN based photonic crystal cavity

    SciTech Connect

    La China, F. Intonti, F.; Caselli, N.; Lotti, F.; Vinattieri, A.; Gurioli, M.; Vico Triviño, N.; Carlin, J.-F.; Butté, R.; Grandjean, N.

    2015-09-07

    We report a full optical deep sub-wavelength imaging of the vectorial components of the electric local density of states for the confined modes of a modified GaN L3 photonic crystal nanocavity. The mode mapping is obtained with a scanning near-field optical microscope operating in a resonant forward scattering configuration, allowing the vectorial characterization of optical passive samples. The optical modes of the investigated cavity emerge as Fano resonances and can be probed without the need of embedded light emitters or evanescent light coupling into the nanocavity. The experimental maps, independently measured in the two in-plane polarizations, turn out to be in excellent agreement with numerical predictions.

  14. Calculated crystal-field parameters for rare-earth impurities in noble metals

    NASA Astrophysics Data System (ADS)

    Steinbeck, Lutz; Richter, Manuel; Eschrig, Helmut; Nitzsche, Ulrike

    1994-06-01

    From first-principles density-functional calculations of the charge distribution the crystal-field (CF) parameters for 4f states of Er and Dy impurities in Ag and Au have been evaluated. The calculations are based on an optimized linear combination of atomic orbitals scheme, where the local-density approximation (LDA) is used for the conduction-electron states, while the localized rare-earth 4f states are treated as ``open core shell.'' As the 4f localization cannot be properly described within LDA, a self-interaction correction for the 4f states is included. In this way, any artificial constraints on the 4f charge density employed in earlier first-principles CF calculations are avoided. The calculated CF parameters agree well with recent neutron scattering data.

  15. Calculations of isothermal elastic constants in the phase-field crystal model

    NASA Astrophysics Data System (ADS)

    Pisutha-Arnond, N.; Chan, V. W. L.; Elder, K. R.; Thornton, K.

    2013-01-01

    The phase-field crystal (PFC) method is an emerging coarse-grained atomistic model that can be used to predict material properties. In this work, we describe procedures for calculating isothermal elastic constants using the PFC method. We find that the conventional procedures used in the PFC method for calculating the elastic constants are inconsistent with those defined from a theory of thermoelasticity of stressed materials. Therefore we present an alternative procedure for calculating the elastic constants that are consistent with the definitions from the thermoelasticity theory, and show that the two procedures result in different predictions. Furthermore, we employ a thermodynamic formulation of stressed solids to quantify the differences between the elastic constants obtained from the two procedures in terms of thermodynamic quantities such as the pressure evaluated at the undeformed state.

  16. Crystal-field levels of terbium in pyrochlore compounds R2Ti2O7

    NASA Astrophysics Data System (ADS)

    Klimin, S. A.

    2015-06-01

    Temperature-dependent infrared spectroscopic study of Tb3+ f-f transitions has been performed for pyrochlore compounds Tb2Ti2O7, (Y0.99Tb0.01)2Ti2O7, and (Gd0.9Tb0.1)2Ti2O7. The peculiarities of terbium absorption were explained taking into account the presence of inversion symmetry at the rare-earth site. The energies of crystal-field levels for the ground Tb3+ multiplet were obtained. Spectral anomaly in Tb2Ti2O7 observed in the temperature range T<20 K was assigned to a strong ion-ion interaction between rare-earth ions, which leads to collective CF excitations in the terbium subsystem.

  17. An adaptive time-stepping strategy for solving the phase field crystal model

    SciTech Connect

    Zhang, Zhengru; Ma, Yuan; Qiao, Zhonghua

    2013-09-15

    In this work, we will propose an adaptive time step method for simulating the dynamics of the phase field crystal (PFC) model. The numerical simulation of the PFC model needs long time to reach steady state, and then large time-stepping method is necessary. Unconditionally energy stable schemes are used to solve the PFC model. The time steps are adaptively determined based on the time derivative of the corresponding energy. It is found that the use of the proposed time step adaptivity cannot only resolve the steady state solution, but also the dynamical development of the solution efficiently and accurately. The numerical experiments demonstrate that the CPU time is significantly saved for long time simulations.

  18. Crystal Field Fluctuations in a Frustrated Pyrochlore Antiferromagnet Tb2Ti2O7.

    NASA Astrophysics Data System (ADS)

    Molavian, Hamid R.; Gingras, Michel J. P.

    2006-03-01

    The antiferromagnetic pyrochlore Tb2Ti2O7 presents a challenging puzzle to experimentalists and theorists studying frustrated magnets. Results from muon spin resonance and neutron scattering experiments for Tb2Ti2O7 reveal a paramagnetic structure down to 50 mK despite an antiferromagnetic Curie-Weiss temperature, θCW=-20 K. Crystal field calculations show that the Tb^3+ ion in Tb2Ti2O7 is a ground state doublet with local <111 > anisotropy and is separated from the first excited doublet state by a gap of 20K. We apply the Rayleigh-Schrodinger method to map the four states problem with exchange and dipole-dipole interactions onto an effective Hamiltonian with two states per ion. We give some properties of this effective Hamiltonian and discuss the possible classical and quantum phases of Tb2Ti2O7.

  19. Interacting spin-wave dispersion relations of ferrimagnetic Heisenberg chains with crystal-field anisotropy

    NASA Astrophysics Data System (ADS)

    Solano-Carrillo, E.; Franco, R.; Silva-Valencia, J.

    2010-11-01

    We study the effect of crystal-field anisotropy on the dispersion relations of mixed-spin (S,s) alternating chains by using the interacting spin-wave theory and the density-matrix renormalization group algorithm. For the easy-plane anisotropy case we find that the spin-wave results fail to describe the ground-state properties of the systems under consideration, whereas for the easy-axis anisotropy regime the method demonstrates a surprising efficiency showing, for example for the system (S,s)=(3/2,1/2), a discrepancy from the density-matrix renormalization group of about 0.0006% for the ground-state energy and of 2% for the sublattice magnetizations.

  20. First and second order operator splitting methods for the phase field crystal equation

    SciTech Connect

    Lee, Hyun Geun; Shin, Jaemin; Lee, June-Yub

    2015-10-15

    In this paper, we present operator splitting methods for solving the phase field crystal equation which is a model for the microstructural evolution of two-phase systems on atomic length and diffusive time scales. A core idea of the methods is to decompose the original equation into linear and nonlinear subequations, in which the linear subequation has a closed-form solution in the Fourier space. We apply a nonlinear Newton-type iterative method to solve the nonlinear subequation at the implicit time level and thus a considerably large time step can be used. By combining these subequations, we achieve the first- and second-order accuracy in time. We present numerical experiments to show the accuracy and efficiency of the proposed methods.

  1. Inductive crystal field control in layered metal oxides with correlated electrons

    SciTech Connect

    Balachandran, P. V.; Cammarata, A.; Rondinelli, J. M.; Nelson-Cheeseman, B. B.; Bhattacharya, A.

    2014-07-01

    We show that the NiO{sub 6} crystal field energies can be tailored indirectly via heterovalent A cation ordering in layered (La,A)NiO{sub 4} Ruddlesden–Popper (RP) oxides, where A = Sr, Ca, or Ba, using density functional calculations. We leverage as a driving force the electrostatic interactions between charged [LaO]{sup 1+} and neutral [AO]{sup 0} planes to inductively tune the Ni–O bond distortions, without intentional doping or epitaxial strain, altering the correlated d-orbital energies. We use this strategy to design cation ordered LaCaNiO{sub 4} and LaBaNiO{sub 4} with distortions favoring enhanced Ni e{sub g} orbital polarization, and find local electronic structure signatures analogous to those in RP La-cuprates, i.e., parent phases of the high-temperature superconducting oxides.

  2. Thermal fluctuations and phase diagrams of the phase-field crystal model with pinning.

    PubMed

    Ramos, J A P; Granato, E; Achim, C V; Ying, S C; Elder, K R; Ala-Nissila, T

    2008-09-01

    We study the influence of thermal fluctuations in the phase diagram of a recently introduced two-dimensional phase field crystal model with an external pinning potential. The model provides a continuum description of pinned lattice systems allowing for both elastic deformations and topological defects. We introduce a nonconserved version of the model and determine the ground-state phase diagram as a function of lattice mismatch and strength of the pinning potential. Monte Carlo simulations are used to determine the phase diagram as a function of temperature near commensurate phases. The results show a rich phase diagram with commensurate, incommensurate, and liquidlike phases with a topology strongly dependent on the type of ordered structure. A finite-size scaling analysis of the melting transition for the c(2x2) commensurate phase shows that the thermal correlation length exponent nu and specific heat behavior are consistent with the Ising universality class as expected from analytical arguments.

  3. Motion of a colloidal particle in a nonuniform director field of a nematic liquid crystal

    NASA Astrophysics Data System (ADS)

    Lee, Beom-Kyu; Kim, Sung-Jo; Lev, Bohdan; Kim, Jong-Hyun

    2017-01-01

    We investigate the dynamics of a single spherical particle immersed in a nematic liquid crystal. A nonuniform director field is imposed on the substrate by a stripe alignment pattern with splay deformation. The particle of homeotropic anchoring at the surface is accompanied by hyperbolic hedgehog or Saturn-ring defects. The particle motion is dependent on the defect structure. We study the two types of motions theoretically and confirm the obtained results experimentally. The particle accompanied by a hyperbolic hedgehog defect is pulled to a deformed region to relax the elastic deformation energy. The motion occurs in the direction heading the hyperbolic hedgehog defect of a particle in a twist region. The position exhibits a weak S-shaped change as a function of time. The particle accompanied by a Saturn-ring defect shows insignificant motion due to its relatively small deformation energy.

  4. Extended depth of field using a liquid crystal annular spatial light modulator.

    PubMed

    Klapp, Iftach; Solodar, Asi; Abdulhalim, Ibrahim

    2014-07-01

    A detailed investigation is presented on the tunable extended depth of field (EDOF) method, proposed recently by Klapp et al. [Opt. Lett.39, 1414 (2014)]. This method is based on temporal multiplexing of phase masks, using an annular liquid crystal spatial light modulator possessing a small number of rings. Examples of 3D simulations used to determine the phase profiles in the pupil plane are presented, as well as more detailed experimental results. Both the experimental and numerical results include comprehensive analysis of contrast dependence on both the spatial spectrum of the object and the amount of defocus. In addition, for the first time, we present the EDOF order inversion in the experimental and simulated data. The results show a profound performance of the proposed system and method.

  5. Comparison of Phase Field Crystal and Molecular Dynamics Simulations for a Shrinking Grain

    SciTech Connect

    Radhakrishnan, Balasubramaniam; Gorti, Sarma B; Nicholson, Don M

    2012-01-01

    The Phase-Field Crystal (PFC) model represents the atomic density as a continuous function, whose spatial distribution evolves at diffusional, rather than vibrational time scales. PFC provides a tool to study defect interactions at the atomistic level but over longer time scales than in molecular dynamics (MD). We examine the behavior of the PFC model with the goal of relating the PFC parameters to physical parameters of real systems, derived from MD simulations. For this purpose we model the phenomenon of the shrinking of a spherical grain situated in a matrix. By comparing the rate of shrinking of the central grain using MD and PFC we obtain a relationship between PFC and MD time scales for processes driven by grain boundary diffusion. The morphological changes in the central grain including grain shape and grain rotation are also examined in order to assess the accuracy of the PFC in capturing the evolution path predicted by MD.

  6. Structure fits the purpose: photonic crystal fibers for evanescent-field surface-enhanced Raman spectroscopy.

    PubMed

    Khaing Oo, Maung Kyaw; Han, Yun; Kanka, Jiri; Sukhishvili, Svetlana; Du, Henry

    2010-02-15

    We report numerical simulation and hyperspectral Raman imaging of three index-guiding solid-core photonic crystal fibers (PCFs) of different air-cladding microstructures to assess their respective potential for evanescent-field Raman spectroscopy, with an emphasis on achieving surface-enhanced Raman scattering (SERS) over the entire fiber length. Suspended-core PCF consisting of a silica core surrounded by three large air channels conjoined by a thin silica web is the most robust of the three SERS-active PCFs, with a demonstrated detection sensitivity of 1x10(-10) M R6G in an aqueous solution of only approximately 7.3 microL sampling volume.

  7. A binary phase field crystal study for liquid phase heteroepitaxial growth

    NASA Astrophysics Data System (ADS)

    Lu, Yanli; Peng, Yingying; Chen, Zheng

    2016-09-01

    The liquid phase heteroepitaxial growth on predefined crystalline substrate is studied with binary phase field crystal (PFC) model. The purpose of this paper focuses on changes of the morphology of epitaxial films, influences of substrate vicinal angles on epitaxial growth, characteristics of islands growth on both sides of the substrate as well. It is found that the morphology of epitaxial films undergoes the following transitions: layer-by-layer growth, islands formation, mismatch dislocations nucleation and climb towards the film-substrate interface. Meanwhile, the density of steps and islands has obviously direct ratio relations with the vicinal angles. Also, preferential regions are found when islands grow on both sides of the substrate. For thinner substrate, the arrangement of islands is more orderly and the appearance of preferential growth is more obvious than that of thicker substrate. Also, the existing of preferential regions is much more valid for small substrate vicinal angles in contrast for big substrate vicinal angles.

  8. The onset of layer undulations in smectic A liquid crystals due to a strong magnetic field

    NASA Astrophysics Data System (ADS)

    Contreras, A.; Garcia-Azpeitia, C.; García-Cervera, C. J.; Joo, S.

    2016-08-01

    We investigate the effect of a strong magnetic field on a three dimensional smectic A liquid crystal. We identify a critical field above which the uniform layered state loses stability; this is associated to the onset of layer undulations. In a previous work García-Cervera and Joo (2012 Arch. Ration. Mech. Anal. 203 1-43), García-Cervera and Joo considered the two dimensional case and analyzed the transition to the undulated state via a simple bifurcation. In dimension n  =  3 the situation is more delicate because the first eigenvalue of the corresponding linearized problem is not simple. We overcome the difficulties inherent to this higher dimensional setting by identifying the irreducible representations for natural actions on the functional that take into account the invariances of the problem thus allowing for reducing the bifurcation analysis to a subspace with symmetries. We are able to describe at least two bifurcation branches, highlighting the richer landscape of energy critical states in the three dimensional setting. Finally, we analyze a reduced two dimensional problem, assuming the magnetic field is very strong, and are able to relate this to a model in micromagnetics studied in Alouges et al (2002 ESAIM Control Optim. Calc. Var. 8 31-68), from where we deduce the periodicity property of minimizers.

  9. A Crystal Field Approach to Orbitally Degenerate SMMs: Beyond the Spin-Only Hamiltonian

    NASA Astrophysics Data System (ADS)

    Bhaskaran, Lakshmi; Marriott, Katie; Murrie, Mark; Hill, Stephen

    Single-Molecule Magnets (SMMs) with large magnetization reversal barriers are promising candidates for high-density information storage. Recently, a large uniaxial magnetic anisotropy was observed for a mononuclear trigonal bipyramidal (TBP) [NiIICl3(Me-abco)2] SMM. High-field EPR studies analyzed on the basis of a spin-only Hamiltonian give ¦D¦>400 cm-1, which is close to the spin-orbit coupling parameter λ = 668 cm-1 for NiII, suggesting an orbitally degenerate ground state. The spin-only description is ineffective in this limit, necessitating the development of a model that includes the orbital moment. Here we describe a phenomenological approach that takes into account a full description of crystal field, electron-electron repulsion and spin-orbit coupling effects on the ground state of a NiII ion in a TBP coordination geometry. The model is in good agreement with the high-field EPR experiments, validating its use for spectroscopic studies of orbitally degenerate molecular nanomagnets. This work was supported by the NSF (DMR-1309463).

