Science.gov

Sample records for small silicon clusters

  1. Electronic structure of small silicon clusters

    SciTech Connect

    Wales, D.J.

    1994-03-01

    Predictions of Stone`s tensor-surface-harmonic theory [Mol. Phys. 41, 1339 (1980)] for bonding in small silicon clusters are tested by {ital ab} {ital initio} calculations. Stable geometries, along with the energies and symmetries of the occupied molecular orbitals, may all be rationalized within the model. For example, local energy minima for Si{sub {ital n}}{sup 2{minus}} clusters are found which are isostructural to the borane and carborane clusters B{sub {ital n}}H{sub {ital n}}{sup 2{minus}} and C{sub 2}B{sub {ital n}{minus}2}H{sub {ital n}}. In particular, both Si{sub 12}{sup 2{minus}} and Si{sub 13}{sup 2+} exhibit icosahedral geometries which are true minima.

  2. How are small endohedral silicon clusters stabilized?

    PubMed

    Avaltroni, Fabrice; Steinmann, Stephan N; Corminboeuf, Clémence

    2012-11-21

    Clusters in the (Be, B, C)@Si(n)((0,1,2+)) (n = 6-10) series, isoelectronic to Si(n)(2-), present multiple symmetric structures, including rings, cages and open structures, which the doping atom stabilizes using contrasting bonding mechanisms. The most striking feature of these clusters is the absence of electron transfer (for Be) or even the inversion (for B and C) in comparison to classic endohedral metallofullerenes (e.g. from the outer frameworks towards the enclosed atom). The relatively small cavity of the highly symmetric Si(8) cubic cage benefits more strongly from the encapsulation of a boron atom than from the insertion of a too large beryllium atom. Overall, the maximization of multicenter-type bonding, as visualized by the Localized Orbital Locator (LOL), is the key to the stabilization of the small Si(n) cages. Boron offers the best balance between size, electronegativity and delocalized bonding pattern when compared to beryllium and carbon. PMID:22968417

  3. Equilibrium Geometries and Electronic Structure of Small Silicon Monohydrides Clusters

    NASA Astrophysics Data System (ADS)

    Yang, Jucai; Bai, Xue; Li, Chunping; Xu, Wenguo; Xiao, Wensheng

    The geometries and energies of small silicon monohydride clusters (Si2H-Si10H) have been systematically investigated by density functional theory (DFT) scheme with DZP++ basis sets. Several possible geometric arrangements and electronic states have been considered for each cluster. The results on Si2H-Si4H are in good accordance with previous ab initio calculation. The geometry of ground state of Si2H is found to be a bridged C2v structure, and Si3H to be a bridged C2v, while Si4H a non-bridged Cs symmetry with 2A‧ state. The non-bridged geometries of ground state of Si5H-Si10H have been found to be corresponding to C2v (2B1), C2v (2B1), C5v (2A1), Cs (2A‧‧) (have two types), C1 (not symmetry), and Cs (2A‧), respectively. The results on Si5H, Si6H, Si8H and Si9H are different from previous calculations. Compared silicon clusters (Sin) with silicon monohydrides (SinH) clusters, the addition of a single hydrogen atom cannot cause great changes in the ground state geometries of Si2, Si3, Si4, Si7, Si9, and Si10 clusters, while in the ground state geometries of Si5, Si6 and Si8 clusters the change is great. The dissociation energies calculated indicates that Si4H, Si7H, and Si10H clusters are less stable than others.

  4. Influence of group 10 metals on the growth and subsequent Coulomb explosion of small silicon clusters under strong light pulses.

    PubMed

    Ross, Matt W; Castleman, A W

    2013-03-18

    Growth and ionization patterns of small silicon clusters are studied using ultrafast pulses centered at 624 nm by varying the metal electron source for cluster formation using group 10 transition metals. The silicon-cluster size was observed to change as the electron source was varied from Pdsilicon-cluster growth in the palladium system is attributed to the higher work function of palladium metal, producing less collisions of the laser-induced plasma with the silane. This shows that changing the metal electron source while holding the laser intensity constant affects the degree of dehydrogenation of SiH4 due to the number of collisions in the cluster source. The saturation intensities of each atomic charge state of silicon, resulting from Coulomb explosion of pure silicon clusters, formed with each metal are measured and compared to those calculated by using semi-classical tunneling theory assuming sequential ionization. The ion signal of silicon atomic charge states produced when using palladium as electron source for cluster formation shows a greater degree of ionization enhancement than that observed for the nickel and platinum systems. This is reflected by the smaller-size clusters formed in the palladium system. Based on a plot of the ion signal as a function of laser intensity compared to the simulated ion signal from tunneling theory, the ionization enhancement of silicon high-charge states is found to increase by varying the electron source from Ni

  5. Determination of ionization energies of small silicon clusters with vacuum?ultraviolet (VUV) radiation

    SciTech Connect

    Kostko, Oleg; Leone, Stephen R.; Duncan, Michael A.; Ahmed, Musahid

    2009-09-23

    In this work we report on single photon vacuum ultraviolet photoionization of small silicon clusters (n=1-7) produced via laser ablation of Si. The adiabatic ionization energies (AIE) are extracted from experimental photoionization efficiency (PIE) curves with the help of Frank?Condon simulations, used to interpret the shape and onset of the PIE curves. The obtained AIEs are (all energies are in eV): Si (8.13+-0.05), Si2 (7.92+-0.05), Si3 (8.12+-0.05), Si4 (8.2+-0.1), Si5 (7.96+-0.07), Si6 (7.8+-0.1), and Si7 (7.8+-0.1). Most of the experimental AIE values are in good agreement with ab initio electronic structure calculations. To explain observed deviations between the experimental and theoretical AIEs for Si4 and Si6, a theoretical search of different isomers of these species is performed. Electronic structure calculations aid in the interpretation of the a2PIu state of Si2+ dimer in the PIE spectrum. Time dependent density functional theory (TD-DFT) calculations are performed to reveal the energies of electronically excited states in the cations for a number of Si clusters.

  6. A density functional study of small sized silver-doped silicon clusters: Ag2Sin (n = 1-13)

    NASA Astrophysics Data System (ADS)

    Yang, Cai; Hao Jia, Song; Ma, Mao Fen; Zhang, Shuai; Lu, Cheng; Li, Gen Quan

    2015-11-01

    The structures and electronic properties for global minimum geometric structures of small-sized neutral Ag2Sin (n = 1-13) clusters have been investigated using the CALYPSO structure searching method coupled with density functional theory calculations. A great deal of low-energy geometric isomers are optimised at the B3LYP / GENECP theory level. The optimised structures suggest that the ground state Ag2Sin clusters are visibly distorted compared with the corresponding pure silicon clusters and favor a three-dimensional configuration. Starting with Ag2Si12, one Ag atom is fully encapsulated by the Si outer cages. Based on the averaged binding energy, fragmentation energy, second-order energy difference and HOMO-LUMO energy gap, it is seen that Ag2Si2 and Ag2Si5 are tested to be the most stable clusters, and the chemical stabilities of pure Sin+2 clusters can be reduced to some extent after doping two Ag atoms. Additionally, natural population and natural electronic configuration are discussed and the results reveal that charges transfer from the Ag atoms to the silicon frames and the spd hybridisations are present in all Ag2Sin clusters. Lastly, the results of natural bonds show that the Ag-Si bond in Ag2Sin clusters is dominated by small ionic character. Supplementary material in the form of one pdf file available from the Journal web page at http://dx.doi.org/10.1140/epjd/e2015-60404-1

  7. Two-component density functional theory calculations of positron lifetimes for small vacancy clusters in silicon

    NASA Astrophysics Data System (ADS)

    Makhov, D. V.; Lewis, Laurent J.

    2005-05-01

    The positron lifetimes for various vacancy clusters in silicon are calculated within the framework of the two-component electron-positron density functional theory. The effect of the trapped positron on the electron density and on the relaxation of the structure is investigated. Our calculations show that, contrary to the usual assumption, the positron-induced forces do not compensate in general for electronic inward forces. Thus, geometry optimization is required in order to determine positron lifetime accurately. For the monovacancy and the divacancy, the results of our calculations are in good agreement with the experimental positron lifetimes, suggesting that this approach gives good estimates of positron lifetimes for larger vacancy clusters, required for their correct identification with positron annihilation spectroscopy. As an application, our calculations show that fourfold trivacancies and symmetric fourfold tetravacancies have positron lifetimes similar to monovacancies and divacancies, respectively, and can thus be confused in the interpretation of positron annihilation experiments.

  8. Implantation and post-annealing characteristics when impinging small B n clusters into silicon at low fluence

    NASA Astrophysics Data System (ADS)

    Liang, J. H.; Han, H. M.

    2005-01-01

    This study investigated the similarities and differences between B1 monomer and Bn cluster ion implantation into silicon. Small polyatomic boron ions ( Bn- , n = 1-4) with the same atomic boron kinetic energy (20 keV/atom) and atomic fluence (5 × 1013 atoms/cm2) were used. In the simulation, the widely-used SRIM computer code was employed to calculate the as-implanted boron and damage depth profiles of B1 monomer ion implantation in order to make comparisons with experimental results. In the experimental one, the B1 monomer and Bn cluster ions extracted from a tandem accelerator were used to perform ion implantation. Post-annealing methods included one-step (RTA) and two-step (FA + RTA) treatments, where RTA denoted high-temperature rapid thermal annealing at 1050 °C for 10 s and FA represented low-temperature furnace annealing at 550 °C for 1 h. The results revealed that all four as-implanted range parameters (average range, longitudinal range straggling, skewness, kurtosis) increase and tend to saturate as the cluster size increases when compared to those of SRIM-calculated results for the B1 implant. Furthermore, the peculiar damage structures produced by different Bn cluster ions lead to various behaviors in both diffusing and activating boron atoms.

  9. Structures and stabilities of small silicon clusters: Ab initio molecular-orbital calculations of Si7-Si11

    NASA Astrophysics Data System (ADS)

    Zhu, Xiaolei; Zeng, X. C.

    2003-02-01

    Ab initio all-electron molecular-orbital calculations have been carried out to study the structure and relative stability of small silicon clusters (Sin, n=7-11). A number of low-energy geometric isomers are optimized at the second-order Møller-Plesset (MP2) MP2/6-31G(d) level. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The total energies of stable isomers are computed at the coupled-cluster single and double substitutions (including triple excitations) [CCSD(T)] CCSD(T)/6-31G(d) level. The calculated binding energies per atom at both the MP2/6-31G(d) and CCSD(T)/6-31G(d) levels agree with the experiments. For Si7, Si8, and Si10, the lowest-energy structures are the same as those predicted previously from the all-electron optimization at the Hartree-Fock (HF) HF/6-31G(d) level [Raghavachari and Rohlfing, J. Chem. Phys. 89, 2219 (1988)]. For Si9, the lowest-energy isomer is same as that predicted based on density-functional plane-wave pseudopotential method [Vasiliev, Ogut, and Chelikowsky, Phys. Rev. Lett. 78, 4805 (1997)]. Particular attention has been given to Si11 because several low-energy geometric isomers were found nearly isoenergetic. On the basis of MP2/6-311G(2d)//CCSD(T)/6-311G(2d) calculation, we identified that the C2v isomer, a tricapped trigonal prism with two additional caps on side trigonal faces, is most likely the global-minimum structure. However, another competitive geometric isomer for the global minimum is also found on basis of the MP2/6-311G(2d)//CCSD(T)/6-311G(2d) calculation. Additionally, calculations of the binding energy and the cluster polarizability offer more insights into relatively strong stability of two magic-number clusters Si6 and Si10.

  10. Structure, stability, and dissociation of small ionic silicon oxide clusters [SiO{sub n}{sup +}(n = 3, 4)]: Insight from density functional and topological exploration

    SciTech Connect

    Sen, Kaushik; Ghosh, Deepanwita; Pakhira, Srimanta; Banu, Tahamida; Das, Abhijit K.

    2013-12-21

    The structures, energies, isomerization, and decomposition pathways of small ionic silicon oxide clusters, SiO{sub n}{sup +} (n = 3, 4), on doublet and quartet energy surfaces are investigated by density functional theory. New structural isomers of these ionic clusters have been obtained with this systematic study. The energy ordering of the isomeric cluster ions on doublet spin surface is found to follow the same general trend as that of the neutral ones, while it differs on the quartet surface. Our computational results reveal the energetically most preferred decomposition pathways of the ionic clusters on both spin surfaces. To comprehend the reaction mechanism, bonding evolution theory has also been employed using atoms in molecules formalism. The possible reasons behind the structural deformation of some isomers on quartet surface have also been addressed. Our results are expected to provide important insight into the decomposition mechanism and relative stability of the SiO{sub n}{sup +} clusters on both the energy surfaces.

  11. Electronic transport properties of silicon clusters

    NASA Astrophysics Data System (ADS)

    Matsuura, Yukihito

    2016-02-01

    The electronic transport properties of silicon clusters were examined via theoretical calculations using the first-principles method. Additionally, p-type doping and n-type doping were analyzed by calculating conductance and current of boron- and phosphorus-doped silicon clusters. The p-type doping and n-type doping provided a new transmission peak at an energy level around the Fermi level to increase conductance. Furthermore, simultaneous boron and phosphorus doping resulted in noticeable rectifying characteristics, with the current drive in forward bias being three times higher than that in the reverse bias. A p-n junction was achieved even on a molecular scale.

  12. Small copper-doped silicon clusters CuSin (n = 4-10) and their anions: structures, thermochemistry, and electron affinities.

    PubMed

    Lin, Lin; Yang, Jucai

    2015-06-01

    The structures and energies of copper-doped small silicon clusters CuSi n (n = 4-10) and their anions were investigated systematically using CCSD(T)/aug-cc-pVTZ-DK//MP2/6-31G(2df,p), G4//MP2/6-31G(2df,p), and the B3LYP/6-311+G* basis set. The performance of the methods used for the prediction of energetic and thermodynamic properties was evaluated. Comparing experimental [Xu et al. (2012) J Chem Phys 136:104308] and theoretical calculations, it was concluded that the CCSD(T) results are very accurate and exhibit the best performance; the mean absolute deviation from experimental data was 0.043 eV. The excellent agreement of vertical detachment energy (VDE) between experimental results and CCSD(T) calculations indicates that the ground state structures of CuSi n (-) (n = 4-10) presented in this paper are reliable. For CuSi10, assigning 2.90±0.08 eV to the experimental adiabatic electron affinity (AEA) and 3.90±0.08 eV to the VDE is more reasonable than to 3.46±0.08 eV and 3.62±0.08 eV, respectively, based on the CCSD(T) calculations and the previous photoelectron spectrum of CuSi10 (-) (Xu et al., op. cit.). The AEAs of CuSi n (n = 4-10), excluding CuSi7, are in excellent agreement with experimental data, showing that the ground state structures of CuSi n (n = 4-6, 8-10) reported in this paper are reliable. CuSi10 is suggested to be the smallest endohedral ground state structure. However, adding an additional electron to CuSi10 pulls out the Cu atom from the center location, forming an exohedral ground state structure of CuSi10 (-). The charge transfer and dissociation energy of Cu from CuSi n and their anions determined to examine the nature of bonding and their relative stabilities. PMID:26003428

  13. Thermochemical property estimation of hydrogenated silicon clusters.

    PubMed

    Adamczyk, Andrew J; Broadbelt, Linda J

    2011-08-18

    The thermochemical properties for selected hydrogenated silicon clusters (Si(x)H(y), x = 3-13, y = 0-18) were calculated using quantum chemical calculations and statistical thermodynamics. Standard enthalpy of formation at 298 K and standard entropy and constant pressure heat capacity at various temperatures, i.e., 298-6000 K, were calculated for 162 hydrogenated silicon clusters using G3//B3LYP. The hydrogenated silicon clusters contained ten to twenty fused Si-Si bonds, i.e., bonds participating in more than one three- to six-membered ring. The hydrogenated silicon clusters in this study involved different degrees of hydrogenation, i.e., the ratio of hydrogen to silicon atoms varied widely depending on the size of the cluster and/or degree of multifunctionality. A group additivity database composed of atom-centered groups and ring corrections, as well as bond-centered groups, was created to predict thermochemical properties most accurately. For the training set molecules, the average absolute deviation (AAD) comparing the G3//B3LYP values to the values obtained from the revised group additivity database for standard enthalpy of formation and entropy at 298 K and constant pressure heat capacity at 500, 1000, and 1500 K were 3.2%, 1.9%, 0.40%, 0.43%, and 0.53%, respectively. Sensitivity analysis of the revised group additivity parameter database revealed that the group parameters were able to predict the thermochemical properties of molecules that were not used in the training set within an AAD of 3.8% for standard enthalpy of formation at 298 K. PMID:21728331

  14. Multiple Aromaticity and Antiaromaticity in Silicon Clusters

    SciTech Connect

    Zhai, Hua JIN.; Kuznetsov, A E.; Boldyrev, Alexander I.; Wang, Lai S.

    2004-12-10

    A series of silicon clusters four atoms, but with different charge states (Si42+, Si4, Si42-, and NaSi4-), are studied using photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series are interpreted in terms of aromaticity and antiaromaticity, allowing prediction of how structures of how structures of the four-atom silicon cluster change upon addition or reduction of two electrons. It is shown that Si42+ is square planar, analogous to the recently discovered aromatic A142- cluster. Upon addition of two electrons, the neutral Si4 becomes ?-antiaromatic, resulting in a rhombus distortion. Adding two more electrons to Si4 leads to two energetically close structures of Si42-: either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of the doubly charged Si42-, a stabilizing cation Na+ was used to produce Si42- in the gas phase in the form of Na+ [Si42-], which was characterized experimentally using photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+ [Si42] species is highly unusual in chemistry.

  15. Multiple Aromaticity and Antiaromaticity in Silicon Clusters

    SciTech Connect

    Zhai, Hua JIN.; Kuznetsov, A E.; Boldyrev, Alexander I.; Wang, Lai S.

    2004-12-17

    A series of silicon clusters containing four atoms but with different charge states (Si{sub 4}{sup 2+}, Si{sub 4}, Si{sub 4}{sup 2-}, and NaSi{sub 4}{sup -}) were studied by photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series were interpreted in terms of aromaticity and antiaromaticity, which allowed the prediction of how structures of the four-atom silicon clusters change upon addition or removal of two electrons. It is shown that Si{sub 4}{sup 2+} is square-planar, analogous to the recently discovered aromatic Al{sub 4}{sup 2-} cluster. Upon addition of two electrons, neutral Si{sub 4} becomes {sigma}-antiaromatic and exhibits a rhombus distortion. Adding two more electrons to Si{sub 4} leads to two energetically close structures of Si{sub 4}{sup 2-}: either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of doubly charged Si{sub 4}{sup 2-}, a stabilizing cation (Na{sup +}) was used to produce Si{sub 4}{sup 2-} in the gas phase in the form of Na{sup +}[Si{sub 4}{sup 2-}], which was characterized experimentally by photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na{sup +}[Si{sub 4}{sup 2-}] species is highly unusual.

  16. Quantum chemical study of small palladium clusters

    NASA Astrophysics Data System (ADS)

    Efremenko, Irena; Sheintuch, Moshe

    1998-09-01

    The extended Hückel method with an electrostatic two-body correction has been used to find the structure of small Pd n clusters for n=2-13. Twins formation, with metal-metal bond lengths slightly smaller than those of bulk palladium, was found to be the preferential direction for cluster growth in the absence of external field. In accordance with the experimental results, these close-packed particles show a significant split in the valence d-zone. The energetic gap between HOMO and LUMO narrows from 3.217 to 0.68 eV as the cluster grows from two to 13 atoms. The LUMO has a bonding character toward Pd-Pd bonds, whereas HOMO is antibonding, so one can suggest that both donating and accepting interactions are favorable for strengthening of clusters. Occupation of 5s and 5p orbitals increases during cluster growth, while the net charge on the outer atoms remains very small. The results obtained for two to six atomic clusters are in good agreement with first principle calculations. Very close similarity with Rh cluster growth was observed.

  17. Stability and migration of small copper clusters in amorphous dielectrics

    SciTech Connect

    Guzman, David M.; Onofrio, Nicolas; Strachan, Alejandro

    2015-05-21

    We use density functional theory (DFT) to study the thermodynamic stability and migration of copper ions and small clusters embedded in amorphous silicon dioxide. We perform the calculations over an ensemble of statistically independent structures to quantify the role of the intrinsic atomic-level variability in the amorphous matrix affect the properties. The predicted formation energy of a Cu ion in the silica matrix is 2.7 ± 2.4 eV, significantly lower the value for crystalline SiO{sub 2}. Interestingly, we find that Cu clusters of any size are energetically favorable as compared to isolated ions; showing that the formation of metallic clusters does not require overcoming a nucleation barrier as is often assumed. We also find a broad distribution of activation energies for Cu migration, from 0.4 to 1.1 eV. This study provides insights into the stability of nanoscale metallic clusters in silica of interest in electrochemical metallization cell memories and optoelectronics.

  18. The structure of small metal clusters

    NASA Technical Reports Server (NTRS)

    Bauschlicher, C. W., Jr.; Pettersson, L. G. M.

    1986-01-01

    One metal atom surrounded by its 12 nearest neighbors is considered for both D(3d) (face-centered cubic-like) and D(3h) (hexagonal close-packed-like) geometries. For Al and Be, the neutral cluster and the positive and negative ions are considered for idealized (all bonds equal) and distorted geometries. The D(3d) geometry is found to be the lowest for Be13, while the D(3h) geometry is lower for Al13. This is the reverse of what is expected based upon the bulk metal structures, Be(hcp) and Al(fcc). Al13 is found to have only small distortions, while Be13 shows large distortions for both the D(3d) and D(3h) geometries. The ions have geometries which are similar to those found for the neutral systems. Both all-electron and effective core potential calculations were carried out on the X13 clusters; the agreement is very good.

  19. The dynamics of small excitable ion channel clusters

    NASA Astrophysics Data System (ADS)

    Shuai, J. W.; Jung, P.

    2006-06-01

    Through computational modeling we predict that small sodium ion channel clusters on small patches of membrane can encode electric signals most efficiently at certain magic cluster sizes. We show that this effect can be traced back to algebraic features of small integers and are universal for channels with a simple gating dynamics. We further explore physiologic conditions under which such effects can occur.

  20. Physics and chemistry of small clusters

    SciTech Connect

    Jena, P.; Rao, B.K.; Khanna, S.N.

    1987-01-01

    This book contains papers on physical and chemical phenomena of solid clusters. The papers cover the atomic and electronic structure, dynamics, stability, fragmentation, optical properties, interaction with adsorbates, astrochemistry and van der Waals forces of clusters. (LSP)

  1. Recent advances in small molecule OLED-on-silicon microdisplays

    NASA Astrophysics Data System (ADS)

    Ghosh, Amalkumar P.; Ali, Tariq A.; Khayrullin, Ilyas; Vazan, Fridrich; Prache, Olivier F.; Wacyk, Ihor

    2009-08-01

    High resolution OLED-on-silicon microdisplay technology is unique and challenging since it requires very small subpixel dimensions (~ 2-5 microns). eMagin's OLED microdisplay is based on white top emitter architecture using small molecule organic materials. The devices are fabricated using high Tg materials. The devices are hermetically sealed with vacuum deposited thin film layers. LCD-type color filters are patterned using photolithography methods to generate primary R, G, B colors. Results of recent improvements in the OLED-on-silicon microdisplay technology, with emphasis on efficiencies, lifetimes, grey scale and CIE color coordinates for SVGA and SXGA resolution microdisplays is presented.

  2. Transportation Cluster Volume 3 [Small Power Sources].

    ERIC Educational Resources Information Center

    Pennsylvania State Dept. of Justice, Harrisburg. Bureau of Correction.

    The document is one of seven volumes of instructional materials developed around a cluster of Transportation Industries. Primarily technical in focus, they are designed to be used in a cluster-concept program and to integrate with a regular General Education Development (G.E.D.) program so that students may attain an employable skill level and a…

  3. Statistical fragmentation of small neutral carbon clusters

    SciTech Connect

    Diaz-Tendero, S.; Alcami, M.; Martin, F.; Hervieux, P.-A.

    2005-03-01

    We present a statistical fragmentation study of the C{sub 5}, C{sub 7}, and C{sub 9} carbon clusters using the Metropolis Monte Carlo and Weisskopf methods. We show that inclusion of several isomeric forms as well as rotational effects is essential to reproduce the experimental observations. We have found that, for cluster excitation energies around 10 eV, several fragmentation channels are efficiently populated, but the dominant one always corresponds to C{sub n-3}/C{sub 3}. For high enough excitation energies, we observe first-order phase transitions corresponding to a complete breakup of the cluster.

  4. Structural evolution of small ruthenium cluster anions

    SciTech Connect

    Waldt, Eugen; Hehn, Anna-Sophia; Ahlrichs, Reinhart; Kappes, Manfred M.; Schooss, Detlef

    2015-01-14

    The structures of ruthenium cluster anions have been investigated using a combination of trapped ion electron diffraction and density functional theory computations in the size range from eight to twenty atoms. In this size range, three different structural motifs are found: Ru{sub 8}{sup −}–Ru{sub 12}{sup −} have simple cubic structures, Ru{sub 13}{sup −}–Ru{sub 16}{sup −} form double layered hexagonal structures, and larger clusters form close packed motifs. For Ru{sub 17}{sup −}, we find hexagonal close packed stacking, whereas octahedral structures occur for Ru{sub 18}{sup −}–Ru{sub 20}{sup −}. Our calculations also predict simple cubic structures for the smaller clusters Ru{sub 4}{sup −}–Ru{sub 7}{sup −}, which were not accessible to electron diffraction measurements.

  5. The photoluminescence mechanism of ultra-small gold clusters.

    PubMed

    Wu, Liangliang; Fang, Weihai; Chen, Xuebo

    2016-07-14

    The understanding of the photoluminescence mechanism of ultra-small gold clusters has seriously lagged behind a wealth of experimental syntheses and optical characterization. Multi-configurational quantum chemical calculations disclose that the optical properties of these clusters are predominantly regulated by the number of diamagnetic electrons and the topological features formed by aurophilic interactions. PMID:27306561

  6. Optical properties of silicon clusters deposited on the basal plane of graphite

    NASA Astrophysics Data System (ADS)

    Dinh, L. N.; Chase, L. L.; Balooch, M.; Terminello, L. J.; Tench, R. J.; Wooten, F.

    1994-04-01

    Laser ablation was used to deposit of silicon on highly oriented pyrolytic graphite surfaces in an ultra high-vacuum environment equipped with Auger electron spectroscopy (AES), scanning tunneling microscopy (STM) and luminescence spectroscopy. For deposition of up to several monolayers, post annealing produced silicon clusters, whose size distribution was determined vs annealing time and temperature using STM. Pure silicon clusters ranging from 1 to 10 nm showed no detectable photoluminescence in visible range. Exposure to oxygen at 10(exp -6) Torr and for up to 8 hours showed adsorption on the surface of the clusters without silicon oxide formation and no detectable luminescence. Hydrogen termination of these clusters was accomplished by exposing them to atomic hydrogen beam but did not result in any photoluminescence. Prolonged exposure of these clusters to ambient air, however, resulted in strong photoluminescence spectra with color ranging from red to greenish-blue depending on average cluster size. Auger electron spectra revealed the existence of partially oxidized silicon clusters. This luminescence could be due to either an oxide phase or to changes in electronic structure of the clusters as a result of quantum confinement effect.

  7. Small Clusters of para-HYDROGEN

    NASA Astrophysics Data System (ADS)

    Guardiola, Rafael; Navarro, Jesús

    2008-06-01

    This paper presents a systematic study of clusters formed by parahydrogen molecules, considered as elementary particles. Among the analyzed properties there are the energy and the one-body distribution. The Many-Body method used for the analysis is the well known Diffusion Monte Carlo (DMC) with importance sampling trial functions including two- and three-body Jastrow correlations. Results are presented for the two current interactions known as Buck and Silvera.

  8. Structural properties of small rhodium clusters

    SciTech Connect

    Soon, Yee Yeen; Yoon, Tiem Leong; Lim, Thong Leng

    2015-04-24

    We report a systematic study of the structural properties of rhodium clusters at the atomistic level. A novel global-minimum search algorithm, known as parallel tempering multicanonical basin hopping plus genetic algorithm (PTMBHGA), is used to obtain the geometrical structures with lowest minima at the semi-empirical level where Gupta potential is used to describe the atomic interaction among the rhodium atoms. These structures are then re-optimized at the density functional theory (DFT) level with exchange-correlation energy approximated by Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA). The structures are optimized for different spin multiplicities. The ones with lowest energies will be taken as ground-state structures. In most cases, we observe only minor changes in the geometry and bond length of the clusters as a result of DFT-level re-optimization. Only in some limited cases, the initial geometries obtained from the PTMBHGA are modified by the re-optimization. The variation of structural properties, such as ground-state geometry, symmetry and binding energy, with respect to the cluster size is studied and agreed well with other results available in the literature.