  10. Spectroscopy of Charge Carriers and Traps in Field-Doped Single Crystal Organic Semiconductors

    SciTech Connect

    Zhu, Xiaoyang

    2014-12-10

    The proposed research aims to achieve quantitative, molecular level understanding of charge carriers and traps in field-doped crystalline organic semiconductors via in situ linear and nonlinear optical spectroscopy, in conjunction with transport measurements and molecular/crystal engineering. Organic semiconductors are emerging as viable materials for low-cost electronics and optoelectronics, such as organic photovoltaics (OPV), organic field effect transistors (OFETs), and organic light emitting diodes (OLEDs). Despite extensive studies spanning many decades, a clear understanding of the nature of charge carriers in organic semiconductors is still lacking. It is generally appreciated that polaron formation and charge carrier trapping are two hallmarks associated with electrical transport in organic semiconductors; the former results from the low dielectric constants and weak intermolecular electronic overlap while the latter can be attributed to the prevalence of structural disorder. These properties have lead to the common observation of low charge carrier mobilities, e.g., in the range of 10-5 - 10-3 cm2/Vs, particularly at low carrier concentrations. However, there is also growing evidence that charge carrier mobility approaching those of inorganic semiconductors and metals can exist in some crystalline organic semiconductors, such as pentacene, tetracene and rubrene. A particularly striking example is single crystal rubrene (Figure 1), in which hole mobilities well above 10 cm2/Vs have been observed in OFETs operating at room temperature. Temperature dependent transport and spectroscopic measurements both revealed evidence of free carriers in rubrene. Outstanding questions are: what are the structural features and physical properties that make rubrene so unique? How do we establish fundamental design principles for the development of other organic semiconductors of high mobility? These questions are critically important but not comprehensive, as the nature of

  11. Experimental measurement of the near tip strain field in an iron-silicon single crystal

    NASA Astrophysics Data System (ADS)

    Shield, T. W.; Kim, K.-S.

    1994-05-01

    EXPERIMENTAL RESULTS are presented for the plastic deformation field near a crack (200 μm wide notch) tip in an iron-3% silicon single crystal. The specimen was loaded in four point bending and the measurements were made at zero load after extensive plastic deformation had occurred. Results are given for a crack on the (011) plane with its tip along the [01|T] direction. The surface deformation field was measured using moire microscopy and a grating on the specimen surface. The in-plane Almansi strain components have been obtained by digitally processing the moire fringes. A well-structured asymptotic field has been found at a distance of 350-500 μm from the notch tip, where the maximum plastic strain is about 9%. The asymptotic field is observed to be composed of many distinct angular sectors. Three (six symmetric) of these sectors are found to have approximately constant strains. In a fourth (two symmetric) sector, the surface strains are approximately 1/ r singular. Between these sectors there are interconnecting transition sectors. The location of the stress state on the yield surface and the active slip systems in each sector are identified by assuming that the plastic strain rates are normal to a Schmid law yield surface. The slip systems identified in this manner show excellent agreement with direct observations of the slip texture on the surface and dislocation etch pits in the interior of the specimen. The experimental strain measurements also show that the constant strain sectors are regions in which unloading occurs. Because of this unloading, the crack tip stress and deformation state is substantially different from an HRR type field which assumes proportional loading. This strong nonproportional loading is thought to be caused by the presence of material anisotropy. The nonproportional loading also provides a large amount of crack tip shielding that is evidence of a toughening mechanism that results from the presence of material anisotropy.

  12. Dynamics of a director reorientation and optical response of polymer films filled with a liquid crystal under strong electric field

    NASA Astrophysics Data System (ADS)

    Pasechnik, S. V.; Shmeleva, D. V.; Chopik, A. P.; Vakulenko, A. A.; Zakharov, A. V.

    2016-08-01

    The dynamics of the director reorientation in nematic liquid crystals (NLC) confined by cylindrical cavities of porous polymeric films under strong electric field E with has been investigated theoretically. The main attention was paid to the specific mode of field application characterized by abrupt changes of the applied voltage's polarity. In experiments with porous films filled with a liquid crystal 5CB such mode resulted in appearance of strong peak -like decreasing of an optical transparence of the films. Two mechanisms of such unusual response based on assumption of electrically induced motion of ions and overall motion of a liquid were considered and applied to explain experimental results.

  13. Magnetic Order and Crystal Field Excitations in Er2Ru2O7: A Neutron Scattering Study

    SciTech Connect

    Ehlers, Georg; Gardner, Jason

    2009-01-01

    The magnetic pyrochlore Er{sub 2}Ru{sub 2}O{sub 7} has been studied with neutron scattering and susceptibility measurements down to a base temperature of 270 mK. For the low temperature phase in which the Er sublattice orders, new magnetic Bragg peaks are reported which can be indexed with integer (hkl) for a face centered cubic cell. Inelastic measurements reveal a wealth of crystal field levels of the Er ion and a copious amount of magnetic scattering below 15 meV. The three lowest groups of crystal field levels are at 6.7, 9.1 and 18.5 meV.

  14. Crystal field states of Tb3 + in the pyrochlore spin liquid Tb2Ti2O7 from neutron spectroscopy

    NASA Astrophysics Data System (ADS)

    Princep, A. J.; Walker, H. C.; Adroja, D. T.; Prabhakaran, D.; Boothroyd, A. T.

    2015-06-01

    We report time-of-flight neutron scattering measurements of the magnetic spectrum of Tb3 + in Tb2Ti2O7 . The data, which extend up to 120 meV and have calibrated intensity, enable us to consolidate and extend previous studies of the single-ion crystal field spectrum. We successfully refine a model for the crystal field potential in Tb2Ti2O7 without relying on data from other rare-earth titanate pyrochlores, and we confirm that the ground state is a non-Kramers doublet with predominantly |±4 > components. We compare the model critically with earlier models.

  15. Transferability of coarse-grained force field for nCB liquid crystal systems.

    PubMed

    Zhang, Jianguo; Guo, Hongxia

    2014-05-01

    In this paper, the transferability of the coarse-grained (CG) force field originally developed for the liquid crystal (LC) molecule 5CB ( Zhang et al. J. Phys. Chem. B 2012 , 116 , 2075 - 2089 ) was investigated by its homologues 6CB and 8CB molecules. Note that, to construct the 5CB CG force field, we combined the structure-based and thermodynamic quantities-based methods and at the same time attempted to use several fragment molecular systems to derive the CG nonbonded interaction parameters. The resultant 5CB CG force field exhibits a good transferability to some extent. For example, not only the experimental densities, the local packing of atom groups, and the antiparallel arrangements of nearest neighboring molecules, but also the unique LC mesophases as well as the nematic-isotropic phase transition temperatures of 6CB and 8CB were reproduced. Meanwhile, the limitations of this 5CB CG force field were also observed, such as the phase transition from nematic to smectic was postponed to the lower temperature and the resulting smectic phase structure is single-layer-like instead of partially interdigitated bilayer-like as observed in underlying atomistic model. Apparently, more attention should be paid when applying a CG force field to the state point which is quite different from which the force field is explicitly parametrized for. The origin of the above limitations can be potentially traced back to the inherent simplifications and some approximations often adopted in the creation process of CG force field, for example, choosing symmetric CG potentials which do not explicitly include electrostatic interactions and are parametrized by reproducing the target properties of the specific nematic 5CB phase at 300 K and 1 atm, as well as using soft nonbonded potential and excluding torsion barriers. Moreover, although by construction this CG force field could inevitably incorporate both thermodynamic and local structural information on the nematic 5CB phase, the

  16. Twofold and Fourfold Symmetric Anisotropic Magnetoresistance Effect in a Model with Crystal Field

    NASA Astrophysics Data System (ADS)

    Kokado, Satoshi; Tsunoda, Masakiyo

    2015-09-01

    We theoretically study the twofold and fourfold symmetric anisotropic magnetoresistance (AMR) effects of ferromagnets. We here use the two-current model for a system consisting of a conduction state and localized d states. The localized d states are obtained from a Hamiltonian with a spin-orbit interaction, an exchange field, and a crystal field. From the model, we first derive general expressions for the coefficient of the twofold symmetric term (C2) and that of the fourfold symmetric term (C4) in the AMR ratio. In the case of a strong ferromagnet, the dominant term in C2 is proportional to the difference in the partial densities of states (PDOSs) at the Fermi energy (EF) between the dɛ and dγ states, and that in C4 is proportional to the difference in the PDOSs at EF among the dɛ states. Using the dominant terms, we next analyze the experimental results for Fe4N, in which |C2| and |C4| increase with decreasing temperature. The experimental results can be reproduced by assuming that the tetragonal distortion increases with decreasing temperature.

  17. Anisotropy of the local field of the light wave in cholesteric liquid crystals

    NASA Astrophysics Data System (ADS)

    Aver'yanov, E. M.

    2010-05-01

    The experimental values of the L j components of the Lorentz tensor have been obtained for the first time for the quasinematic layer of the cholesteric phase and in the smectic phase A for homologs of cholesteric fatty ethers using the dispersion dependences of the refraction indices for the planar texture of cholesteric liquid crystals (CLCs). The dependence of the L j components on the homolog number, mesophase temperature, the magnitude of birefringence, and the change in the orientational ordering of molecules in the cholesteric phase and at the cholesteric-smectic A phase transition was determined. Isotropization of the Lorentz tensor L and the local field tensor f was found for CLCs when the birefringence of LCs and the anisotropy of molecular polarizability decreased simultaneously. The anisotropy Δ f was found to be negative for the quasinematic layer of CLCs and the smectic phase in the visible range of the spectrum. The values of L j , obtained with known local field models for CLCs and smectics A, gave positive Δ f irrespective of the chemical structure of molecules, optical anisotropy of LCs, and the spectral region, which contradicts to the experiment.

  18. Nonlinear driven response of a phase-field crystal in a periodic pinning potential.

    PubMed

    Achim, C V; Ramos, J A P; Karttunen, M; Elder, K R; Granato, E; Ala-Nissila, T; Ying, S C

    2009-01-01

    We study numerically the phase diagram and the response under a driving force of the phase field crystal model for pinned lattice systems introduced recently for both one- and two-dimensional systems. The model describes the lattice system as a continuous density field in the presence of a periodic pinning potential, allowing for both elastic and plastic deformations of the lattice. We first present results for phase diagrams of the model in the absence of a driving force. The nonlinear response to a driving force on an initially pinned commensurate phase is then studied via overdamped dynamic equations of motion for different values of mismatch and pinning strengths. For large pinning strength the driven depinning transitions are continuous, and the sliding velocity varies with the force from the threshold with power-law exponents in agreement with analytical predictions. Transverse depinning transitions in the moving state are also found in two dimensions. Surprisingly, for sufficiently weak pinning potential we find a discontinuous depinning transition with hysteresis even in one dimension under overdamped dynamics. We also characterize structural changes of the system in some detail close to the depinning transition.

  19. Linear magnetoresistance and zero-field anomalies in HfNiSn single crystals

    NASA Astrophysics Data System (ADS)

    Steinke, Lucia; Kistner-Morris, Jedediah J.; Deng, Haiming; Geschwind, Gayle; Aronson, Meigan C.

    The Half-Heusler compound HfNiSn is probably best known as a candidate material for thermoelectric applications, and studies of its properties have mainly focused on polycrystalline samples and thin films. However, magnetotransport studies of HfNiSn show unusual transport properties like linear magnetoresistance (LMR), where single-crystalline samples of HfNiSn exhibit unexpected LMR at very low fields. In this work, we optimized the solution growth of HfNiSn to obtain high-quality single crystals, where electrical transport measurements show that it is a compensated semimetal below ~ 200 K, where the Hall voltage is zero. At higher temperatures, we see a finite Hall contribution from activated excess carriers. In the semimetallic regime, we observe transport anomalies like resistive signals that strongly depend on contact configuration, and LMR below 5 K. Both low-field DC and low frequency AC magntization measurements show pronounced diamagnetic behavior and the onset of paramagnetism below 4 K. High-frequency diamagnetic screening may be attributed to a decreased skin depth with decreased resistance, but this scenario seems unlikely in HfNiSn since the measured resistance increases steeply at the lowest temperatures This research was supported by the Army Research Office.

  20. Quantum spin-1 anisotropic ferromagnetic Heisenberg model in a crystal field: a variational approach.

    PubMed

    Carvalho, D C; Plascak, J A; Castro, L M

    2013-09-01

    A variational approach based on Bogoliubov inequality for the free energy is employed in order to treat the quantum spin-1 anisotropic ferromagnetic Heisenberg model in the presence of a crystal field. Within the Bogoliubov scheme an improved pair approximation has been used. The temperature-dependent thermodynamic functions have been obtained and provide much better results than the previous simple mean-field scheme. In one dimension, which is still nonintegrable for quantum spin-1, we get the exact results in the classical limit, or near-exact results in the quantum case, for the free energy, magnetization, and quadrupole moment, as well for the transition temperature. In two and three dimensions the corresponding global phase diagrams have been obtained as a function of the parameters of the Hamiltonian. First-order transition lines, second-order transition lines, tricritical and tetracritical points, and critical endpoints have been located through the analysis of the minimum of the Helmholtz free energy and a Landau-like expansion in the approximated free energy. Only first-order quantum transitions have been found at zero temperature. Limiting cases, such as isotropic Heisenberg, Blume-Capel, and Ising models, have been analyzed and compared to previous results obtained from other analytical approaches as well as from Monte Carlo simulations.

  1. Transferability of empirical crystal-field parameters of Ni(II) complexes of different symmetries

    NASA Astrophysics Data System (ADS)

    Beltrán, F. Gómez; Sordo, J. A.; Pueyo, L.

    1982-10-01

    A method of systematic linearization of the crystal-field matrices appropriate for obtaining empirical parameters of transition metal complexes of any symmetry is presented and applied to forty complexes of Ni 2+ of Oh and D4 h symmetries. The method is a generalization of that proposed by L. Pueyo, M. Bermejo, and J. W. Richardson ( J. Solid State Chem.31, 217 (1980)) for complexes of Oh symmetry and incorporates the spin-orbit coupling in a very simple manner. Using this method, classical parameters, such as 10 Dq, and punctual quantities, such as the ligand perturbing charges qi, were obtained for these complexes. The former are transferable within 10% if (a) there are not big changes in the metal-ligand distances and (b) the chemical environments of the ligand atoms are comparable. However, the punctual parameters show variations of 20% or more. Electronic repulsion integrals seem to be nicely transferable by means of addition rules based on the hypothesis of isotropic repulsion in the low-symmetry field. Since one of the fitting parameters is a scaling factor of the Rnl( r) metal function, the process of optimization generates an empirical representation, Rnl( λr), of the locally perturbed metallic state.

  2. Defected-core photonic crystal fiber magnetic field sensor based on Sagnac interferometer

    NASA Astrophysics Data System (ADS)

    Liu, Qiang; Li, Shuguang; Dou, Chao; Wang, Xinyu

    2017-03-01

    A high-sensitivity magnetic field sensor based on photonic crystal fiber (PCF) Sagnac interferometer is proposed by finite element method (FEM). The elliptical-hole PCF is injected with water-based magnetic fluid. The fiber core is introduced in an elliptical hole to act as a defected core. The sensitivities are 13.25 and -14.32 nm/Oe at the wavelength of 1750 and 1780 nm, respectively, as the magnetic field H is 100 Oe. The simulation result shows that the sensitivity is extremely high near the wavelength of 1765 nm, because the group birefringence Bg is about zero at that wavelength. The above theory is examined in a different configuration by the transmission mode. The average sensitivities are 0.265, 1.63 and -1.915 nm/Oe, respectively, for the dip wavelength A, B and C as the fiber length is 5 cm. The detecting window is 60Oe. We also demonstrate that the sensitivity can be greatly enhanced by 1-2 orders of magnitude by introducing a defected core.