  9. Small-scale Conformity of the Virgo Cluster Galaxies

    NASA Astrophysics Data System (ADS)

    Lee, Hye-Ran; Lee, Joon Hyeop; Jeong, Hyunjin; Park, Byeong-Gon

    2016-06-01

    We investigate the small-scale conformity in color between bright galaxies and their faint companions in the Virgo Cluster. Cluster member galaxies are spectroscopically determined using the Extended Virgo Cluster Catalog and the Sloan Digital Sky Survey Data Release 12. We find that the luminosity-weighted mean color of faint galaxies depends on the color of adjacent bright galaxy as well as on the cluster-scale environment (gravitational potential index). From this result for the entire area of the Virgo Cluster, it is not distinguishable whether the small-scale conformity is genuine or if it is artificially produced due to cluster-scale variation of galaxy color. To disentangle this degeneracy, we divide the Virgo Cluster area into three sub-areas so that the cluster-scale environmental dependence is minimized: A1 (central), A2 (intermediate), and A3 (outermost). We find conformity in color between bright galaxies and their faint companions (color–color slope significance S ∼ 2.73σ and correlation coefficient {cc}∼ 0.50) in A2, where the cluster-scale environmental dependence is almost negligible. On the other hand, the conformity is not significant or very marginal (S ∼ 1.75σ and {cc}∼ 0.27) in A1. The conformity is not significant either in A3 (S ∼ 1.59σ and {cc}∼ 0.44), but the sample size is too small in this area. These results are consistent with a scenario in which the small-scale conformity in a cluster is a vestige of infallen groups and these groups lose conformity as they come closer to the cluster center.

  10. Small-scale Conformity of the Virgo Cluster Galaxies

    NASA Astrophysics Data System (ADS)

    Lee, Hye-Ran; Lee, Joon Hyeop; Jeong, Hyunjin; Park, Byeong-Gon

    2016-06-01

    We investigate the small-scale conformity in color between bright galaxies and their faint companions in the Virgo Cluster. Cluster member galaxies are spectroscopically determined using the Extended Virgo Cluster Catalog and the Sloan Digital Sky Survey Data Release 12. We find that the luminosity-weighted mean color of faint galaxies depends on the color of adjacent bright galaxy as well as on the cluster-scale environment (gravitational potential index). From this result for the entire area of the Virgo Cluster, it is not distinguishable whether the small-scale conformity is genuine or if it is artificially produced due to cluster-scale variation of galaxy color. To disentangle this degeneracy, we divide the Virgo Cluster area into three sub-areas so that the cluster-scale environmental dependence is minimized: A1 (central), A2 (intermediate), and A3 (outermost). We find conformity in color between bright galaxies and their faint companions (color–color slope significance S ˜ 2.73σ and correlation coefficient {cc}˜ 0.50) in A2, where the cluster-scale environmental dependence is almost negligible. On the other hand, the conformity is not significant or very marginal (S ˜ 1.75σ and {cc}˜ 0.27) in A1. The conformity is not significant either in A3 (S ˜ 1.59σ and {cc}˜ 0.44), but the sample size is too small in this area. These results are consistent with a scenario in which the small-scale conformity in a cluster is a vestige of infallen groups and these groups lose conformity as they come closer to the cluster center.

  11. Tangling clustering instability for small particles in temperature stratified turbulence

    NASA Astrophysics Data System (ADS)

    Elperin, T.; Kleeorin, N.; Liberman, M.; Rogachevskii, I.

    2013-08-01

    We study tangling clustering instability of inertial particles in a temperature stratified turbulence with small finite correlation time. It is shown that the tangling mechanism in the temperature stratified turbulence strongly increases the degree of compressibility of particle velocity field. This results in the strong decrease of the threshold for the excitation of the tangling clustering instability even for small particles. The tangling clustering instability in the temperature stratified turbulence is essentially different from the inertial clustering instability that occurs in non-stratified isotropic and homogeneous turbulence. While the inertial clustering instability is caused by the centrifugal effect of the turbulent eddies, the mechanism of the tangling clustering instability is related to the temperature fluctuations generated by the tangling of the mean temperature gradient by the velocity fluctuations. Temperature fluctuations produce pressure fluctuations and cause particle accumulations in regions with increased instantaneous pressure. It is shown that the growth rate of the tangling clustering instability is by sqrtRe (ell _0 / L_T)^2 / (3 Ma)^4 times larger than that of the inertial clustering instability, where Re is the Reynolds number, Ma is the Mach number, ℓ0 is the integral turbulence scale, and LT is the characteristic scale of the mean temperature variations. It is found that depending on the parameters of the turbulence and the mean temperature gradient there is a preferential particle size at which the particle clustering due to the tangling clustering instability is more effective. The particle number density inside the cluster after the saturation of this instability can be by several orders of magnitude larger than the mean particle number density. It is also demonstrated that the evaporation of droplets drastically changes the tangling clustering instability, e.g., it increases the instability threshold in the droplet radius. The

  12. Small Si clusters on surfaces of carbon nanotubes

    SciTech Connect

    Meng, Lijun; Zhang, Kaiwang; Stocks, George Malcolm; Zhong, Jianxin

    2006-01-01

    Structures of small Si clusters, Sin, on surfaces of carbon nanotubes have been studied by molecular dynamics simulation. We show that the lowest-energy structures of Sin are three-dimensional clusters rather than thin Si sheets covering the surface of a nanotube. As n increases from 10 to 30, Sin undergoes structural transitions from a tent-like structure (with nanotube surface as its base) to a cage-like structure (without interior atoms) and further to a spherical compact structure (with interior atoms). Our results are different from the structures of small Si clusters found in a free space without Si-nanotube interaction.

  13. Small Dust Cluster Probes within a Dusty Plasma

    NASA Astrophysics Data System (ADS)

    Kong, Jie; Qiao, Ke; Matthews, Lorin; Hyde, Truell

    2014-10-01

    Small-number dust particle clusters are often seen in dusty plasmas. Interestingly, such clusters can often be used as in-situ probes providing plasma diagnostics. The number of dust particles, as well as the cluster size and shape, can be easily controlled employing a glass box placed on the powered lower electrode within a GEC rf reference chamber to provide confinement of the dust. Adjusting the rf power alters the plasma conditions creating structural changes within the cluster. This effect can be used to probe the relationship between the rf power and other plasma parameters of interest. This experiment employs the sloshing and breathing modes of small cluster oscillations to examine the relationship between the system's rf power and the plasma screening length inside the glass box as well as determine the particle charge. Experimental results indicate that both the screening length and the dust charge decrease as the rf power inside the box increases.

  14. Ab initio Monte Carlo investigation of small lithium clusters.

    SciTech Connect

    Srinivas, S.

    1999-06-16

    Structural and thermal properties of small lithium clusters are studied using ab initio-based Monte Carlo simulations. The ab initio scheme uses a Hartree-Fock/density functional treatment of the electronic structure combined with a jump-walking Monte Carlo sampling of nuclear configurations. Structural forms of Li{sub 8} and Li{sub 9}{sup +} clusters are obtained and their thermal properties analyzed in terms of probability distributions of the cluster potential energy, average potential energy and configurational heat capacity all considered as a function of the cluster temperature. Details of the gradual evolution with temperature of the structural forms sampled are examined. Temperatures characterizing the onset of structural changes and isomer coexistence are identified for both clusters.

  15. Ultra-small rhenium clusters supported on graphene

    PubMed Central

    Miramontes, Orlando; Bonafé, Franco; Santiago, Ulises; Larios-Rodriguez, Eduardo; Velázquez-Salazar, Jesús J.; Mariscal, Marcelo M.; Yacaman, Miguel José

    2015-01-01

    The adsorption of very small rhenium clusters (2 – 13 atoms) supported on graphene was studied with high annular dark field - scanning transmission electron microscopy (HAADF-STEM). The atomic structure of the clusters was fully resolved with the aid of density functional calculations and STEM simulations. It was found that octahedral and tetrahedral structures work as seeds to obtain more complex morphologies. Finally, a detailed analysis of the electronic structure suggested that a higher catalytic effect can be expected in Re clusters when adsorbed on graphene than in isolated ones. PMID:25721176

  16. Variation in optical-absorption edge in SiN{sub x} layers with silicon clusters

    SciTech Connect

    Efremov, M. D. Volodin, V. A.; Marin, D. V.; Arzhannikova, S. A.; Kamaev, G. N.; Kochubei, S. A.; Popov, A. A.

    2008-02-15

    Using optical methods, data on optical constants are obtained for silicon nitride films synthesized by plasma-chemical vapor deposition (PCVD). Models for calculating the permittivity in the model of inhomogeneous phase mixture of silicon and silicon nitride are considered. It is found that the optical-absorption edge (E{sub g}) and the photoluminescence peak shift to longer wavelengths with increasing nitrogen atomic fraction x in sin{sub x} films. When x approaches the value 4/3 characteristic for stoichiometric silicon nitride Si{sub 3}N{sub 4}, a nonlinear sharp increase in E{sub g} is observed. Using Raman scattering, Si-Si bonds are revealed, which confirms the direct formation of silicon clusters during the film deposition. The relation between the composition of nonstoichiometric silicon nitride films, values of permittivity, and the optical-band width is established for light transmission.

  17. Reactions of silicon atoms and small clusters with CO: Experimental and theoretical characterization of Si{sub n}CO (n=1-5), Si{sub 2}(CO){sub 2}, c-Si{sub 2}({mu}-O)({mu}-CSi), and c-Si{sub 2}({mu}-O)({mu}-CCO) in solid argon

    SciTech Connect

    Zhou Mingfei; Jiang Ling; Xu Qiang

    2004-12-01

    Reactions of silicon atoms and small clusters with carbon monoxide molecules in solid argon have been studied using matrix isolation infrared absorption spectroscopy. In addition to the previously reported SiCO monocarbonyl, Si{sub 2}(CO){sub 2} and Si{sub n}CO (n=2-5) carbonyl molecules were formed spontaneously on annealing and were characterized on the basis of isotopic substitution and theoretical calculations. It was found that Si{sub 2}CO, Si{sub 3}CO, and Si{sub 5}CO are bridge-bonded carbonyl compounds, whereas Si{sub 4}CO is a terminal-bonded carbonyl molecule. The Si{sub 2}(CO){sub 2} and Si{sub 3}CO molecules photochemically rearranged to the more stable c-Si{sub 2}({mu}-O)({mu}-CCO) and c-Si{sub 2}({mu}-O) ({mu}-CSi) isomers where Si{sub 2} is inserted into the CO triple bond.

  18. Effect of the Viscosity of Silicone Oil on the Aggregation Behavior of C:F Clusters on a Silicone Oil Liquid Substrate

    NASA Astrophysics Data System (ADS)

    Deng, Yan-Hong; Ye, Chao; Yuan, Yuan; Liu, Hui-Min; Cui, Jin

    2011-04-01

    We investigate the effect of silicone oil viscosity on the aggregation behavior of C:F clusters deposited on silicone oil liquid substrates with viscous coefficients of 100, 350 and 500mm2/s by C4F8 dual-frequency capacitively coupled plasma. The aggregated C:F clusters all exhibit a branch-like fractal structure. However, the fractal dimension decreases from 1.67 to 1.45 with the silicone oil viscous coefficient increasing from 100mm2/s to 500 mm2/s. Owing to the fractal dimension of 1.67 and 1.45, corresponding to the diffusion-limited-aggregation (DLA) model and the cluster-cluster-aggregation (CCA) model respectively, the results show that the increase of silicone oil viscosity can lead to the change of C:F clusters aggregating on a silicone oil liquid substrate from DLA to CCA growth.

  19. Theoretical Study of Electron Scattering By Small Clusters and Adsorbates

    NASA Technical Reports Server (NTRS)

    Huo, Winifred M.; Sheehy, J. A.

    1994-01-01

    Current interest in clusters stems from their role as novel materials as well as a possible extension of cluster results to bulk systems. Experimental investigations on clusters have been carried out using laser spectroscopy, microwave spectroscopy, heavy-particle collisions, as well as electron collisions with earlier experimental work on electron attachment and ionization having been reviewed previously. Recently, Mark and coworkers studied the decay channels of cluster ions following electron impact ionization. Rauth et al. reported the formation of the superhalogen ion SF7(-) and other nonstoichiometric cluster ions in their study of electron attachment to SF6 clusters. Kresin et al. measured the absolute electron-impact depletion cross section of metal clusters Na8, Na(20), and Na(40). They found that the inelastic scattering cross section increased with cluster size and was considerably greater than the hard sphere collision cross sections. They hypothesized that electron attachment and collision-induced fragmentation were the dominant physical processes responsible for this effect. For the two smaller clusters, they also found a sharp increase in the cross section near threshold. Most theoretical studies of clusters have been devoted to their electronic structures, vibrational relaxation, and predissociation while investigations of electron scattering from clusters has been lacking. In view of this, we recently undertook an ab initio study of electron scattering from small Be clusters and BeCO. Beryllium was chosen because it is readily amenable to ab t'nitio calculations. Moreover, the electronic structure of Be clusters has been studied extensively, showing that the Be-Be bond is relatively weak in comparison with a normal chemical bond. Our investigation focuses on how the cross sections change with cluster size and geometry. The range of energy studied, 0.05 - 5.0 eV, is chosen because of the ubiquitous resonance in the low-energy scattering of Be. Hence

  20. Global geometry optimization of silicon clusters described by three empirical potentials

    NASA Astrophysics Data System (ADS)

    Yoo, S.; Zeng, X. C.

    2003-07-01

    The "basic-hopping" global optimization technique developed by Wales and Doye is employed to study the global minima of silicon clusters Sin(3⩽n⩽30) with three empirical potentials: the Stillinger-Weber (SW), the modified Stillinger-Weber (MSW), and the Gong potentials. For the small-sized SW and Gong clusters (3⩽n⩽15), it is found that the global minima obtained based on the basin-hopping method are identical to those reported by using the genetic algorithm [Iwamatsu, J. Chem. Phys. 112, 10976 (2000)], as well as with those by using molecular dynamics and the steepest-descent quench (SDQ) method [Feuston, Kalia, and Vashishta, Phys. Rev. B 37, 6297 (1988)]. However, for the mid-sized SW clusters (16⩽n⩽20), the global minima obtained differ from those based on the SDQ method, e.g., the appearance of the endohedral atom with fivefold coordination starting at n=17, as opposed to n=19. For larger SW clusters (20⩽n⩽30), it is found that the "bulklike" endohedral atom with tetrahedral coordination starts at n=20. In particular, the overall structural features of SW Si21, Si23, Si25, and Si28 are nearly identical to the MSW counterparts. With the SW Si21 as the starting structure, a geometric optimization at the B3LYP/6-31G(d) level of density-functional theory yields an isomer similar to the ground-state- isomer of Si21 reported by Pederson et al. [Phys. Rev. B 54, 2863 (1996)].

  1. Structure and stability of a silicon cluster on sequential doping with carbon atoms

    NASA Astrophysics Data System (ADS)

    AzeezullaNazrulla, Mohammed; Joshi, Krati; Israel, S.; Krishnamurty, Sailaja

    2016-02-01

    SiC is a highly stable material in bulk. On the other hand, alloys of silicon and carbon at nanoscale length are interesting from both technological as well fundamental view point and are being currently synthesized by various experimental groups (Truong et. al., 2015 [26]). In the present work, we identify a well-known silicon cluster viz., Si10 and dope it sequentially with carbon atoms. The evolution of electronic structure (spin state and the structural properties) on doping, the charge redistribution and structural properties are analyzed. It is interesting to note that the ground state SiC clusters prefer to be in the lowest spin state. Further, it is seen that carbon atoms are the electron rich centres while silicon atoms are electron deficient in every SiC alloy cluster. The carbon-carbon bond lengths in alloy clusters are equivalent to those seen in fullerene molecules. Interestingly, the carbon atoms tend to aggregate together with silicon atoms surrounding them by donating the charge. As a consequence, very few Si-Si bonds are noted with increasing concentrations of C atoms in a SiC alloy. Physical and chemical stability of doped clusters is studied by carrying out finite temperature behaviour and adsorbing O2 molecule on Si9C and Si8C2 clusters, respectively.

  2. Distribution and Chemical State of Cu-rich Clusters in Silicon: Preprint

    SciTech Connect

    Buonassisi, T.; Marcus, M. A.; Istratov, A. A.; Heuer, M.; Ciszek, T. F.; Lai, B.; Cai, Z.; Weber, E. R.

    2004-08-01

    the chemical state and distribution of Cu-rich clusters were determined in four different silicon-based materials with varying contamination pathways and degrees of oxygen concentration, including as-grown multicrystalline silicon. In all four samples, Cu3Si was the only chemical state observed. Cu3Si clusters were observed at structural defects within all four materials; XBIC measurements revealed that the presence of Cu3Si corresponds to increased recombination activity. Oxidized Cu compounds are not likely to form in silicon. The +1 eV edge shift in the -XAS absorption spectrum of Cu3Si relative to Cu metal is believed to be an indication of a degree of covalent bonding between Cu atoms and their silicon neighbors.

  3. Sputtered metal and silicon cluster ions: collision-induced fragmentation and neutralization

    NASA Astrophysics Data System (ADS)

    Begemann, W.; Hector, R.; Liu, Y. Y.; Tiggesbäumker, J.; Meiwes-Broer, K. H.; Lutz, H. O.

    1989-03-01

    Mass separated metal and silicon cluster ion beams M{/n +, -} are produced by sputtering and undergo fragmenting and/or neutralizing collisions at different kinetic energies (100 1800 eV) in Ar and SF6. Fragment patterns induced by rare gas collisions open a way to determine ionization potentials and electron affinities of clusters. These values are compared to known experimental and theoretical data. For negatively charged clusters the absorption in gas targets is mainly due to neutralization, the cross sections varying with cluster material, number of atoms and collision partner from 10 Å2 to about 50 Å2.

  4. Photoionization dynamics of glycine adsorbed on a silicon cluster: ''On-the-fly'' simulations

    SciTech Connect

    Shemesh, Dorit; Baer, Roi; Seideman, Tamar; Gerber, R. Benny

    2005-05-08

    Dynamics of glycine chemisorbed on the surface of a silicon cluster is studied for a process that involves single-photon ionization, followed by recombination with the electron after a selected time delay. The process is studied by ''on-the-fly'' molecular dynamics simulations, using the semiempirical parametric method number 3 (PM3) potential energy surface. The system is taken to be in the ground state prior to photoionization, and time delays from 5 to 50 fs before the recombination are considered. The time evolution is computed over 10 ps. The main findings are (1) the positive charge after ionization is initially mostly distributed on the silicon cluster. (2) After ionization the major structural changes are on the silicon cluster. These include Si-Si bond breaking and formation and hydrogen transfer between different silicon atoms. (3) The transient ionization event gives rise to dynamical behavior that depends sensitively on the ion state lifetime. Subsequent to 45 fs evolution in the charged state, the glycine molecule starts to rotate on the silicon cluster. Implications of the results to various processes that are induced by transient transition to a charged state are discussed. These include inelastic tunneling in molecular devices, photochemistry on conducting surfaces, and electron-molecule scattering.

  5. Characteristics of multiprocessing MCNP5 on small personal computer clusters

    SciTech Connect

    Robinson, Sean M.; McConn, Ronald J.; Pagh, Richard T.; Schweppe, John E.; Siciliano, Edward R.

    2006-06-05

    The feasibility and efficiency of performing MCNP5 calculations with a small, heterogeneous computing cluster built from Microsoft® Windows™ Personal Computers (PCs) are explored. The performance increases that may be expected with such clusters are estimated. Our results show that the speed increase from additional slave PCs is nearly linear up to 10 processors. Guidance is given as to the specific advantages of changing various parameters present in the system. Implementing load balancing, and reducing the overhead from the MCNP rendezvous mechanism add to heterogeneous cluster efficiency. Hyper-threading technology and matching the total number of slave processes to the total number of logical processors also yield modest speed increases in the range below 7 processors. Because of the ease of acquisition of heterogeneous desktop computers, and the peak in efficiency at the level of a few physical processors, a strong case is made for the use of small clusters as a tool for producing MCNP5 calculations rapidly, and detailed instructions for constructing such clusters are provided.

  6. Monoxides of small terbium clusters: A density functional theory investigation

    SciTech Connect

    Zhang, G. L.; Yuan, H. K. Chen, H.; Kuang, A. L.; Li, Y.; Wang, J. Z.; Chen, J.

    2014-12-28

    To investigate the effect of oxygen atom on the geometrical structures, electronic, and magnetic properties of small terbium clusters, we carried out the first-principles calculations on Tb{sub n}O (n = 1-14) clusters. The capping of an oxygen atom on one trigonal-facet of Tb{sub n} structures is always favored energetically, which can significantly improve the structural stability. The far-infrared vibrational spectroscopies are found to be different from those of corresponding bare clusters, providing a distinct signal to detect the characteristic structures of Tb{sub n}O clusters. The primary effect of oxygen atom on magnetic properties is to change the magnetic orderings among Tb atoms and to reduce small of local magnetic moments of the O-coordinated Tb atoms, both of which serve as the key reasons for the experimental magnetic evolution of an oscillating behavior. These calculations are consistent with, and help to account for, the experimentally observed magnetic properties of monoxide Tb{sub n}O clusters [C. N. Van Dijk et al., J. Appl. Phys. 107, 09B526 (2010)].

  7. Development of a small-scale computer cluster

    NASA Astrophysics Data System (ADS)

    Wilhelm, Jay; Smith, Justin T.; Smith, James E.

    2008-04-01

    An increase in demand for computing power in academia has necessitated the need for high performance machines. Computing power of a single processor has been steadily increasing, but lags behind the demand for fast simulations. Since a single processor has hard limits to its performance, a cluster of computers can have the ability to multiply the performance of a single computer with the proper software. Cluster computing has therefore become a much sought after technology. Typical desktop computers could be used for cluster computing, but are not intended for constant full speed operation and take up more space than rack mount servers. Specialty computers that are designed to be used in clusters meet high availability and space requirements, but can be costly. A market segment exists where custom built desktop computers can be arranged in a rack mount situation, gaining the space saving of traditional rack mount computers while remaining cost effective. To explore these possibilities, an experiment was performed to develop a computing cluster using desktop components for the purpose of decreasing computation time of advanced simulations. This study indicates that small-scale cluster can be built from off-the-shelf components which multiplies the performance of a single desktop machine, while minimizing occupied space and still remaining cost effective.

  8. Networks of neuroblastoma cells on porous silicon substrates reveal a small world topology.

    PubMed

    Marinaro, Giovanni; La Rocca, Rosanna; Toma, Andrea; Barberio, Marianna; Cancedda, Laura; Di Fabrizio, Enzo; Decuzzi, Paolo; Gentile, Francesco

    2015-02-01

    The human brain is a tightly interweaving network of neural cells where the complexity of the network is given by the large number of its constituents and its architecture. The topological structure of neurons in the brain translates into its increased computational capabilities, low energy consumption, and nondeterministic functions, which differentiate human behavior from artificial computational schemes. In this manuscript, we fabricated porous silicon chips with a small pore size ranging from 8 to 75 nm and large fractal dimensions up to Df ∼ 2.8. In culturing neuroblastoma N2A cells on the described substrates, we found that those cells adhere more firmly to and proliferate on the porous surfaces compared to the conventional nominally flat silicon substrates, which were used as controls. More importantly, we observed that N2A cells on the porous substrates create highly clustered, small world topology patterns. We conjecture that neurons with a similar architecture may elaborate information more efficiently than in random or regular grids. Moreover, we hypothesize that systems of neurons on nano-scale geometry evolve in time to form networks in which the propagation of information is maximized. PMID:25515929

  9. Thermodynamic and morphological analysis of large silicon self-interstitial clusters using atomistic simulations

    SciTech Connect

    Chuang, Claire Y.; Sinno, Talid; Sattler, Andreas

    2015-04-07

    We study computationally the formation of thermodynamics and morphology of silicon self-interstitial clusters using a suite of methods driven by a recent parameterization of the Tersoff empirical potential. Formation free energies and cluster capture zones are computed across a wide range of cluster sizes (2 < N{sub i} < 150) and temperatures (0.65 < T/T{sub m} < 1). Self-interstitial clusters above a critical size (N{sub i} ∼ 25) are found to exhibit complex morphological behavior in which clusters can assume either a variety of disordered, three-dimensional configurations, or one of two macroscopically distinct planar configurations. The latter correspond to the well-known Frank and perfect dislocation loops observed experimentally in ion-implanted silicon. The relative importance of the different cluster morphologies is a function of cluster size and temperature and is dictated by a balance between energetic and entropic forces. The competition between these thermodynamic forces produces a sharp transition between the three-dimensional and planar configurations, and represents a type of order-disorder transition. By contrast, the smaller state space available to smaller clusters restricts the diversity of possible structures and inhibits this morphological transition.

  10. Processes involved in the formation of silver clusters on silicon surface

    NASA Astrophysics Data System (ADS)

    Bhattacharyya, S. R.; Chini, T. K.; Datta, D.; Hippler, R.; Shyjumon, I.; Smirnov, B. M.

    2008-12-01

    We analyze scanning electron microscopy measurements for structures formed in the deposition of solid silver clusters onto a silicon(100) substrate and consider theoretical models of cluster evolution onto a surface as a result of diffusion and formation of aggregates of merged clusters. Scanning electron microscopy (SEM) data are presented in addition to energy dispersive X-ray spectrometry (EDX) measurements of the these films. Solid silver clusters are produced by a DC magnetron sputtering source with a quadrupole filter for selection of cluster sizes (4.1 and 5.6 nm or 1900 and 5000 atoms per cluster in this experiment); the energy of cluster deposition is 0.7 eV/atom. Rapid thermal annealing of the grown films allows analysis of their behavior at high temperatures. The results exhibit formation of cluster aggregates via the diffusion of deposited solid clusters along the surface; an aggregate consists of up to hundreds of individual clusters. This process is essentially described by the diffusion-limited aggregation (DLA) model, and thus a grown porous film consists of cluster aggregates joined by bridges. Subsequent annealing of this film leads to its melting at temperatures lower than to the melting point of bulk silver. Analysis of evaporation of this film at higher temperatures gives a binding energy in bulk silver of ɛ0= (2.74 ± 0.03) eV/atom.

  11. Raman Vibrational Shifts of Small Clusters of Hydrogen Isotopologues.

    PubMed

    Schmidt, Matthew; Fernández, José M; Faruk, Nabil; Nooijen, Marcel; Le Roy, Robert J; Morilla, Juan H; Tejeda, Guzmán; Montero, Salvador; Roy, Pierre-Nicholas

    2015-12-17

    Raman vibrational shifts of small parahydrogen (pH2), orthodeuterium (oD2), and paratritium (pT2) clusters with respect to the free molecules are calculated by combining a first order perturbation theory approach with Langevin equation Path Integral Ground State (LePIGS) simulations [ J. Phys. Chem. A 2013 , 117 , 7461 ]. Our theoretical predictions are compared to existing cryogenic free jet expansion results for pure (pH2)N clusters [ Phys. Rev. Lett. 2004 , 92 , 223401 ] and to new measurements for (oD2)N clusters reported here. This method has been successfully used before to predict the Raman vibrational shifts of (pH2)N clusters [ J. Chem. Phys. 2014 , 141 , 014310 ]. The 6-D interaction potential of Hinde [ J. Chem. Phys. 2008 , 128 , 154308 ] is reduced to 1-D using the Adiabatic Hindered Rotor approximation to yield effective pair potentials for both molecules being in the ground vibrational state, and for one of them carrying one quantum of vibrational excitation. These reduced 1-D potentials are fitted to a Morse Long Range analytic form for later convenience. Good agreement between experiment and theory is found for the smaller clusters, but significant deviations remain for the larger ones. PMID:26517305

  12. On the performance limiting behavior of defect clusters in commercial silicon solar cells

    SciTech Connect

    Sopori, B.L.; Chen, W.; Jones, K.; Gee, J.

    1998-09-01

    The authors report the observation of defect clusters in high-quality, commercial silicon solar cell substrates. The nature of the defect clusters, their mechanism of formation, and precipitation of metallic impurities at the defect clusters are discussed. This defect configuration influences the device performance in a unique way--by primarily degrading the voltage-related parameters. Network modeling is used to show that, in an N/P junction device, these regions act as shunts that dissipate power generated within the cell.

  13. Ionization Thresholds of Small Carbon Clusters: Tunable VUVExperiments and Theory

    SciTech Connect

    Belau, Leonid; Wheeler, Steven E.; Ticknor, Brian W.; Ahmed,Musahid; Leone, Stephen R.; Allen, Wesley D.; Schaefer III, Henry F.; Duncan, Michael A.