  3. Effects of a High Magnetic Field on the Microstructure of Ni-Based Single-Crystal Superalloys During Directional Solidification

    NASA Astrophysics Data System (ADS)

    Xuan, Weidong; Lan, Jian; Liu, Huan; Li, Chuanjun; Wang, Jiang; Ren, Weili; Zhong, Yunbo; Li, Xi; Ren, Zhongming

    2017-08-01

    High magnetic fields are widely used to improve the microstructure and properties of materials during the solidification process. During the preparation of single-crystal turbine blades, the microstructure of the superalloy is the main factor that determines its mechanical properties. In this work, the effects of a high magnetic field on the microstructure of Ni-based single-crystal superalloys PWA1483 and CMSX-4 during directional solidification were investigated experimentally. The results showed that the magnetic field modified the primary dendrite arm spacing, γ' phase size, and microsegregation of the superalloys. In addition, the size and volume fractions of γ/ γ' eutectic and the microporosity were decreased in a high magnetic field. Analysis of variance (ANOVA) results showed that the effect of a high magnetic field on the microstructure during directional solidification was significant ( p < 0.05). Based on both experimental results and theoretical analysis, the modification of microstructure was attributed to thermoelectric magnetic convection occurring in the interdendritic regions under a high magnetic field. The present work provides a new method to optimize the microstructure of Ni-based single-crystal superalloy blades by applying a high magnetic field.

  4. High-magnetic-field transport properties and irreversibility line of single crystals (2212) and ceramics of BPSCCO (2223)

    NASA Astrophysics Data System (ADS)

    Pradhan, A. K.; Chen, C.; Wanklyn, B. M.

    1995-02-01

    We have measured the high-magnetic-field (up to 15 T) transport properties of (BiPb) 2Sr 2Ca 1Cu 2O y (BPSCCO) single crystals and (2223) ceramics. We have extracted the melting line of BPSCCO crystals from the measurements of precise I- V curves when the field is applied parallel to the c-axis and shown that the melting line coincides with the irreversibility line measured by the magnetisation technique. This is explained for the case of weak pinning. The depinning line was found to be 10 to 12 K above the melting line. The observed hump in the magnetoresistance curve above 2 T predicts the depinning of flux from the weak pinning centres when the temperature approaches the depinning temperature. The basic behavior of ceramic BPSCCO, although having a higher Tc, is similar to its single crystal analogue with (2212) phase except for the enhanced depinning line.

  5. Glass bead size and morphology characteristics in support of Crystal Mist field experiments

    SciTech Connect

    Einfeld, W.

    1995-03-01

    One of the tasks of the Lethality Group within US Army Space and Strategic Defense Command (USASSDC) is the development of a capability to simulate various missile intercept scenarios using computer codes. Currently under development within USASSDC and its various contractor organizations is a group of codes collected under a master code called PEGEM for Post Event Ground Effects Model. Among the various components of the code are modules which are used to predict atmospheric dispersion and transport of particles or droplets following release at the altitude specified in the missile intercept scenario. The atmospheric transport code takes into account various source term data from the intercept such as: initial cloud size; droplet or particle size distribution; and, total mass of agent released. An ongoing USASSDC experimental program termed Crystal Mist involved release of precision glass beads under various altitude and meteorological conditions to assist in validation and refinement of various codes that are components of PEGEM used to predict particle atmospheric transport and diffusion following a missile intercept. Here, soda-lime glass beads used in the Crystal Mist series of atmospheric transport and diffusion tests were characterized by scanning electron microscopy and automated image processing routines in order to fully define their size distributions and morphology. Four bead size classifications ranging from a median count diameter of 45 to 200 micrometers were found to be approximately spherical and to fall within the supplier`s sizing specifications. Log-normal functions fit to the measured size distributions resulted in geometric standard deviations ranging from 1.08 to 1.12, thereby fulfilling the field trial requirements for a relatively narrow bead size distribution.

  6. Effect of carbon substitution on low magnetic field AC losses in MgB 2 single crystals

    NASA Astrophysics Data System (ADS)

    Ciszek, M.; Rogacki, K.; Karpiński, J.

    2011-11-01

    The DC magnetization and AC magnetic susceptibilities were measured for MgB2 single crystals, unsubstituted and carbon substituted with the composition of Mg(B0.94C0.06)2. AC magnetic losses were derived from the AC susceptibility data as a function of the AC amplitude and the DC bias magnetic field. From the DC magnetization loops critical current densities were derived as a function of temperature and DC field. Results show that the substitution with carbon decreases critical current densities at low external magnetic fields, in contrast to the well known effect of an increase of the critical current densities at higher magnetic fields.

  7. Effect of an electric field on the orientation of a liquid crystal in a cell with a nonuniform director distribution

    SciTech Connect

    Aksenova, E. V. Karetnikov, A. A.; Karetnikov, N. A.; Kovshik, A. P.; Ryumtsev, E. I.; Sakhatskii, A. S.; Svanidze, A. V.

    2016-05-15

    The electric field-induced reorientation of a nematic liquid crystal in cells with a planar helicoidal or a homeoplanar structure of a director field is studied theoretically and experimentally. The dependences of the capacitances of these systems on the voltage in an applied electric field below and above the Fréedericksz threshold are experimentally obtained and numerically calculated. The calculations use the director distribution in volume that is obtained by direct minimization of free energy at various voltages. The inhomogeneity of the electric field inside a cell is taken into account. The calculation results are shown to agree with the experimental data.

  8. Diffusion-controlled anisotropic growth of stable and metastable crystal polymorphs in the phase-field crystal model.

    PubMed

    Tegze, G; Gránásy, L; Tóth, G I; Podmaniczky, F; Jaatinen, A; Ala-Nissila, T; Pusztai, T

    2009-07-17

    We use a simple density functional approach on a diffusional time scale, to address freezing to the body-centered cubic (bcc), hexagonal close-packed (hcp), and face-centered cubic (fcc) structures. We observe faceted equilibrium shapes and diffusion-controlled layerwise crystal growth consistent with two-dimensional nucleation. The predicted growth anisotropies are discussed in relation with results from experiment and atomistic simulations. We also demonstrate that varying the lattice constant of a simple cubic substrate, one can tune the epitaxially growing body-centered tetragonal structure between bcc and fcc, and observe a Mullins-Sekerka-Asaro-Tiller-Grinfeld-type instability.

  9. LABORATORY AND FIELD EVALUATION OF CRYSTALLIZED DOW 704 OIL ON THE PERFORMANCE OF THE PM2.5 WINS FRACTIONATOR

    EPA Science Inventory

    Subsequent to the PM2.5 FRM's 1997 promulgation, technicians at the CT Dept. of Env. Protection observed that the DOW 704 diffusion oil used in the method's WINS fractionator would occasionally crystallize during field use - particularly under wintertime conditions. While the f...

  10. Can the electron magnetic resonance (EMR) techniques measure the crystal (ligand) field parameters?

    NASA Astrophysics Data System (ADS)

    Rudowicz, C.; Sung, H. W. F.

    2001-07-01

    In this paper, the question posed in the title is critically examined on the basis of the available literature evidence implying the positive answer. The distinction between, on the one hand, the actual crystal field (CF) or equivalently ligand field (LF) related quantities and, on the other hand, the actual zero-field splitting (ZFS) or equivalently fine structure (FS) quantities, is elucidated. The origin and possible roots of the incorrect terminology consisting in mixing up the two physically distinct quantities at different levels are examined. Aspects concerning Hamiltonians, parameters, energy level splitting, and nature of the operators involved are taken into account. Problems with the various notations for the operators and parameters used in the electron magnetic resonance (EMR) area are also identified and reviewed. A large number of cases of incorrect terminology and other inconsistencies identified in the course of a comprehensive literature survey are analyzed and systematically classified. Implications of the confusion in question, which go beyond the simple semantic issues, are discussed. The results of the survey reveal that the two most serious categories of this confusion lead to misinterpretation of the experimental EMR data. Several examples of serious misinterpretations found in the books, reviews, and original papers are discussed. The incorrect terminology contributes also to misleading keyword classifications of papers in journals as well as references in scientific literature databases. Thus, the database searches may produce unreliable outcomes. Examples of such outcomes are also shown. It is concluded that, in order to prevent further proliferation of the incorrect terminology and thus to increase reliability of the published EMR data, a concerted effort within the EMR community is indispensable. Various ways in this regard at the international level are suggested.

  11. Chemical pressure effect in magnetic frustrated pyrochlore Nd2Pb2O7: A crystal-field analysis

    NASA Astrophysics Data System (ADS)

    Swarnakar, Debasish; Jana, Yatramohan; Alam, Jahangir; Nandi, Saikat

    2017-09-01

    Variation of chemical pressure at R-site due to substitution of nonmagnetic cation of varying size at the M-site makes a fine tuning between the crystal-field and molecular field to adopt exotic ground states in the frustrated magnetic R2M2O7 pyrochlore structures. Presence of larger cation at M-site increases the lattice parameter or nearest-neighbor bond distance between magnetic R-spins, and causes subtle changes to the local oxygen environment surrounding each R-ion, thereby reduces the chemical pressure at R-sites which leads to a dramatic change in the crystal-field and molecular field at R-site. To explore the effect of chemical pressure, the experimental results of powder magnetic susceptibility and isothermal magnetization of a geometrically frustrated compound, Nd2Pb2O7 containing largest cation, e.g. lead (Pb), at M4+-sites are simulated and analyzed employing a D3d crystal-field (CF) and anisotropic molecular field at R-sites in the self-consistent mean-field approach. The second-ordered axial parameter B20 and total CF splitting of the ground multiplet 4I9/2 of Nd3+-ions became the lowest among the isomorphic Nd-pyrochlore compounds, implying reduced effect of the crystal-field at Nd sites. Nd2Pb2O7 has strong [111] Ising anisotropy. Relative strength and values of the exchange tensor among nearest-neighbor Nd3+-spins in Nd2Pb2O7 and Nd2Zr2O7 result in a very close competition of anti-ferromagnetic and ferromagnetic interactions.

  12. Anisotropy of Lamb and SH waves propagation in langasite single crystal plates under the influence of dc electric field.

    PubMed

    Burkov, S I; Zolotova, O P; Sorokin, B P

    2012-03-01

    Paper is presented the results of computer simulation. Effect of the homogeneous dc electric field influence on the propagation of zero and first order Lamb and SH waves in piezoelectric langasite single crystal plates for a lot of cuts and directions have been calculated. Crystalline directions and cuts with maximal and minimal influence of dc electric field have indicated. Effect of hybridization of plate modes has been discussed.

  13. Magma Differentiation and Storage Inferred from Crystal Textures at Harrat Rahat Volcanic Field, Kingdom of Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Witter, M. R.; Mahood, G. A.; Stelten, M. E.; Downs, D. T.; Zahran, H. M.

    2015-12-01

    We present results of a petrographic study of Harrat Rahat volcanic field in western Saudi Arabia as part of a collaborative project between the U.S.G.S. and the Saudi Geological Survey. Lavas range in composition from alkali basalt to trachyphonolite. Basalts have <2-10 vol.% phenocrysts of euhedral olivine and plagioclase (± minor clinopyroxene). In intermediate lavas, phenocrysts (<5 vol.%) of olivine and plagioclase are resorbed, and plagioclase also exhibits sieve textures and strong zoning, indicative of complex magmatic histories. Trachyphonolite lavas have 0-35 vol.% large phenocrysts of anorthoclase and trace fayalitic olivine but are characterized by a size distribution of crystals that is seriate in hand specimen, so that most exceeded 45% crystals at the time of eruption. Some contain groundmass alkali amphibole. Crystal size distributions (CSD) of crystal-rich trachyphonolites produce simple linear trends (see below), which are interpreted as signifying that all the crystals are related through a common nucleation and growth history, at more or less constant pressure. Linear CSDs indicate no loss of small crystals due to reheating of magmas by recharge, no gain of small crystals due to late-stage nucleation on ascent or degassing, and no addition of large phenocrysts by crystal accumulation or magma mixing. Experimental studies demonstrate that silica-undersaturated evolved magmas like those erupted at Harrat Rahat can form by fractionation of alkali basalts at crustal depths greater than ~25 km. The observed phenocryst assemblage in the trachyphonolites, however, forms at shallow depths, ~2-4 km, according to MELTS modeling. Coupled with CSD data, this suggests that deep extraction events yield crystal-poor trachyphonolite magmas that rise to the upper crust where they undergo crystallization. Extensive shallow crystallization of trachyphonolites may have triggered eruptions by causing vapor saturation, which lowers magma density via vesiculation and

  14. Yolk/Shell Colloidal Crystals Incorporating Movable Cores with Their Motion Controlled by an External Electric Field.

    PubMed

    Watanabe, Kanako; Ishii, Haruyuki; Konno, Mikio; Imhof, Arnout; van Blaaderen, Alfons; Nagao, Daisuke

    2017-01-10

    Yolk/shell particles composed of a submicrometer-sized movable core and a silica shell are promising building blocks for novel optical colloidal crystals, because the locations of cores in the shell compartment can be reversibly changed by using external stimuli. Two dimensional arrays of yolk/shell particles incorporating movable cores were prepared by a self-assembly method. The movable cores of colloidal crystals in water could be observed with an optical microscope under application of external electric field. The motions of inner silica cores depended on the electric field strength and frequency and were categorized into three cases: (1) Random Brownian motion, (2) anisotropic motion of cores moving in a direction orthogonal to a field, and (3) suppressed motion fixed in the center of shell compartment. Random Brownian motion of cores was scarcely affected by field strength when a high frequency (in the MHz range) electric field was applied. On the other hand, an increase in field strength at low-frequency fields (kHz) transiently changed the core motion from (1) to (2) and a further increase in field strength changed it from (2) to (3). When the silica core was incorporated in a large void a stronger electric field was needed to suppress its motion than when it was in a small void. The high responsivity to electric fields in a low-frequency range indicated the importance of electric double layer (EDL) interaction between core and inner shell in controlling the core location in yolk/shell colloidal crystals. It was also shown that movable titania cores in yolk/shell particles required a low-frequency field with a high strength to change from the random to anisotropic motion. The result suggested that the electrostatic interaction between EDLs of the silica core and the inner silica wall could be stronger than that between EDLs of the titania core and the silica shell.

  15. Bimodal random crystal field distribution effects on the ferrimagnetic mixed spin-1/2 > and spin-3/2 Blume-Capel model

    NASA Astrophysics Data System (ADS)

    Yigit, Ali; Albayrak, Erhan

    2013-03-01

    The effects of bimodal random crystal field on the phase diagrams and magnetization curves of ferrimagnetic mixed spin-1/2 and spin-3/2 Blume-Capel model are examined by using the effective field theory with correlations for honeycomb lattice. The phase diagrams are obtained on the (Δ,kT/|J|), (Δ,Tcomp) and (p,kT/|J|) planes for given values of p and Δ, respectively. The model exhibits only the second-order phase transitions as in the Blume-Capel model with constant crystal fields. In addition, it was found that the model presents one or two compensation temperatures for appropriate values of random crystal field for given probability in contrast to constant crystal field case. Therefore, it is shown that the random crystal field considerably affects the thermal variations of net and sublattice magnetizations.

  16. Single Crystal Paleointensity Analyses of Olivine-Diogenites: Implications for a Past Vestan Magnetic Field

    NASA Astrophysics Data System (ADS)

    Tarduno, J. A.; Cottrell, R. D.

    2012-04-01

    Pioneering studies of meteorites and recent investigations have presented paleomagnetic data suggesting some parent bodies had dynamos. With this background, meteorites of the Howardite-Eucrite-Diogenite (HED) group of anchondrites, linked to the differentiated asteroid 4 Vesta, represent promising targets for magnetic investigation. Prior studies of HED meteorites have yielded contrasting results. Cisowski [1] reported low paleofields (1-5 μT) from two unbrecciated eucrites, whereas Morden [2] reported paleointensities of up to 37 μT from Thellier analyses of the brecciated Millbillillie eucrite; the latter were interpreted as indicative of a past dynamo. The age of the Millbillillie magnetization might be approximately 3.55 Ga [3] when the meteorite was heated by impact. Fu and Weiss [4-5] have recently reported a study of fusion crust of the Millibillie eucrite, supporting the conclusion that the meteorite preserves an ancient magnetization, but with very low (2-3 μT) paleointensity values. Here we discuss results from Northwest Africa (NWA) 5480, which is a olivine-diogenite (or harzburgite), 57 vol% olivine and 42 vol% orthopyroxene. Olivine is found in bands that have been interpreted as magmatic flow within the Vestan mantle [6-7]. We use single crystal paleointensity analyses [8]. Olivine grains 1-2 mm in size were separated for analyses. We specifically exclude grains with large visible inclusions as these may be multidomain magnetic minerals which relax on relatively short timescales. Magnetic hysteresis measurements suggest that olivine hosts single to pseudo-single domain magnetic inclusions suitable for paleointensity analyses. Thermal demagnetization reveals removal of several scattered magnetizations at low unblocking temperatures, followed by stable decay at higher temperatures. Thellier-Coe paleointensity data suggest a field of approximately 36 μT. These preliminary data, if confirmed, imply a Vestan dynamo because alternative primary magnetic

  17. Effect of Bending on the Electrical Characteristics of Flexible Organic Single Crystal-based Field-effect Transistors.

    PubMed

    Ho, Man-Tzu; Tao, Yu-Tai

    2016-11-07

    The charge transport in an organic semiconductor depends highly on the molecular packing in the crystal, which influences the electronic coupling immensely. However, in soft electronics, in which organic semiconductors play a critical role, the devices will be bent or folded repeatedly. The effect of bending on the crystal packing and thus the charge transport is crucial to the performance of the device. In this manuscript, we describe the protocol to bend a single crystal of 5,7,12,16-tetrachloro-6,13-diazapentacene (TCDAP) in the field-effect transistor configuration and to obtain reproducible I-V characteristics upon bending the crystal. The results show that bending a field-effect transistor prepared on a flexible substrate results in nearly reversible yet opposite trends in charge mobility, depending on the bending direction. The mobility increases when the device is bent toward the top gate/dielectric layer (upward, compressive state) and decreases when bent toward the crystal/substrate side (downward, tensile state). The effect of bending curvature was also observed, with greater mobility change resulting from higher bending curvature. It is suggested that the intermolecular π-π distance changes upon bending, thereby influencing the electronic coupling and the subsequent carrier transport ability.

  18. The influence of detector size relative to field size in small-field photon-beam dosimetry using synthetic diamond crystals as sensors

    NASA Astrophysics Data System (ADS)

    Ade, N.; Nam, T. L.

    2015-08-01

    The choice of a detector for small-field dosimetry remains a challenge due to the size/volume effect of detectors in small fields. Aimed at selecting a suitable crystal type and detector size for small-field dosimetry, this study investigates the relationship between detector and field size by analysing output factors (OFs) measured with a Diode E (reference detector), a Farmer chamber and synthetic diamond detectors of various types and sizes in the dosimetry of a 6 MV photon beam with small fields between 0.3×0.3 cm2 and 10×10 cm2. The examined diamond sensors included two HPHT samples (HP1 and HP2) and six polycrystalline CVD specimens of optical grade (OG) and detector grade (DG) qualities with sizes between 0.3 and 1.0 cm. Each diamond was encapsulated in a tissue-equivalent probe housing which can hold crystals of various dimensions up to 1.0×1.0×0.1 cm3 and has different exposure geometries ('edge-on' and 'flat-on') for impinging radiation. The HPHT samples were found to show an overall better performance compared to the CVD crystals with the 'edge-on' orientation being a preferred geometry for OF measurement especially for very small fields. For instance, down to a 0.4×0.4 cm2 field a maximum deviation of 1.9% was observed between the OFs measured with Diode E and HP2 in the 'edge-on' orientation compared to a 4.6% deviation in the 'flat-on' geometry. It was observed that for fields below 4×4 cm2, the dose deviation between the OFs measured with the detectors and Diode E increase with increasing detector size. It was estimated from an established relationship between the dose deviation and the ratio of detector size to field size for the detectors that the dose deviation probably due to the volume averaging effect would be >3% when the detector size is >3/4 of the field size. A sensitivity value of 223 nC Gy-1 mm-3 was determined in a 0.5×0.5 cm2 field with HP2 compared to a value of 159.2 nC Gy-1 mm-3 obtained with the diode. The results of this

  19. Electro-optical properties of a nematic liquid crystal cell by double-side fringe-field switching

    NASA Astrophysics Data System (ADS)

    Xiang, C. Y.; Sun, X. W.

    2004-09-01

    The electro-optical and response properties of a nematic liquid crystal cell driven by double-side fringe-field switching [C. Y. Xiang, X. W. Sun, and X. J. Yin, Appl. Phys. Lett. 83, 5154 (2003)] have been studied. The transmission-voltage curve of the double-side fringe-field cell can be modeled by the change of the maximum twist angle. It is shown that the liquid crystal layer is linearly twisted before the transmission reaches the maximum. The threshold voltage of double-side fringe-field switching is the same as that of single-side fringe-field switching. The turn-on and turn-off response times, showing four time improvement over the single-side fringe-field switching, have been derived. The experimental turn-off times of the double-sides fringe-field switching and single-side fringe-field switching cell match with the theoretical prediction. The experimental turn-on time of the double-side fringe-field switching cell shows four times improvement over the single-side fringe-field one as predicted by the theoretical analysis.

  20. Electric-field-induced transport of microspheres in the isotropic and chiral nematic phase of liquid crystals

    NASA Astrophysics Data System (ADS)

    Oh, Jiyoung; Gleeson, Helen F.; Dierking, Ingo

    2017-02-01

    The application of an electric field to microspheres suspended in a liquid crystal causes particle translation in a plane perpendicular to the applied field direction. Depending on applied electric field amplitude and frequency, a wealth of different motion modes may be observed above a threshold, which can lead to linear, circular, or random particle trajectories. We present the stability diagram for these different translational modes of particles suspended in the isotropic and the chiral nematic phase of a liquid crystal and investigate the angular velocity, circular diameter, and linear velocity as a function of electric field amplitude and frequency. In the isotropic phase a narrow field amplitude-frequency regime is observed to exhibit circular particle motion whose angular velocity increases with applied electric field amplitude but is independent of applied frequency. The diameter of the circular trajectory decreases with field amplitude as well as frequency. In the cholesteric phase linear as well as circular particle motion is observed. The former exhibits an increasing velocity with field amplitude, while decreasing with frequency. For the latter, the angular velocity exhibits an increase with field amplitude and frequency. The rotational sense of the particles on a circular trajectory in the chiral nematic phase is independent of the helicity of the liquid crystalline structure, as is demonstrated by employing a cholesteric twist inversion compound.

  1. Electric-field-induced transport of microspheres in the isotropic and chiral nematic phase of liquid crystals.

    PubMed

    Oh, Jiyoung; Gleeson, Helen F; Dierking, Ingo

    2017-02-01

    The application of an electric field to microspheres suspended in a liquid crystal causes particle translation in a plane perpendicular to the applied field direction. Depending on applied electric field amplitude and frequency, a wealth of different motion modes may be observed above a threshold, which can lead to linear, circular, or random particle trajectories. We present the stability diagram for these different translational modes of particles suspended in the isotropic and the chiral nematic phase of a liquid crystal and investigate the angular velocity, circular diameter, and linear velocity as a function of electric field amplitude and frequency. In the isotropic phase a narrow field amplitude-frequency regime is observed to exhibit circular particle motion whose angular velocity increases with applied electric field amplitude but is independent of applied frequency. The diameter of the circular trajectory decreases with field amplitude as well as frequency. In the cholesteric phase linear as well as circular particle motion is observed. The former exhibits an increasing velocity with field amplitude, while decreasing with frequency. For the latter, the angular velocity exhibits an increase with field amplitude and frequency. The rotational sense of the particles on a circular trajectory in the chiral nematic phase is independent of the helicity of the liquid crystalline structure, as is demonstrated by employing a cholesteric twist inversion compound.

  2. The diagram of phase-field crystal structures: an influence of model parameters in a two-mode approximation

    NASA Astrophysics Data System (ADS)

    Ankudinov, V.; Galenko, P. K.

    2017-04-01

    Effect of phase-field crystal model (PFC-model) parameters on the structure diagram is analyzed. The PFC-model is taken in a two-mode approximation and the construction of structure diagram follows from the free energy minimization and Maxwell thermodynamic rule. The diagram of structure’s coexistence for three dimensional crystal structures [Body-Centered-Cubic (BCC), Face-Centered-Cubic (FCC) and homogeneous structures] are constructed. An influence of the model parameters, including the stability parameters, are discussed. A question about the structure diagram construction using the two-mode PFC-model with the application to real materials is established.

  3. Terahertz field enhancement via coherent superposition of the pulse sequences after a single optical-rectification crystal

    NASA Astrophysics Data System (ADS)

    Sajadi, Mohsen; Wolf, Martin; Kampfrath, Tobias

    2014-03-01

    Terahertz electromagnetic pulses are frequently generated by optical rectification of femtosecond laser pulses. In many cases, the efficiency of this process is known to saturate with increasing intensity of the generation beam because of two-photon absorption. Here, we demonstrate two routes to reduce this effect in ZnTe(110) crystals and enhance efficiency, namely, by (i) recycling the generation pulses and by (ii) splitting each generation pulse into two pulses before pumping the crystal. In both methods, the second pulse arrives ˜1 ns after the first one, sufficiently long for optically generated carriers to relax. Enhancement is achieved by coherently superimposing the two resulting terahertz fields.

  4. Terahertz field enhancement via coherent superposition of the pulse sequences after a single optical-rectification crystal

    SciTech Connect

    Sajadi, Mohsen Wolf, Martin; Kampfrath, Tobias

    2014-03-03

    Terahertz electromagnetic pulses are frequently generated by optical rectification of femtosecond laser pulses. In many cases, the efficiency of this process is known to saturate with increasing intensity of the generation beam because of two-photon absorption. Here, we demonstrate two routes to reduce this effect in ZnTe(110) crystals and enhance efficiency, namely, by (i) recycling the generation pulses and by (ii) splitting each generation pulse into two pulses before pumping the crystal. In both methods, the second pulse arrives ∼1 ns after the first one, sufficiently long for optically generated carriers to relax. Enhancement is achieved by coherently superimposing the two resulting terahertz fields.

  5. On a phase field approach for martensitic transformations in a crystal plastic material at a loaded surface

    NASA Astrophysics Data System (ADS)

    Schmitt, Regina; Kuhn, Charlotte; Müller, Ralf

    2017-07-01

    A continuum phase field model for martensitic transformations is introduced, including crystal plasticity with different slip systems for the different phases. In a 2D setting, the transformation-induced eigenstrain is taken into account for two martensitic orientation variants. With aid of the model, the phase transition and its dependence on the volume change, crystal plastic material behavior, and the inheritance of plastic deformations from austenite to martensite are studied in detail. The numerical setup is motivated by the process of cryogenic turning. The resulting microstructure qualitatively coincides with an experimentally obtained martensite structure. For the numerical calculations, finite elements together with global and local implicit time integration scheme are employed.

  6. Viewing Angle Characteristics of Transflective Display in a Homogeneously Aligned Liquid Crystal Cell Driven by Fringe-Field

    NASA Astrophysics Data System (ADS)

    Jung, Tae Bong; Song, Je Hoon; Seo, Dae-Shik; Lee, Seung Hee

    2004-09-01

    We have studied the optimal cell configuration for a fringe-field driven transflective liquid crystal display that exhibits high image quality. The cell is composed of two half-plate compensation films, liquid crystal, and two parallel polarizers in the transmissive region. Viewing angle characteristics of the device mainly depends on the orientation of the polarizer axis. The measured contrast ratio in an optimized configuration is greater than 5 in polar angles of over 50° in all directions and in those over 80° in certain azimuthal cross-sectional planes.

  7. Interband coherence response to electric fields in crystals: Berry-phase contributions and disorder effects

    NASA Astrophysics Data System (ADS)

    Culcer, Dimitrie; Sekine, Akihiko; MacDonald, Allan H.

    2017-07-01

    by scattering that is sensitive to the presence of the Fermi surface. To demonstrate the rich physics captured by our theory, we explicitly solve for some electric-field response properties of simple model systems that are known to be dominated by interband coherence contributions. At the same time we discuss an extensive list of complicated problems that cannot be solved analytically. Our goal is to stimulate progress in computational transport theory for electrons in crystals.

  8. Incorporating physically-based microstructures in materials modeling: Bridging phase field and crystal plasticity frameworks

    DOE PAGES

    Lim, Hojun; Abdeljawad, Fadi; Owen, Steven J.; ...

    2016-04-25

    Here, the mechanical properties of materials systems are highly influenced by various features at the microstructural level. The ability to capture these heterogeneities and incorporate them into continuum-scale frameworks of the deformation behavior is considered a key step in the development of complex non-local models of failure. In this study, we present a modeling framework that incorporates physically-based realizations of polycrystalline aggregates from a phase field (PF) model into a crystal plasticity finite element (CP-FE) framework. Simulated annealing via the PF model yields ensembles of materials microstructures with various grain sizes and shapes. With the aid of a novel FEmore » meshing technique, FE discretizations of these microstructures are generated, where several key features, such as conformity to interfaces, and triple junction angles, are preserved. The discretizations are then used in the CP-FE framework to simulate the mechanical response of polycrystalline α-iron. It is shown that the conformal discretization across interfaces reduces artificial stress localization commonly observed in non-conformal FE discretizations. The work presented herein is a first step towards incorporating physically-based microstructures in lieu of the overly simplified representations that are commonly used. In broader terms, the proposed framework provides future avenues to explore bridging models of materials processes, e.g. additive manufacturing and microstructure evolution of multi-phase multi-component systems, into continuum-scale frameworks of the mechanical properties.« less

  9. Incorporating physically-based microstructures in materials modeling: Bridging phase field and crystal plasticity frameworks

    SciTech Connect

    Lim, Hojun; Abdeljawad, Fadi; Owen, Steven J.; Hanks, Byron W.; Foulk, James W.; Battaile, Corbett C.