    2007-07-31

    Small carbon clusters (Cn, n = 2-15) are produced in amolecular beam by pulsed laser vaporization and studied with vacuumultraviolet (VUV) photoionization mass spectrometry. The required VUVradiation in the 8-12 eV range is provided by the Advanced Light Source(ALS) at the Lawrence Berkeley National Laboratory. Mass spectra atvarious ionization energies reveal the qualitative relative abundances ofthe neutral carbon clusters produced. By far the most abundant species isC3. Using the tunability of the ALS, ionization threshold spectra arerecorded for the clusters up to 15 atoms in size. The ionizationthresholds are compared to those measured previously with charge-transferbracketing methods. To interpret the ionization thresholds for differentcluster sizes, new ab initio calculations are carried out on the clustersfor n = 4-10. Geometric structures are optimized at the CCSD(T) levelwith cc-pVTZ (or cc-pVDZ) basis sets, and focal point extrapolations areapplied to both neutral and cation species to determine adiabatic andvertical ionization potentials. The comparison of computed and measuredionization potentials makes it possible to investigate the isomericstructures of the neutral clusters produced in this experiment. Themeasurements are inconclusive for the n = 4-6 species because ofunquenched excited electronic states. However, the data provide evidencefor the prominence of linear structures for the n = 7, 9, 11, 13 speciesand the presence of cyclic C10.

  14. Chemical shifts of small heterogeneous Ar/Xe clusters

    SciTech Connect

    Lindblad, A.; Rander, T.; Bradeanu, I.; Oehrwall, G.; Bjoerneholm, O.; Mucke, M.; Ulrich, V.; Lischke, T.; Hergenhahn, U.

    2011-03-15

    Heterogeneous rare-gas clusters produced by a coexpansion of an argon/xenon mixture have been studied using synchrotron-radiation-based photoelectron spectroscopy. Both valence and Xe 4d{sub 5/2} core-level photoelectron spectra were recorded for three different concentrations of the primary argon/xenon mixture and, for those mixtures, spectra were recorded at several different stagnation conditions. The studied size regime of the mixed clusters ranges from large, similar to those studied in an earlier paper [Phys. Rev. A 69, 031210(R) (2004)], to very small--as reflected in the cluster line shapes and chemical shifts. The chemical shifts obtained from a curve fitting procedure similar to that used in our earlier paper are discussed in terms of the mixed cluster structure which can be expected from equilibrium considerations and the Lennard-Jones parameters of the constituent atoms. Molecular dynamics simulations of the vertical polarization shifts allow more specific assignments of ''on-top'' sites and interfacial sites.

  15. Auger parameter and Wagner plot studies of small copper clusters

    NASA Astrophysics Data System (ADS)

    Moretti, Giuliano; Palma, Amedeo; Paparazzo, Ernesto; Satta, Mauro

    2016-04-01

    We discuss application of the Auger parameter and Wagner plot concepts to the study of small copper clusters deposited on various supports such as C(graphite), SiO2 and Al2O3. We demonstrate that the cluster size and the electronic properties of the support influence the shifts of both the binding energy of the Cu 2p3/2 transition and the kinetic energy of the Cu L3M45M45; 1G Auger transition. We find that the Cu L3M45M45; 1G-2p3/2 Auger parameter and Wagner plot allow one to single out and measure both initial- and final-state effects with a detail which is superior to that achieved in photoemission studies.

  16. Hollow Polyhedral Structures in Small Gold Sulfide Clusters

    SciTech Connect

    Pei, Dr. Yong; Shao, Nan; Li, Prof. Hui; Jiang, Deen; Zeng, X.C.

    2011-01-01

    Using ab initio methods, we investigate the structural evolution of a family of gold-sulfide cluster anions (Au{sub m}S{sub n}{sup -}). We show that this family of clusters exhibits simple size-evolution rules and novel hollow polyhedron structures. The highly stable Au{sub m}S{sub n}{sup -} species such as Au{sub 6}S{sub 4}{sup -}, Au{sub 9}S{sub 5}{sup -}, Au{sub 9}S{sub 6}{sup -}, Au{sub 10}S{sub 6}{sup -}, Au{sub 11}S{sub 6}{sup -}, Au{sub 12}S{sub 8}{sup -}, and Au{sub 13}S{sub 8}{sup -} detected in the recent ion mobility mass spectrometry experiment of Au{sub 25}(SCH{sub 2}CH{sub 2}Ph){sub 18} (Angel et al. ACS Nano2010, 4, 4691) are found to possess either quasi-tetrahedron, pyramidal, quasi-triangular prism, or quasi-cuboctahedron structures. The formation of these polyhedron structures are attributed to the high stability of the S-Au-S structural unit. A unique 'edge-to-face' growth mechanism is proposed to understand the structural evolution of the small Au{sub m}S{sub n}{sup -} cluster. A 3:2 ratio rule of Au/S is suggested for the formation of a hollow polyhedron structure among small-sized Au{sub m}S{sub m} clusters.

  17. Electronic Principles Governing the Stability and Reactivity of Ligated Metal and Silicon Encapsulated Transition Metal Clusters

    NASA Astrophysics Data System (ADS)

    Abreu, Marissa Baddick

    A thorough understanding of the underlying electronic principles guiding the stability and reactivity of clusters has direct implications for the identification of stable clusters for incorporation into clusters-assembled materials with tunable properties. This work explores the electronic principles governing the stability and reactivity of two types of clusters: ligated metal clusters and silicon encapsulated transition metal clusters. In the first case, the reactivity of iodine-protected aluminum clusters, Al13Ix - (x=0-4) and Al14Iy- (y=0-5), with the protic species methanol was studied. The symmetrical ground states of Al13Ix- showed no reactivity with methanol but reactivity was achieved in a higher energy isomer of Al 13I2- with iodines on adjacent aluminum atoms -- complementary Lewis acid-base active sites were induced on the opposite side of the cluster capable of breaking the O-H bond in methanol. Al 14Iy- (y=2-5) react with methanol, but only at the ligated adatom site. Reaction of methanol with Al14 - and Al14I- showed that ligation of the adatom was necessary for the reaction to occur there -- revealing the concept of a ligand-activated adatom. In the second case, the study focused heavily on CrSi12, a silicon encapsulated transition metal cluster whose stability and the reason for that stability has been debated heavily in the literature. Calculations of the energetic properties of CrSi n (n=6-16) revealed both CrSi12 and CrSi14 to have enhanced stability relative to other clusters; however CrSi12 lacks all the traditional markers of a magic cluster. Molecular orbital analysis of each of these clusters showed the CNFEG model to be inadequate in describing their stability. Because the 3dz2 orbital of Cr is unfilled in CrSi12, this cluster has only 16 effective valence electrons, meaning that the 18-electron rule is not applicable. The moderate stability of CrSi 12 can be accounted for by the crystal-field splitting of the 3d orbitals, which pushes the

  18. Irradiation-induced defect clustering and amorphization in silicon carbide

    SciTech Connect

    Weber, William J.; Gao, Fei

    2010-12-01

    Previous computer simulations of multiple 10 keV Si cascades in 3C-SiC demonstrated that many damage-state properties exhibit relatively smooth, but noticeably different, dose dependencies. Recent analysis of these archived damage-state properties reveals more complex relationships between system energy, swelling, energy per defect, relative disorder, elastic modulus and elastic constant, C11. These relationships provide evidence for the onset of defect clustering and amorphization processes, both of which appear to be driven by local energy and elastic instabilities from the accumulation of defects. The results provide guidance on experimental approaches to reveal the onset of these processes.

  19. Resonant mixing of optical orbital and spin angular momentum by using chiral silicon nanosphere clusters.

    PubMed

    Al-Jarro, Ahmed; Biris, Claudiu G; Panoiu, Nicolae C

    2016-04-01

    We present an in-depth analysis of the resonant intermixing between optical orbital and spin angular momentum of Laguerre-Gaussian (LG) beams, mediated by chiral clusters made of silicon nanospheres. In particular, we establish a relationship between the spin and orbital quantum numbers characterizing the LG beam and the order q of the rotation symmetry group q of the cluster of nanospheres for which resonantly enhanced coupling between the two components of the optical angular momentum is observed. Thus, similar to the case of diffraction grating-mediated transfer of linear momentum between optical beams, we demonstrate that clusters of nanospheres that are invariant to specific rotation transformations can efficiently transfer optical angular momentum between LG beams with different quantum numbers. We also discuss the conditions in which the resonant interaction between LG beams and a chiral cluster of nanospheres leads to the generation of superchiral light. PMID:27136989

  20. Robustness of cluster synchronous patterns in small-world networks with inter-cluster co-competition balance

    NASA Astrophysics Data System (ADS)

    Zhang, Jianbao; Ma, Zhongjun; Chen, Guanrong

    2014-06-01

    All edges in the classical Watts and Strogatz's small-world network model are unweighted and cooperative (positive). By introducing competitive (negative) inter-cluster edges and assigning edge weights to mimic more realistic networks, this paper develops a modified model which possesses co-competitive weighted couplings and cluster structures while maintaining the common small-world network properties of small average shortest path lengths and large clustering coefficients. Based on theoretical analysis, it is proved that the new model with inter-cluster co-competition balance has an important dynamical property of robust cluster synchronous pattern formation. More precisely, clusters will neither merge nor split regardless of adding or deleting nodes and edges, under the condition of inter-cluster co-competition balance. Numerical simulations demonstrate the robustness of the model against the increase of the coupling strength and several topological variations.

  1. Robustness of cluster synchronous patterns in small-world networks with inter-cluster co-competition balance

    SciTech Connect

    Zhang, Jianbao; Ma, Zhongjun; Chen, Guanrong

    2014-06-15

    All edges in the classical Watts and Strogatz's small-world network model are unweighted and cooperative (positive). By introducing competitive (negative) inter-cluster edges and assigning edge weights to mimic more realistic networks, this paper develops a modified model which possesses co-competitive weighted couplings and cluster structures while maintaining the common small-world network properties of small average shortest path lengths and large clustering coefficients. Based on theoretical analysis, it is proved that the new model with inter-cluster co-competition balance has an important dynamical property of robust cluster synchronous pattern formation. More precisely, clusters will neither merge nor split regardless of adding or deleting nodes and edges, under the condition of inter-cluster co-competition balance. Numerical simulations demonstrate the robustness of the model against the increase of the coupling strength and several topological variations.

  2. GW and Bethe-Salpeter study of small water clusters

    NASA Astrophysics Data System (ADS)

    Blase, Xavier; Boulanger, Paul; Bruneval, Fabien; Fernandez-Serra, Marivi; Duchemin, Ivan

    2016-01-01

    We study within the GW and Bethe-Salpeter many-body perturbation theories the electronic and optical properties of small (H2O)n water clusters (n = 1-6). Comparison with high-level CCSD(T) Coupled-Cluster at the Single Double (Triple) levels and ADC(3) Green's function third order algebraic diagrammatic construction calculations indicates that the standard non-self-consistent G0W0@PBE or G0W0@PBE0 approaches significantly underestimate the ionization energy by about 1.1 eV and 0.5 eV, respectively. Consequently, the related Bethe-Salpeter lowest optical excitations are found to be located much too low in energy when building transitions from a non-self-consistent G0W0 description of the quasiparticle spectrum. Simple self-consistent schemes, with update of the eigenvalues only, are shown to provide a weak dependence on the Kohn-Sham starting point and a much better agreement with reference calculations. The present findings rationalize the theory to experiment possible discrepancies observed in previous G0W0 and Bethe-Salpeter studies of bulk water. The increase of the optical gap with increasing cluster size is consistent with the evolution from gas to dense ice or water phases and results from an enhanced screening of the electron-hole interaction.

  3. Hippocampal cells encode places by forming small anatomical clusters.

    PubMed

    Nakamura, N H; Fukunaga, M; Akama, K T; Soga, T; Ogawa, S; Pavlides, C

    2010-03-31

    The hippocampus has been hypothesized to function as a "spatial" or "cognitive" map, however, the functional cellular organization of the spatial map remains a mystery. The majority of electrophysiological studies, thus far, have supported the view of a random-type organization in the hippocampus. However, using immediate early genes (IEGs) as an indicator of neuronal activity, we recently observed a cluster-type organization of hippocampal principal cells, whereby a small number ( approximately 4) of nearby cells were activated in rats exposed to a restricted part of an environment. To determine the fine structure of these clusters and to provide a 3D image of active hippocampal cells that encode for different parts of an environment, we established a functional mapping of IEGs zif268 and Homer1a, using in situ hybridization and 3D-reconstruction imaging methods. We found that, in rats exposed to the same location twice, there were significantly more double IEG-expressing cells, and the clusters of nearby cells were more "tightly" formed, in comparison to rats exposed to two different locations. We propose that spatial encoding recruits specific cell ensembles in the hippocampus and that with repeated exposure to the same place the ensembles become better organized to more accurately represent the "spatial map." PMID:20060034

  4. Hippocampal cells encode places by forming small anatomical clusters

    PubMed Central

    Nakamura, Nozomu H.; Fukunaga, Masaki; Akama, Keith T.; Soga, Tomoko; Ogawa, Sonoko; Pavlides, Constantine

    2010-01-01

    The hippocampus has been hypothesized to function as a “spatial” or “cognitive” map, however, the functional cellular organization of the spatial map remains a mystery. The majority of electrophysiological studies, thus far, have supported the view of a random-type organization in the hippocampus. However, using immediate early genes (IEGs) as an indicator of neuronal activity, we recently observed a cluster-type organization of hippocampal principal cells, whereby a small number (~4) of nearby cells were activated in animals exposed to a restricted part of an environment. To determine the fine structure of these clusters and to provide a 3D image of active hippocampal cells that encode for different parts of an environment, we established a functional mapping of immediate early genes (IEGs) zif268 and Homer1a, using in situ hybridization and 3D-reconstruction imaging methods. We found that, in animals exposed to the same location twice, there were significantly more double IEG-expressing cells, and the clusters of nearby cells were more “tightly” formed, in comparison to animals exposed to two different locations. We propose that spatial encoding recruits specific cell ensembles in the hippocampus and that with repeated exposure to the same place the ensembles become better organized to more accurately represent the “spatial map”. PMID:20060034

  5. A study of the self-aligned nanometre scale palladium clusters on silicon formation process

    NASA Astrophysics Data System (ADS)

    Gavrilov, S.; Lemeshko, S.; Shevyakov, V.; Roschin, V.

    1999-06-01

    The possibility of the self-aligned formation of Pd/Pd2Si/Si nanostructures on a single-crystal silicon substrate is shown. A porous anodic oxide film of Al was used as a mask which determines the size and shape of the nanostructures. A thin Al film was first deposited on the silicon substrate and then transformed in a nanoporous oxide by the well known anodic treatment procedure in a sulfuric acid and water solution. It is shown by atomic force microscopy that nanoscale Pd clusters with diameters equal to the size of pores in anodic Al remain at the surface of silicon substrate after cathode deposition of Pd into the pores, vacuum thermal annealing and chemical etching of the Al2O3 mask. In addition, we determine the dependencies of the size and shape of the nanoclusters on the mask formation regimes and the Pd deposition conditions.

  6. Isolation and Versatile Derivatization of an Unsaturated Anionic Silicon Cluster (Siliconoid)

    PubMed Central

    Willmes, Philipp; Leszczyńska, Kinga; Heider, Yannic; Abersfelder, Kai; Zimmer, Michael; Huch, Volker

    2016-01-01

    Abstract The characteristic features of bulk silicon surfaces are echoed in the related partially substituted—and thus unsaturated—neutral silicon clusters (siliconoids). The incorporation of siliconoids into more‐extended frameworks is promising owing to their unique electronic features, but further developments in this regard are limited by the notable absence of functionalized siliconoid derivatives until now. Herein we report the isolation and full characterization of the lithium salt of an anionic R5Si6‐siliconoid, thus providing the missing link between silicon‐based Zintl anions and siliconoid clusters. Proof‐of‐principle for the high potential of this species for the efficient transfer of the intact unsaturated R5Si6 moiety is demonstrated by clean reactions with representative electrophiles of Groups 13, 14, and 15. PMID:26800440

  7. Phase Behavior of Thermodynamically Small Clusters of Colloidal Particles

    NASA Astrophysics Data System (ADS)

    Thyagarajan, Raghuram; Maroudas, Dimitrios; Ford, David

    The self-assembly of finite clusters of colloidal particles into crystalline objects is a topic of technological interest, as a route to produce photonic crystals and other metamaterials. Such assembly problems also are fundamentally interesting because they involve thermodynamically small systems, with number of particles between 10 and 1000 that is far below the bulk limit. In contrast to bulk systems, these colloidal assemblies exhibit phase coexistence over a finite range of physical conditions. Here, we report the results of a computational study of phase behavior of small colloidal clusters interacting via the Asakura-Oosawa depletion pair potential. We conducted Monte Carlo simulations for various levels of the osmotic pressure that controls the strength of the interparticle interactions, using potential energy histograms to identify distinct phases. Over a narrow but finite range of the osmotic pressure, we find bimodal distributions in the potential energy space that are indicative of coexistence between fluid-like and crystalline configurations. We also report systematic quantitative comparisons of the phase behavior observed here with results from a Fokker-Planck order-parameter approach.

  8. Irradiation-induced defect clustering and amorphization in silicon carbide

    SciTech Connect

    Weber, William J; Gao, Fei

    2010-01-01

    Previous computer simulations of multiple 10 keV Si cascades in 3C-SiC demonstrated that many damage-state properties exhibit relatively smooth, but noticeably different, dose dependencies. A more recent analysis of these damage-state properties, which includes additional data at low and intermediate doses, reveals more complex relationships between system energy, swelling, energy per defect, relative disorder, elastic modulus and elastic constant, C11. These relationships provide evidence for the onset of both defect clustering and solid-state amorphization, which appear to be driven by local energy and elastic instabilities from the accumulation of defects. The results provide guidance on experimental approaches to reveal the onset of these processes.

  9. Ionization potentials of small lithium clusters (Lin) and hydrogenated lithium clusters (LinH)

    NASA Astrophysics Data System (ADS)

    Wheeler, Steven E.; Schaefer, Henry F.

    2005-05-01

    We present accurate ionization potentials (IPs) for small lithium clusters and hydrogenated lithium clusters (n=1-4), computed using coupled-cluster singles and doubles theory augmented with a perturbative correction for connected triple excitations [CCSD(T)] with the correlation-consistent weighted core-valence quadruple-ζ basis set (cc-pwCVQZ). In some cases the full CCSDT method has been used. Comparison of computed binding energies with experiment for the pure cationic lithium clusters reveals excellent agreement, demonstrating that previous discrepancies between computed and experimentally derived atomization energies for the corresponding neutral clusters are due to the use of an inaccurate experimental IP for Li4. The experimental IP for Li4 falls 0.43eV below our theoretical adiabatic value of 4.74eV, which should be a lower bound to the measured IP. Our recommended zero-point corrected adiabatic IPs for Li, Li2, Li3, Li4, LiH, Li2H, Li3H, and Li4H are 5.39, 5.14, 4.11, 4.74, 7.69, 3.98, 4.69, and 4.05eV, respectively. Zero-point vibrationally corrected CCSD(T) atomization energies per atom for Li2+, Li3+, Li4+, LiH+, Li2H+, Li3H+, and Li4H+ are 0.64, 0.96, 0.90, 0.056, 1.62, 1.40, and 1.40eV, respectively.

  10. Understanding the structural transformation, stability of medium-sized neutral and charged silicon clusters

    PubMed Central

    Ding, Li Ping; Zhang, Fang Hui; Zhu, Yong Sheng; Lu, Cheng; Kuang, Xiao Yu; Lv, Jian; Shao, Peng

    2015-01-01

    The structural and electronic properties for the global minimum structures of medium-sized neutral, anionic and cationic Sinμ (n = 20–30, μ = 0, −1 and +1) clusters have been studied using an unbiased CALYPSO structure searching method in conjunction with first-principles calculations. A large number of low-lying isomers are optimized at the B3PW91/6-311 + G* level of theory. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The growth behaviors clearly indicate that a structural transition from the prolate to spherical-like geometries occurs at n = 26 for neutral silicon clusters, n = 27 for anions and n = 25 for cations. These results are in good agreement with the available experimental and theoretical predicted findings. In addition, no significant structural differences are observed between the neutral and cation charged silicon clusters with n = 20–24, both of them favor prolate structures. The HOMO-LUMO gaps and vertical ionization potential patterns indicate that Si22 is the most chemical stable cluster, and its dynamical stability is deeply discussed by the vibrational spectra calculations. PMID:26526519

  11. Thermal radiation and fragmentation pathways of photo-excited silicon clusters.

    PubMed

    Ferrari, Piero; Janssens, Ewald; Lievens, Peter; Hansen, Klavs

    2015-12-14

    The fragmentation of laser heated silicon clusters was studied by time-of-flight mass spectrometry. For Si(n)(+) (n = 5-19, 21), the lowest energy fragmentation pathways were identified as the metastable decay channel occurring after the primary acceleration of the ions. The radiative cooling of laser excited Si(n)(+) (n = 5-9, 11, and 13) was quantified via its quenching effect on the amount of metastable fragmentation. The quenching varied strongly with cluster size, from no observable amount for Si7(+) to a cooling constant of 3 ⋅ 10(5) s(-1) for Si13(+). In addition, based on the observed fragmentation channels, the ionization energies and the relative binding energies of the clusters were partially ordered, and several ionization energies have been bracketed more precisely. PMID:26671381

  12. Small area silicon diffused junction x-ray detectors

    SciTech Connect

    Walton, J.T.; Pehl, R.H.; Larsh, A.E.

    1981-10-01

    The low temperature performance of silicon diffused junction detectors in the measurement of low energy x-rays is reported. The detectors have an area of 0.04 cm/sup 2/ and a thickness of 100 ..mu..m. The spectral resolutions of these detectors were found to be in close agreement with expected values indicating that the defects introduced by the high temperature processing required in the device fabrication were not deleteriously affecting the detection of low energy x-rays. Device performance over a temperature range of 77 to 150/sup 0/K is given. These detectors were designed to detect low energy x-rays in the presence of minimum ionizing electrons. The successful application of silicon diffused junction technology to x-ray detector fabrication may facilitate the development of other novel silicon x-ray detector designs.

  13. Small signal modeling of high electron mobility transistors on silicon and silicon carbide substrate with consideration of substrate loss mechanism

    NASA Astrophysics Data System (ADS)

    Sahoo, A. K.; Subramani, N. K.; Nallatamby, J. C.; Sylvain, L.; Loyez, C.; Quere, R.; Medjdoub, F.

    2016-01-01

    In this paper, we present a comparative study on small-signal modeling of AlN/GaN/AlGaN double hetero-structure high electron mobility transistors (HEMTs) grown on silicon (Si) and silicon carbide (SiC) substrate. The traditional small signal equivalent circuit model is modified to take into account the transmission loss mechanism of coplanar waveguide (CPW) line which cannot be neglected at high frequencies. CPWs and HEMTs-on-AlN/GaN/AlGaN epitaxial layers are fabricated on both the Si and SiC substrates. S-parameter measurements at room temperature are performed over the frequency range from 0.5 GHz to 40 GHz. Transmission loss of CPW lines are modeled with a distributed transmission line (TL) network and an equivalent circuit model is included in the small-signal transistor model topology. Measurements and simulations are compared and found to be in good agreement.

  14. Physical electrostatics of small field emitter arrays/clusters

    NASA Astrophysics Data System (ADS)

    Forbes, Richard G.

    2016-08-01

    This paper aims to improve qualitative understanding of electrostatic influences on apex field enhancement factors (AFEFs) for small field emitter arrays/clusters. Using the "floating sphere at emitter-plate potential" (FSEPP) model, it re-examines the electrostatics and mathematics of three simple systems of identical post-like emitters. For the isolated emitter, various approaches are noted. An adequate approximation is to consider only the effects of sphere charges and (for significantly separated emitters) image charges. For the 2-emitter system, formulas are found for charge-transfer ("charge-blunting") effects and neighbor-field effects, for widely spaced and for "sufficiently closely spaced" emitters. Mutual charge-blunting is always the dominant effect, with a related (negative) fractional AFEF-change δtwo. For sufficiently small emitter spacing c, |δtwo| varies approximately as 1/c; for large spacing, |δtwo| decreases as 1/c3. In a 3-emitter equispaced linear array, differential charge-blunting and differential neighbor-field effects occur, but differential charge-blunting effects are dominant, and cause the "exposed" outer emitters to have higher AFEF (γ0) than the central emitter (γ1). Formulas are found for the exposure ratio Ξ = γ0/γ1, for large and for sufficiently small separations. The FSEPP model for an isolated emitter has accuracy around 30%. Line-charge models (LCMs) are an alternative, but an apparent difficulty with recent LCM implementations is identified. Better descriptions of array electrostatics may involve developing good fitting equations for AFEFs derived from accurate numerical solution of Laplace's equation, perhaps with equation form(s) guided qualitatively by FSEPP-model results. In existing fitting formulas, the AFEF-reduction decreases exponentially as c increases, which is different from the FSEPP-model formulas. This discrepancy needs to be investigated, using systematic Laplace-based simulations and appropriate results

  15. Fourier Transform Vibrational Spectroscopy of Pure Carbon and Silicon-Carbon Clusters.

    NASA Astrophysics Data System (ADS)

    Withey, Paul Andrew

    Fourier transform infrared studies of pure carbon and silicon-carbon clusters produced by vacuum ultraviolet (VUV) photolysis and by the newly developed method of laser evaporation have resulted in the identification of new vibrational information for the C_4, SiC_4 and C_9 clusters. For the first time, the far-infrared bending vibration of C_4 has been observed at a frequency of 172.4 cm^{-1} and confirmed by ^{13}C isotopic data in agreement with predictions of theoretical ab initio calculations for the linear geometry. Along with the earlier observation of the antisymmetric stretching mode at 1543.4 cm^{-1}, the characterization of the infrared active fundamentals of C_4 under the strict linear geometry is now complete. With the exception of C_3, C _4 remains the only pure carbon cluster to be detected in the far-infrared by direct observation. An analysis of the products of the VUV photolysis of a mixture of silane (SiH_4) and 1,3-butadiene rm (C_4H_6) has resulted in the first identification of a vibration of SiC_4 at 2080.1 cm^ {-1} assigned to the nu _1 stretching mode. Prior to this, only rotational transitions for this cluster had been observed. SiC _4 is one of the few molecules to be identified in the circumstellar shell of an evolved carbon star, and the detection of the first vibrational frequency may facilitate its further detection in astronomical sources. A new technique employing laser evaporation of a graphite rod, designed specifically for the detection of the vibrational spectrum of C_9, has resulted in the confirmation of an absorption at 1998.0 cm^{-1} assigned to the nu_6(sigma_{u}) stretching fundamental. Another band at 1601.0 cm^{-1} is tentatively assigned to the nu_7(sigma_ {u}) vibration of the linear C _9 cluster. Laser evaporation has many advantages over high temperature evaporation and it is expected that this method may be beneficial in the observation of vibrational spectra of other molecular species, such as the pure silicon

  16. MODELING THE VERY SMALL SCALE CLUSTERING OF LUMINOUS RED GALAXIES

    SciTech Connect

    Watson, Douglas F.; Berlind, Andreas A.; McBride, Cameron K.; Masjedi, Morad

    2010-01-20

    We model the small-scale clustering of luminous red galaxies (LRGs) in the Sloan Digital Sky Survey. Specifically, we use the halo occupation distribution formalism to model the projected two-point correlation function of LRGs on scales well within the sizes of their host halos (0.016 h {sup -1} Mpc <= r <= 0.42 h {sup -1} Mpc). We start by varying P(N|M), the probability distribution that a dark matter halo of mass M contains N LRGs, and assuming that the radial distribution of satellite LRGs within halos traces the Navarro-Frenk-White (NFW) dark matter density profile. We find that varying P(N|M) alone is not sufficient to match the small-scale data. We next allow the concentration of satellite LRG galaxies to differ from that of dark matter and find that this is also not sufficient. Finally, we relax the assumption of an NFW profile and allow the inner slope of the density profile to vary. We find that this model provides a good fit to the data and the resulting value of the slope is -2.17 +- 0.12. The radial density profile of satellite LRGs within halos is thus not compatible with that of the underlying dark matter, but rather is closer to an isothermal distribution.

  17. Microstructure of amorphous-silicon-based solar cell materials by small-angle x-ray scattering. Annual subcontract report, 6 April 1994--5 April 1995

    SciTech Connect

    Williamson, D.L.

    1995-08-01

    The general objective of this research is to provide detailed microstructural information on the amorphous-silicon-based, thin-film materials under development for improved multijunction solar cells. The experimental technique used is small-angle x-ray scattering (SAXS) providing microstructural data on microvoid fractions, sizes, shapes, and their preferred orientations. Other microstructural features such as alloy segregation, hydrogen-rich clusters and alloy short-range order are probed.

  18. Characterizing the Small Scale Structure in Clusters of Galaxies

    NASA Technical Reports Server (NTRS)

    Forman, William R.

    2001-01-01

    We studied galaxy clusters Abell 119, Abell 754, and Abell 1750, using data from the ASCA and ROSAT satellites. In addition, we completed the paper "Merging Binary Clusters". In this paper we study three prominent bi-modal X-ray clusters: A3528, A1750 and A3395. Since the sub-clusters in these systems have projected separations of 0.93, 1.00 and 0.67 Mpc respectively, we examine their X-ray and optical observations to investigate the dynamics and possible merging of these sub-clusters. Using data taken with ROSAT and ASCA, we analyze the temperature and surface brightness distributions. We also analyze the velocity distributions of the three clusters using new measurements supplemented with previously published data. We examined both the overall cluster properties as well as the two sub-cluster elements in each. These results were then applied to the determination of the overall cluster masses, that demonstrate excellent consistency between the various methods used. While the characteristic parameters of the sub-clusters are typical of isolated objects, our temperature results for the regions between the two sub-clusters clearly confirm the presence of merger activity that is suggested by the surface brightness distributions. These three clusters represent a progression of equal-sized sub-cluster mergers, starting from initial contact to immediately before first core passage.

  19. Growth and properties of silicon heterostructures with buried nanosize Mg2Si clusters

    NASA Astrophysics Data System (ADS)

    Galkin, N. G.; Galkin, K. N.

    2005-06-01

    The technology of solid-phase growth of nanosize islands of magnesium suicide on Si (111) 7x7 with narrow distributions of lateral size and height (60 - 80 and 5 - 7 nanometers, respectively) and density of up to 2x 109 sm-2 is proposed. A 20-50 nm thick Si layer has been grown upon these islands. Basing on the data of AES, EELS, AFM and JR spectroscopy, a conclusion is made that the Mg2Si islands remain in depth of the Si layer. The suggestion is made that sizes, density and crystal structure of the buried magnesium suicide clusters preserves. It is shown, that the system of three as-grown layers of buried clusters has smoother surface than the one layer system. The contribution of the Mg2Si clusters into the dielectric function is observed at the energy 0.8-1.2 eV, it is maximal if the clusters are localized on the silicon surface. It is shown, that with increase of the number of Mg2Si cluster layers their contribution increases into the effective number of electrons per a unit cell and effective dielectric function of the sample.

  20. First principle study of magnetic and electronic properties of single X (X = Al, Si) atom added to small carbon clusters (C n X, n = 2-10)

    NASA Astrophysics Data System (ADS)

    Afshar, M.; Hoseini, S. S.; Sargolzaei, M.

    2016-07-01

    In this paper, the magnetic and electronic properties of single aluminum and silicon atom added to small carbon clusters (C n X; X = Al, Si; n = 2-10) are studied in the framework of generalized-gradient approximation using density functional theory. The calculations were performed for linear, two dimensional and three dimensional clusters based on full-potential local-orbital (FPLO) method. The total energies, HOMO-LUMO energy gap and total magnetic moments of the most stable structures are presented in this work. The calculations show that C n Si clusters have more stability compared to C n Al clusters. In addition, our magnetic calculations were shown that the C n Al isomers are magnetic objects whereas C n Si clusters are nonmagnetic objects.

  1. Small scale clustering of late forming dark matter

    NASA Astrophysics Data System (ADS)

    Agarwal, S.; Corasaniti, P.-S.; Das, S.; Rasera, Y.

    2015-09-01

    We perform a study of the nonlinear clustering of matter in the late-forming dark matter (LFDM) scenario in which dark matter results from the transition of a nonminimally coupled scalar field from radiation to collisionless matter. A distinct feature of this model is the presence of a damped oscillatory cutoff in the linear matter power spectrum at small scales. We use a suite of high-resolution N-body simulations to study the imprints of LFDM on the nonlinear matter power spectrum, the halo mass and velocity functions and the halo density profiles. The model largely satisfies high-redshift matter power spectrum constraints from Lyman-α forest measurements, while it predicts suppressed abundance of low-mass halos (˜109- 1010 h-1 M⊙ ) at all redshifts compared to a vanilla Λ CDM model. The analysis of the LFDM halo velocity function shows a better agreement than the Λ CDM prediction with the observed abundance of low-velocity galaxies in the local volume. Halos with mass M ≳1011 h-1 M⊙ show minor departures of the density profiles from Λ CDM expectations, while smaller-mass halos are less dense, consistent with the fact that they form later than their Λ CDM counterparts.

  2. Neurovascular Modeling: Small-Batch Manufacturing of Silicone Vascular Replicas

    PubMed Central

    Chueh, J.Y.; Wakhloo, A.K.; Gounis, M.J.

    2009-01-01

    BACKGROUND AND PURPOSE Realistic, population based cerebrovascular replicas are required for the development of neuroendovascular devices. The objective of this work was to develop an efficient methodology for manufacturing realistic cerebrovascular replicas. MATERIALS AND METHODS Brain MR angiography data from 20 patients were acquired. The centerline of the vasculature was calculated, and geometric parameters were measured to describe quantitatively the internal carotid artery (ICA) siphon. A representative model was created on the basis of the quantitative measurements. Using this virtual model, we designed a mold with core-shell structure and converted it into a physical object by fused-deposit manufacturing. Vascular replicas were created by injection molding of different silicones. Mechanical properties, including the stiffness and luminal coefficient of friction, were measured. RESULTS The average diameter, length, and curvature of the ICA siphon were 4.15 ± 0.09 mm, 22.60 ± 0.79 mm, and 0.34 ± 0.02 mm-1 (average ± standard error of the mean), respectively. From these image datasets, we created a median virtual model, which was transformed into a physical replica by an efficient batch-manufacturing process. The coefficient of friction of the luminal surface of the replica was reduced by up to 55% by using liquid silicone rubber coatings. The modulus ranged from 0.67 to 1.15 MPa compared with 0.42 MPa from human postmortem studies, depending on the material used to make the replica. CONCLUSIONS Population-representative, smooth, and true-to-scale silicone arterial replicas with uniform wall thickness were successfully built for in vitro neurointerventional device-testing by using a batch-manufacturing process. PMID:19321626

  3. Silicon avalanche photodiode operation and lifetime analysis for small satellites.

    PubMed

    Tan, Yue Chuan; Chandrasekara, Rakhitha; Cheng, Cliff; Ling, Alexander

    2013-07-15

    Silicon avalanche photodiodes (APDs) are sensitive to operating temperature fluctuations and are also susceptible to radiation flux expected in satellite-based quantum experiments. We introduce a low power voltage adjusting mechanism to overcome the effects of in-orbit temperature fluctuations. We also present data on the performance of Si APDs after irradiation (γ-ray and proton beam). Combined with an analysis of expected orbital irradiation, we propose that a Si APD in a 400 km equatorial orbit may operate beyond the lifetime of the satellite. PMID:23938543

  4. Silicon oxide cluster formation and stability in the laser ablation of SiO targets.

    PubMed

    Jadraque, María; Santos, Magna; Díaz, Luís; Alvarez-Ruiz, Jesús; Martín, Margarita

    2009-10-15

    The formation mechanism and stability of silicon oxide clusters observed in the ablation of SiO targets at 266 nm were investigated by time-of-flight mass spectrometry, laser-induced fluorescence (LIF), and DFT calculations. Neutral and positively charged Si(n)(+/0) and Si(n)O(m)H(0,1)(+) clusters were identified in the plume, but neutral Si(n)O(m) could not be observed. The time distribution of SiO in the plume measured by postionization with an ArF laser (Delta lambda approximately 1 nm, tau approximately 14 ns) and mass spectrometric detection was compared with that obtained by LIF with narrowband dye laser selective excitation of one specific rovibronic transition in SiO. Postionization leads to a multicomponent distribution that extends up to times near 100 micros after ablation, whereas LIF measurements obtain time distributions shorter than 20 micros. DFT calculations of several Si(n)O(m)(0/+) were performed, showing that one photon absorption of the postionization laser makes available low-energy dissociation channels of the neutrals, whereas two photon absorption is required for ionization. DFT calculations were carried out for stoichiometric H-containing clusters Si(n)O(n)H(+) (n = 1-4). For n = 1,2, the optimized geometries involve bonding of hydrogen to one oxygen atom in the clusters; for n = 3 and 4, the structures containing H-Si bonds are more stable. PMID:19810756

  5. Effect of particle clustering of silver nanoparticles on ultrathin silicon solar cell

    NASA Astrophysics Data System (ADS)

    Shokeen, Poonam; Jain, Amit; Kapoor, Avinashi; Gupta, Vinay

    2016-07-01

    Particle clustering is a major concern for uniform dispersal of nanoparticles in various deposition procedures. Well separated uniform distribution of metal nanoparticles is essential for effective coupling of surface plasmons. This work experimentally and theoretically, discusses the effect of nanoparticle clustering on the light trapping efficiency of silver nanoparticles. Pulsed laser deposition system has been used for deposition of silver nanoparticles, and substrate heating has been used to promote uniform distribution of nanoparticles. Pre-heated substrate depositions are compared with corresponding post-annealed samples. XRD, FESEM, Photoluminescence and UV-visible spectroscopy have been used to study the variations in their structural and optical properties. Mono-dispersal of silver nanoparticles for pre-heated substrates results in sharper surface plasmon resonance in comparison to post-annealed samples. Mie theory is used to estimate the particle size of the nanoparticles and findings are in accordance with quantitative analysis of FESEM images. Finite-difference time domain technique is used to discuss the effect of particle distribution on an ultrathin film silicon solar cell. Device degradation is observed as a result of clustering of silver nanoparticles. Hence, mono-dispersal of plasmonic nanostructures is important for required results and pre-heated deposition of metal nanoparticles by pulsed laser deposition can effectively solve the problem of particle clustering.

  6. Structural determination of niobium-doped silicon clusters by far-infrared spectroscopy and theory.

    PubMed

    Li, Xiaojun; Claes, Pieterjan; Haertelt, Marko; Lievens, Peter; Janssens, Ewald; Fielicke, André

    2016-02-17

    In this work, the structures of cationic SinNb(+) (n = 4-12) clusters are determined using the combination of infrared multiple photon dissociation (IR-MPD) and density functional theory (DFT) calculations. The experimental IR-MPD spectra of the argon complexes of SinNb(+) are assigned by comparison to the calculated IR spectra of low-energy structures of SinNb(+) that are identified using the stochastic 'random kick' algorithm in conjunction with the BP86 GGA functional. It is found that the Nb dopant tends to bind in an apex position of the Sin framework for n = 4-9 and in surface positions with high coordination numbers for n = 10-12. For the larger doped clusters, it is suggested that multiple isomers coexist and contribute to the experimental spectra. The structural evolution of SinNb(+) clusters is similar to V-doped silicon clusters (J. Am. Chem. Soc., 2010, 132, 15589-15602), except for the largest size investigated (n = 12), since V takes an endohedral position in Si12V(+). The interaction with a Nb atom, with its partially unfilled 4d orbitals leads to a significant stability enhancement of the Sin framework as reflected, e.g. by high binding energies and large HOMO-LUMO gaps. PMID:26853772

  7. Electronic Structure and Geometries of Small Compound Metal Clusters

    SciTech Connect

    1999-04-14

    During the tenure of the DOE grant DE-FG05-87EI145316 we have concentrated on equilibrium geometries, stability, and the electronic structure of transition metal-carbon clusters (met-cars), clusters designed to mimic the chemistry of atoms, and reactivity of homo-nuclear metal clusters and ions with various reactant molecules. It is difficult to describe all the research the authors have accomplished as they have published 38 papers. In this report, they outline briefly the salient features of their work on the following topics: (1) Designer Clusters: Building Blocks for a New Class of Solids; (2) Atomic Structure, Stability, and Electronic Properties of Metallo-Carbohedrenes; (3) Reactivity of Metal Clusters with H{sub 2} and NO; and (4) Anomalous Spectroscopy of Li{sub 4} Clusters.

  8. Properties of small Ar sub N-1 K/+/ ionic clusters

    NASA Technical Reports Server (NTRS)

    Etters, R. D.; Danilowicz, R.; Dugan, J.

    1977-01-01

    A self-consistent formalism is developed that, based upon a many-body potential, dynamically determines the thermodynamic properties of ionic clusters without an a priori designation of the equilibrium structures. Aggregates consisting of a single closed shell K(+) ion and N-1 isoelectronic argon atoms were studied. The clusters form crystallites at low temperatures, and melting transitions and spontaneous dissociations are indicated. The results confirm experimental evidence that shows that ionic clusters become less stable with increasing N. The crystallite structures formed by four different clusters are isosceles triangle, skewed form, octahedron with ion in the middle, and icosahedron with the ion in the middle.

  9. Computer simulation structure and vibrations of small metal cluster on the Cu (111) surface

    SciTech Connect

    Borisova, Svetlana D. Rusina, Galina G.

    2015-10-27

    Vibrational properties of the small tetrahedral cluster of Co on the Cu (111) surface are studied by using tight-binding second moment approximation interatomic interaction potentials. It was shown that interaction of the clusters with substrate leads to arising of frustrated translation and frustrated rotation in-plane polarized vibrational modes localized on the cluster atoms. The Co{sub 4} cluster on the surface the high frequency modes remain strongly localized and mixed with the nearest neighbor atoms vibrations.

  10. Raman spectra of silicon carbide small particles and nanowires

    NASA Astrophysics Data System (ADS)

    Wieligor, Monika; Wang, Yuejian; Zerda, T. W.

    2005-04-01

    Two manufacturing protocols of silicon carbide (SiC) nanowires are discussed. The Raman spectra of produced SiC nanowires are compared with spectra of SiC powders of various grain sizes. The temperature and pressure dependence of the Raman spectra for powders is similar to that of bulk crystals, but is different for nanowires. Frequency shifts, band broadenings and the presence of shoulders are discussed in terms of crystal size, character of defects and their population. The concentration of defects in synthesized nanowires depends on the sintering method. Raman intensity enhancement of the LO phonon was observed when the wavelength of the excitation laser was changed from 780 to 514 nm.

  11. Direct observation of small cluster mobility and ripening

    NASA Technical Reports Server (NTRS)

    Heinemann, K.; Poppa, H.

    1976-01-01

    Direct evidence is reported for the simultaneous occurrence of Ostwald ripening and short-distance cluster mobility during annealing of discontinuous metal films on clean amorphous substrates. The annealing characteristics of very thin particulate deposits of silver on amorphized clean surfaces of single-crystalline thin graphite substrates have been studied by in situ transmission electron microscopy (TEM) under controlled environmental conditions in the temperature range from 25 to 450 C. It was possible to monitor all stages of the experiments by TEM observation of the same specimen area. Slow Ostwald ripening was found to occur over the entire temperature range, but the overriding surface transport mechanism was short-distance cluster mobility. This was concluded from in situ observations of individual particles during annealing and from measurements of cluster size distributions, cluster number densities, area coverages, and mean cluster diameters.

  12. Negatively charged subnanometer-sized silicon clusters and their reversible migration into AFI zeolite pores studied with X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy

    NASA Astrophysics Data System (ADS)

    Choo, Cheow-keong; Sakamoto, Takashi; Tanaka, Katsumi; Nakata, Ryouhei; Asakawa, Tetsuo

    1999-02-01

    Subnanometer sized silicon clusters were deposited on AFI zeolite (AlPO 4-5: one-dimensional channel diameter <0.73 nm) by pulsed laser ablation of silicon wafer. Their electronic structures were elucidated in situ by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Core level Si 2p spectra were analyzed into five components, Si(I) to Si(V). Si(I) and Si(II) species selectively increased with a constant ratio during pulsed laser silicon ablation. Their binding energies (BEs) were below 99.5 eV implying negatively charged states. Charge transfer occurred between silicon clusters and framework oxygen and phosphor ions. It was interpreted that the stability of negative charge is due to large electron affinity of silicon clusters. The intensity of XPS signals decreased as a function of time and at the same time the channels were blocked. These results were interpreted due to migration of silicon clusters into zeolite pores. The estimated activation energy (57 kJ/mol) suggests that rate-determining step of the migration is reflected by a weak adsorbed state of silicon clusters similar to physisorbed state. The silicon clusters were partially oxidized at 573 K, which was interpreted as a driving force of backward migration from zeolite pores to the external surface. The composition of silicon cluster was discussed based on homogeneous dispersion of single species.

  13. Vibration-rotation-tunneling dynamics in small water clusters

    SciTech Connect

    Pugliano, N.

    1992-11-01

    The goal of this work is to characterize the intermolecular vibrations of small water clusters. Using tunable far infrared laser absorption spectroscopy, large amplitude vibration-rotation-tunneling (VRT) dynamics in vibrationally excited states of the water dimer and the water trimer are investigated. This study begins with the measurement of 12 VRT subbands, consisting of approximately 230 transitions, which are assigned to an 82.6 cm[sup [minus]1] intermolecular vibration of the water dimer-d[sub 4]. Each of the VRT subbands originate from K[sub a][double prime]=0 and terminate in either K[sub a][prime]=0 or 1. These data provide a complete characterization of the tunneling dynamics in the vibrationally excited state as well as definitive symmetry labels for all VRT energy levels. Furthermore, an accurate value for the A[prime] rotational constant is found to agree well with its corresponding ground state value. All other excited state rotational constants are fitted, and discussed in terms of the corresponding ground state constants. In this vibration, the quantum tunneling motions are determined to exhibit large dependencies with both the K[sub a][prime] quantum number and the vibrational coordinate, as is evidenced by the measured tunneling splittings. The generalized internal-axis-method treatment which has been developed to model the tunneling dynamics, is considered for the qualitative description of each tunneling pathway, however, the variation of tunneling splittings with vibrational excitation indicate that the high barrier approximation does not appear to be applicable for this vibrational coordinate. The data are consistent with a motion possessing a[prime] symmetry, and the vibration is assigned as the [nu][sub 12] acceptor bending coordinate. This assignment is in agreement with the vibrational symmetry, the resultsof high level ab initio calculations, and preliminary data assigned to the analogous vibration in the D[sub 2]O-DOH isotopomer.

  14. Vibration-rotation-tunneling dynamics in small water clusters

    SciTech Connect

    Pugliano, N.

    1992-11-01

    The goal of this work is to characterize the intermolecular vibrations of small water clusters. Using tunable far infrared laser absorption spectroscopy, large amplitude vibration-rotation-tunneling (VRT) dynamics in vibrationally excited states of the water dimer and the water trimer are investigated. This study begins with the measurement of 12 VRT subbands, consisting of approximately 230 transitions, which are assigned to an 82.6 cm{sup {minus}1} intermolecular vibration of the water dimer-d{sub 4}. Each of the VRT subbands originate from K{sub a}{double_prime}=0 and terminate in either K{sub a}{prime}=0 or 1. These data provide a complete characterization of the tunneling dynamics in the vibrationally excited state as well as definitive symmetry labels for all VRT energy levels. Furthermore, an accurate value for the A{prime} rotational constant is found to agree well with its corresponding ground state value. All other excited state rotational constants are fitted, and discussed in terms of the corresponding ground state constants. In this vibration, the quantum tunneling motions are determined to exhibit large dependencies with both the K{sub a}{prime} quantum number and the vibrational coordinate, as is evidenced by the measured tunneling splittings. The generalized internal-axis-method treatment which has been developed to model the tunneling dynamics, is considered for the qualitative description of each tunneling pathway, however, the variation of tunneling splittings with vibrational excitation indicate that the high barrier approximation does not appear to be applicable for this vibrational coordinate. The data are consistent with a motion possessing a{prime} symmetry, and the vibration is assigned as the {nu}{sub 12} acceptor bending coordinate. This assignment is in agreement with the vibrational symmetry, the resultsof high level ab initio calculations, and preliminary data assigned to the analogous vibration in the D{sub 2}O-DOH isotopomer.

  15. Theoretical studies of the electronic structure of small metal clusters

    NASA Technical Reports Server (NTRS)

    Jordan, K. D.

    1982-01-01

    Theoretical studies of the electronic structure of metal clusters, in particular clusters of Group IIA and IIB atoms were conducted. Early in the project it became clear that electron correlation involving d orbitals plays a more important role in the binding of these clusters than had been previously anticipated. This necessitated that computer codes for calculating two electron integrals and for constructing the resulting CI Hamiltonions be replaced with newer, more efficient procedures. Program modification, interfacing and testing were performed. Results of both plans are reported.

  16. Nano Vacancy Clusters and Trap Limited Diffusion of Si Interstitials in Silicon

    SciTech Connect

    Prof. Wei-Kan Chu

    2010-05-05

    The objective of this project is to develop a method to characterize nano vacancy clusters and the dynamics of their formation in ion-irradiated silicon. It will impact (1) semiconductor device processing involving ion implantation, and (2) device design concerning irradiation hardness in harsh environments. It also aims to enhance minority participation in research and curricula on emerging materials and ion beam science. Vacancy defects are of scientific and technological importance since they are ubiquitous when the host materials are exposed to particle irradiation. Studies on vacancy clustering in the past decades were mainly theoretical and the approach heavily relied on the total-energy calculation methods. The lack of experimental data is mainly due to the formidable task in measuring the cluster size and density using modern metrological techniques, including transmission electron microscopy and positron annihilation spectroscopy. To surmount these challenges, we proposed a novel approach to tackle the metrological problems on the nano vacancy clusters, especially in determining densities and sizes of the nano vacancies based on the premise that the vacancy-clusters act as diffusion-trapping centers. For a silicon substrate containing vacancyclusters, the diffusion of interstitials (from the surface) can be classified into three phases: (1) an ultrafast phase-I in which the trapping centers have little effect on the diffusion of interstitials; (2) a prolonged phase-II in which the loss rate of interstitials by trapping balances the influx of interstitials from the surface; and (3) a phase-III diffusion in which surface influx of interstitials depletes the trapping centers and interstitials consequently propagate deeper into the bulk. By measuring diffusion profiles of Si interstitials as a function of diffusion time, void sizes and void densities can be obtained through fitting. Experimentally, our approach to characterize voids is realized through three

  17. IDENTIFICATION OF A NEW RELATIVELY OLD STAR CLUSTER IN THE SMALL MAGELLANIC CLOUD

    SciTech Connect

    Piatti, Andres E.

    2012-09-10

    We present results on the age and metallicity estimates of the astonishingly unstudied Small Magellanic Cloud (SMC) cluster (ESO 51-SC09), from CCD BVI photometry obtained at the ESO NTT with the EMMI attached. ESO 51-SC09 turns out to be a relatively small cluster (FWHM = 10 {+-} 1 pc) located {approx}4 Degree-Sign northward from the galaxy center. We report for the first time a mean cluster age of 7.0 {+-} 1.3 Gyr and a mean cluster metallicity of [Fe/H] = -1.00 {+-} 0.15 dex, concluding that ESO 51-SC09 belongs to the group of the oldest SMC clusters. We found that the cluster is projected onto an older (age {approx}10-13 Gyr) and more metal-poor ([Fe/H] = -1.3 {+-} 0.2 dex) dominant field stellar population, so that the cluster could have reached its current location because of its orbital motion.

  18. Electronic and magnetic properties of small rhodium clusters

    SciTech Connect

    Soon, Yee Yeen; Yoon, Tiem Leong; Lim, Thong Leng

    2015-04-24

    We report a theoretical study of the electronic and magnetic properties of rhodium-atomic clusters. The lowest energy structures at the semi-empirical level of rhodium clusters are first obtained from a novel global-minimum search algorithm, known as PTMBHGA, where Gupta potential is used to describe the atomic interaction among the rhodium atoms. The structures are then re-optimized at the density functional theory (DFT) level with exchange-correlation energy approximated by Perdew-Burke-Ernzerhof generalized gradient approximation. For the purpose of calculating the magnetic moment of a given cluster, we calculate the optimized structure as a function of the spin multiplicity within the DFT framework. The resultant magnetic moments with the lowest energies so obtained allow us to work out the magnetic moment as a function of cluster size. Rhodium atomic clusters are found to display a unique variation in the magnetic moment as the cluster size varies. However, Rh{sub 4} and Rh{sub 6} are found to be nonmagnetic. Electronic structures of the magnetic ground-state structures are also investigated within the DFT framework. The results are compared against those based on different theoretical approaches available in the literature.

  19. Measurement of small angle based on a (1 0 0) silicon wafer and heterodyne interferometer

    NASA Astrophysics Data System (ADS)

    Hsieh, Meng-Chang; Lin, Jiun-You; Chen, Yu-Fong; Chang, Chia-Ou

    2016-06-01

    In this paper, a new optical material application and a heterodyne interferometer are proposed for measuring small angles. In the proposed interferometer, the optical material is a (1 0 0) silicon wafer applied to compose a new architecture of small angle sensor. The small angle measurement used the phase difference which is dependent on the incident angle at the silicon wafer surface to deduce the angular variation. The proposed architecture is simple and uses the common path method to compare test and reference signals; thus, small angles can be easily and accurately measured by estimating the phase difference. The experimental results demonstrate the feasibility of this method. The angular resolution and sensitivity levels superior to 7 × 10-5° (1.3 × 10-6 rad) and 150 (deg/deg), respectively, were attainable in a dynamic range of 0.45°.

  20. Nonlinear effects in infrared action spectroscopy of silicon and vanadium oxide clusters: experiment and kinetic modeling.

    PubMed

    Calvo, Florent; Li, Yejun; Kiawi, Denis M; Bakker, Joost M; Parneix, Pascal; Janssens, Ewald

    2015-10-21

    For structural assignment of gas phase compounds, infrared action spectra are usually compared to computed linear absorption spectra. However, action spectroscopy is highly nonlinear owing to the necessary transfer of the excitation energy and its subsequent redistribution leading to statistical ionization or dissociation. Here, we examine by joint experiment and dedicated modeling how such nonlinear effects affect the spectroscopic features in the case of selected inorganic clusters. Vibrational spectra of neutral silicon clusters are recorded by tunable IR-UV two-color ionization while IR spectra for cationic vanadium oxide clusters are obtained by IR multiphoton absorption followed by dissociation of the bare cluster or of its complex with Xe. Our kinetic modeling accounts for vibrational anharmonicities, for the laser interaction through photon absorption and stimulated emission rates, as well as for the relevant ionization or dissociation rates, all based on input parameters from quantum chemical calculations. Comparison of the measured and calculated spectra indicates an overall agreement as far as trends are concerned, except for the photodissociation of the V3O7(+)-Xe messenger complex, for which anharmonicities are too large and poorly captured by the perturbative anharmonic model. In all systems studied, nonlinear effects are essentially manifested by variations in the intensities as well as spectral broadenings. Differences in some band positions originate from inaccuracies of the quantum chemical data rather than specific nonlinear effects. The simulations further yield information on the average number of photons absorbed, which is otherwise unaccessible information: several to several tens of photons need to be absorbed to observe a band through dissociation, while three to five photons can be sufficient for detection of a band via IR-UV ionization. PMID:26208251

  1. The microstructure of dislocation clusters in industrial directionally solidified multicrystalline silicon

    SciTech Connect

    Kivambe, Maulid M.; Stokkan, Gaute; Ervik, Torunn; Lohne, Otto; Ryningen, Birgit

    2011-09-15

    The microstructure of commonly occurring dislocation patterns in industrial directionally solidified multicrystalline silicon has been systematically studied by light microscopy, electron backscatter diffraction, and transmission electron microscopy. The work has been focused on dislocation clusters on wafers near the top of cast blocks. In near {l_brace}111{r_brace} grain surface, dislocation arrays parallel to {l_brace}110{r_brace} plane traces are lying in parallel rows of {l_brace}111{r_brace} planes inclined to the surface, in mainly <112>30 deg. orientation. The dislocation configuration suggests that the microstructure may result from a recovery process. The dislocations formed during crystal growth and cooling have undergone transformations at high temperature in order to achieve low energy configurations for minimization of dislocation and crystal energy.

  2. Hydrogen reverses the clustering tendency of carbon in amorphous silicon oxycarbide.

    PubMed

    Ding, Hepeng; Demkowicz, Michael J

    2015-01-01

    Amorphous silicon oxycarbide (SiOC) is of great technological interest. However, its atomic-level structure is not well understood. Using density functional theory calculations, we show that the clustering tendency of C atoms in SiOC is extremely sensitive to hydrogen (H): without H, the C-C interaction is attractive, leading to enrichment of aggregated SiC4 tetrahedral units; with hydrogen, the C-C interaction is repulsive, leading to enrichment of randomly distributed SiCO3 tetrahedral units. Our results suggest that conflicting experimental characterizations of C distributions may be due to differing amounts of H present in the samples investigated. Our work also opens a path for tailoring the properties of SiOC by using the total H content to control the C distribution. PMID:26269200

  3. Percolation with excluded small clusters and Coulomb blockade in a granular system

    NASA Astrophysics Data System (ADS)

    Ioselevich, A. S.; Lyubshin, D. S.