    2016-04-25

    Here, the mechanical properties of materials systems are highly influenced by various features at the microstructural level. The ability to capture these heterogeneities and incorporate them into continuum-scale frameworks of the deformation behavior is considered a key step in the development of complex non-local models of failure. In this study, we present a modeling framework that incorporates physically-based realizations of polycrystalline aggregates from a phase field (PF) model into a crystal plasticity finite element (CP-FE) framework. Simulated annealing via the PF model yields ensembles of materials microstructures with various grain sizes and shapes. With the aid of a novel FE meshing technique, FE discretizations of these microstructures are generated, where several key features, such as conformity to interfaces, and triple junction angles, are preserved. The discretizations are then used in the CP-FE framework to simulate the mechanical response of polycrystalline α-iron. It is shown that the conformal discretization across interfaces reduces artificial stress localization commonly observed in non-conformal FE discretizations. The work presented herein is a first step towards incorporating physically-based microstructures in lieu of the overly simplified representations that are commonly used. In broader terms, the proposed framework provides future avenues to explore bridging models of materials processes, e.g. additive manufacturing and microstructure evolution of multi-phase multi-component systems, into continuum-scale frameworks of the mechanical properties.

  10. Heteromorphism and crystallization paths of katungites, Navajo volcanic field, Arizona, USA

    SciTech Connect

    Laughlin, A.W.; Charles, R.W.; Aldrich, M.J. Jr.

    1986-01-01

    A swarm of thin, isochemical but heteromorphic dikes crops out in the valley of Hasbidito Creek in NE Arizona. The swarm is part of the dominantly potassic, mid-Tertiary Navajo volcanic field of the Colorado Plateau. Whole-rock chemical analyses of five samples from four of the dikes indicate that they are chemically identical to the katungites of Uganda. These dikes show the characteristic seriate-porphyritic texture of lamprophyres. Samples of an olivine-melilitite dike from the same swarm lack this texture and the chemical analysis, while similar to those of the other dikes, shows effects from the incorporation of xenocrystic olivine. Over 20 mineral phases have been identified in the Arizona samples and as many as 18 phases may occur in a single sample. The major phases are phlogopite, olivine, perovskite, opaque oxides, +- melilite and +- clinopyroxene. Based upon the modal mineralogies and textures of ten dike samples, we recognize five general non-equilibrium assemblages. Comparison of these assemblages with recent experimental results shows that they represent various combinations of complete and incomplete reactions. Reaction relations were determined by entering melt and phase compositions into the computer program GENMIX to obtain balanced reactions. By combining petrographic observations with mineral chemical data, balanced reactions from GENMIX, and the recently determined phase diagrams we are able to trace crystallization paths for the katungite magma.

  11. Touch sensors based on planar liquid crystal-gated-organic field-effect transistors

    NASA Astrophysics Data System (ADS)

    Seo, Jooyeok; Lee, Chulyeon; Han, Hyemi; Lee, Sooyong; Nam, Sungho; Kim, Hwajeong; Lee, Joon-Hyung; Park, Soo-Young; Kang, Inn-Kyu; Kim, Youngkyoo

    2014-09-01

    We report a tactile touch sensor based on a planar liquid crystal-gated-organic field-effect transistor (LC-g-OFET) structure. The LC-g-OFET touch sensors were fabricated by forming the 10 μm thick LC layer (4-cyano-4'-pentylbiphenyl - 5CB) on top of the 50 nm thick channel layer (poly(3-hexylthiophene) - P3HT) that is coated on the in-plane aligned drain/source/gate electrodes (indium-tin oxide - ITO). As an external physical stimulation to examine the tactile touch performance, a weak nitrogen flow (83.3 μl/s) was employed to stimulate the LC layer of the touch device. The LC-g-OFET device exhibited p-type transistor characteristics with a hole mobility of 1.5 cm2/Vs, but no sensing current by the nitrogen flow touch was measured at sufficiently high drain (VD) and gate (VG) voltages. However, a clear sensing current signal was detected at lower voltages, which was quite sensitive to the combination of VD and VG. The best voltage combination was VD = -0.2 V and VG = -1 V for the highest ratio of signal currents to base currents (i.e., signal-to-noise ratio). The change in the LC alignment upon the nitrogen flow touch was assigned as the mechanism for the present LC-g-OFET touch sensors.

  12. ±J model on the Bethe lattice with crystal field interaction

    NASA Astrophysics Data System (ADS)

    Albayrak, Erhan

    2014-04-01

    ±J model was exploited on the Bethe lattice for the spin-1 Blume-Capel model. Either ferromagnetic (J>0) or antiferromagnetic (J<0) interactions are assumed between the nearest-neighbor (NN) spins on the Bethe lattice for given probabilities p or 1-p, respectively. The phase diagrams of the model were calculated exactly on the temperature (T)-probability (p) or crystal field (D) planes for given coordination numbers q=3, 4 and 6. In addition to the first- and second-order phase transitions, another kind phase transition with zero magnetization and quadrupolar order-parameter presenting a jump was obtained above the second-order phase transition temperature which seems to be indicating the spin glass phase. When the lines of these three phases combine is a multicritical point and the first two combine is a tricritical point. In the (T,p) planes, the lower critical dimensions (the value of q) of obtaining any phase transition increase with increasing negative D values.

  13. An energy stable, hexagonal finite difference scheme for the 2D phase field crystal amplitude equations

    NASA Astrophysics Data System (ADS)

    Guan, Zhen; Heinonen, Vili; Lowengrub, John; Wang, Cheng; Wise, Steven M.

    2016-09-01

    In this paper we construct an energy stable finite difference scheme for the amplitude expansion equations for the two-dimensional phase field crystal (PFC) model. The equations are formulated in a periodic hexagonal domain with respect to the reciprocal lattice vectors to achieve a provably unconditionally energy stable and solvable scheme. To our knowledge, this is the first such energy stable scheme for the PFC amplitude equations. The convexity of each part in the amplitude equations is analyzed, in both the semi-discrete and fully-discrete cases. Energy stability is based on a careful convexity analysis for the energy (in both the spatially continuous and discrete cases). As a result, unique solvability and unconditional energy stability are available for the resulting scheme. Moreover, we show that the scheme is point-wise stable for any time and space step sizes. An efficient multigrid solver is devised to solve the scheme, and a few numerical experiments are presented, including grain rotation and shrinkage and grain growth studies, as examples of the strength and robustness of the proposed scheme and solver.

  14. Crystal electric field excitations in quasicrystal approximant TbCd6 studied by inelastic neutron scattering

    NASA Astrophysics Data System (ADS)

    Das, Pinaki; Flint, R.; Kong, T.; Canfield, P. C.; Kreyssig, A.; Goldman, A. I.; de Boissieu, M.; Lory, P.-F.; Beutier, G.; Hiroto, T.

    All of the known quasicrystals with local moments exhibit frustration and spin glass-like behavior at low temperature. The onset of the spin freezing temperature is believed to be affected by the crystal electric field (CEF) splitting of the local moments. The quasicrystal approximant TbCd6 and its related icosahedral quasicrystal phase, i-Tb-Cd, form a set of model systems to explore how magnetism evolves from a conventional lattice (approximant phase) to an aperiodic quasicrystal. Though TbCd6 shows long-range antiferromagnetic ordering (TN = 24 K), only spin glass like behavior is observed in i-Tb-Cd with a spin freezing temperature of TF = 6 K. To investigate further, we have performed inelastic neutron scattering measurements on powder samples of TbCd6 and observed two distinct CEF excitations at low energies which points to a high degeneracy of the CEF levels related to the Tb surrounding with almost icosahedral symmetry. Work at Ames Laboratory was supported by the DOE, BES, Division of Materials Sciences & Engineering, under Contract No. DE-AC02-07CH11358. This research used resources at Institut Laue-Langevin, France.

  15. Crystal electric field effects and thermal expansion of rare-earth hexaborides

    NASA Astrophysics Data System (ADS)

    Novikov, V. V.; Pilipenko, E. S.; Bud'ko, S. L.

    2017-02-01

    Anomalies in the magnetic contribution to the thermal expansion coefficients ∆β(T)of the CeB6, PrB6, and NdB6 hexaborides in the range of 5-300 K were found by comparison with diamagnetic LaB6. The characteristic of the anomalies was the same in all the studied borides: a distinct peak at low temperatures, followed by a broad maximum at higher temperatures (50-100 K), then a decrease and transition to the region of negative values as the temperature increases further. The features of ∆β(T) are explained by the effects of the magnetic order (sharp low temperature peaks) and the crystal electric field (CEF). The βCEF(T) dependencies were calculated using Raman and neutron scattering data on the splitting of the rare-earth (RE) ions R3+ f-level by the CEF. A satisfactory consistency between the values of βCEF(T) and ∆β(T)was obtained for the studied hexaborides. Additionally, we determined the values of the Grüneisen parameter γi that correspond to the transition between the ground and excited multiplets of R3+ ions f-level splitting.

  16. Phase field crystal study on the grain boundary porosity induced by the Kirkendall effect

    NASA Astrophysics Data System (ADS)

    Lu, Guang-Ming; Lu, Yan-Li; Hu, Ting-Ting; Chen, Zheng

    2016-03-01

    Grain boundary (GB) porosity strongly degrades the bonding quality of interfaces and affects the physical and mechanical properties of solid polycrystalline materials. In this paper, the formation and evolution mechanisms of porosity at the grain boundary were investigated using the binary phase field crystal simulation method. Simulated results indicate that the Kirkendall effect existing in the interdiffusion of substitutional binary alloys can result in GB porosity. For the low-angle grain boundary interdiffusion system, the porosity initially forms at the isolated dislocation core, evolving from circle to irregular polygon. For the large-angle GB interdiffusion system, the porosity initially forms at the dislocation core close to the diffusion plane, and then evolves toward the dislocation cores away from the diffusion plane. The porosities finally connect as a continuous slit that splits up the GB. The results also show that the diffusion of fast diffusers along the GB is obviously enhanced with the mobility ratio of species A and B increasing. Our simulation results agree well with theoretical and experimental results.

  17. Incorporating physically-based microstructures in materials modeling: Bridging phase field and crystal plasticity frameworks

    SciTech Connect

    Lim, Hojun; Abdeljawad, Fadi; Owen, Steven J.; Hanks, Byron W.; Foulk, James W.; Battaile, Corbett C.

    2016-04-25

    Here, the mechanical properties of materials systems are highly influenced by various features at the microstructural level. The ability to capture these heterogeneities and incorporate them into continuum-scale frameworks of the deformation behavior is considered a key step in the development of complex non-local models of failure. In this study, we present a modeling framework that incorporates physically-based realizations of polycrystalline aggregates from a phase field (PF) model into a crystal plasticity finite element (CP-FE) framework. Simulated annealing via the PF model yields ensembles of materials microstructures with various grain sizes and shapes. With the aid of a novel FE meshing technique, FE discretizations of these microstructures are generated, where several key features, such as conformity to interfaces, and triple junction angles, are preserved. The discretizations are then used in the CP-FE framework to simulate the mechanical response of polycrystalline α-iron. It is shown that the conformal discretization across interfaces reduces artificial stress localization commonly observed in non-conformal FE discretizations. The work presented herein is a first step towards incorporating physically-based microstructures in lieu of the overly simplified representations that are commonly used. In broader terms, the proposed framework provides future avenues to explore bridging models of materials processes, e.g. additive manufacturing and microstructure evolution of multi-phase multi-component systems, into continuum-scale frameworks of the mechanical properties.

  18. GW correlation effects on plutonium quasiparticle energies: changes in crystal-field splitting

    SciTech Connect

    Albers, Robert C; Chantis, Athanasios N; Svane, Axel; Christensen, Niels E

    2009-01-01

    We present results for the electronic structure of plutonium by using a recently developed quasiparticle self-consistent GW method (QSGW). We consider a paramagnetic solution without spin-orbit interaction as a function of volume for the face-centered cubic (fcc) unit cell. We span unit-cell volumes ranging from 10% greater than the equilibrium volume of the 8 phase to 90 % of the equivalent for the a phase of Pu. The self-consistent GW quasiparticle energies are compared to those obtained within the Local Density Approximation (LDA). The goal of the calculations is to understand systematic trends in the effects of electronic correlations on the quasiparticle energy bands of Pu as a function of the localization of the J orbitals. We show that correlation effects narrow the f bands in two significantly different ways. Besides the expected narrowing of individual f bands (flatter dispersion), we find that an even more significant effect on the f bands is a decrease in the crystal-field splitting of the different bands

  19. Appearance of singularities of optical fields under torsion of crystals containing threefold symmetry axes.

    PubMed

    Skab, Ihor; Vasylkiv, Yurij; Zapeka, Bohdan; Savaryn, Viktoriya; Vlokh, Rostyslav

    2011-07-01

    We present an analysis of the effect of torsion stresses on the spatial distribution of optical birefringence in crystals of different point symmetry groups. The symmetry requirements needed so that the optical beam carries dislocations of the phase front are evaluated for the case when the crystals are twisted and the beam closely corresponds to a plane wave. It is shown that the torsion stresses can produce screw-edge, pure screw, or pure edge dislocations of the phase front in the crystals belonging to cubic and trigonal systems. The conditions for appearance of canonical and noncanonical vortices in the conditions of crystal torsion are analyzed.

  20. Quantitative scheme for full-field polarization rotating fluorescence microscopy using a liquid crystal variable retarder

    NASA Astrophysics Data System (ADS)

    Lesoine, John F.; Youn Lee, Ji; Krogmeier, Jeffrey R.; Kang, Hyeonggon; Clarke, Matthew L.; Chang, Robert; Sackett, Dan L.; Nossal, Ralph; Hwang, Jeeseong

    2012-05-01

    We present a quantitative scheme for full-field polarization rotating fluorescence microscopy. A quarter-wave plate, in combination with a liquid crystal variable retarder, provides a tunable method to rotate polarization states of light prior to its being coupled into a fluorescence microscope. A calibration of the polarization properties of the incident light is performed in order to correct for elliptical polarization states. This calibration allows the response of the sample to linear polarization states of light to be recovered. Three known polarization states of light can be used to determine the average fluorescent dipole orientations in the presence of a spatially varying dc offset or background polarization-invariant fluorescence signal. To demonstrate the capabilities of this device, we measured a series of full-field fluorescence polarization images from fluorescent analogs incorporated in the lipid membrane of Burkitts lymphoma CA46 cells. The fluorescent lipid-like analogs used in this study are molecules that are labeled by either a DiI (1,1'-Dioctadecyl 3,3,3',3'-Tetramethylindocarbocyanine) fluorophore in its head group or a Bodipy (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) molecule in its acyl chain. A spatially varying contrast in the normalized amplitude was observed on the cell surface, where the orientation of the DiI molecules is tangential to the cell membrane. The internally labeled cellular structures showed zero response to changes in linear polarization, and the net linear polarization amplitude for these regions was zero. This instrument provides a low cost calibrated method that may be coupled to existing fluorescence microscopes to perform investigations of cellular processes that involve a change in molecular orientations.

  1. Broken symmetry phase transition in solid p-H 2, o-D 2 and HD: crystal field effects

    NASA Astrophysics Data System (ADS)

    Freiman, Yu. A.; Hemley, R. J.; Jezowski, A.; Tretyak, S. M.

    1999-04-01

    We report the effect of the crystal field (CF) on the broken symmetry phase transition (BSP) in solid parahydrogen, orthodeuterium, and hydrogen deuteride. The CF was calculated taking into account a distortion from the ideal HCP structure. We find that, in addition to the molecular field generated by the coupling terms in the intermolecular potential, the Hamiltonian of the system contains a crystal-field term, originating from single-molecular terms in the intermolecular potential. Ignoring the CF is the main cause of the systematic underestimation of the transition pressure, characteristic of published theories of the BSP transition. The distortion of the lattice that gives rise to the negative CF in response to the applied pressure is in accord with the general Le Chatelier-Braun principle.

  2. Fast fringe-field switching of a liquid crystal cell by two-dimensional confinement with virtual walls

    PubMed Central

    Choi, Tae-Hoon; Oh, Seung-Won; Park, Young-Jin; Choi, Yeongyu; Yoon, Tae-Hoon

    2016-01-01

    We report a simple method for reducing the response time of a fringe-field switching liquid crystal cell by using two-dimensional confinement of the liquid crystals. Through both numerical calculations and experiments, we show that the switching speed can be increased by several fold in a fringe-field switching cell by simply using a rubbing angle of zero, which causes virtual walls to be built when an electric field is applied between the interdigitated electrodes and the common electrode, without requiring additional fabrication steps or complicated drive schemes. Furthermore, the devices fabricated with this method exhibit a reduced color shift and excellent dynamic stability, even with a high applied voltage and under external pressure. PMID:27301651

  3. Crystallization of spin superlattices with pressure and field in the layered magnet SrCu2(BO3)2

    PubMed Central

    Haravifard, S.; Graf, D.; Feiguin, A. E.; Batista, C. D.; Lang, J. C.; Silevitch, D. M.; Srajer, G.; Gaulin, B. D.; Dabkowska, H. A.; Rosenbaum, T. F.

    2016-01-01

    An exact mapping between quantum spins and boson gases provides fresh approaches to the creation of quantum condensates and crystals. Here we report on magnetization measurements on the dimerized quantum magnet SrCu2(BO3)2 at cryogenic temperatures and through a quantum-phase transition that demonstrate the emergence of fractionally filled bosonic crystals in mesoscopic patterns, specified by a sequence of magnetization plateaus. We apply tens of Teslas of magnetic field to tune the density of bosons and gigapascals of hydrostatic pressure to regulate the underlying interactions. Simulations help parse the balance between energy and geometry in the emergent spin superlattices. The magnetic crystallites are the end result of a progression from a direct product of singlet states in each short dimer at zero field to preferred filling fractions of spin-triplet bosons in each dimer at large magnetic field, enriching the known possibilities for collective states in both quantum spin and atomic systems. PMID:27320787

  4. Giant reversible rotating cryomagnetocaloric effect in KEr (MoO4)2 induced by a crystal-field anisotropy

    NASA Astrophysics Data System (ADS)

    Tkáč, V.; Orendáčová, A.; Čižmár, E.; Orendáč, M.; Feher, A.; Anders, A. G.

    2015-07-01

    Magnetocaloric properties of KEr(MoO4)2 single crystals were investigated using magnetization and specific heat measurements in the magnetic field applied along the easy and hard axis. Large conventional magnetocaloric effect was found around 10 K (-Δ Smax =14 J/kg K for 5 T) in the field applied along the easy axis. What is more, a huge magnetic anisotropy in the a b plane leads to a large anisotropy of magnetocaloric effect, -Δ SR ,max =10 and 13 J/kg K obtained by a simple rotating of the single crystal within the a b plane in the constant magnetic field 2 and 5 T, respectively. Large Δ SR values with no hysteresis losses and rather wide working temperature spans imply that KEr(MoO4)2 may serve as a promising candidate for the implementation of a compact rotary magnetic cryorefrigerator.

  5. Fast fringe-field switching of a liquid crystal cell by two-dimensional confinement with virtual walls

    NASA Astrophysics Data System (ADS)

    Choi, Tae-Hoon; Oh, Seung-Won; Park, Young-Jin; Choi, Yeongyu; Yoon, Tae-Hoon

    2016-06-01

    We report a simple method for reducing the response time of a fringe-field switching liquid crystal cell by using two-dimensional confinement of the liquid crystals. Through both numerical calculations and experiments, we show that the switching speed can be increased by several fold in a fringe-field switching cell by simply using a rubbing angle of zero, which causes virtual walls to be built when an electric field is applied between the interdigitated electrodes and the common electrode, without requiring additional fabrication steps or complicated drive schemes. Furthermore, the devices fabricated with this method exhibit a reduced color shift and excellent dynamic stability, even with a high applied voltage and under external pressure.

  6. Crystallization of spin superlattices with pressure and field in the layered magnet SrCu2(BO3)2

    NASA Astrophysics Data System (ADS)

    Haravifard, S.; Graf, D.; Feiguin, A. E.; Batista, C. D.; Lang, J. C.; Silevitch, D. M.; Srajer, G.; Gaulin, B. D.; Dabkowska, H. A.; Rosenbaum, T. F.

    2016-06-01

    An exact mapping between quantum spins and boson gases provides fresh approaches to the creation of quantum condensates and crystals. Here we report on magnetization measurements on the dimerized quantum magnet SrCu2(BO3)2 at cryogenic temperatures and through a quantum-phase transition that demonstrate the emergence of fractionally filled bosonic crystals in mesoscopic patterns, specified by a sequence of magnetization plateaus. We apply tens of Teslas of magnetic field to tune the density of bosons and gigapascals of hydrostatic pressure to regulate the underlying interactions. Simulations help parse the balance between energy and geometry in the emergent spin superlattices. The magnetic crystallites are the end result of a progression from a direct product of singlet states in each short dimer at zero field to preferred filling fractions of spin-triplet bosons in each dimer at large magnetic field, enriching the known possibilities for collective states in both quantum spin and atomic systems.

  7. X-ray dynamical diffraction from single crystals with arbitrary shape and strain field: A universal approach to modeling

    NASA Astrophysics Data System (ADS)

    Yan, Hanfei; Li, Li

    2014-01-01

    The effects of dynamical diffraction in single crystals engender many unique diffraction phenomena that cannot be interpreted by the kinematical-diffraction theory, yet knowledge of them is vital to resolving a vast variety of scientific problems ranging from crystal optics to strain measurements in crystalline specimens. Although the fundamental dynamical-diffraction theory was established decades ago, modeling it remains a challenge in a general case wherein the crystal has complex boundaries and mixed diffraction geometries (Bragg or Laue). Here, we propose a universal approach for modeling x-ray dynamical diffraction from a single crystal with arbitrary shape and strain field that is based on the integral representation of the Takagi-Taupin equations. Using it, we can construct the solution iteratively via a converging series, independent of the diffraction geometry. Moreover, the integral equations offer additional insights into the diffraction physics that are not readily apparent in its differential counterparts. To demonstrate this approach, we studied the dynamical diffraction from a slab of single crystal with both Bragg and Laue diffraction excited on the entrance boundaries, a problem that is difficult to model by other methods. We also explored the mirage effect caused by the presence of a linear strain field and compared it to the Eikonal theory. Lastly, we derived a dynamical-diffraction equation correlating the structural properties of a particle to its far-field Bragg-diffraction pattern, shedding light on how dynamical diffraction affects these kinematical-diffraction-based inverse techniques for reconstructing the shape and the strain field.

  8. Toward polarizable AMOEBA thermodynamics at fixed charge efficiency using a dual force field approach: application to organic crystals.

    PubMed

    Nessler, Ian J; Litman, Jacob M; Schnieders, Michael J

    2016-11-09

    First principles prediction of the structure, thermodynamics and solubility of organic molecular crystals, which play a central role in chemical, material, pharmaceutical and engineering sciences, challenges both potential energy functions and sampling methodologies. Here we calculate absolute crystal deposition thermodynamics using a novel dual force field approach whose goal is to maintain the accuracy of advanced multipole force fields (e.g. the polarizable AMOEBA model) while performing more than 95% of the sampling in an inexpensive fixed charge (FC) force field (e.g. OPLS-AA). Absolute crystal sublimation/deposition phase transition free energies were determined using an alchemical path that grows the crystalline state from a vapor reference state based on sampling with the OPLS-AA force field, followed by dual force field thermodynamic corrections to change between FC and AMOEBA resolutions at both end states (we denote the three step path as AMOEBA/FC). Importantly, whereas the phase transition requires on the order of 200 ns of sampling per compound, only 5 ns of sampling was needed for the dual force field thermodynamic corrections to reach a mean statistical uncertainty of 0.05 kcal mol(-1). For five organic compounds, the mean unsigned error between direct use of AMOEBA and the AMOEBA/FC dual force field path was only 0.2 kcal mol(-1) and not statistically significant. Compared to experimental deposition thermodynamics, the mean unsigned error for AMOEBA/FC (1.4 kcal mol(-1)) was more than a factor of two smaller than uncorrected OPLS-AA (3.2 kcal mol(-1)). Overall, the dual force field thermodynamic corrections reduced condensed phase sampling in the expensive force field by a factor of 40, and may prove useful for protein stability or binding thermodynamics in the future.

  9. Study of Crystal-field Effects in Rare-earth (RE) - Transition-metal Intermetallic Compounds and in RE-based Laser Crystals

    NASA Astrophysics Data System (ADS)

    Magnani, Nicola

    2003-09-01

    Rare-earth (RE) based compounds and alloys are of great interest both for their fundamental physical properties and for applications. In order to tailor the required compounds for a specific task, one must be able to predict the energy level structure and transition intensities for any magnetic ion in any crystalline environment. The crystal-field (CF) analysis is one of the most powerful theoretical methods to deal with the physics of magnetic ions. In the present work, this technique is used to analyze peculiar physical properties of some materials employed in the production of new-generation solid-state laser and high-performance permanent magnets.

  10. Modeling local structure using crystal field and spin Hamiltonian parameters: the tetragonal FeK3+-OI2- defect center in KTaO3 crystal

    NASA Astrophysics Data System (ADS)

    Gnutek, P.; Y Yang, Z.; Rudowicz, C.

    2009-11-01

    The local structure and the spin Hamiltonian (SH) parameters, including the zero-field-splitting (ZFS) parameters D and (a+2F/3), and the Zeeman g factors g_{\\parallel } and g_{\\perp } , are theoretically investigated for the FeK3+-OI2- center in KTaO3 crystal. The microscopic SH (MSH) parameters are modeled within the framework of the crystal field (CF) theory employing the CF analysis (CFA) package, which also incorporates the MSH modules. Our approach takes into account the spin-orbit interaction as well as the spin-spin and spin-other-orbit interactions omitted in previous studies. The superposition model (SPM) calculations are carried out to provide input CF parameters for the CFA/MSH package. The combined SPM-CFA/MSH approach is used to consider various structural models for the FeK3+-OI2- defect center in KTaO3. This modeling reveals that the off-center displacement of the Fe3+ ions, Δ1(Fe3+), combined with an inward relaxation of the nearest oxygen ligands, Δ2(O2-), and the existence of the interstitial oxygen OI2- give rise to a strong tetragonal crystal field. This finding may explain the large ZFS experimentally observed for the FeK3+-OI2- center in KTaO3. Matching the theoretical MSH predictions with the available structural data as well as electron magnetic resonance (EMR) and optical spectroscopy data enables predicting reasonable ranges of values of Δ1(Fe3+) and Δ2(O2-) as well as the possible location of OI2- ligands around Fe3+ ions in KTaO3. The defect structure model obtained using the SPM-CFA/MSH approach reproduces very well the ranges of the experimental SH parameters D, g_{\\parallel } and g_{\\perp } and importantly yields not only the correct magnitude of D but also the sign, unlike previous studies. More reliable predictions may be achieved when experimental data on (a+2F/3) and/or crystal field energy levels become available. Comparison of our results with those arising from alternative models existing in the literature indicates

  11. DC-field-assisted grating formation and nonlinear diffractions in methyl-red dye-doped blue phase liquid crystals.

    PubMed

    Khoo, Iam Choon

    2015-01-01

    We report the observation of enhanced nonlinear optical responses of methyl-red-doped blue-phase liquid crystals by application of a DC field. We have observed strong multi-order nonlinear grating diffractions characterized by a nonlinear index coefficient n(2)∼0.5  cm(2)/W using unfocused CW laser power of ∼1  mW and a DC field of a few V/μm. The underlying mechanisms are crystalline lattice and director axis reorientations by torques exerted by the DC field and photo-excited dye molecules.

  12. C-axis Resistivity of Superconductive FeSe Single Crystals: Upper Critical Field and its Angular Behavior

    NASA Astrophysics Data System (ADS)

    Sadakov, A. V.; Romanova, T. A.; Knyazev, D. A.; Chareev, D. A.; Martovitsky, V. P.

    We report out-of-plane magnetotransport ρc(B, T) measurements for a high quality superconducting FeSe single crystals in magnetic fields up to 9 Tesla. Samples, grown from the flux under a permanent gradient of temperature with [001] crystallographic orientation were put in magnetic field parallel to ab-plane. The samples were rotated around c-axis, and its superconducting transitions R(H) were measured for each fixed angle in several temperatures. We show that Hc2 is anisotropic in these relatively small fields, with Hc2||a/Hc2||b being ∼1.2 for T=8.3K.

  13. Magnetic anisotropy and crystalline electric field effects in RRh{sub 4}B{sub 4} single crystals.

    SciTech Connect

    Zhou, H.; Lambert, S. E.; Maple, M. B.; Dunlap, B. D.; Materials Science Division; Univ. of California at San Diego

    2009-08-01

    Research on polycrystalline RRh{sub 4}B{sub 4} samples has shown that crystalline electric field (CEF) effects play an important role in these compounds. The successful synthesis of single crystal samples of RRh{sub 4}B{sub 4} with R = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu has provided an opportunity to further investigate CEF effects in these materials. Magnetization and magnetic susceptibility measurements on the RRh{sub 4}B{sub 4} single crystals revealed strong magnetic anisotropy, and the experimental results could be described well by CEF calculations based on the parameters derived from an analysis of experimental data for ErRh{sub 4}B{sub 4} single crystals. The easy directions of magnetization of these compounds are consistent with the signs of the Stevens factor {alpha}J of the CEF Hamiltonian. A strong influence of magnetic anisotropy on superconductivity was also observed.

  14. Thermal Characterization, Crystal Field Analysis and In-Band Pumped Laser Performance of Er Doped NaY(WO4)2 Disordered Laser Crystals

    PubMed Central

    Serrano, María Dolores; Cascales, Concepción; Han, Xiumei; Zaldo, Carlos; Jezowski, Andrzej; Stachowiak, Piotr; Ter-Gabrielyan, Nikolay; Fromzel, Viktor; Dubinskii, Mark

    2013-01-01

    Undoped and Er-doped NaY(WO4)2 disordered single crystals have been grown by the Czochralski technique. The specific heat and thermal conductivity (κ) of these crystals have been characterized from T = 4 K to 700 K and 360 K, respectively. It is shown that κ exhibits anisotropy characteristic of single crystals as well as a κ(T) behavior observed in glasses, with a saturation mean free phonon path of 3.6 Å and 4.5 Å for propagation along a and c crystal axes, respectively. The relative energy positions and irreducible representations of Stark Er3+ levels up to 4G7/2 multiplet have been determined by the combination of experimental low (<10 K) temperature optical absorption and photoluminescence measurements and simulations with a single-electron Hamiltonian including both free-ion and crystal field interactions. Absorption, emission and gain cross sections of the 4I13/2↔4I15/2 laser related transition have been determined at 77 K. The 4I13/2 Er3+ lifetime (τ) was measured in the temperature range of 77–300 K, and was found to change from τ (77K) ≈ 4.5 ms to τ (300K) ≈ 3.5 ms. Laser operation is demonstrated at 77 K and 300 K by resonantly pumping the 4I13/2 multiplet at λ≈1500 nm with a broadband (FWHM≈20 nm) diode laser source perfectly matching the 77 K crystal 4I15/2 → 4I13/2 absorption profile. At 77 K as much as 5.5 W of output power were obtained in π-polarized configuration with a slope efficiency versus absorbed pump power of 57%, the free running laser wavelength in air was λ≈1611 nm with the laser output bandwidth of 3.5 nm. The laser emission was tunable over 30.7 nm, from 1590.7 nm to 1621.4 nm, for the same π-polarized configuration. PMID:23555664

  15. Thermal characterization, crystal field analysis and in-band pumped laser performance of Er doped NaY(WO(4))(2) disordered laser crystals.