    2010-01-01

    We consider dc-conductivity σ of a mixture of small conducting and insulating grains slightly below the percolation threshold, where finite clusters of conducting grains are characterized by a wide spectrum of sizes. The charge transport is controlled by tunneling of carriers between neighboring conducting clusters via short “links“ consisting of one insulating grain. Upon lowering temperature small clusters (up to some T-dependent size) become Coulomb blockaded, and are avoided, if possible, by relevant hopping paths. We introduce a relevant percolational problem of next-nearest-neighbors (NNN) conductivity with excluded small clusters and demonstrate (both numerically and analytically) that σ decreases as power law of the size of excluded clusters. As a physical consequence, the conductivity is a power-law function of temperature in a wide intermediate temperature range. We express the corresponding index through known critical indices of the perco lation theory and confirm this relation numerically.

  4. Magic Numbers in Small Iron Clusters: A First-Principles Study

    SciTech Connect

    Kim, Eunja; Mohrland, Andrew B.; Weck, Philippe F.; Pang, Tao; Czerwinski, Kenneth R.; Tomanek, David

    2014-10-03

    We perform ab initio spin-polarized density functional calculations of Fen aggregates with n ≤ 17 atoms to reveal the origin of the observed magic numbers, which indicate particularly high stability of clusters with 7, 13 and 15 atoms. Our results clarify the controversy regarding the ground state geometry of clusters such as Fe5and indicate that magnetism plays an important role in determining the stability and magic numbers in small iron clusters.

  5. Mechanical and fracture behaviors of defective silicon nanowires: combined effects of vacancy clusters, temperature, wire size, and shape

    NASA Astrophysics Data System (ADS)

    Kuo, Jenn-Kun; Huang, Pei-Hsing; Wu, Wei-Te; Lu, Chi-Ming

    2014-03-01

    Abstract The coupled effects of vacancy clusters (VCs), temperature, wire size, and geometry on the mechanical and fracture behaviors of defective silicon nanowires (Si NWs) were investigated using molecular dynamics modeling with Tersoff potential. The formation energies ( E v ) of a monovacancy (3.933 eV) and a tetrahedron vacancy (10.189 eV) obtained in this study agree well with experimental results and ab initio calculation. Simulation results show that the slip deformations of defective Si NWs are triggered at the wire's surface and edge due to the number of dangling bonds on the wire's surface being much greater than that inside a vacancy defect. VC defects barely affect to the value of Young's modulus, but substantially weaken the ultimate strength of wires with a small cross-sectional size. With decreasing wire size and increasing operation temperature, significant reductions in Young's modulus and fracture strength were observed. The average Young's modulus for square NWs was about 3.7 % higher than that of wires with a circular shape due to the surface facet effect. A brittle-to-ductile transition (BDT) occurred for [001]-oriented Si NWs with a lateral size≤5.43 nm and an operation temperature T≥300 K.

  6. The role of micro size computing clusters for small physics groups

    NASA Astrophysics Data System (ADS)

    Shevel, A. Y.

    2014-06-01

    A small physics group (3-15 persons) might use a number of computing facilities for the analysis/simulation, developing/testing, teaching. It is discussed different types of computing facilities: collaboration computing facilities, group local computing cluster (including colocation), cloud computing. The author discuss the growing variety of different computing options for small groups and does emphasize the role of the group owned computing cluster of micro size.

  7. Diffusion of Small He Clusters in Bulk and Grain Boundaries in α-Fe

    SciTech Connect

    Deng, Huiqiu; Hu, W. Y.; Gao, Fei; Heinisch, Howard L.; Hu, Shenyang Y.; Li, Yulan; Kurtz, Richard J.

    2013-11-02

    The diffusion properties of He interstitials and He clusters in the bulk and grain boundaries (GBs) of α-Fe have been studied using molecular dynamics with a newly developed Fe-He potential. The low migration energy barrier for a single He interstitial in the bulk is consistent with that obtained using ab initio methods. Small He clusters can migrate at low temperatures, but at higher temperatures they will kick out a self-interstitial atom (SIA) and become trapped by the vacancy, forming an He-vacancy complex. It is of great interest to note that small Henvacancy clusters (n<5) in the bulk are able to absorb an SIA, and the clusters become mobile again. Trapping and de-trapping of He clusters by emitting and absorbing an SIA represent an important dynamic process that provides a mechanism for the diffusion of He clusters and the nucleation of He bubbles in bulk Fe, particularly under irradiation in which numerous SIAs and vacancies are constantly being produced. A single He interstitial can migrate one-dimensionally or two-dimensionally within GBs, depending on the GB structure. Small interstitial Hen clusters 2 (n ~ 1 - 10) can easily kick out an SIA, and become trapped by the vacancy, while the SIA quickly diffuses away from the clusters, disappearing into the GB, such that de-trapping of the He clusters by absorbing an SIA is less likely to occur. This suggests that small He clusters may be treated as relatively immobile defects in GBs. The different behavior of He clusters in the bulk compared to their behavior in GBs may explain the different He bubble sizes experimentally observed in the bulk and in GBs in reduced activation ferritic/martensitic steels that have been simultaneously neutron irradiated and He implanted.

  8. Accumulation of structural defects in silicon irradiated with PF{sub n}{sup +} cluster ions with medium energies

    SciTech Connect

    Azarov, A. Yu. Titov, A. I.

    2007-01-15

    The method of Rutherford backscattering spectrometry in combination with channeling is used to study the accumulation of structural defects in silicon at room temperature as a result of irradiation with P{sup +} and F{sup +} atomic ions and also with cluster PF{sub n}{sup +} ions (n = 1, ..., 4) with the energy of 2.1 keV/amu and with identical generation rate of primary defects. The conditions for correct comparison of the results of bombardment with atomic and cluster ions composed of atoms of various types are suggested. It is found that the characteristics of accumulation of structural defects in silicon in the case of bombardment with PF{sub n}{sup +} cluster ions differ widely from those under irradiation with both atomic ions that are involved in the cluster ion (P{sup +} and F{sup +}) and with atomic heavy ions that have atomic mass close to that of the mass of a PF{sub n}{sup +} cluster. It is shown that, with irradiation conditions being the same, cluster ions produce much more radiation defects in the surface region than do atomic ions; i.e., a molecular effect is observed. Plausible mechanisms of this phenomenon are considered.

  9. Thermal expansion in small metal clusters and its impact on the electric polarizability

    PubMed

    Kummel; Akola; Manninen

    2000-04-24

    The thermal expansion coefficients of Na(N) clusters with 8clusters are obtained from ab initio Born-Oppenheimer local-density-approximation molecular dynamics. Thermal expansion of small metal clusters is considerably larger than that in the bulk and is size dependent. We demonstrate that the average static electric dipole polarizability of Na clusters depends linearly on the mean interatomic distance and only to a minor extent on the detailed ionic configuration when the overall shape of the electron density is enforced by electronic shell effects. Taking thermal expansion into account brings theoretical and experimental polarizabilities into quantitative agreement. PMID:11019216

  10. Preferential Sampling and Small-Scale Clustering of Gyrotactic Microswimmers in Turbulence.

    PubMed

    Gustavsson, K; Berglund, F; Jonsson, P R; Mehlig, B

    2016-03-11

    Recent studies show that spherical motile microorganisms in turbulence subject to gravitational torques gather in down-welling regions of the turbulent flow. By analyzing a statistical model we analytically compute how shape affects the dynamics, preferential sampling, and small-scale spatial clustering. We find that oblong organisms may spend more time in up-welling regions of the flow, and that all organisms are biased to regions of positive fluid-velocity gradients in the upward direction. We analyze small-scale spatial clustering and find that oblong particles may either cluster more or less than spherical ones, depending on the strength of the gravitational torques. PMID:27015512

  11. Preferential Sampling and Small-Scale Clustering of Gyrotactic Microswimmers in Turbulence

    NASA Astrophysics Data System (ADS)

    Gustavsson, K.; Berglund, F.; Jonsson, P. R.; Mehlig, B.

    2016-03-01

    Recent studies show that spherical motile microorganisms in turbulence subject to gravitational torques gather in down-welling regions of the turbulent flow. By analyzing a statistical model we analytically compute how shape affects the dynamics, preferential sampling, and small-scale spatial clustering. We find that oblong organisms may spend more time in up-welling regions of the flow, and that all organisms are biased to regions of positive fluid-velocity gradients in the upward direction. We analyze small-scale spatial clustering and find that oblong particles may either cluster more or less than spherical ones, depending on the strength of the gravitational torques.

  12. Clustering Methods with Qualitative Data: a Mixed-Methods Approach for Prevention Research with Small Samples.

    PubMed

    Henry, David; Dymnicki, Allison B; Mohatt, Nathaniel; Allen, James; Kelly, James G

    2015-10-01

    Qualitative methods potentially add depth to prevention research but can produce large amounts of complex data even with small samples. Studies conducted with culturally distinct samples often produce voluminous qualitative data but may lack sufficient sample sizes for sophisticated quantitative analysis. Currently lacking in mixed-methods research are methods allowing for more fully integrating qualitative and quantitative analysis techniques. Cluster analysis can be applied to coded qualitative data to clarify the findings of prevention studies by aiding efforts to reveal such things as the motives of participants for their actions and the reasons behind counterintuitive findings. By clustering groups of participants with similar profiles of codes in a quantitative analysis, cluster analysis can serve as a key component in mixed-methods research. This article reports two studies. In the first study, we conduct simulations to test the accuracy of cluster assignment using three different clustering methods with binary data as produced when coding qualitative interviews. Results indicated that hierarchical clustering, K-means clustering, and latent class analysis produced similar levels of accuracy with binary data and that the accuracy of these methods did not decrease with samples as small as 50. Whereas the first study explores the feasibility of using common clustering methods with binary data, the second study provides a "real-world" example using data from a qualitative study of community leadership connected with a drug abuse prevention project. We discuss the implications of this approach for conducting prevention research, especially with small samples and culturally distinct communities. PMID:25946969

  13. Thick Silicon Double-Sided Strip Detectors for Low-Energy Small-Animal SPECT

    PubMed Central

    Shokouhi, Sepideh; McDonald, Benjamin S.; Durko, Heather L.; Fritz, Mark A.; Furenlid, Lars R.; Peterson, Todd E.

    2010-01-01

    This work presents characterization studies of thick silicon double-sided strip detectors for a high-resolution small-animal SPECT. The dimension of these detectors is 60.4 mm × 60.4 mm × 1 mm. There are 1024 strips on each side that give the coordinates of the photon interaction, with each strip processed by a separate ASIC channel. Our measurement shows that intrinsic spatial resolution equivalent to the 59 μm strip pitch is attainable. Good trigger uniformity can be achieved by proper setting of a 4-bit DAC in each ASIC channel to remove trigger threshold variations. This is particularly important for triggering at low energies. The thick silicon DSSD (Double-sided strip detector) shows high potential for small-animal SPECT. PMID:20686626

  14. Comparison of Cluster Calculation with Different Software—The Case of Small Clusters

    NASA Astrophysics Data System (ADS)

    Goel, Neetu; Gautam, Seema; Priyanka, Dharamvir, Keya

    2011-12-01

    The electronic and optical properties of gold clusters Au n of size n = 1 = 15 were examined using a) Gaussian Software (GS) b) Siesta Code (SC). The structural properties were analyzed using GS and SC as well as Gupta Potential. Electronic properties like binding energy per atom ( E b / n ), Homo-Lumo Gap ( E g ), ionization potentials, (Adiabatic (AIP)), Chemical Potential(μ), hardness(η) and Softness(S) were investigated using GS and SC within the frame work of DFT formalism. Optical properties like static dipole polarizability of Au n were examined using GS only. Geometry optimization starting from a number of initial geometries were performed for each cluster size and obtained confirmed global minima till size n = 12. Interestingly, for cluster size n, at which 2D to 3D occurs in Au n , different in all different meanslike GS, SC and GP.

  15. Performance of small cluster surveys and the clustered LQAS design to estimate local-level vaccination coverage in Mali

    PubMed Central

    2012-01-01

    Background Estimation of vaccination coverage at the local level is essential to identify communities that may require additional support. Cluster surveys can be used in resource-poor settings, when population figures are inaccurate. To be feasible, cluster samples need to be small, without losing robustness of results. The clustered LQAS (CLQAS) approach has been proposed as an alternative, as smaller sample sizes are required. Methods We explored (i) the efficiency of cluster surveys of decreasing sample size through bootstrapping analysis and (ii) the performance of CLQAS under three alternative sampling plans to classify local VC, using data from a survey carried out in Mali after mass vaccination against meningococcal meningitis group A. Results VC estimates provided by a 10 × 15 cluster survey design were reasonably robust. We used them to classify health areas in three categories and guide mop-up activities: i) health areas not requiring supplemental activities; ii) health areas requiring additional vaccination; iii) health areas requiring further evaluation. As sample size decreased (from 10 × 15 to 10 × 3), standard error of VC and ICC estimates were increasingly unstable. Results of CLQAS simulations were not accurate for most health areas, with an overall risk of misclassification greater than 0.25 in one health area out of three. It was greater than 0.50 in one health area out of two under two of the three sampling plans. Conclusions Small sample cluster surveys (10 × 15) are acceptably robust for classification of VC at local level. We do not recommend the CLQAS method as currently formulated for evaluating vaccination programmes. PMID:23057445

  16. Radiation-induced mobility of small defect clusters in covalent materials

    NASA Astrophysics Data System (ADS)

    Jiang, Hao; He, Li; Morgan, Dane; Voyles, Paul M.; Szlufarska, Izabela

    2016-07-01

    Although defect clusters are detrimental to the electronic and mechanical properties of semiconductor materials, annihilation of such clusters is limited by their lack of thermal mobility due to high migration barriers. Here, we find that small clusters in bulk SiC (a covalent material of importance for both electronic and nuclear applications) can become mobile at room temperature under the influence of electron radiation. So far, direct observation of radiation-induced diffusion of defect clusters in bulk materials has not yet been demonstrated. This finding was made possible by low-angle annular dark-field scanning transmission electron microscopy combined with a nonrigid registration technique to remove sample instability, which enables atomic resolution imaging of small migrating defect clusters. We show that the underlying mechanism of this athermal diffusion is a ballistic collision between incoming electrons and cluster atoms. Our findings suggest that defect clusters may be mobile under certain irradiation conditions, changing the current understanding of the cluster annealing process in irradiated covalent materials.

  17. Age determination of 15 old to intermediate-age small Magellanic cloud star clusters

    SciTech Connect

    Parisi, M. C.; Clariá, J. J.; Piatti, A. E.; Geisler, D.; Leiton, R.; Carraro, G.; Costa, E.; Grocholski, A. J.; Sarajedini, A. E-mail: claria@oac.uncor.edu E-mail: dgeisler@astro-udec.cl E-mail: gcarraro@eso.org E-mail: grocholski@phys.lsu.edu

    2014-04-01

    We present color-magnitude diagrams in the V and I bands for 15 star clusters in the Small Magellanic Cloud (SMC) based on data taken with the Very Large Telescope (VLT, Chile). We selected these clusters from our previous work, wherein we derived cluster radial velocities and metallicities from calcium II infrared triplet (CaT) spectra also taken with the VLT. We discovered that the ages of six of our clusters have been appreciably underestimated by previous studies, which used comparatively small telescopes, graphically illustrating the need for large apertures to obtain reliable ages of old and intermediate-age SMC star clusters. In particular, three of these clusters, L4, L6, and L110, turn out to be among the oldest SMC clusters known, with ages of 7.9 ± 1.1, 8.7 ± 1.2, and 7.6 ± 1.0 Gyr, respectively, helping to fill a possible 'SMC cluster age gap'. Using the current ages and metallicities from Parisi et al., we analyze the age distribution, age gradient, and age-metallicity relation (AMR) of a sample of SMC clusters measured homogeneously. There is a suggestion of bimodality in the age distribution but it does not show a constant slope for the first 4 Gyr, and we find no evidence for an age gradient. Due to the improved ages of our cluster sample, we find that our AMR is now better represented in the intermediate/old period than we had derived in Parisi et al., where we simply took ages available in the literature. Additionally, clusters younger than ∼4 Gyr now show better agreement with the bursting model of Pagel and Tautvaišienė, but we confirm that this model is not a good representation of the AMR during the intermediate/old period. A more complicated model is needed to explain the SMC chemical evolution in that period.

  18. Age Determination of 15 Old to Intermediate-age Small Magellanic Cloud Star Clusters

    NASA Astrophysics Data System (ADS)

    Parisi, M. C.; Geisler, D.; Carraro, G.; Clariá, J. J.; Costa, E.; Grocholski, A. J.; Sarajedini, A.; Leiton, R.; Piatti, A. E.

    2014-04-01

    We present color-magnitude diagrams in the V and I bands for 15 star clusters in the Small Magellanic Cloud (SMC) based on data taken with the Very Large Telescope (VLT, Chile). We selected these clusters from our previous work, wherein we derived cluster radial velocities and metallicities from calcium II infrared triplet (CaT) spectra also taken with the VLT. We discovered that the ages of six of our clusters have been appreciably underestimated by previous studies, which used comparatively small telescopes, graphically illustrating the need for large apertures to obtain reliable ages of old and intermediate-age SMC star clusters. In particular, three of these clusters, L4, L6, and L110, turn out to be among the oldest SMC clusters known, with ages of 7.9 ± 1.1, 8.7 ± 1.2, and 7.6 ± 1.0 Gyr, respectively, helping to fill a possible "SMC cluster age gap." Using the current ages and metallicities from Parisi et al., we analyze the age distribution, age gradient, and age-metallicity relation (AMR) of a sample of SMC clusters measured homogeneously. There is a suggestion of bimodality in the age distribution but it does not show a constant slope for the first 4 Gyr, and we find no evidence for an age gradient. Due to the improved ages of our cluster sample, we find that our AMR is now better represented in the intermediate/old period than we had derived in Parisi et al., where we simply took ages available in the literature. Additionally, clusters younger than ~4 Gyr now show better agreement with the bursting model of Pagel & Tautvaišienė, but we confirm that this model is not a good representation of the AMR during the intermediate/old period. A more complicated model is needed to explain the SMC chemical evolution in that period.

  19. Small-Pore Molecular Sieves SAPO-34 with Chabazite Structure: Theoretical Study of Silicon Incorporation and Interrelated Catalytic Activity

    NASA Astrophysics Data System (ADS)

    Wang, Hong; Lewis, James; Liu, Zhongmin

    2011-03-01

    The catalytic conversion of methonal to olefin (MTO) has attracted attention both in industrial and academic fields. Strong evidence shows that small-pore molecular sieves with certain amount silicon incorporated (SAPO) present promising high catalytic activity in MTO conversion. Using DFT, we study the structural and electronic properties of chabazite SAPO-34. Although there are extensively experimental results show that silicon incorporation does not change the overall structure as the original AlPO structure, local structural changes are still created by silicon substitution, which probably accounted for the high catalytic activity. It is noted that the catalytic activity of SAPO-34 presents increasing trend along with the silicon incorporation amount increasing and maintain a flat peak even with more silicon incorporated. Hence, there is an optimal silicon incorporation amount which possibly yields the highest catalytic MTO conversion.

  20. Energy efficiency enhancements for semiconductors, communications, sensors and software achieved in cool silicon cluster project

    NASA Astrophysics Data System (ADS)

    Ellinger, Frank; Mikolajick, Thomas; Fettweis, Gerhard; Hentschel, Dieter; Kolodinski, Sabine; Warnecke, Helmut; Reppe, Thomas; Tzschoppe, Christoph; Dohl, Jan; Carta, Corrado; Fritsche, David; Tretter, Gregor; Wiatr, Maciej; Detlef Kronholz, Stefan; Mikalo, Ricardo Pablo; Heinrich, Harald; Paulo, Robert; Wolf, Robert; Hübner, Johannes; Waltsgott, Johannes; Meißner, Klaus; Richter, Robert; Michler, Oliver; Bausinger, Markus; Mehlich, Heiko; Hahmann, Martin; Möller, Henning; Wiemer, Maik; Holland, Hans-Jürgen; Gärtner, Roberto; Schubert, Stefan; Richter, Alexander; Strobel, Axel; Fehske, Albrecht; Cech, Sebastian; Aßmann, Uwe; Pawlak, Andreas; Schröter, Michael; Finger, Wolfgang; Schumann, Stefan; Höppner, Sebastian; Walter, Dennis; Eisenreich, Holger; Schüffny, René

    2013-07-01

    An overview about the German cluster project Cool Silicon aiming at increasing the energy efficiency for semiconductors, communications, sensors and software is presented. Examples for achievements are: 1000 times reduced gate leakage in transistors using high-fc (HKMG) materials compared to conventional poly-gate (SiON) devices at the same technology node; 700 V transistors integrated in standard 0.35 μm CMOS; solar cell efficiencies above 19% at < 200 W/m2 irradiation; 0.99 power factor, 87% efficiency and 0.088 distortion factor for dc supplies; 1 ns synchronization resolution via Ethernet; database accelerators allowing 85% energy savings for servers; adaptive software yielding energy reduction of 73% for e-Commerce applications; processors and corresponding data links with 40% and 70% energy savings, respectively, by adaption of clock frequency and supply voltage in less than 20 ns; clock generator chip with tunable frequency from 83-666 MHz and 0.62-1.6 mW dc power; 90 Gb/s on-chip link over 6 mm and efficiency of 174 fJ/mm; dynamic biasing system doubling efficiency in power amplifiers; 60 GHz BiCMOS frontends with dc power to bandwidth ratio of 0.17 mW/MHz; driver assistance systems reducing energy consumption by 10% in cars Contribution to the Topical Issue “International Semiconductor Conference Dresden-Grenoble - ISCDG 2012”, Edited by Gérard Ghibaudo, Francis Balestra and Simon Deleonibus.

  1. Age and metallicity of star clusters in the Small Magellanic Cloud from integrated spectroscopy

    NASA Astrophysics Data System (ADS)

    Dias, Bruno; Coelho, Paula; Kerber, Leandro; Barbuy, Beatriz; Idiart, Thais

    2010-04-01

    Analysis of integrated spectra of star clusters in the Magellanic Clouds can bring important information for studies on the chemical evolution of the Clouds. The aim of the present work is to derive ages and metallicities from integrated spectra of 15 star clusters in the Small Magellanic Cloud (SMC), some of them not studied so far. Making use of a full spectrum fitting technique, we compared the integrated spectra of the sample clusters to three different sets of single stellar population models available in the literature. We derived ages and metallicities for the sample clusters employing the codes STARLIGHT and ULySS. Out of the 15 clusters in our sample, 9 are old/intermediate age clusters and 6 are young clusters. We point out the results for the newly identified as old/intermediate age clusters HW1, NGC 152, Lindsay 3 and 11. We also confirm old ages for NGC 361, NGC 419 and Kron 3, and the oldest well-known SMC cluster NGC 121.

  2. Dating star clusters in the Small Magellanic Cloud by means of integrated spectra

    NASA Astrophysics Data System (ADS)

    Ahumada, A. V.; Clariá, J. J.; Bica, E.; Dutra, C. M.

    2002-10-01

    In this study flux-calibrated integrated spectra in the range (3600-6800) Å are presented for 16 concentrated star clusters in the Small Magellanic Cloud (SMC), approximately half of which constitute unstudied objects. We have estimated ages and foreground interstellar reddening values from the comparison of the line strengths and continuum distribution of the cluster spectra with those of template cluster spectra with known parameters. Most of the sample clusters are young blue clusters (6-50 Myr), while L 28, NGC 643 and L 114 are found to be intermediate-age clusters (1-6 Gyr). One well known SMC cluster (NGC 416) was observed for comparison purposes. The sample includes clusters in the surroundings and main body of the SMC, and the derived foreground reddening values are in the range 0.00 <= E(B-V) <= 0.15. The present data also make up a cluster spectral library at SMC metallicity. Based on observations made at Complejo Astronómico El Leoncito, which is operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba and San Juan, Argentina.

  3. STRUCTURAL PARAMETERS OF SEVEN SMALL MAGELLANIC CLOUD INTERMEDIATE-AGE AND OLD STAR CLUSTERS

    SciTech Connect

    Glatt, Katharina; Grebel, Eva K.; Kayser, Andrea; Gallagher, John S.; Harbeck, Daniel; Nota, Antonella; Sabbi, Elena; Sirianni, Marco; Clementini, Gisella; Tosi, Monica; Da Costa, Gary; Koch, Andreas

    2009-11-15

    We present structural parameters for the seven intermediate-age and old star clusters NGC 121, Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC). We fit King profiles and Elson, Fall, and Freeman profiles to both surface-brightness and star-count data taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. Clusters older than {approx}1 Gyr show a spread in cluster core radii that increases with age, while the youngest clusters have relatively compact cores. No evidence for post-core-collapse clusters was found. We find no correlation between core radius and distance from the SMC center, although consistent with other studies of dwarf galaxies, some relatively old and massive clusters have low densities. The oldest SMC star cluster, the only globular NGC121, is the most elliptical object of the studied clusters. No correlation is seen between ellipticity and distance from the SMC center. The structures of these massive intermediate-age (1-8 Gyr) SMC star clusters thus appear to primarily result from internal evolutionary processes.

  4. Structure and stability of silicon nanoclusters passivated by hydrogen and oxygen: evolutionary algorithm and first- principles study

    NASA Astrophysics Data System (ADS)

    Baturin, V. S.; Lepeshkin, S. V.; Matsko, N. L.; Uspenskii, Yu A.

    2016-02-01

    We investigate the structural and thermodynamical properties of small silicon clusters. Using the graph theory applied to previously obtained structures of Si10H2m clusters we trace the connection between geometry and passivation degree. The existing data on these clusters and structures of Si10O4n clusters obtained here using evolutionary calculations allowed to analyze the features of Si10H2m clusters in hydrogen atmosphere and Si10O4n clusters in oxygen atmosphere. We have shown the basic differences between structures and thermodynamical properties of silicon clusters, passivated by hydrogen and silicon oxide clusters.

  5. Anion-radical oxygen centers in small (AgO)n clusters: Density functional theory predictions

    NASA Astrophysics Data System (ADS)

    Trushin, Egor V.; Zilberberg, Igor L.

    2013-02-01

    Anion-radical form of the oxygen centers O- is predicted at the DFT level for small silver oxide particles having the AgO stoichiometry. Model clusters (AgO)n appear to be ferromagnetic with appreciable spin density at the oxygen centers. In contrast to these clusters, the Ag2O model cluster have no unpaired electrons in the ground state. The increased O/Ag ratio in the oxide particles is proved to be responsible for the spin density at oxygen centers.

  6. Analysis of radiation-induced small Cu particle cluster formation in aqueous CuCl2

    USGS Publications Warehouse

    Jayanetti, Sumedha; Mayanovic, Robert A.; Anderson, Alan J.; Bassett, William A.; Chou, I.-Ming

    2001-01-01

    Radition-induced small Cu particle cluster formation in aqueous CuCl2 was analyzed. It was noticed that nearest neighbor distance increased with the increase in the time of irradiation. This showed that the clusters approached the lattice dimension of bulk copper. As the average cluster size approached its bulk dimensions, an increase in the nearest neighbor coordination number was found with the decrease in the surface to volume ratio. Radiolysis of water by incident x-ray beam led to the reduction of copper ions in the solution to themetallic state.

  7. Probing the Unique Size-Dependent Properties of Small Au Clusters, Au Alloy Clusters, and CO Chemisorbed Au Clusters in the Gas Phase

    SciTech Connect

    Zhai, Hua-jin; Li, Xi; Wang, Lai S.

    2007-04-01

    When materials are reduced in size to the nanometer scale, their physical and chemical properties undergo major changes and become size-dependent, forming the foundation for nanoscience and nanotechnology. Gold nanoparticles and small gold clusters have been the focus of intensive research activities lately. The modern “goldrush” is largely motivated by the recent discoveries that (i) nanogold shows unexpected catalytic properties for a wide spectrum of chemical reactions [1], (ii) nanogold enables selective binding to biomolecules such as DNA and thus can serve as biosensors [2], (iii) gold has important potential applications in nanoelectronics [3,4], and (iv) gold clusters and gold-containing compounds possess unique chemical properties [5]. All these golden discoveries have made gold a surprising and rewarding subject of investigation in nanoscience and cluster science. Indeed, some of our oldest notions regarding gold, such as its inertness, are being changed dramatically by the recent findings in nanogold.

  8. Interactions of small platinum clusters with the TiC(001) surface

    SciTech Connect

    Mao, Jianjun; Li, Shasha; Chu, Xingli; Yang, Zongxian

    2015-11-14

    Density functional theory calculations are used to elucidate the interactions of small platinum clusters (Pt{sub n}, n = 1–5) with the TiC(001) surface. The results are analyzed in terms of geometric, energetic, and electronic properties. It is found that a single Pt atom prefers to be adsorbed at the C-top site, while a Pt{sub 2} cluster prefers dimerization and a Pt{sub 3} cluster forms a linear structure on the TiC(001). As for the Pt{sub 4} cluster, the three-dimensional distorted tetrahedral structure and the two-dimensional square structure almost have equal stability. In contrast with the two-dimensional isolated Pt{sub 5} cluster, the adsorbed Pt{sub 5} cluster prefers a three-dimensional structure on TiC(001). Substantial charge transfer takes place from TiC(001) surface to the adsorbed Pt{sub n} clusters, resulting in the negatively charged Pt{sub n} clusters. At last, the d-band centers of the absorbed Pt atoms and their implications in the catalytic activity are discussed.

  9. Interactions of small platinum clusters with the TiC(001) surface

    NASA Astrophysics Data System (ADS)

    Mao, Jianjun; Li, Shasha; Chu, Xingli; Yang, Zongxian

    2015-11-01

    Density functional theory calculations are used to elucidate the interactions of small platinum clusters (Ptn, n = 1-5) with the TiC(001) surface. The results are analyzed in terms of geometric, energetic, and electronic properties. It is found that a single Pt atom prefers to be adsorbed at the C-top site, while a Pt2 cluster prefers dimerization and a Pt3 cluster forms a linear structure on the TiC(001). As for the Pt4 cluster, the three-dimensional distorted tetrahedral structure and the two-dimensional square structure almost have equal stability. In contrast with the two-dimensional isolated Pt5 cluster, the adsorbed Pt5 cluster prefers a three-dimensional structure on TiC(001). Substantial charge transfer takes place from TiC(001) surface to the adsorbed Ptn clusters, resulting in the negatively charged Ptn clusters. At last, the d-band centers of the absorbed Pt atoms and their implications in the catalytic activity are discussed.

  10. All-Electron Scalar Relativistic Calculations on the Adsorption of Small Gold Clusters Toward Methanol Molecule.

    PubMed

    Kuang, Xiang-Jun; Wang, Xin-Qiang; Liu, Gao-Bin

    2015-02-01

    Under the framework of DFT, an all-electron scalar relativistic calculation on the adsorption of Aun (n = 1-13) clusters toward methanol molecule has been performed with the generalized gradient approximation at PW91 level. Our calculation results reveal that the small gold cluster would like to bond with oxygen of methanol molecule at the edge of gold cluster plane. After adsorption, the chemical activities of hydroxyl group and methyl group are enhanced to some extent. The even-numbered AunCH3OH cluster with closed-shell electronic configuration is relatively more stable than the neighboring odd-numbered AunCH3OH cluster with open-shell electronic configuration. All the AunCH3OH clusters prefer low spin multiplicity (M = 1 for even-numbered AuNCH3OH clusters, M = 2 for odd-numbered AunCH3OH clusters) and the magnetic moments are mainly contributed by gold atoms. The odd-even alterations of magnetic moments and electronic configurations can be observed clearly and may be simply understood in terms of the electron pairing effect. PMID:26353643

  11. Big Fish in Small Ponds: massive stars in the low-mass clusters of M83

    SciTech Connect

    Andrews, J. E.; Calzetti, D.; McElwee, Sean; Chandar, R.; Elmegreen, B. G.; Kennicutt, R. C.; Kim, Hwihyun; Krumholz, Mark R.; Lee, J. C.; Whitmore, B.; O'Connell, R. W. E-mail: callzetti@astro.umass.edu

    2014-09-20

    We have used multi-wavelength Hubble Space Telescope WFC3 data of the starbursting spiral galaxy M83 in order to measure variations in the upper end of the stellar initial mass function (uIMF) using the production rate of ionizing photons in unresolved clusters with ages ≤ 8 Myr. As in earlier papers on M51 and NGC 4214, the uIMF in M83 is consistent with a universal IMF, and stochastic sampling of the stellar populations in the ∼<10{sup 3} M {sub ☉} clusters are responsible for any deviations in this universality. The ensemble cluster population, as well as individual clusters, also imply that the most massive star in a cluster does not depend on the cluster mass. In fact, we have found that these small clusters seem to have an over-abundance of ionizing photons when compared to an expected universal or truncated IMF. This also suggests that the presence of massive stars in these clusters does not affect the star formation in a destructive way.

  12. Arbuscular mycorrhizal fungal communities are phylogenetically clustered at small scales

    PubMed Central

    Horn, Sebastian; Caruso, Tancredi; Verbruggen, Erik; Rillig, Matthias C; Hempel, Stefan

    2014-01-01

    Next-generation sequencing technologies with markers covering the full Glomeromycota phylum were used to uncover phylogenetic community structure of arbuscular mycorrhizal fungi (AMF) associated with Festuca brevipila. The study system was a semi-arid grassland with high plant diversity and a steep environmental gradient in pH, C, N, P and soil water content. The AMF community in roots and rhizosphere soil were analyzed separately and consisted of 74 distinct operational taxonomic units (OTUs) in total. Community-level variance partitioning showed that the role of environmental factors in determining AM species composition was marginal when controlling for spatial autocorrelation at multiple scales. Instead, phylogenetic distance and spatial distance were major correlates of AMF communities: OTUs that were more closely related (and which therefore may have similar traits) were more likely to co-occur. This pattern was insensitive to phylogenetic sampling breadth. Given the minor effects of the environment, we propose that at small scales closely related AMF positively associate through biotic factors such as plant-AMF filtering and interactions within the soil biota. PMID:24824667

  13. Arbuscular mycorrhizal fungal communities are phylogenetically clustered at small scales.

    PubMed

    Horn, Sebastian; Caruso, Tancredi; Verbruggen, Erik; Rillig, Matthias C; Hempel, Stefan

    2014-11-01

    Next-generation sequencing technologies with markers covering the full Glomeromycota phylum were used to uncover phylogenetic community structure of arbuscular mycorrhizal fungi (AMF) associated with Festuca brevipila. The study system was a semi-arid grassland with high plant diversity and a steep environmental gradient in pH, C, N, P and soil water content. The AMF community in roots and rhizosphere soil were analyzed separately and consisted of 74 distinct operational taxonomic units (OTUs) in total. Community-level variance partitioning showed that the role of environmental factors in determining AM species composition was marginal when controlling for spatial autocorrelation at multiple scales. Instead, phylogenetic distance and spatial distance were major correlates of AMF communities: OTUs that were more closely related (and which therefore may have similar traits) were more likely to co-occur. This pattern was insensitive to phylogenetic sampling breadth. Given the minor effects of the environment, we propose that at small scales closely related AMF positively associate through biotic factors such as plant-AMF filtering and interactions within the soil biota. PMID:24824667

  14. All-electron scalar relativistic calculation of water molecule adsorption onto small gold clusters.

    PubMed

    Kuang, Xiang-Jun; Wang, Xin-Qiang; Liu, Gao-Bin

    2011-08-01

    An all-electron scalar relativistic calculation was performed on Au( n )H(2)O (n = 1-13) clusters using density functional theory (DFT) with the generalized gradient approximation at PW91 level. The calculation results reveal that, after adsorption, the small gold cluster would like to bond with oxygen and the H(2)O molecule prefers to occupy the single fold coordination site. Reflecting the strong scalar relativistic effect, Au( n ) geometries are distorted slightly but still maintain a planar structure. The Au-Au bond is strengthened and the H-O bond is weakened, as manifested by the shortening of the Au-Au bond-length and the lengthening of the H-O bond-length. The H-O-H bond angle becomes slightly larger. The enhancement of reactivity of the H(2)O molecule is obvious. The Au-O bond-lengths, adsorption energies, VIPs, HLGs, HOMO (LUMO) energy levels, charge transfers and the highest vibrational frequencies of the Au-O mode for Au( n )H(2)O clusters exhibit an obvious odd-even oscillation. The most favorable adsorption between small gold clusters and the H(2)O molecule takes place when the H(2)O molecule is adsorbed onto an even-numbered Au( n ) cluster and becomes an Au( n )H(2)O cluster with an even number of valence electrons. The odd-even alteration of magnetic moments is observed in Au( n )H(2)O clusters and may serve as material with a tunable code capacity of "0" and "1" by adsorbing a H(2)O molecule onto an odd or even-numbered small gold cluster. PMID:21140279

  15. Measurement of the spectral signature of small carbon clusters at near and far infrared wavelengths

    NASA Technical Reports Server (NTRS)

    Tarter, J.; Saykally, R.

    1991-01-01

    A significant percentage of the carbon inventory of the circumstellar and interstellar media may be in the form of large refractory molecules (or small grains) referred to as carbon clusters. At the small end, uneven numbers of carbon atoms seem to be preferred, whereas above 12 atoms, clusters containing an even number of carbon atoms appear to be preferred in laboratory chemistry. In the lab, the cluster C-60 appears to be a particularly stable form and has been nicknamed Bucky Balls because of its resemblance to a soccer ball and to geodesic domes designed by Buckminster Fuller. In order to investigate the prevalence of these clusters, and their relationship to the polycyclic aromatic hydrocarbons (PAHs) that have become the newest focus of IR astronomy, it is necessary to determine the spectroscopic characteristics of these clusters at near and far infrared wavelengths. Described here is the construction of a near to far IR laser magnetic resonance spectrometer that has been built at the University of California Berkeley in order to detect and characterize these spectra. The equipment produces carbon clusters by laser evaporation of a graphitic target. The clusters are then cooled in a supersonic expansion beam in order to simulate conditions in the interstellar medium (ISM). The expansion beam feeds into the spectrometer chamber and permits concentrations of clusters sufficiently high as to permit ultra-high resolution spectroscopy at near and far IR wavelengths. The first successful demonstration of this apparatus occurred last year when the laboratory studies permitted the observational detection of C-5 in the stellar outflow surrounding IRC+10216 in the near-IR. Current efforts focus on reducing the temperature of the supersonic expansion beam that transport the C clusters evaporated from a graphite target into the spectrometer down to temperatures as low as 1 K.

  16. Effects of vacancy cluster defects on electrical and thermodynamic properties of silicon crystals.

    PubMed

    Huang, Pei-Hsing; Lu, Chi-Ming

    2014-01-01

    A first-principle plane-wave pseudopotential method based on the density function theory (DFT) was employed to investigate the effects of vacancy cluster (VC) defects on the band structure and thermoelectric properties of silicon (Si) crystals. Simulation results showed that various VC defects changed the energy band and localized electron density distribution of Si crystals and caused the band gap to decrease with increasing VC size. The results can be ascribed to the formation of a defect level produced by the dangling bonds, floating bonds, or high-strain atoms surrounding the VC defects. The appearance of imaginary frequencies in the phonon spectrum of defective Si crystals indicates that the defect-region structure is dynamically unstable and demonstrates phase changes. The phonon dispersion relation and phonon density of state were also investigated using density functional perturbation theory. The obtained Debye temperature ( θ D ) for a perfect Si crystal had a minimum value of 448 K at T = 42 K and a maximum value of 671 K at the high-temperature limit, which is consistent with the experimental results reported by Flubacher. Moreover, the Debye temperature decreased with increases in the VC size. VC defects had minimal effects on the heat capacity (C v ) value when temperatures were below 150 K. As the temperature was higher than 150 K, the heat capacity gradually increased with increasing temperature until it achieved a constant value of 11.8 cal/cell · K. The heat capacity significantly decreased as the VC size increased. For a 2 × 2 × 2 superlattice Si crystal containing a hexagonal ring VC (HRVC10), the heat capacity decreased by approximately 17%. PMID:24526923

  17. Radiotherapy electron beams collimated by small tubular applicators: characterization by silicon and diamond diodes.

    PubMed

    Bagalà, P; Di Venanzio, C; Falco, M D; Guerra, A S; Marinelli, Marco; Milani, E; Pimpinella, M; Pompili, F; Prestopino, G; Santoni, R; Tonnetti, A; Verona, C; Verona-Rinati, G

    2013-11-21

    High-energy electron beams generated by linear accelerators, typically in the range 6 to 20 MeV, are used in small field sizes for radiotherapy of localized superficial tumors. Unshielded silicon diodes (Si-D) are commonly considered suitable detectors for relative dose measurements in small electron fields due to their high spatial resolution. Recently, a novel synthetic single crystal diamond diode (SCDD) showed suitable properties for standard electron beams and small photon beams dosimetry. The aim of the present study is twofold: to characterize 6 to 15 MeV small electron beams shaped by using commercial tubular applicators with 2, 3, 4 and 5 cm diameter and to assess the dosimetric performance under such irradiation conditions of the novel SCDD dosimeter by comparison with commercially available dosimeters, namely a Si-D and a plane–parallel ionization chamber. Percentage depth dose curves, beam profiles and output factors (OFs) were measured. A good agreement among the dosimeters was observed in all of the performed measurements. As for the tubular applicators, two main effects were evidenced: (i) OFs larger than unity were measured for a number of field sizes and energies, with values up to about 1.3, that is an output 30% greater than that obtained at the 10 × 10 cm2 reference field; (ii) for each diameter of the tubular applicator a noticeable increase of the OF values was observed with increasing beam energy, up to about 100% in the case of the smaller applicator. This OF behavior is remarkably different from what typically observed for small blocked fields having the same size and energy as those used in this study. OFs for tubular applicators depend considerably on the field size, so interpolation is unadvisable to predict the linear accelerator output for such applicators whereas reliable high-resolution detectors, as the silicon and diamond diodes used in this work allow OF measurements with uncertainties of about 1%. PMID:24200743

  18. A comprehensive photometric study of dynamically evolved small van den Bergh-Hagen open clusters

    NASA Astrophysics Data System (ADS)

    Piatti, Andrés E.

    2016-09-01

    We present results from Johnson UBV, Kron-Cousins RI and Washington CT1T2 photometries for seven van den Bergh-Hagen (vdBH) open clusters, namely, vdBH 1, 10, 31, 72, 87, 92, and 118. The high-quality, multi-band photometric data sets were used to trace the cluster stellar density radial profiles and to build colour-magnitude diagrams (CMDs) and colour-colour (CC) diagrams from which we estimated their structural parameters and fundamental astrophysical properties. The clusters in our sample cover a wide age range, from ˜ 60 Myr up to 2.8 Gyr, are of relatively small size (˜ 1 - 6 pc) and are placed at distances from the Sun which vary between 1.8 and 6.3 kpc, respectively. We also estimated lower limits for the cluster present-day masses as well as half-mass relaxation times (tr). The resulting values in combination with the structural parameter values suggest that the studied clusters are in advanced stages of their internal dynamical evolution (age/tr ˜ 20 - 320), possibly in the typical phase of those tidally filled with mass segregation in their core regions. Compared to open clusters in the solar neighbourhood, the seven vdBH clusters are within more massive (˜ 80 - 380M$⊙$), with higher concentration parameter values (c ˜ 0.75-1.15) and dynamically evolved ones.

  19. Industry-Higher Education Collaborations within Small Business Clusters: Evidence from UK Case Studies.

    ERIC Educational Resources Information Center

    Matlay, Harry

    2000-01-01

    Analyzes instances of business-higher education collaboration within small business clusters in Britain, detailing the extent, contents, and intensity of that collaboration. Suggests that the leaders must committed to and believe in the benefits of collaborative relationships for them to be successful. (JOW)

  20. Site segregation in model clusters of small bimetallic RuGe and RuSn aggregates

    SciTech Connect

    Goursot, A.; Pedocchi, L.; Coq, B.

    1994-09-01

    RuGe and RuSn model clusters in both fixed bulk and relaxed structures are studied for stability during relaxation. Distortion causes a reduction in coordination of the Sn or Ge atom. Relaxation causes the Ge to have a small preference for the corner site (low coordination), but the Sn has a large corner site preference. 45 refs., 1 fig., 4 tabs.

  1. Small-Molecule Library Synthesis on Silicon-Functionalized SynPhase Lanterns

    PubMed Central

    Duvall, Jeremy R.; Vrcic, Anita; Marcaurelle, Lisa A.

    2011-01-01

    Silicon-functionalized SynPhase Lanterns are useful for the combinatorial synthesis of small-molecule libraries. Lanterns bearing an alkyl tethered diisopropylarylsilane are first activated with triflic acid to afford the corresponding diisopropylsilyl triflate, which is then reacted with a library scaffold bearing a free alcohol. Once the scaffold has been loaded onto the solid phase, a variety of transformations can be run, including amine cappings, cross-coupling reactions and amide bond formation. These reactions can yield a variety of products when run sequentially using split-pool synthesis strategies. Upon completion of the solid-phase transformations, the small-molecules are released from the Lanterns using HF/pyridine. Using the techniques described within, libraries can be made ranging from a few compounds to >10,000 members in a highly efficient manner. PMID:22679566

  2. Small-Molecule Library Synthesis on Silicon-Functionalized SynPhase Lanterns.

    PubMed

    Duvall, Jeremy R; Vrcic, Anita; Marcaurelle, Lisa A

    2010-01-01

    Silicon-functionalized SynPhase Lanterns are useful for the combinatorial synthesis of small-molecule libraries. Lanterns bearing an alkyl tethered diisopropylarylsilane are first activated with triflic acid to afford the corresponding diisopropylsilyl triflate, which is then reacted with a library scaffold bearing a free alcohol. Once the scaffold has been loaded onto the solid phase, a variety of transformations can be run, including amine cappings, cross-coupling reactions and amide bond formation. These reactions can yield a variety of products when run sequentially using split-pool synthesis strategies. Upon completion of the solid-phase transformations, the small-molecules are released from the Lanterns using HF/pyridine. Using the techniques described within, libraries can be made ranging from a few compounds to >10,000 members in a highly efficient manner. PMID:22679566

  3. Vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters

    SciTech Connect

    Ahmed, Musahid; Ahmed, Musahid; Wilson, Kevin R.; Belau, Leonid; Kostko, Oleg

    2008-05-12

    In this work we report on thevacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters. Clusters of methanol with water are generated via co-expansion of the gas phase constituents in a continuous supersonic jet expansion of methanol and water seeded in Ar. The resulting clusters are investigated by single photon ionization with tunable vacuumultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Protonated methanol clusters of the form (CH3OH)nH + (n=1-12) dominate the mass spectrum below the ionization energy of the methanol monomer. With an increase in water concentration, small amounts of mixed clusters of the form (CH3OH)n(H2O)H + (n=2-11) are detected. The only unprotonated species observed in this work are the methanol monomer and dimer. Appearance energies are obtained from the photoionization efficiency (PIE) curves for CH3OH +, (CH 3OH)2 +, (CH3OH)nH + (n=1-9), and (CH 3OH)n(H2O)H + (n=2-9 ) as a function of photon energy. With an increase in the water content in the molecular beam, there is an enhancement of photoionization intensity for methanol dimer and protonated methanol monomer at threshold. These results are compared and contrasted to previous experimental observations.

  4. Vacuum-Ultraviolet (VUV) Photoionization of Small Methanol and Methanol-Water Clusters

    SciTech Connect

    Kostko, Oleg; Belau, Leonid; Wilson, Kevin R.; Ahmed, Musahid

    2008-04-24

    In this work, we report on the vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters. Clusters of methanol with water are generated via co-expansion of the gas phase constituents in a continuous supersonic jet expansion of methanol and water seeded in Ar. The resulting clusters are investigated by single photon ionization with tunable vacuum-ultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Protonated methanol clusters of the form (CH3OH)nH+(n = 1-12) dominate the mass spectrum below the ionization energy of the methanol monomer. With an increase in water concentration, small amounts of mixed clusters of the form (CH3OH n(H2O)H+ (n = 2-11) are detected. The only unprotonated species observed in this work are the methanol monomer and dimer. Appearance energies are obtained from the photoionization efficiency (PIE) curves for CH3OH+, (CH3OH)2+, (CH3OH)nH+ (n = 1-9), and (CH3OH)n(H2O)H+ (n = 2-9) as a function of photon energy. With an increasein the water content in the molecular beam, there is an enhancement of photoionization intensity for the methanol dimer and protonated methanol monomer at threshold. These results are compared and contrasted to previous experimental observations.

  5. AGE DETERMINATION OF SIX INTERMEDIATE-AGE SMALL MAGELLANIC CLOUD STAR CLUSTERS WITH HST/ACS

    SciTech Connect

    Glatt, Katharina; Kayser, Andrea; Grebel, Eva K.; Sabbi, Elena; Gallagher, John S. III; Harbeck, Daniel; Nota, Antonella; Sirianni, Marco; Clementini, Gisella; Tosi, Monica; Koch, Andreas; Da Costa, Gary

    2008-10-15

    We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color-magnitude diagrams (CMDs) extend {approx}3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC 419 where the Padova isochrones provided the best fit. Using Dartmouth isochrones we derive ages of 7.5 {+-} 0.5 Gyr (Lindsay 1), 6.5 {+-} 0.5 Gyr (Kron 3), 6 {+-} 0.5 Gyr (NGC 339), 6 {+-} 0.5 Gyr (NGC 416), and 6.5 {+-} 0.5 Gyr (Lindsay 38). The CMD of NGC 419 shows several main-sequence turnoffs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC 419. We confirm that the SMC contains several intermediate-age populous star clusters with ages unlike those of the Large Magellanic Cloud and the Milky Way. Interestingly, our intermediate-age star clusters have a metallicity spread of {approx}0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally-concentrated blue straggler star candidates in NGC 416, while these are not present for the other clusters. Using the red clump magnitudes, we find that the closest cluster, NGC 419 ({approx}50 kpc), and the farthest cluster, Lindsay 38 ({approx}67 kpc), have a relative distance of {approx}17 kpc, which confirms the large depth of the SMC. The three oldest SMC clusters (NGC 121, Lindsay 1, and Kron 3) lie in the northwestern

  6. Quantum monte carlo study of the energetics of small hydrogenated and fluoride lithium clusters.

    PubMed

    Moreira, N L; Brito, B G A; Rabelo, J N Teixeira; Cândido, Ladir

    2016-06-30

    An investigation of the energetics of small lithium clusters doped either with a hydrogen or with a fluorine atom as a function of the number of lithium atoms using fixed-node diffusion quantum Monte Carlo (DMC) simulation is reported. It is found that the binding energy (BE) for the doped clusters increases in absolute values leading to a more stable system than for the pure ones in excellent agreement with available experimental measurements. The BE increases for pure, remains almost constant for hydrogenated, and decreases rapidly toward the bulk lithium for the fluoride as a function of the number of lithium atoms in the clusters. The BE, dissociation energy as well as the second difference in energy display a pronounced odd-even oscillation with the number of lithium atoms. The electron correlation inverts the odd-even oscillation pattern for the doped in comparison with the pure clusters and has an impact of 29%-83% to the BE being higher in the pure cluster followed by the hydrogenated and then by the fluoride. The dissociation energy and the second difference in energy indicate that the doped cluster Li3 H is the most stable whereas among the pure ones the more stable are Li2 , Li4 , and Li6 . The electron correlation energy is crucial for the stabilization of Li3 H. © 2016 Wiley Periodicals, Inc. PMID:26992447

  7. The nature and role of the gold-krypton interactions in small neutral gold clusters.

    PubMed

    Mancera, Luis A; Benoit, David M

    2015-03-26

    We investigate the nature and role of krypton embedding in small neutral gold clusters. For some of these clusters, we observe a particular site-dependent character of the Kr binding that does not completely follow the criterion of binding at low-coordinated sites, widely accepted for interaction of a noble gas with closed-shell metal systems such as metal surfaces. We aim at understanding the effect of low dimensionality and open-shell electronic structure of the odd-numbered clusters on the noble gas-metal cluster interaction. First, we investigate the role of attractive and repulsive forces, and the frontier molecular orbitals. Second, we investigate the Au-Kr interaction in terms of reactivity and bonding character. We use a reactivity index derived from Fukui formalism, and criteria provided by the electron localization function (ELF), in order to classify the type of bonding. We carry out this study on the minimum energy structures of neutral gold clusters, as obtained using pseudo potential plane-wave density functional theory (DFT). A model is proposed that includes the effect of attractive electrostatic, van der Waals and repulsive forces, together with effects originating from orbital overlap. This satisfactorily explains minimum configurations of the noble gas-gold cluster systems, the site preference of the noble gas atoms, and changes in electronic properties. PMID:25742369

  8. Small-angle neutron scattering data on C{sub 60} clusters in weakly polar solutions of fullerenes

    SciTech Connect

    Tropin, T. V. Avdeev, M. V.; Aksenov, V. L.

    2007-05-15

    Solutions of fullerence C{sub 60} in carbon disulfide CS{sub 2} have been investigated by small-angle neutron scattering. Combination of solubility, contrast, and incoherent scattering make it possible to measure and analyze the relatively small scattering cross section of this system. Along with single fullerene molecules, a small amount of large fullerene clusters (more than 100 A in size) is found in these solutions. The formation of these clusters depends on the procedure of solution preparation. The size distribution functions of clusters are compared with the results of the phenomenological cluster model of fullerene solubility.

  9. Magnetism, structure and chemical order in small CoPd clusters: A first-principles study

    NASA Astrophysics Data System (ADS)

    Mokkath, Junais Habeeb

    2014-01-01

    The structural, electronic and magnetic properties of small ComPdn(N=m+n=8,m=0-N) nanoalloy clusters are studied in the framework of a generalized-gradient approximation to density-functional theory. The optimized cluster structures have a clear tendency to maximize the number of nearest-neighbor CoCo pairs. The magnetic order is found to be ferromagnetic-like (FM) for all the ground-state structures. Antiferromagnetic-like spin arrangements were found in some low-lying isomers. The average magnetic moment per atom μ increases approximately linearly with Co content. A remarkable enhancement of the local Co moments is observed as a result of Pd doping. This is a consequence of the increase in the number of Co d holes, due to CoPd charge transfer, combined with the reduced local coordination. The influence of spin-orbit interactions on the cluster properties is also discussed.

  10. Novel Silicon-Carbon Nanostructures: Electronic structure study on the stability of Si60C2n Clusters.

    NASA Astrophysics Data System (ADS)

    Srinivasan, A.

    2005-03-01

    The formalism of generalized gradient approximation to density functional theory has been used to study the electronic and geometric structures of Si60C2n fullerene-like nanostructures. In our previous work, we have shown that the additions of carbon atoms increase the stability of smaller silicon cages [1]. In this talk, we will present our results on the addition of two and four carbon atoms on the surface of the Si60 cages by substitution as also inside the cage at various symmetry orientations. Full geometry optimizations have been performed using the Hay-Wadt basis set without any symmetry constraints using the Gaussian 03 suite of programs [2]. Binding energies, ionization potentials, electron affinities and the ``band'' gaps of the stable silicon-carbon fullerene like nanostructures will be presented and discussed in detail. In general, we find that the optimized silicon-carbon fullerene-like cages have increased stability compared to the bare Si60 cage. Possibilities of adding larger carbon clusters to the Si60 structure will also be discussed. *Work supported, in part, by the Welch Foundation, Houston, Texas (Grant No. Y-1525). [1] M. N. Huda and A. K. Ray, Phys. Rev. A 69, 011201(R) (2004); Eur. Phys. J. D 31, 63 (2004). [2] Gaussian 03, M. J. Frisch et al. Gaussian Inc., Pittsburgh, PA.

  11. First results in the application of silicon photomultiplier matrices to small animal PET

    NASA Astrophysics Data System (ADS)

    Llosá, G.; Belcari, N.; Bisogni, M. G.; Collazuol, G.; Marcatili, S.; Boscardin, M.; Melchiorri, M.; Tarolli, A.; Piemonte, C.; Zorzi, N.; Barrillon, P.; Bondil-Blin, S.; Chaumat, V.; de La Taille, C.; Dinu, N.; Puill, V.; Vagnucci, J.-F.; Del Guerra, A.

    2009-10-01

    A very high resolution small animal PET scanner that employs matrices of silicon photomultipliers as photodetectors is under development at the University of Pisa and INFN Pisa. The first SiPM matrices composed of 16 (4×4)1 mm×1 mm pixel elements on a common substrate have been produced at FBK-irst, and are being evaluated for this application. The MAROC2 ASIC developed at LAL-Orsay has been employed for the readout of the SiPM matrices. The devices have been tested with pixelated and continuous LYSO crystals. The results show the good performance of the matrices and lead to the fabrication of matrices with 64 SiPM elements.