    PubMed

    Serrano, María Dolores; Cascales, Concepción; Han, Xiumei; Zaldo, Carlos; Jezowski, Andrzej; Stachowiak, Piotr; Ter-Gabrielyan, Nikolay; Fromzel, Viktor; Dubinskii, Mark

    2013-01-01

    Undoped and Er-doped NaY(WO4)2 disordered single crystals have been grown by the Czochralski technique. The specific heat and thermal conductivity (κ) of these crystals have been characterized from T = 4 K to 700 K and 360 K, respectively. It is shown that κ exhibits anisotropy characteristic of single crystals as well as a κ(T) behavior observed in glasses, with a saturation mean free phonon path of 3.6 Å and 4.5 Å for propagation along a and c crystal axes, respectively. The relative energy positions and irreducible representations of Stark Er(3+) levels up to (4)G(7/2) multiplet have been determined by the combination of experimental low (<10 K) temperature optical absorption and photoluminescence measurements and simulations with a single-electron Hamiltonian including both free-ion and crystal field interactions. Absorption, emission and gain cross sections of the (4)I(13/2)↔(4)I(15/2) laser related transition have been determined at 77 K. The (4)I(13/2) Er(3+) lifetime (τ) was measured in the temperature range of 77-300 K, and was found to change from τ (77K) ≈ 4.5 ms to τ (300K) ≈ 3.5 ms. Laser operation is demonstrated at 77 K and 300 K by resonantly pumping the (4)I(13/2) multiplet at λ≈1500 nm with a broadband (FWHM≈20 nm) diode laser source perfectly matching the 77 K crystal (4)I(15/2) → (4)I(13/2) absorption profile. At 77 K as much as 5.5 W of output power were obtained in π-polarized configuration with a slope efficiency versus absorbed pump power of 57%, the free running laser wavelength in air was λ≈1611 nm with the laser output bandwidth of 3.5 nm. The laser emission was tunable over 30.7 nm, from 1590.7 nm to 1621.4 nm, for the same π-polarized configuration.

  16. Void growth and coalescence in triaxial stress fields in irradiated FCC single crystals

    NASA Astrophysics Data System (ADS)

    Ling, Chao; Tanguy, Benoît; Besson, Jacques; Forest, Samuel; Latourte, Felix

    2017-08-01

    Void growth and coalescence, known as main mechanisms of ductile fracture, are investigated for irradiated FCC single crystals. Finite element simulations of voided unit cells are performed with a single crystal plasticity model accounting for strain hardening and softening associated with irradiation-induced defects. The simulations predict a rather brittle overall behavior for the voided irradiated single crystal at high stress triaxiality, with a large amount of local plastic deformation, which is consistent with experimental observations reported in the literature for stainless steels irradiated in fast reactors. Compared with unirradiated single crystals, irradiated crystals exhibit a higher void growth rate leading to an earlier void coalescence, which is caused by a stronger plastic slip localization in the region near the voids.

  17. On beam shaping of the field radiated by a line source coupled to finite or infinite photonic crystals.

    PubMed

    Ceccuzzi, Silvio; Jandieri, Vakhtang; Baccarelli, Paolo; Ponti, Cristina; Schettini, Giuseppe

    2016-04-01

    Comparison of the beam-shaping effect of a field radiated by a line source, when an ideal infinite structure constituted by two photonic crystals and an actual finite one are considered, has been carried out by means of two different methods. The lattice sums technique combined with the generalized reflection matrix method is used to rigorously investigate the radiation from the infinite photonic crystals, whereas radiation from crystals composed of a finite number of rods along the layers is analyzed using the cylindrical-wave approach. A directive radiation is observed with the line source embedded in the structure. With an increased separation distance between the crystals, a significant edge diffraction appears that provides the main radiation mechanism in the finite layout. Suitable absorbers are implemented to reduce the above-mentioned diffraction and the reflections at the boundaries, thus obtaining good agreement between radiation patterns of a localized line source coupled to finite and infinite photonic crystals, when the number of periods of the finite structure is properly chosen.

  18. Exchange field on the rare earth Sm3+ in a single crystal perovskite SmMnO3

    NASA Astrophysics Data System (ADS)

    Cheng, J.-G.; Zhou, J.-S.; Goodenough, J. B.; Su, Y. T.; Sui, Y.; Ren, Y.

    2011-09-01

    Single crystal SmMnO3 has been grown by the floating-zone method. We have measured the magnetization and specific heat in magnetic fields oriented along three principal crystal axes of precisely oriented single crystals. Below TN of the Mn3+-ion array, the magnetic moments of the Sm3+ ions are progressively oriented antiparellel to the weak canted-spin ferromagnetic moment of the antiferromagnetic (AF) Mn3+-ion array due to an internal exchange field Hin ∥ c. On cooling through a compensation temperature Tcomp ≈ 9 K, the dominant moment parallel to c changes from the canted-spin Mn3+ ions to the Sm3+ moments. A spin reversal in an Hc ≥ 1 T changes the magnetic field splitting of the Kramers doublet on the Sm3+ ions from Hin - Hc to Hin + Hc, where Hc is a field applied along the c axis. This change, monitored by the Schottky contribution to the specific heat, creates an abrupt change at Tt = Tcomp ± δ. We have found no evidence that the transition at Tt is first-order despite its abrupt nature.

  19. Magnetic properties of a ferrimagnetic mixed (1,3/2) spin chain with inhomogeneous crystal-field anisotropy

    NASA Astrophysics Data System (ADS)

    Solano-Carrillo, E.; Franco, R.; Silva-Valencia, J.

    2010-07-01

    Using molecular-field theory and density-matrix renormalization group calculations we investigated the magnetic properties of a ferrimagnetic mixed (1,3/2) Ising spin chain with inhomogeneous crystal-field anisotropy. Our analysis introduces a clear physical mechanism for the appearance of the magnetic plateaus in the system and for the quantum phase transitions which are present. We consider two cases of interest: when the crystal field anisotropy D1 is present only on the spin-1 ions, and when D is present only on the spin-3/2 ions. This latter case turns out to be the more interesting one since a plateau at {1}/{5} of the saturation magnetization is formed by means of two physically distinct mechanisms. The magnetic change between these two phases is gradual, varying over the region 1/2crystal field anisotropy is present only on the spin-1 ions is favorable since the overall free energy of the system is lower.

  20. Effect of crystal-field splitting and interband hybridization on the metal-insulator transitions of strongly correlated systems

    NASA Astrophysics Data System (ADS)

    Poteryaev, Alexander I.; Ferrero, Michel; Georges, Antoine; Parcollet, Olivier

    2008-07-01

    We investigate a quarter-filled two-band Hubbard model involving a crystal-field splitting, which lifts the orbital degeneracy as well as an interorbital hopping (interband hybridization). Both terms are relevant to the realistic description of correlated materials such as transition-metal oxides. The nature of the Mott metal-insulator transition is clarified and is found to depend on the magnitude of the crystal-field splitting. At large values of the splitting, a transition from a two-band to a one-band metal is first found as the on-site repulsion is increased and is followed by a Mott transition for the remaining band, which follows the single-band (Brinkman-Rice) scenario well documented previously within dynamical mean-field theory. At small values of the crystal-field splitting, a direct transition from a two-band metal to a Mott insulator with partial orbital polarization is found, which takes place simultaneously for both orbitals. This transition is characterized by a vanishing of the quasiparticle weight for the majority orbital but has a first-order character for the minority orbital. It is pointed out that finite-temperature effects may easily turn the metallic regime into a bad metal close to the orbital polarization transition in the metallic phase.

  1. Doping-driven orbital-selective Mott transition in multi-band Hubbard models with crystal field splitting

    NASA Astrophysics Data System (ADS)

    Yilin, Wang; Li, Huang; Liang, Du; Xi, Dai

    2016-03-01

    We have studied the doping-driven orbital-selective Mott transition in multi-band Hubbard models with equal band width in the presence of crystal field splitting. Crystal field splitting lifts one of the bands while leaving the others degenerate. We use single-site dynamical mean-field theory combined with continuous time quantum Monte Carlo impurity solver to calculate a phase diagram as a function of total electron filling N and crystal field splitting Δ. We find a large region of orbital-selective Mott phase in the phase diagram when the doping is large enough. Further analysis indicates that the large region of orbital-selective Mott phase is driven and stabilized by doping. Such models may account for the orbital-selective Mott transition in some doped realistic strongly correlated materials. Project supported by the National Natural Science Foundation of China (Grant No. 2011CBA00108) and the National Basic Research Program of China (Grant No. 2013CB921700).

  2. Analysis of the effect of symmetric/asymmetric CUSP magnetic fields on melt/crystal interface during Czochralski silicon growth

    NASA Astrophysics Data System (ADS)

    Daggolu, Parthiv; Ryu, Jae Woo; Galyukov, Alex; Kondratyev, Alexey

    2016-10-01

    With the use of 300 mm silicon wafers for industrial semiconductor device manufacturing, the Czochralski (Cz) crystal growth process has to be optimized to achieve higher quality and productivity. Numerical studies based on 2D global thermal models combined with 3D simulation of melt convection are widely used today to save time and money in the process development. Melt convection in large scale Cz Si growth is controlled by a CUSP or transversal magnetic field (MF) to suppress the melt turbulence. MF can be optimized to meet necessary characteristics of the growing crystal, in terms of point defects, as MF affects the melt/crystal interface geometry and allows adjustment of the pulling rate. Among the different knobs associated with the CUSP magnetic field, the nature of its configuration, going from symmetric to asymmetric, is also reported to be an important tool for the control of crystallization front. Using a 3D unsteady model of the CGSim software, we have studied these effects and compared with several experimental results. In addition, physical mechanisms behind these observations are explored through a detailed modeling analysis of the effect of an asymmetric CUSP MF on convection features governing the heat transport in the silicon melt.

  3. Electric field induced structural colour tuning of a silver/titanium dioxide nanoparticle one-dimensional photonic crystal

    PubMed Central

    Aluicio-Sarduy, Eduardo; Callegari, Simone; Figueroa del Valle, Diana Gisell; Desii, Andrea; Kriegel, Ilka

    2016-01-01

    Summary An electric field is employed for the active tuning of the structural colour in photonic crystals, which acts as an effective external stimulus with an impact on light transmission manipulation. In this work, we demonstrate structural colour in a photonic crystal device comprised of alternating layers of silver nanoparticles and titanium dioxide nanoparticles, exhibiting spectral shifts of around 10 nm for an applied voltage of only 10 V. The accumulation of charge at the metal/dielectric interface with an applied electric field leads to an effective increase of the charges contributing to the plasma frequency in silver. This initiates a blue shift of the silver plasmon band with a simultaneous blue shift of the photonic band gap as a result of the change in the silver dielectric function (i.e. decrease of the effective refractive index). These results are the first demonstration of active colour tuning in silver/titanium dioxide nanoparticle-based photonic crystals and open the route to metal/dielectric-based photonic crystals as electro-optic switches. PMID:27826514

  4. Particles at fluid-fluid interfaces: A new Navier-Stokes-Cahn-Hilliard surface-phase-field-crystal model

    PubMed Central

    Aland, Sebastian; Lowengrub, John; Voigt, Axel

    2013-01-01

    Colloid particles that are partially wetted by two immiscible fluids can become confined to fluid-fluid interfaces. At sufficiently high volume fractions, the colloids may jam and the interface may crystallize. The fluids together with the interfacial colloids form an emulsion with interesting material properties and offer an important route to new soft materials. A promising approach to simulate these emulsions was presented in Aland et al. [Phys. Fluids 23, 062103 (2011)], where a Navier-Stokes-Cahn-Hilliard model for the macroscopic two-phase fluid system was combined with a surface phase-field-crystal model for the microscopic colloidal particles along the interface. Unfortunately this model leads to spurious velocities which require very fine spatial and temporal resolutions to accurately and stably simulate. In this paper we develop an improved Navier-Stokes-Cahn-Hilliard-surface phase-field-crystal model based on the principles of mass conservation and thermodynamic consistency. To validate our approach, we derive a sharp interface model and show agreement with the improved diffuse interface model. Using simple flow configurations, we show that the new model has much better properties and does not lead to spurious velocities. Finally, we demonstrate the solid-like behavior of the crystallized interface by simulating the fall of a solid ball through a colloid-laden multiphase fluid. PMID:23214691

  5. Particles at fluid-fluid interfaces: A new Navier-Stokes-Cahn-Hilliard surface- phase-field-crystal model.

    PubMed

    Aland, Sebastian; Lowengrub, John; Voigt, Axel

    2012-10-01

    Colloid particles that are partially wetted by two immiscible fluids can become confined to fluid-fluid interfaces. At sufficiently high volume fractions, the colloids may jam and the interface may crystallize. The fluids together with the interfacial colloids form an emulsion with interesting material properties and offer an important route to new soft materials. A promising approach to simulate these emulsions was presented in Aland et al. [Phys. Fluids 23, 062103 (2011)], where a Navier-Stokes-Cahn-Hilliard model for the macroscopic two-phase fluid system was combined with a surface phase-field-crystal model for the microscopic colloidal particles along the interface. Unfortunately this model leads to spurious velocities which require very fine spatial and temporal resolutions to accurately and stably simulate. In this paper we develop an improved Navier-Stokes-Cahn-Hilliard-surface phase-field-crystal model based on the principles of mass conservation and thermodynamic consistency. To validate our approach, we derive a sharp interface model and show agreement with the improved diffuse interface model. Using simple flow configurations, we show that the new model has much better properties and does not lead to spurious velocities. Finally, we demonstrate the solid-like behavior of the crystallized interface by simulating the fall of a solid ball through a colloid-laden multiphase fluid.

  6. Electric field induced structural colour tuning of a silver/titanium dioxide nanoparticle one-dimensional photonic crystal.

    PubMed

    Aluicio-Sarduy, Eduardo; Callegari, Simone; Figueroa Del Valle, Diana Gisell; Desii, Andrea; Kriegel, Ilka; Scotognella, Francesco

    2016-01-01

    An electric field is employed for the active tuning of the structural colour in photonic crystals, which acts as an effective external stimulus with an impact on light transmission manipulation. In this work, we demonstrate structural colour in a photonic crystal device comprised of alternating layers of silver nanoparticles and titanium dioxide nanoparticles, exhibiting spectral shifts of around 10 nm for an applied voltage of only 10 V. The accumulation of charge at the metal/dielectric interface with an applied electric field leads to an effective increase of the charges contributing to the plasma frequency in silver. This initiates a blue shift of the silver plasmon band with a simultaneous blue shift of the photonic band gap as a result of the change in the silver dielectric function (i.e. decrease of the effective refractive index). These results are the first demonstration of active colour tuning in silver/titanium dioxide nanoparticle-based photonic crystals and open the route to metal/dielectric-based photonic crystals as electro-optic switches.