  12. Small interfering RNA delivery by polyethylenimine-functionalised porous silicon nanoparticles.

    PubMed

    Hasanzadeh Kafshgari, M; Alnakhli, M; Delalat, B; Apostolou, S; Harding, F J; Mäkilä, E; Salonen, J J; Kuss, B J; Voelcker, N H

    2015-12-01

    In this study, thermally hydrocarbonised porous silicon nanoparticles (THCpSiNPs) capped with polyethylenimine (PEI) were fabricated, and their potential for small interfering RNA (siRNA) delivery was investigated in an in vitro glioblastoma model. PEI coating following siRNA loading enhanced the sustained release of siRNA, and suppressed burst release effects. The positively-charged surface improved the internalisation of the nanoparticles across the cell membrane. THCpSiNP-mediated siRNA delivery reduced mRNA expression of the MRP1 gene, linked to the resistence of glioblastoma to chemotherapy, by 63% and reduced MRP1-protein levels by 70%. MRP1 siRNA loaded nanoparticles did not induce cytotoxicity in glioblastoma cells, but markedly reduced cell proliferation. In summary, the results demonstrated that non-cytotoxic cationic THCpSiNPs are promising vehicles for therapeutic siRNA delivery. PMID:26343506

  13. Guided basin-hopping search of small boron clusters with density functional theory

    SciTech Connect

    Ng, Wei Chun; Yoon, Tiem Leong; Lim, Thong Leng

    2015-04-24

    The search for the ground state structures of Boron clusters has been a difficult computational task due to the unique metalloid nature of Boron atom. Previous research works had overcome the problem in the search of the Boron ground-state structures by adding symmetry constraints prior to the process of locating the local minima in the potential energy surface (PES) of the Boron clusters. In this work, we shown that, with the deployment of a novel computational approach that incorporates density functional theory (DFT) into a guided global optimization search algorithm based on basin-hopping, it is possible to directly locate the local minima of small Boron clusters in the PES at the DFT level. The ground-state structures search algorithm as proposed in this work is initiated randomly and needs not a priori symmetry constraint artificially imposed throughout the search process. Small sized Boron clusters so obtained compare well to the results obtained by similar calculations in the literature. The electronic properties of each structures obtained are calculated within the DFT framework.

  14. What Protects the Core When the Thiolated Gold Cluster is Extremely Small?

    SciTech Connect

    Jiang, Deen; Chen, Wei; Whetten, Robert L; Chen, Zhongfang

    2009-01-01

    The title question is motivated by the fact that extremely small thiolated-gold clusters such as Au{sub 20}(SR){sub 16} have been isolated, but their undetermined structures cannot be fully rationalized by the present knowledge derived from single-crystal structures of larger clusters. One needs to go beyond the linear monomer (RSAuSR) and V-shaped dimer (RSAuSRAuSR) motifs that were found to protect larger clusters. We hypothesize that the U-shaped trimer motif (RSAuSRAuSRAuSR) is required to protect the core of some extremely small thiolated-gold clusters, which have about 20 or fewer Au atoms. We test this hypothesis by proposing structural models for Au{sub 10}(SR){sub 8} based on two trimer motifs protecting a tetrahedral Au{sub 4} core and for Au{sub 20}(SR){sub 16} based on four trimer motifs protecting an Au{sub 8} core.

  15. A detector head design for small-animal PET with silicon photomultipliers (SiPM).

    PubMed

    Moehrs, Sascha; Del Guerra, Alberto; Herbert, Deborah J; Mandelkern, Mark A

    2006-03-01

    Small-animal PET systems are now striving for sub-millimetre resolution. Current systems based upon PSPMTs and finely pixellated scintillators can be pushed to higher resolution, but at the expense of other performance parameters and a rapidly escalating cost. Moreover, depth of interaction (DOI) information is usually difficult to assess in such systems, even though this information is highly desirable to reduce the parallax error, which is often the dominant error for such high-resolution systems. In this study we propose a high-resolution detector head for a small-animal PET imaging system with intrinsic DOI information. Instead of a pixellated scintillator, our design is based upon the classic Anger camera principle, i.e. the head is constructed of modular layers each consisting of a continuous slab of scintillator, viewed by a new type of compact silicon photodetector. The photodetector is the recently developed silicon photomultiplier (SiPM) that as well as being very compact has many other attractive properties: high gain at low bias voltage, excellent single-photoelectron resolution and fast timing. A detector head of about 4 x 4 cm2 in area is proposed, constructed from three modular layers of the type described above. We perform a simulation study, using the Monte Carlo simulation package Geant4. The simulation results are used to optimize the geometry of the detector head and characterize its performance. Additionally, hit estimation algorithms are studied to determine the interaction position of annihilation photons correctly over the whole detector surface. The resulting detector has a nearly uniform efficiency for 511 keV photons of approximately 70% and an intrinsic spatial resolution of less than approximately 0.4 mm full width at half maximum (fwhm). PMID:16481681

  16. A detector head design for small-animal PET with silicon photomultipliers (SiPM)

    NASA Astrophysics Data System (ADS)

    Moehrs, Sascha; DelGuerra, Alberto; Herbert, Deborah J.; Mandelkern, Mark A.

    2006-03-01

    Small-animal PET systems are now striving for sub-millimetre resolution. Current systems based upon PSPMTs and finely pixellated scintillators can be pushed to higher resolution, but at the expense of other performance parameters and a rapidly escalating cost. Moreover, depth of interaction (DOI) information is usually difficult to assess in such systems, even though this information is highly desirable to reduce the parallax error, which is often the dominant error for such high-resolution systems. In this study we propose a high-resolution detector head for a small-animal PET imaging system with intrinsic DOI information. Instead of a pixellated scintillator, our design is based upon the classic Anger camera principle, i.e. the head is constructed of modular layers each consisting of a continuous slab of scintillator, viewed by a new type of compact silicon photodetector. The photodetector is the recently developed silicon photomultiplier (SiPM) that as well as being very compact has many other attractive properties: high gain at low bias voltage, excellent single-photoelectron resolution and fast timing. A detector head of about 4 × 4 cm2 in area is proposed, constructed from three modular layers of the type described above. We perform a simulation study, using the Monte Carlo simulation package Geant4. The simulation results are used to optimize the geometry of the detector head and characterize its performance. Additionally, hit estimation algorithms are studied to determine the interaction position of annihilation photons correctly over the whole detector surface. The resulting detector has a nearly uniform efficiency for 511 keV photons of ~70% and an intrinsic spatial resolution of less than ~0.4 mm full width at half maximum (fwhm).

  17. Infrared spectra of small anionic water clusters from density functional theory and wavefunction theory calculations.

    PubMed

    Gong, Zu-Yong; Duan, Sai; Tian, Guangjun; Jiang, Jun; Xu, Xin; Luo, Yi

    2015-05-21

    We performed systematic theoretical studies on small anionic water/deuterated water clusters W/D(-)(N=2-6) at both density functional theory (B3LYP) and wavefunction theory (MP2) levels. The focus of the study is to examine the convergence of calculated infrared (IR) spectra with respect to the increasing number of diffuse functions. It is found that at the MP2 level for larger clusters (n = 4-6), only one extra diffuse function is needed to obtain the converged relative IR intensities, while two or three more sets of extra diffuse functions are needed for smaller clusters. Such behaviour is strongly associated with the convergence of the electronic structure of corresponding clusters at the MP2 level. It is striking to observe that at the B3LYP level, the calculated relative IR intensities for all the clusters under investigations are diverse and show no trend of convergence upon increasing the number of diffuse functions. Moreover, the increasing contribution from the extra diffuse functions to the dynamic IR dipole moment indicates that the B3LYP electronic structure also fails to converge. These results manifest that MP2 is a preferential theoretical method, as compared to the widely used B3LYP, for the IR intensity of dipole bounded electron systems. PMID:25903989

  18. Integrated spectral properties of 22 small angular diameter galactic open clusters

    NASA Astrophysics Data System (ADS)

    Ahumada, A. V.; Clariá, J. J.; Bica, E.

    2007-10-01

    Aims:Flux-calibrated integrated spectra of a sample of 22 Galactic open clusters of small angular diameter are presented. With one exception (ESO 429-SC2), all objects have Galactic longitudes in the range 208° < l < 33°. The spectra cover the range ≈3600-6800 Å, with a resolution of ≈14 Å. The properties of the present cluster sample are compared with those of well-studied clusters located in two 90° sectors, centred at l = 257° and l = 347°. The dissolution rate of Galactic open clusters in these two sectors is examined. Methods: Using the equivalent widths of the Balmer lines and comparing line intensities and continuum distribution of the cluster spectra with those of template cluster spectra with known properties, we derive both foreground reddening values and ages. Thus, we provide information independent of that determined through colour-magnitude diagrams. Results: The derived E(B-V) values for the whole sample vary from 0.0 in ESO 445-SC74 to 1.90 in Pismis 24, while the ages range from ~3 Myr (NGC 6604 and BH 151) to ~3.5 Gyr (Ruprecht 2). For six clusters (Dolidze 34, ESO 429-SC2, ESO 445-SC74, Ruprecht 2, BH 151 and Hogg 9) the foreground E(B-V) colour excesses and ages are determined for the first time. The results obtained for the remaining clusters show, in general terms, good agreement with previous photometric results. Conclusions: The age and reddening distributions of the present sample match those of known clusters in the two selected Galactic sectors. The present results would favour a major dissolution rate of star clusters in these two sectors. Two new solar-metallicity templates are defined corresponding to the age groups of (4-5) Myr and 30 Myr among those of Piatti et al. (2002, MNRAS, 335, 233). The Piatti et al. templates of 20 Myr and (3-4) Gyr are here redefined. Based on observations made at Complejo Astronómico El Leoncito, which is operated under agreement between the Consejo Nacional de Investigaciones Científicas y T

  19. Atomic structure, alloying behavior, and magnetism in small Fe-Pt clusters

    NASA Astrophysics Data System (ADS)

    Chittari, Bheema Lingam; Kumar, Vijay

    2015-09-01

    We report results of the atomic structure, alloying behavior, and magnetism in F emP tn(m +n =2 -10 ) clusters using projector augmented wave (PAW) pseudopotential method and spin-polarized generalized gradient approximation (GGA) for the exchange-correlation energy. These results are compared with those obtained by using HCTH exchange-correlation functional and LANL2DZ basis set in the Gaussian program and the overall trends are found to be similar. As in bulk Fe-Pt alloys, clusters with equal composition of Fe and Pt have the largest binding energy and the largest heat of nanoalloy formation for a given number of atoms in the cluster. There are some deviations due to the different symmetries in clusters and in cases where the total number of atoms is odd. The lowest energy isomers tend to maximize bonds between unlike atoms with Fe (Pt) atoms occupying high (low) coordination sites in the core (surface) of the cluster. The binding energy, heat of formation, and the second order difference of the total energy show F e2P t2 , F e4P t4 , and F e4P t6 clusters to be the most stable ones among the different clusters we have studied. The magnetic moments on Fe atoms are high in Pt-rich clusters as well as in small Fe-rich clusters and decrease as the aggregation of Fe atoms and the cluster size increases. The maximum value of the magnetic moments on Fe atoms is ˜3.8 μB , whereas for Pt atoms it is 1 μB. These are quite high compared with the values for bulk Fe as well as bulk FePt and F e3Pt phases while bulk Pt is nonmagnetic. There is significant charge transfer from those Fe atoms that interact directly with Pt atoms. We discuss the hybridization between the electronic states of Pt and Fe atoms as well as the variation in the magnetic moments on Fe and Pt atoms. Our results provide insight into the understanding of the nanoalloy behavior of Fe-Pt and we hope that this would help to design Fe based nanoalloys and their assemblies with high magnetic moments for

  20. Role of spin-orbit interaction in the ultrafast demagnetization of small iron clusters

    NASA Astrophysics Data System (ADS)

    Stamenova, Maria; Simoni, Jacopo; Sanvito, Stefano

    2016-07-01

    The ultrafast demagnetization of small iron clusters initiated by an intense optical excitation is studied from the time-dependent spin density functional theory (TDSDFT). In particular we investigate the effect of the spin-orbit interaction on the onset of the demagnetization process. It is found that demagnetization occurs locally, in the vicinity of the atomic sites, and the initial rate of spin loss, coherent with the laser field, is proportional to the square of the ionic spin-orbit coupling strength λ . A simplified quantum spin model comprising spin-orbit interaction and a time-dependent magnetic field is found to be the minimal model able to reproduce our ab initio results. The model predicts the λ2 dependence of the onset rate of demagnetization when it is solved either analytically for the small t regime, or numerically integrated in the time domain. Our findings are supported by additional TDSDFT simulations of clusters made of Co and Ni.

  1. Cooperative network clustering and task allocation for heterogeneous small satellite network

    NASA Astrophysics Data System (ADS)

    Qin, Jing

    The research of small satellite has emerged as a hot topic in recent years because of its economical prospects and convenience in launching and design. Due to the size and energy constraints of small satellites, forming a small satellite network(SSN) in which all the satellites cooperate with each other to finish tasks is an efficient and effective way to utilize them. In this dissertation, I designed and evaluated a weight based dominating set clustering algorithm, which efficiently organizes the satellites into stable clusters. The traditional clustering algorithms of large monolithic satellite networks, such as formation flying and satellite swarm, are often limited on automatic formation of clusters. Therefore, a novel Distributed Weight based Dominating Set(DWDS) clustering algorithm is designed to address the clustering problems in the stochastically deployed SSNs. Considering the unique features of small satellites, this algorithm is able to form the clusters efficiently and stably. In this algorithm, satellites are separated into different groups according to their spatial characteristics. A minimum dominating set is chosen as the candidate cluster head set based on their weights, which is a weighted combination of residual energy and connection degree. Then the cluster heads admit new neighbors that accept their invitations into the cluster, until the maximum cluster size is reached. Evaluated by the simulation results, in a SSN with 200 to 800 nodes, the algorithm is able to efficiently cluster more than 90% of nodes in 3 seconds. The Deadline Based Resource Balancing (DBRB) task allocation algorithm is designed for efficient task allocations in heterogeneous LEO small satellite networks. In the task allocation process, the dispatcher needs to consider the deadlines of the tasks as well as the residue energy of different resources for best energy utilization. We assume the tasks adopt a Map-Reduce framework, in which a task can consist of multiple

  2. Adsorption and reactions of O2 and D2 on small free palladium clusters in a cluster-molecule scattering experiment

    NASA Astrophysics Data System (ADS)

    Andersson, Mats; Rosén, Arne

    2010-08-01

    The adsorption of oxygen and hydrogen (deuterium) on small neutral palladium clusters was investigated in a cluster beam experiment. The beam passes through two low-pressure reaction cells, and the clusters, with and without adsorbed molecules, are detected using laser ionization and mass spectrometry. Both H2 and O2 adsorb efficiently on the palladium clusters with only moderate variations with cluster size in the investigated range, i.e. between 8 and 28 atoms. The co-adsorption of H2 and O2 results in the formation of H2O, detected as a decrease in the number of adsorbed oxygen atoms with an increasing number of collisions with H2 molecules. A comparison is done with an earlier similar study of clusters of Pt. Furthermore a comparison is done with what is known for sticking and reactivity of surfaces.

  3. Study of small carbon and semiconductor clusters using negative ion threshold photodetachment spectroscopy

    SciTech Connect

    Arnold, C.C.

    1994-08-01

    The bonding and electronics of several small carbon and semiconductor clusters containing less than ten atoms are probed using negative ion threshold photodetachment (zero electron kinetic energy, or ZEKE) spectroscopy. ZEKE spectroscopy is a particularly advantageous technique for small cluster study, as it combines mass selection with good spectroscopic resolution. The ground and low-lying electronic states of small clusters in general can be accessed by detaching an electron from the ground anion state. The clusters studied using this technique and described in this work are C{sub 6}{sup {minus}}/C{sub 6}, Si{sub n}{sup {minus}}/Si{sub n} (n = 2, 3, 4), Ge{sub 2}{sup {minus}}/Ge{sub 2}, In{sub 2}P{sup {minus}}/In{sub 2}P,InP{sub 2}{sup {minus}}/InP{sup 2}, and Ga{sub 2}As{sup {minus}}. The total photodetachment cross sections of several other small carbon clusters and the ZEKE spectrum of the I{sup {minus}}{center_dot}CH{sub 3}I S{sub N}2 reaction complex are also presented to illustrate the versatility of the experimental apparatus. Clusters with so few atoms do not exhibit bulk properties. However, each specie exhibits bonding properties that relate to the type of bonding found in the bulk. C{sub 6}, as has been predicted, exhibits a linear cumulenic structure, where double bonds connect all six carbon atoms. This double bonding reflects how important {pi} bonding is in certain phases of pure carbon (graphite and fullerenes). The symmetric stretch frequencies observed in the C{sub 6}{sup {minus}} spectra, however, are in poor agreement with the calculated values. Also observed as sharp structure in total photodetachment cross section scans was an excited anion state bound by only {approximately}40 cm{sup {minus}1} relative to the detachment continuum. This excited anion state appears to be a valence bound state, possible because of the high electron affinity of C{sub 6}, and the open shell of the anion.

  4. Infrared Communications for Small Spacecraft: From a Wireless Bus to Cluster Concepts

    NASA Technical Reports Server (NTRS)

    Webb, Suzanne C.; Schneider, Wolfger; Darrin, M. Ann G.; Boone, Bradley G.; Luers, Philip J.; Day, John H. (Technical Monitor)

    2001-01-01

    Nanosatellites operating singly or in clusters are anticipated for future space science missions. To implement this new communications paradigm, we are approaching cluster communications by first developing an infrared (IR) intra-craft wireless bus capability, following initially the MIL-STD-1553B protocol. Benefits of an IR wireless bus are low mass, size, power, and cost, simplicity of implementation, ease of use, minimum EMI, and efficient and reliable data transfer. Our goals are to maximize the reliable link margin in order to afford greater flexibility in receiver placement, which will ease technology insertion. We have developed a concept demonstration using a high-speed visible-band silicon PIN photodiode and a high-efficiency visible LED operating at a data rate up to 4 Mb/sec. In designing an internal IR wireless bus, we have characterized various candidate materials, emitters, and geometries, assuming a single reflection. Thus, we have measured the bidirectional reflectance distribution function (BRDF) for five different materials characteristic of typical spacecraft structures, which range from nearly Lambertian to highly specular. We have fit our data to empirical BRDF functions and modeled the detected irradiance anywhere in the plane of incidence for a divergent (LED) emitter. We have also determined the angular limits on the link geometry to remain within the required bit error rate by determining the received signal-to-noise ratio (SNR) for minimum values of irradiance received at the detector.

  5. Infrared communications for small spacecraft: from a wireless bus to cluster concepts

    NASA Astrophysics Data System (ADS)

    Walts, Suzanne C.; Schneider, Wolfger; Darrin, Margaret A. G.; Boone, Bradley G.; Luers, Philip J.

    2001-08-01

    Nanosatellites operating singly or in clusters are anticipated for future space science missions. To implement this new communications paradigm, we are approaching cluster communications by first developing an infrared (IR) intra- craft wireless bus capability, following initially the MIL- STD-1553B protocol. Benefits of an IR wireless bus are low mass, size, power, and cost, simplicity of implementation, ease of use, minimum EMI, and efficient and reliable data transfer. Our goals are to maximize the reliable link margin in order to afford greater flexibility in receiver placement, which will ease technology insertion. We have developed a concept demonstration using a high-speed visible-band silicon PIN photodiode and a high-efficiency visible LED operating at a data rate up to 4 Mb/sec. In designing an internal IR wireless bus, we have characterized various candidate materials, emitters, and geometries, assuming a single reflection. Thus, we have measured the bidirectional reflectance distribution function (BRDF) for five different materials characteristic of typical spacecraft structures, which range from nearly Lambertian to highly specular. We have fit our data to empirical BRDF functions and modeled the detected irradiance anywhere in the plane of incidence for a divergent emitter. We have also determined the angular limits on the link geometry to remain within the required bit error rate by determining the received signal-to-noise ratio for minimum values of irradiance received at the detector.

  6. Atomic characterization of Au clusters in vapor-liquid-solid grown silicon nanowires

    SciTech Connect

    Chen, Wanghua; Roca i Cabarrocas, Pere; Pareige, Philippe; Castro, Celia; Xu, Tao; Grandidier, Bruno; Stiévenard, Didier

    2015-09-14

    By correlating atom probe tomography with other conventional microscope techniques (scanning electron microscope, scanning transmission electron microscope, and scanning tunneling microscopy), the distribution and composition of Au clusters in individual vapor-liquid-solid grown Si nanowires is investigated. Taking advantage of the characteristics of atom probe tomography, we have developed a sample preparation method by inclining the sample at certain angle to characterize the nanowire sidewall without using focused ion beam. With three-dimensional atomic scale reconstruction, we provide direct evidence of Au clusters tending to remain on the nanowire sidewall rather than being incorporated into the Si nanowires. Based on the composition measurement of Au clusters (28% ± 1%), we have demonstrated the supersaturation of Si atoms in Au clusters, which supports the hypothesis that Au clusters are formed simultaneously during nanowire growth rather than during the cooling process.

  7. BINARY QUASARS IN THE SLOAN DIGITAL SKY SURVEY: EVIDENCE FOR EXCESS CLUSTERING ON SMALL SCALES

    SciTech Connect

    Hennawi, J F; Strauss, M A; Oguri, M; Inada, N; Richards, G T; Pindor, B; Schneider, D P; Becker, R H; Gregg, M D; Hall, P B; Johnston, D E; Fan, X; Burles, S; Schlegel, D J; Gunn, J E; Lupton, R; Bahcall, N A; Brunner, R J; Brinkman, J

    2005-11-10

    We present a sample of 218 new quasar pairs with proper transverse separations R{sub prop} < 1 h{sup -1} Mpc over the redshift range 0.5 < z < 3.0, discovered from an extensive follow up campaign to find companions around the Sloan Digital Sky Survey and 2dF Quasar Redshift Survey quasars. This sample includes 26 new binary quasars with separations R{sub prop} < 50 h{sup -1} kpc ({theta} < 10''), more than doubling the number of such systems known. We define a statistical sample of binaries selected with homogeneous criteria and compute its selection function, taking into account sources of incompleteness. The first measurement of the quasar correlation function on scales 10 h{sup -1} kpc < R{sub prop} < 400 h{sup -1} kpc is presented. For R{sub prop} {approx}< 40 h{sup -1} kpc, we detect an order of magnitude excess clustering over the expectation from the large scale (R{sub prop} {approx}> 3 h{sup -1} Mpc) quasar correlation function, extrapolated down as a power law to the separations probed by our binaries. The excess grows to {approx}30 at R{sub prop} {approx} 10 h{sup -1} kpc, and provides compelling evidence that the quasar autocorrelation function gets progressively steeper on sub-Mpc scales. This small scale excess can likely be attributed to dissipative interaction events which trigger quasar activity in rich environments. Recent small scale measurements of galaxy clustering and quasar-galaxy clustering are reviewed and discussed in relation to our measurement of small scale quasar clustering.

  8. Characterisation of a cobalt-60 small-beam animal irradiator using a realtime silicon pixelated detector

    NASA Astrophysics Data System (ADS)

    Porumb, C. S.; Davies, J. B.; Perevertaylo, V.; Rosenfeld, A. B.; Petasecca, M.

    2016-04-01

    The paper presents a study performed by the Centre for Medical Radiation Physics (CMRP) using a high spatial and temporal resolution silicon pixelated detector named MagicPlate-512. The study focuses on the characterisation of three pencil beams from a low-dose rate, 6 TBq, cobalt-60 source, in terms of percentage depth dose, beam profiles, output factor and shutter timing. Where applicable, the findings were verified against radiochromic EBT3 film and ionization chambers. It was found that the results of the MagicPlate-512 and film agreed within 0.9 mm for penumbra and full-width at half-maximum measurements of the beam profiles, and within 0.75% for percentage depth dose study. The dose rate of the cobalt-60 source was determined to be (10.65 ± 0.03) cGy/min at 1.5 cm depth in Solid Water. A significant asymmetry of the small pencil beam profile was found, which is due to the irregular machining of the small collimator. The average source shutter speed was calculated to be 26 cm/s. The study demonstrates that the MagicPlate-512 dosimetry system, developed at CMRP, is capable of beam characterisation even in cases of very low dose rate sources.

  9. Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients.

    PubMed

    Fei, Hongzhan; Wiedenbeck, Michael; Yamazaki, Daisuke; Katsura, Tomoo

    2013-06-13

    Water has been thought to affect the dynamical processes in the Earth's interior to a great extent. In particular, experimental deformation results suggest that even only a few tens of parts per million of water by weight enhances the creep rates in olivine by orders of magnitude. However, those deformation studies have limitations, such as considering only a limited range of water concentrations and very high stresses, which might affect the results. Rock deformation can also be understood as an effect of silicon self-diffusion, because the creep rates of minerals at temperatures as high as those in the Earth's interior are limited by self-diffusion of the slowest species. Here we experimentally determine the silicon self-diffusion coefficient DSi in forsterite at 8 GPa and 1,600 K to 1,800 K as a function of water content CH2O from less than 1 to about 800 parts per million of water by weight, yielding the relationship, DSi ≈ (CH2O)(1/3). This exponent is strikingly lower than that obtained by deformation experiments (1.2; ref. 7). The high nominal creep rates in the deformation studies under wet conditions may be caused by excess grain boundary water. We conclude that the effect of water on upper-mantle rheology is very small. Hence, the smooth motion of the Earth's tectonic plates cannot be caused by mineral hydration in the asthenosphere. Also, water cannot cause the viscosity minimum zone in the upper mantle. And finally, the dominant mechanism responsible for hotspot immobility cannot be water content differences between their source and surrounding regions. PMID:23765497

  10. Hydrolysis of Sulfur Dioxide in Small Clusters of Sulfuric Acid: Mechanistic and Kinetic Study.

    PubMed

    Liu, Jingjing; Fang, Sheng; Wang, Zhixiu; Yi, Wencai; Tao, Fu-Ming; Liu, Jing-Yao

    2015-11-17

    The deposition and hydrolysis reaction of SO2 + H2O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO2-(H2SO4)n-(H2O)m (m = 1,2; n = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO2 as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO2 + H2O + H2SO4 - H2O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H2SO4-(H2O)2 forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO2 + (H2SO4)-H2O > SO2 + (H2SO4)2-H2O > SO2 + H2SO4-H2O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO2). Consequently, sulfuric acid promotes the hydrolysis of SO2 both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO2. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO2, leading to the formation and growth of new particles. PMID:26450714

  11. Internal electric fields in small water clusters [(H2O)n; n = 2-6].

    PubMed

    Sen, Saumik; Boda, Manjusha; Venkat Lata, S; Naresh Patwari, G

    2016-06-22

    The electric field experienced by a water molecule within a water cluster depends on its position relative to the rest of the water molecules. The stabilization energies and the red-shifts in the donor O-H stretching vibrations in the water clusters increase with the cluster size concomitant with the increase in the electric field experienced by the donor O-H of a particular water molecule due to the hydrogen bonding network. The red-shifts in O-H stretching frequencies show a spread of about ±100 cm(-1) against the corresponding electric fields. Deviations from linearity were marked in the region of 100-160 MV cm(-1), which can be attributed to the strain in the hydrogen bonding network, especially for structures with DDAA and DDA motifs. The linear Stark effect holds up to 200 MV cm(-1) of internal electric field for the average red-shifts in the O-H stretching frequencies, with a Stark tuning rate of 2.4 cm(-1) (MV cm(-1))(-1), suggesting the validity of the classical model in small water clusters. PMID:27270616

  12. Theoretical study of IR and photoelectron spectra of small gallium-arsenide clusters

    SciTech Connect

    Pouchan, Claude; Marchal, Rémi; Hayashi, Shinsuke

    2015-01-22

    Relative stabilities of small Ga{sub n}As{sub m} clusters, as well as their structural electronic and vibrational properties, were computed and analysed using a CCSD(T) reference method since experimental data in this area are sparse or unknown. With the aim of investigating larger clusters, we explored several DFT functionals and basis sets able to mimic the reliable CCSD(T) approach. Among them, the PBE0/SBKJC+sp,d appears as the most efficient to describe the structural and vibrational properties since average differences of about 0.042Å and 5.1cm{sup −1} were obtained for bond lengths and fundamental vibrational frequencies, respectively for the first small clusters [1] of the series found from our GSAM method [2]. As further test, this model is used in order to investigate and revisit an experimental IR spectrum of Ga{sub n}As{sub m} mixture previously published by Li et al. [3]. More complicated is the difficulty which arises in the electronic description due to the presence of numerous low lying electronic states nearly degenerated to correctly describe the electronic structure. The case of Ga{sub 2}As will be discussed and the photoelectron spectra of the Ga{sub 2}As anion reanalyzed on the ground of our calculations [4] comparatively to the experimental spectra obtained by Neumark and co-workers [5].

  13. Method and apparatus for detecting and/or imaging clusters of small scattering centers in the body

    DOEpatents

    Perez-Mendez, Victor; Sommer, Frank G.