  7. Path length tunable light-matter interaction in magnetic nanofluid based field-induced photonic crystal-glass structure

    NASA Astrophysics Data System (ADS)

    Laskar, Junaid M.; Raj, Baldev; Philip, John

    2016-10-01

    The ability to control the light-matter interaction and the simultaneous tuning of both the structural order and disorder in materials, although important in photonics, remain major challenges. In this paper, we demonstrate that path length dictates light-matter interaction for the same crystal structure, formed by the ordering of magnetic nanoparticle self-assembled columns inside magnetic nanofluid under applied field. When the path length is shorter (L=80 μ {{m}}), the condition for maintaining temporal coherence for the constructive interference is therefore satisfied, resulting in the formation of a concentric diffraction ring pattern; while for a longer path length (L=1 {{mm}}), only a corona ring of scattered light is observed. Analysis of diffraction ring pattern suggests the formation of 3D hexagonal crystal structure, where the longitudinal and lateral inter-column spacings are 5.281 μm and 7.344 μm, respectively. Observation of speckles and diffuse scattering background within the diffraction ring pattern confirms the presence of certain degree of crystal disorder, which can be tuned by controlling the applied field strength, nanoparticle size and particle volume fraction. Our results provide a new approach to develop next generation of tunable photonic devices, e.g. tunable random laser, based on simultaneous harnessing of the properties of disordered photonic glass and 3D photonic crystal.

  8. Touch sensors based on planar liquid crystal-gated-organic field-effect transistors

    SciTech Connect

    Seo, Jooyeok; Lee, Chulyeon; Han, Hyemi; Lee, Sooyong; Nam, Sungho; Kim, Youngkyoo; Kim, Hwajeong; Lee, Joon-Hyung; Park, Soo-Young; Kang, Inn-Kyu

    2014-09-15

    We report a tactile touch sensor based on a planar liquid crystal-gated-organic field-effect transistor (LC-g-OFET) structure. The LC-g-OFET touch sensors were fabricated by forming the 10 μm thick LC layer (4-cyano-4{sup ′}-pentylbiphenyl - 5CB) on top of the 50 nm thick channel layer (poly(3-hexylthiophene) - P3HT) that is coated on the in-plane aligned drain/source/gate electrodes (indium-tin oxide - ITO). As an external physical stimulation to examine the tactile touch performance, a weak nitrogen flow (83.3 μl/s) was employed to stimulate the LC layer of the touch device. The LC-g-OFET device exhibited p-type transistor characteristics with a hole mobility of 1.5 cm{sup 2}/Vs, but no sensing current by the nitrogen flow touch was measured at sufficiently high drain (V{sub D}) and gate (V{sub G}) voltages. However, a clear sensing current signal was detected at lower voltages, which was quite sensitive to the combination of V{sub D} and V{sub G}. The best voltage combination was V{sub D} = −0.2 V and V{sub G} = −1 V for the highest ratio of signal currents to base currents (i.e., signal-to-noise ratio). The change in the LC alignment upon the nitrogen flow touch was assigned as the mechanism for the present LC-g-OFET touch sensors.

  9. Dependence of Pentacene Crystal Growth on Dielectric Roughness for Fabrication of Flexible Field-Effect Transistors

    SciTech Connect

    Yang, H.; Yang, C; Kim, S; Jang, M; Park, C

    2010-01-01

    The dependence of pentacene nanostructures on gate dielectric surfaces were investigated for flexible organic field-effect transistor (OFET) applications. Two bilayer types of polymer/aluminum oxide (Al{sub 2}O{sub 3}) gate dielectrics were fabricated on commercial Al foils laminated onto a polymer back plate. Some Al foils were directly used as gate electrodes, and others were smoothly polished by an electrolytic etching. These Al surfaces were then anodized and coated with poly({alpha}-methyl styrene) (PAMS). For PAMS/Al{sub 2}O{sub 3} dielectrics onto etched Al foils, surface roughness up to 1 nm could be reached, although isolated dimples with a lateral diameter of several micrometers were still present. On PAMS/Al{sub 2}O{sub 3} dielectrics (surface roughness >40 nm) containing mechanical grooves of Al foil, average hole mobility ({mu}FET) of 50 nm thick pentacene-FETs under the low operating voltages (|V| < 6 V) was {approx}0.15 cm{sup 2} V{sup -1} s{sup -1}. In contrast, pentacene-FETs employing the etched Al gates exhibited {mu}FET of 0.39 cm{sup 2} V{sup -1} s{sup -1}, which was comparable to that of reference samples with PAMS/Al{sub 2}O{sub 3} dielectrics onto flat sputtered Al gates. Conducting-probe atomic force microscopy and two-dimensional X-ray diffraction of pentacene films with various thicknesses revealed different out-of-plane and in-plane crystal orderings of pentacene, depending on the surface roughness of the gate dielectrics.

  10. Growth of K(Cl, Br) crystals from aqueous solutions in an X-ray field

    SciTech Connect

    Anishchik, V. M.; Val'ko, N. G. Voina, V. V.; Vorontsov, A. S.

    2008-07-15

    The influence of X-rays of different wavelength on the degree of structural quality of K(Cl, Br) crystals of mixed composition grown from aqueous solutions is considered. It is found by the methods of chemical etching, X-ray analysis, pycnometric density, and atomic-force microscopy that X-ray irradiation leads to the formation of crystals with a more perfect substructure and surface morphology and a decrease of the dislocation density and concentration of pores and cavities containing the mother liquor. It is shown that X-ray irradiation can promote preferential incorporation of less electronegative components into the crystal lattice of a solid solution.

  11. Metastable Copper-Phthalocyanine Single-Crystal Nanowires and Their Use in Fabricating High-Performance Field-Effect Transistors

    SciTech Connect

    Xiao, Kai; Li, Rongjin; Tao, Jing; Payzant, E Andrew; Ivanov, Ilia N; Puretzky, Alexander A; Hu, Wenping; Geohegan, David B

    2009-01-01

    This paper describes a simple, vapor-phase route to the synthesis of metastable α-phase copper-phthalocyanine (CuPc) single-crystal nanowires through control of the growth temperature. The influence of the growth temperature on the crystal structures, morphology, and size of the CuPc nanostructures was explored by XRD, optical absorption and Transmission Electron Microscopy (TEM). α-CuPc nanowires were successfully incorporated as active semiconductors in field-effect transistors (FETs). Single nanowire devices exhibited the carrier mobilities and current on/off ratios as high as 0.4 cm2/Vs and > 104, respectively, rendering them useful for organic photovoltaic cells, organic light-emitting diodes, field-effect transistors, memories and gas sensors

  12. Reversed Crystal-Field Splitting and Spin-Orbital Ordering in α-Sr2CrO4

    NASA Astrophysics Data System (ADS)

    Ishikawa, Takashi; Toriyama, Tatsuya; Konishi, Takehisa; Sakurai, Hiroya; Ohta, Yukinori

    2017-03-01

    The origin of successive phase transitions observed in the layered perovskite α-Sr2CrO4 is studied by the density-functional-theory-based electronic structure calculation and mean-field analysis of the proposed low-energy effective model. We find that, despite the fact that the CrO6 octahedron is elongated along the c-axis of the crystal structure, the crystal-field level of nondegenerate 3dxy orbitals of the Cr ion is lower in energy than that of doubly degenerate 3dyz and 3dxz orbitals, giving rise to the orbital degrees of freedom in the system with a 3d2 electron configuration. We show that the higher (lower) temperature phase transition is caused by the ordering of the orbital (spin) degrees of freedom.

  13. k Dependence of the crystal-field splittings of 4f states in rare-earth systems.

    PubMed

    Vyalikh, D V; Danzenbächer, S; Kucherenko, Yu; Kummer, K; Krellner, C; Geibel, C; Holder, M G; Kim, T K; Laubschat, C; Shi, M; Patthey, L; Follath, R; Molodtsov, S L

    2010-12-03

    The occupation, energy separation, and order of the crystal-field-split 4f states are crucial for the understanding of the magnetic properties of rare-earth systems. We provide the experimental evidence that crystal-field-split 4f states exhibit energy dispersion in momentum space leading to variations of energy spacings between them and even of their energy sequence across the Brillouin zone. These observations were made by performing angle-resolved photoemission experiments on YbRh(2)Si(2) and properly simulated within a simple model based on results obtained by inelastic neutron scattering experiments and band structure calculations. Our findings should be generally applicable to rare-earth systems and have considerable impact on the understanding of magnetism and related phenomena.

  14. An updated version of the computational package SIMPRE that uses the standard conventions for Stevens crystal field parameters.

    PubMed

    Baldoví, José J; Clemente-Juan, Juan M; Coronado, Eugenio; Gaita-Ariño, Alejandro; Palii, Andrew

    2014-10-05

    The crystal field approach used by SIMPRE is analyzed, verifying the exactness of the results concerning energy levels and magnetic properties calculated by the package. To coincide with the prevailing conventions, we reformulate the presentation of the crystal field parameters, so that the results are now, also from a formal point of view, strictly correct. New calculations are presented to test the influence of neglecting the excited J states, a common but critical approximation employed by SIMPRE. For that, we examine the case of Er(trensal) complex (H3 trensal = 2,2',2″-tris(salicylideneimino)triethylamine) where the influence of this approximation is found to be minimal. A patched version of the code, SIMPRE 1.1, and an updated version of the user manual are now available. Finally, we comment on "Software package SIMPRE - revisited," which apparently revisits a software package without inspecting or using the code. Copyright © 2014 Wiley Periodicals, Inc.

  15. Spatial inhomogeneity in RFeAs(O,F)(R = Pr, Nd) determined from rare earth crystal field excitations.

    SciTech Connect

    Goremychkin, E. A.; Osborn, R.; Wang, C. H.; Lumsden, M. D.; McGuire, M. A.; Sefat, A. S.; Sales, B. C.; Mandrus, D.; Ronnow, H. M.; Su, Y.; Christianson, A. D.

    2011-06-27

    We report inelastic neutron-scattering measurements of crystal-field transitions in PrFeAsO, PrFeAsO{sub 0.87}F{sub 0.13}, and NdFeAsO{sub 0.85}F{sub 0.15}. Doping with fluorine produces additional crystal-field excitations, providing evidence that there are two distinct charge environments around the rare-earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4f electrons of the Pr{sup 3+} and Nd{sup 3+} ions have nonmagnetic and magnetic ground states, respectively, indicating that the enhancement of T{sub c} compared to LaFeAsO{sub 1-x}F{sub x} is not due to rare-earth magnetism.

  16. Hydrogen-deuterium exchange induced by an electric field in α-Al2O3 single crystals

    NASA Astrophysics Data System (ADS)

    Ramírez, R.; Colera, I.; Gonz&Ález, R.; Savoini, B.; Chen, Y.

    Hydrogen and deuterium are observed in α-Al2O3 crystals in the form of OH- and OD- radicals, respectively, which absorb in the infrared region. Infrared-absorption measurements were used to monitor diffusion of deuterons and protons in α-Al2O3 single crystals under the application of a moderate electric field parallel to the crystallographic c-axis, in the temperature range of 973-1333K. A linear dependence of the percent of exchange with both annealing time and applied voltage is observed, indicating that ionic conduction was taking place. The activation energy for the H+ ↔; D+ exchange was determined to be 2.4 eV, less than half the value obtained by pure thermal means, suggesting that under the application of an electric field the deuteron (proton) diffusion mechanism is different.

  17. Crystal-electric-field excitations and spin dynamics in Ce3Co4Sn13 semimetallic chiral-lattice phase

    NASA Astrophysics Data System (ADS)

    Iwasa, Kazuaki; Otomo, Yuka; Suyama, Kazuya; Tomiyasu, Keisuke; Ohira-Kawamura, Seiko; Nakajima, Kenji; Mignot, Jean-Michel

    2017-05-01

    Inelastic neutron scattering experiments have been conducted to investigate the spin dynamics and crystal-electric-field level scheme of the Ce 4 f electrons in Ce3Co4Sn13 . This compound exhibits a large specific heat at low temperatures and anomalous semimetallic transport in the chiral crystallographic phase below 160 K. Distinctly observed magnetic excitations at approximately 6 and 29 meV are asymmetric in spectral shape and are reproduced by two inequivalent crystal-electric-field splitting schemes, which are deduced from the chiral structure. We have also observed the spin dynamics reflecting antiferromagnetic correlations below 1 meV, which is enhanced with an upturn in the electrical resistivity below 15 K and which yields a low-energy density of state relevant to the large specific heat. We discuss the possibility of a three-dimensional Weyl semimetal state, considering the chiral-lattice symmetry, electronic hybridization, and magnetic correlation.

  18. 4D STUDY OF STRAIN GRADIENTS EVOLUTION IN TWINNED NiMnGa SINGLE CRYSTALS UNDER MAGNETIC FIELD

    SciTech Connect

    Barabash, Rozaliya; Xu, Ruqing; Barabash, Oleg M; Sozinov, Alexei

    2014-01-01

    Time-resolved 3D X-ray microscopy with a submicron beam size was used to follow the evolution of strains in off-stoichiometric NiMnGa twinned crystals near type I (hard) twin boundary under magnetic field. Laminate A/B microstructure was revealed near the twin boundaries in A variant. Large strain gradients are observed in the C variant in the immediate vicinity of the type I twin boundary: the lattice is under large tensile strains ~0.4% along the c- axes within first micron. Distinct a and b lattice parameter evolution with temperature and magnetic field is demonstrated. In an applied magnetic field the strain field was observed at larger distances from the twin boundary and becomes more complex. Stochastic twin boundary motion was observed after the magnetic field reaches a certain critical value.

  19. Tuning the band structures of a one-dimensional width-modulated magnonic crystal by a transverse magnetic field

    SciTech Connect

    Di, K.; Lim, H. S. Zhang, V. L.; Ng, S. C.; Kuok, M. H.; Nguyen, H. T.; Cottam, M. G.

    2014-02-07

    Theoretical studies, based on three independent techniques, of the band structure of a one-dimensional width-modulated magnonic crystal under a transverse magnetic field are reported. The band diagram is found to display distinct behaviors when the transverse field is either larger or smaller than a critical value. The widths and center positions of bandgaps exhibit unusual non-monotonic and large field-tunability through tilting the direction of magnetization. Some bandgaps can be dynamically switched on and off by simply tuning the strength of such a static field. Finally, the impact of the lowered symmetry of the magnetic ground state on the spin-wave excitation efficiency of an oscillating magnetic field is discussed. Our finding reveals that the magnetization direction plays an important role in tailoring magnonic band structures and hence in the design of dynamic spin-wave switches.

  20. Influence of the magnetic and laser fields on the director structures of a ferrocholesteric liquid crystal in homeotropic cells

    NASA Astrophysics Data System (ADS)

    Petrescu, Emil; Bena, Eleonora-Rodica

    We study the influence of the magnetic and laser fields on the director structures of a ferrocholesteric liquid crystal in homeotropic cells. Using the analytical method based on the Euler-Lagrange equations, we find a correlation between the fields intensities and the confinement ratio r=d/p, ( d is the cell thickness and p is the cholesteric pitch) at the limit of the transition from the homeotropic alignment to the translationally invariant configuration (TIC) with uniform in plane twist. We discuss this correlation as a function of the sign of the magnetic and dielectric anisotropies. If both anisotropies are positive and the magnetic field and the laser beam are perpendicular to the cell walls, the magnetic field preserves the homeotropic alignment while the laser beam pushes the system towards the TIC. The control parameters of the transition are the laser beam intensity and the confinement ratio. If the magnetic anisotropy is negative and the dielectric one is positive both fields concur in driving the system towards the TIC. The spinodal surface separating the metastable homeotropic configuration from the instable TIC is an ellipsoid whose halfaxes are smaller that in the case of a pure liquid crystal by a factor depending on the material constants and the cell thickness. We find also the total twist angle across the sample witch can be varied between a lower and an upper limit by changing the light intensity or the magnetic field strength. Our results can be useful in designing magneto-optical devices.