    1982-01-01

    An ultrasonic method and apparatus are provided for detecting and imaging clusters of small scattering centers in the breast wherein periodic pulses are applied to an ultrasound emitting transducer and projected into the body, thereafter being received by at least one receiving transducer positioned to receive scattering from the scattering center clusters. The signals are processed to provide an image showing cluster extent and location.

  14. Method and apparatus for detecting and/or imaging clusters of small scattering centers in the body

    DOEpatents

    Perez-Mendez, V.; Sommer, F.G.

    1982-07-13

    An ultrasonic method and apparatus are provided for detecting and imaging clusters of small scattering centers in the breast wherein periodic pulses are applied to an ultrasound emitting transducer and projected into the body, thereafter being received by at least one receiving transducer positioned to receive scattering from the scattering center clusters. The signals are processed to provide an image showing cluster extent and location. 6 figs.

  15. Diffusion and transformation kinetics of small helium clusters in bulk tungsten

    NASA Astrophysics Data System (ADS)

    Perez, Danny; Vogel, Thomas; Uberuaga, Blas P.

    2014-07-01

    The production of energy through nuclear fusion poses serious challenges related to the stability and performance of materials in extreme conditions. In particular, the constant bombardment of the walls of the reactor with high doses of He ions is known to lead to deleterious changes in their microstructures. These changes follow from the aggregation of He into bubbles that can grow and blister, potentially leading to the contamination of the plasma, or to the degradation of their mechanical properties. We computationally study the behavior of small clusters of He atoms in W in conditions relevant to fusion energy production. Using a wide range of techniques, we investigate the thermodynamics of the clusters and their kinetics in terms of diffusivity, growth, and breakup, as well as mutation into nanobubbles. Our study provides the essential ingredients to model the early stages of He exposure leading up to the nucleation of He bubbles.

  16. A cluster of Hodgkin's disease in a small community: evidence for environmental factors.

    PubMed

    Schwartz, R S; Callen, J P; Silva, J

    1978-07-01

    A cluster of Hodgkin's disease (HD) cases occurred in a small rural town of 1250 people. Ten cases of HD and three cases of non-Hodgkin's lymphoma were identified within or linked to this town since 1954. This town therefore had an average annual incidence and mortality for HD of 29.3 and 16.7 cases per 100,000 population, respectively. Most cases of HD demonstrated case-contact associations, and a distinct geographic distribution. The data suggested that there was an environmental agent responsible for the elevated rates of HD. One unique aspect of this cluster is that this town has only one industry, a large grain elevator. The cases closely surrounded this elevator. We postulate that residents of the town are subject to chronic immune stimulation from mitogenic substances in this environment. These agents may alter immunity in the residents of this community and predispose them to acquiring HD. PMID:685972

  17. The Very Small Scale Clustering of SDSS-II and SDSS-III Galaxies

    NASA Astrophysics Data System (ADS)

    Piscionere, Jennifer

    2015-01-01

    We measure the angular clustering of galaxies from the Sloan Digital Sky Survey Data Release 7 in order to probe the spatial distribution of satellite galaxies within their dark matter halos. Specifically, we measure the angular correlation function on very small scales (7 - 320‧‧) in a range of luminosity threshold samples (absolute r-band magnitudes of -18 up to -21) that are constructed from the subset of SDSS that has been spectroscopically observed more than once (the so-called plate overlap region). We choose to measure angular clustering in this reduced survey footprint in order to minimize the effects of fiber collision incompleteness, which are otherwise substantial on these small scales. We model our clustering measurements using a fully numerical halo model that populates dark matter halos in N-body simulations to create realistic mock galaxy catalogs. The model has free parameters that specify both the number and spatial distribution of galaxies within their host halos. We adopt a flexible density profile for the spatial distribution of satellite galaxies that is similar to the dark matter Navarro-Frenk-White (NFW) profile, except that the inner slope is allowed to vary. We find that the angular clustering of our most luminous samples (Mr < -20 and -21) suggests that luminous satellite galaxies have substantially steeper inner density profiles than NFW. Lower luminosity samples are less constraining, however, and are consistent with satellite galaxies having shallow density profiles. Our results confirm the findings of Watson et al. (2012) while using different clustering measurements and modeling methodology. With the new SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS; Dawson et al., 2013), we can measure how the same class of galaxy evolves over time. The BOSS CMASS sample is of roughly constant stellar mass and number density out to z ˜ 0.6. The clustering of these samples appears to evolve very little with redshift, and each of the

  18. Formation of aluminum films on silicon by ion beam deposition: A comparison with ionized cluster beam deposition

    SciTech Connect

    Zuhr, R.A.; Haynes, T.E.; Galloway, M.D. ); Tanaka, S.; Yamada, A.; Yamada, I. . Ion Beam Engineering Lab.)

    1990-01-01

    The direct ion beam deposition (IBD) technique has been used to study the formation of oriented aluminum films on single crystal silicon substrates. In the IBD process, thin film growth is accomplished by decelerating a magnetically-analyzed ion beam to low energies (10--200 eV) for direct deposition onto the substrate under UHV conditions. The energy of the incident ions can be selected to provide the desired growth conditions, and the mass analysis ensures good beam purity. The aluminum on silicon system is one which has been studied extensively by ionized cluster beam (ICB) deposition. In this work, we have studied the formation of such films by IBD with emphasis on the effects of ion energy, substrate temperature, and surface cleanliness. Oriented films have been grown on Si(111) at temperatures from 40{degree} to 300{degree}C and with ion energies from 30 to 120 eV per ion. Completed films were analyzed by ion scattering, x-ray diffraction, scanning electron microscopy, and optical microscopy. Results achieved for thin films grown by IBD are compared with results for similar films grown by ICB deposition. 15 refs., 3 figs.

  19. Structural Evolution of Anionic Silicon Clusters SiN (20≤N≤45)

    SciTech Connect

    Bai, Jaeil; Cui, Lifeng; Wang, Jin; Yoo, Soohaeng; Li, Xi; Jellinek, Julius; Koehler, Christa; Frauenheim, Thomas; Wang, Lai S.; Zeng, Xiao Cheng

    2006-01-26

    Results of a combined photoelectron spectroscopy and first-principles density-functional study of SiN - clusters in the size range 20 e N e 45 are reported and discussed. Evidence for a prolate-to-near-spherical shape transition at N 27 is presented. It is shown that the tricapped-trigonal-prism (TTP) structural motif Si9 found in most low-lying clusters SiN -, 9 e N e 19, is replaced or augmented by a series of structural motifs consisting of a bulklike ?adamantane? fragment plus a magic-number cluster (Si6, Si7, Si10) or TTP Si9 in low-lying prolate clusters SiN -, N g 20. For 28 e N e 45, almost all low-lying near-spherical clusters SiN -adopt ?stuffed-cage?-like structures where the cages are homologous to carbon fullerenes in the sense that they are composed of only five- and six-membered rings. However the arrangement of the ?stuffing? atoms is not yet diamondlike.

  20. The changes in small metal cluster size with adsorption Be13Xn, X = H, O, S, Cl and F

    NASA Technical Reports Server (NTRS)

    Bauschlicher, C. W., Jr.

    1985-01-01

    The Be-Be and Be-adsorbate distances are optimized for small Be13Xn clusters,s assuming only two independent degrees of freedom. Results for chemisorption into the three-fold hollows are quite similar to those found for small metal clusters on supports. It is predicted that Cl and F will have the same effect on the metal-metal bond lengths, and so will O and S.

  1. In situ investigation of the mobility of small gold clusters on cleaved MgO surfaces

    NASA Technical Reports Server (NTRS)

    Metois, J. J.; Heinemann, K.; Poppa, H.

    1976-01-01

    The mobility of small clusters of gold (about 10 A in diameter) on electron-beam-cleaved MgO surfaces was studied by in situ transmission electron microscopy under controlled vacuum and temperature conditions. During the first 10 min following a deposition at room temperature, over 10 per cent of the crystallites moved over short distances (about 20 A) discontinuously, with a velocity greater than 150 A/sec. Eighty per cent of the mobility events were characterized by the avoidance of proximity of other crystallites, and this was tentatively explained as the result of repulsive elastic forces between the interacting crystallites.

  2. Trapping of hydrogen atoms inside small beryllium clusters and their ions

    NASA Astrophysics Data System (ADS)

    Naumkin, F. Y.; Wales, D. J.

    2016-08-01

    Structure, stability and electronic properties are evaluated computationally for small Ben (n = 5-9) cluster cages accommodating atomic H inside and forming core-shell species. These parameters are predicted to vary significantly upon insertion of H, for ionic derivatives, and with the system size. In particular, the energy barrier for H-atom exit from the cage changes significantly for ions compared to the neutral counterparts. The corresponding effects predicted for cage assemblies suggest the possibility of efficient charge-control of hydrogen release. This, together with a high capacity for storing hydrogen in extended such assemblies might indicate a possible way towards feasible hydrogen-storage solutions.

  3. Effect of three-body interactions on the structure of small clusters

    NASA Technical Reports Server (NTRS)

    Halicioglu, T.; White, P. J.

    1980-01-01

    Minimum energy configurations of microclusters (up to six atoms) have been calculated using two- and three-body interactions. Structural changes were parametrically analyzed as a function of the intensity of three-body forces. The results are qualitative in nature; they indicate, however, that three-body interactions play an important role in the equilibrium structure of microclusters. The effect of the intensity of the three-body interactions on the structure of small clusters is not manifested in a continuous manner. Rather, changes in the energetically most stable structure occur abruptly. The results are in qualitative agreement with experimental observations as well as other calculations.

  4. The Formation of Clusters and Nanocrystals in Er-Implanted Hexagonal Silicon Carbide

    NASA Astrophysics Data System (ADS)

    Kaiser, U.; Muller, D. A.; Chuvilin, A.; Pasold, G.; Witthuhn, W.

    2004-04-01

    Impurity atom cluster and nanocrystal formation in Er-implanted hexagonal SiC were studied using TEM and HAADF-STEM. Short interstitial loops were initially observed to form in the as-implanted layers. After annealing at 1600°C extended matrix defects (wide interstitial loops and voids), Er atom clusters and nanocrystals grew. The wide interstitial loops act as strong sinks capturing diffusing dopants that gather first in lines, then planes, and finally in three-dimensional ErSi2 nanocrystals. The unstrained nanocrystals have a hill-like shape and only two polarity-dependent orientations with respect to the matrix. One-, two-, and three-dimensional Er atom clusters were also identified. For the case of Ge implantation, again the wide interstitial loops act as sinks for the implanted Ge, representing the seeds of the nanocrystal.

  5. Evolution of nanoripples on silicon by gas cluster-ion irradiation

    SciTech Connect

    Lozano, Omar; Chen, Q. Y.; Wadekar, P. V.; Chinta, P. V.; Tilakaratne, B. P.; Wang, X. M.; Wijesundera, D.; Chu, W. K.; Seo, H. W.; Tu, L. W.; Ho, N. J.

    2013-06-15

    Si wafers of (100), (110) and (111) orientations were bombarded by gas cluster ion beam (GCIB) of 3000 Ar-atoms/cluster on average at a series of angles. Similar surface morphology ripples developed in different nanoscales. A simple scaling functional satisfactorily describe the roughness and wavelength of the ripple patterns as a function of dosage and angle of incidence. The ripples are formed orthogonal to the incident cluster-ions at large off-normal angles. An ellipsoidal pattern was created by two consecutive irradiations incident in mutually orthogonal directions with unequal exposure times between each irradiation, from 7:1 to 10:1, beyond which the original ripple imprints would be over-written. This work was inspired by use of the ripples to seed growth of controlled nanostructures without patterning by lithography or predeposition of catalysts.

  6. Ca II Triplet Spectroscopy of Small Magellanic Cloud Red Giants. III. Abundances and Velocities for a Sample of 14 Clusters

    NASA Astrophysics Data System (ADS)

    Parisi, M. C.; Geisler, D.; Clariá, J. J.; Villanova, S.; Marcionni, N.; Sarajedini, A.; Grocholski, A. J.

    2015-05-01

    We obtained spectra of red giants in 15 Small Magellanic Cloud (SMC) clusters in the region of the Ca ii lines with FORS2 on the Very Large Telescope. We determined the mean metallicity and radial velocity with mean errors of 0.05 dex and 2.6 km s-1, respectively, from a mean of 6.5 members per cluster. One cluster (B113) was too young for a reliable metallicity determination and was excluded from the sample. We combined the sample studied here with 15 clusters previously studied by us using the same technique, and with 7 clusters whose metallicities determined by other authors are on a scale similar to ours. This compilation of 36 clusters is the largest SMC cluster sample currently available with accurate and homogeneously determined metallicities. We found a high probability that the metallicity distribution is bimodal, with potential peaks at -1.1 and -0.8 dex. Our data show no strong evidence of a metallicity gradient in the SMC clusters, somewhat at odds with recent evidence from Ca ii triplet spectra of a large sample of field stars. This may be revealing possible differences in the chemical history of clusters and field stars. Our clusters show a significant dispersion of metallicities, whatever age is considered, which could be reflecting the lack of a unique age-metallicity relation in this galaxy. None of the chemical evolution models currently available in the literature satisfactorily represents the global chemical enrichment processes of SMC clusters.

  7. Calculation of the cluster size distribution functions and small-angle neutron scattering data for C60/N-methylpyrrolidone

    NASA Astrophysics Data System (ADS)

    Tropin, T. V.; Jargalan, N.; Avdeev, M. V.; Kyzyma, O. A.; Sangaa, D.; Aksenov, V. L.

    2014-01-01

    The aggregate growth in a C60/N-methylpyrrolidone (NMP) solution has been considered in the framework of the approach developed earlier for describing the cluster growth kinetics in fullerene polar solutions. The final cluster size distribution functions in model solutions have been estimated for two fullerene aggregation models including the influence of complex formation on the cluster growth using extrapolations of the characteristics of the cluster state and distribution parameters. Based on the obtained results, the model curves of small-angle neutron scattering have been calculated for a C60/NMP solution at various values of the model parameters.

  8. Theoretical Study of the Spin Competition in Small-Sized Al Clusters.

    PubMed

    López-Estrada, Omar; Orgaz, Emilio

    2015-12-10

    Stern-Gerlach (SG) experiments on aluminum clusters indicate that some small-sized aggregates exhibit a deflection signal consistent with the existence of magnetic moments. However, in the particular case of Al6 and Al8 clusters, electronic structure investigations show ambiguity on the 0 K ground spin state. In this work extensive computations of the electronic structure have been carried out in order to determine the ground state of these structures. Electron correlation has been introduced at MP2, MP4, and CCSD(T) theory level as well as by DFT computations with different density functionals. DFT-based Born-Oppenheimer molecular dynamics results at different simulation temperatures complete this investigation. One of our main conclusions is that singlet spin states are systematically the more stable configuration at 0 K. These Al clusters exhibit almost degenerate electronic structures at singlet and triplet spin states. The geometries are similar, and the paths connecting both structures allow an intersystem crossing through a spin-orbit coupling mechanism, indicating a dynamical interchange of both spin states at finite temperatures. PMID:26583532

  9. Structural and electronic properties of small CuO m clusters

    NASA Astrophysics Data System (ADS)

    Pouillon, Y.; Massobrio, C.

    2004-03-01

    The bonding between copper and oxygen atoms and its evolution with size has motivated extensive photoelectron spectroscopy measurements (see H. Wu, S. Desai, L.-S. Wang, J. Phys. Chem. A, 101 (1997) 2103-2777). Despite the small sizes involved in the experimental work carried out so far, the structure of the isomers is far from being elucidated. To go beyond qualitative interpretations, we have performed structural optimizations by using the first-principles molecular dynamics framework. The calculations on copper oxide clusters presented here are carried out within density functional theory (DFT), with a plane-wave basis set and generalized gradient corrections. Our results show that except in one case the CuO3 cluster takes a planar geometry, one of these isomers being an ozonide. The most stable isomer of CuO4 also exhibits planar geometry. Results obtained for CuO6 show that the symmetries deduced from the experiments do not correspond to the most stable forms, and are even unstable in some cases. More generally, the three largest clusters in the series are made of the structural blocks corresponding to the shape of the smaller isomers. Ozonides are favoured as the number of atoms increases.

  10. Structural and electronic properties of small bimetallic Ag-Cu clusters

    NASA Astrophysics Data System (ADS)

    Kilimis, D. A.; Papageorgiou, D. G.

    2010-01-01

    The structural and electronic properties of small gas-phase AgmCun clusters with m+n=2-5 atoms are investigated using spin-polarized density functional theory. The LANL2DZ effective core potential and the corresponding basis set are employed while the performance of several exchange-correlation functionals is assessed. For a given cluster size all possible compositions are subject to optimization using a variety of initial structures. The geometry, binding energy, relative stability, ionization potential, electron affinity and HOMO-LUMO gap are reported for the lowest energy structure of every cluster size and composition. The results show that planar structures are favored, triangular for trimers, rhombic for tetramers and trapezoidal for pentamers. Moreover, for tetramers and pentamers we found that silver atoms demonstrate a clear tendency to occupy edge positions. The calculation of electronic properties indicates that although all exchange-correlation functionals predict the same trends, the choice of method is crucial concerning the final quantitative results.

  11. On the structural landscape in endohedral silicon and germanium clusters, M@Si12 and M@Ge12.

    PubMed

    Goicoechea, José M; McGrady, John E

    2015-04-21

    Amongst the endohedral clusters of the tetrel elements, M@En, the 12-vertex species are unique in that three completely different geometries, the icosahedron (Ih, [Ni@Pb12](2-)), the hexagonal prism (HP, Cr@Si12) and the bicapped pentagonal prism (BPP, [Ru@Ge12](3-)) have been identified in stable molecules. We explore here the origins of this structural diversity by comparing stability patterns across isovalent and isoelectronic series, M@Si12, M@Ge12 and [M@Ge12](3-). The BPP structure dominates the structural landscape for high valence electron counts (57-60) while the HP has a rather narrower window of stability around the 54-56 count. Moreover the preference for an HP structure is unique to silicon: in no case is a rigorously D6h-symmetric structure the global minimum for M@Ge12. Distortions from the high-symmetry limits, where present, can be traced to degeneracies or near-degeneracies in the frontier orbital domains. In all cases the structure adopted is that which maximizes the delocalization of electron density between the metal and the cluster cage, such that both components attain stable electronic configurations. PMID:25636138

  12. Theoretical study of small sodium-potassium alloy clusters through genetic algorithm and quantum chemical calculations.

    PubMed

    Silva, Mateus X; Galvão, Breno R L; Belchior, Jadson C

    2014-05-21

    Genetic algorithm is employed to survey an empirical potential energy surface for small Na(x)K(y) clusters with x + y ≤ 15, providing initial conditions for electronic structure methods. The minima of such empirical potential are assessed and corrected using high level ab initio methods such as CCSD(T), CR-CCSD(T)-L and MP2, and benchmark results are obtained for specific cases. The results are the first calculations for such small alloy clusters and may serve as a reference for further studies. The validity and choice of a proper functional and basis set for DFT calculations are then explored using the benchmark data, where it was found that the usual DFT approach may fail to provide the correct qualitative result for specific systems. The best general agreement to the benchmark calculations is achieved with def2-TZVPP basis set with SVWN5 functional, although the LANL2DZ basis set (with effective core potential) and SVWN5 functional provided the most cost-effective results. PMID:24691391

  13. Phase synchronization of bursting neurons in clustered small-world networks

    NASA Astrophysics Data System (ADS)

    Batista, C. A. S.; Lameu, E. L.; Batista, A. M.; Lopes, S. R.; Pereira, T.; Zamora-López, G.; Kurths, J.; Viana, R. L.

    2012-07-01

    We investigate the collective dynamics of bursting neurons on clustered networks. The clustered network model is composed of subnetworks, each of them presenting the so-called small-world property. This model can also be regarded as a network of networks. In each subnetwork a neuron is connected to other ones with regular as well as random connections, the latter with a given intracluster probability. Moreover, in a given subnetwork each neuron has an intercluster probability to be connected to the other subnetworks. The local neuron dynamics has two time scales (fast and slow) and is modeled by a two-dimensional map. In such small-world network the neuron parameters are chosen to be slightly different such that, if the coupling strength is large enough, there may be synchronization of the bursting (slow) activity. We give bounds for the critical coupling strength to obtain global burst synchronization in terms of the network structure, that is, the probabilities of intracluster and intercluster connections. We find that, as the heterogeneity in the network is reduced, the network global synchronizability is improved. We show that the transitions to global synchrony may be abrupt or smooth depending on the intercluster probability.

  14. Evaluation of Matrix9 silicon photomultiplier array for small-animal PET

    PubMed Central

    Du, Junwei; Schmall, Jeffrey P.; Yang, Yongfeng; Di, Kun; Roncali, Emilie; Mitchell, Gregory S.; Buckley, Steve; Jackson, Carl; Cherry, Simon R.

    2015-01-01

    Purpose: The MatrixSL-9-30035-OEM (Matrix9) from SensL is a large-area silicon photomultiplier (SiPM) photodetector module consisting of a 3 × 3 array of 4 × 4 element SiPM arrays (total of 144 SiPM pixels) and incorporates SensL’s front-end electronics board and coincidence board. Each SiPM pixel measures 3.16 × 3.16 mm2 and the total size of the detector head is 47.8 × 46.3 mm2. Using 8 × 8 polished LSO/LYSO arrays (pitch 1.5 mm) the performance of this detector system (SiPM array and readout electronics) was evaluated with a view for its eventual use in small-animal positron emission tomography (PET). Methods: Measurements of noise, signal, signal-to-noise ratio, energy resolution, flood histogram quality, timing resolution, and array trigger error were obtained at different bias voltages (28.0–32.5 V in 0.5 V intervals) and at different temperatures (5 °C–25 °C in 5 °C degree steps) to find the optimal operating conditions. Results: The best measured signal-to-noise ratio and flood histogram quality for 511 keV gamma photons were obtained at a bias voltage of 30.0 V and a temperature of 5 °C. The energy resolution and timing resolution under these conditions were 14.2% ± 0.1% and 4.2 ± 0.1 ns, respectively. The flood histograms show that all the crystals in the 1.5 mm pitch LSO array can be clearly identified and that smaller crystal pitches can also be resolved. Flood histogram quality was also calculated using different center of gravity based positioning algorithms. Improved and more robust results were achieved using the local 9 pixels for positioning along with an energy offset calibration. To evaluate the front-end detector readout, and multiplexing efficiency, an array trigger error metric is introduced and measured at different lower energy thresholds. Using a lower energy threshold greater than 150 keV effectively eliminates any mispositioning between SiPM arrays. Conclusions: In summary, the Matrix9 detector system can resolve

  15. WFPC2 Observations of Star Clusters in the Magellanic Clouds. Report 2; The Oldest Star Clusters in the Small Magellanic Cloud

    NASA Technical Reports Server (NTRS)

    Mighell, Kenneth J.; Sarajedini, Ata; French, Rica S.

    1998-01-01

    We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations in F45OW ( approximately B) and F555W (approximately V) of the intermediate-age populous star clusters NGC 121, NGC 339, NGC 361, NGC 416, and Kron 3 in the Small Magellanic Cloud. We use published photometry of two other SMC populous star clusters, Lindsay 1 and Lindsay 113, to investigate the age sequence of these seven populous star clusters in order to improve our understanding of the formation chronology of the SMC. We analyzed the V vs B-V and M(sub V) vs (B-V)(sub 0) color-magnitude diagrams of these populous Small Magellanic Cloud star clusters using a variety of techniques and determined their ages, metallicities, and reddenings. These new data enable us to improve the age-metallicity relation of star clusters in the Small Magellanic Cloud. In particular, we find that a closed-box continuous star-formation model does not reproduce the age-metallicity relation adequately. However, a theoretical model punctuated by bursts of star formation is in better agreement with the observational data presented herein.

  16. THE CLUSTERED NATURE OF STAR FORMATION. PRE-MAIN-SEQUENCE CLUSTERS IN THE STAR-FORMING REGION NGC 602/N90 IN THE SMALL MAGELLANIC CLOUD

    SciTech Connect

    Gouliermis, Dimitrios A.; Gennaro, Mario; Schmeja, Stefan; Dolphin, Andrew E.; Tognelli, Emanuele; Prada Moroni, Pier Giorgio

    2012-03-20

    Located at the tip of the wing of the Small Magellanic Cloud (SMC), the star-forming region NGC 602/N90 is characterized by the H II nebular ring N90 and the young cluster of pre-main-sequence (PMS) and early-type main-sequence stars NGC 602, located in the central area of the ring. We present a thorough cluster analysis of the stellar sample identified with Hubble Space Telescope/Advanced Camera for Surveys in the region. We show that apart from the central cluster low-mass PMS stars are congregated in 13 additional small, compact sub-clusters at the periphery of NGC 602, identified in terms of their higher stellar density with respect to the average background density derived from star counts. We find that the spatial distribution of the PMS stars is bimodal, with an unusually large fraction ({approx}60%) of the total population being clustered, while the remaining is diffusely distributed in the intercluster area, covering the whole central part of the region. From the corresponding color-magnitude diagrams we disentangle an age difference of {approx}2.5 Myr between NGC 602 and the compact sub-clusters, which appear younger, on the basis of comparison of the brighter PMS stars with evolutionary models, which we accurately calculated for the metal abundance of the SMC. The diffuse PMS population appears to host stars as old as those in NGC 602. Almost all detected PMS sub-clusters appear to be centrally concentrated. When the complete PMS stellar sample, including both clustered and diffused stars, is considered in our cluster analysis, it appears as a single centrally concentrated stellar agglomeration, covering the whole central area of the region. Considering also the hot massive stars of the system, we find evidence that this agglomeration is hierarchically structured. Based on our findings, we propose a scenario according to which the region NGC 602/N90 experiences an active clustered star formation for the last {approx}5 Myr. The central cluster NGC 602 was

  17. Physico-chemical treatment of wastewater from clusters of small scale cotton textile units.

    PubMed

    Pathe, P P; Biswas, A K; Rao, N N; Kaul, S N

    2005-03-01

    Small scale industries can not own individual wastewater treatment facility due to non-availability of land and skilled manpower for operation and maintenance of wastewater treatment plants. A centralized wastewater treatment facility for clusters of small scale industries is appropriate. This concept is gaining popularity in recent years. In India, various textile process operations are undertaken by individual small scale units. The wastewater generated at these units is conveyed to a common effluent treatment facility comprising of equalization, flocculation-clarification, activated sludge process, secondary clarification and finally discharge into inland surface water bodies. The wastewater from small scale cotton textile processing units was highly coloured and alkaline with average BOD and COD concentration of 205 and 790 mg l(-1), respectively. Due to the presence of several dyes, particularly reactive dyes, the biological treatment is often found less effective. Therefore, applicability of various physico-chemical treatment methods needs to be investigated in pursuit of an alternative to biological treatment of textile wastewater. A physico-chemical treatment scheme, involving chemical coagulation-sedimentation, dual media filtration, activated carbon adsorption followed by chemical oxidation was investigated in this paper. The quality of final treated wastewater in terms of BOD and COD was 18-24 and 230-240 mg l(-1), respectively through this scheme. A scheme of treatment comprising coagulation-sedimentation, dual media filtration, activated carbon, chemical oxidation may be considered as an alternative to biological treatment of textile wastewater. PMID:15881028

  18. Encapsulating Metal Clusters and Acid Sites within Small Voids: Synthetic Strategies and Catalytic Consequences

    NASA Astrophysics Data System (ADS)

    Goel, Sarika

    The selective encapsulation of metal clusters within zeolites can be used to prepare clusters that are uniform in diameter and to protect them against sintering and contact with feed impurities, while concurrently allowing active sites to select reactants based on their molecular size, thus conferring enzyme-like specificity to chemical catalysis. The apertures in small and medium-pore zeolites preclude the use of post-synthetic protocols to encapsulate the relevant metal precursors because cationic or anionic precursors with their charge-balancing double layer and gaseous complexes cannot diffuse through their windows or channels. We have developed general strategies to encapsulate metal clusters within small-pore zeolites by using metal precursors stabilized by ammonia or organic amine ligands, which stabilize metal precursors against their premature precipitation at the high temperature and pH conditions required for the hydrothermal synthesis of the target zeolite structures and favor interactions between metal precursors and incipient aluminosilicate nuclei during the self-assembly of microporous frameworks. When synthesis temperatures were higher than 400 K, available ligands were unable to prevent the premature precipitation of the metal precursors. In such cases, encapsulation was achieved instead via interzeolite transformations after successfully encapsulating metal precursors or clusters via post-synthesis exchange or ligand protection into parent zeolites and subsequently converting them into the target structures while retaining the encapsulated clusters or precursors. Such strategies led to the successful selective encapsulation of a wide range of metal clusters (Pt, Pd, Ru, Rh, Ir, Re, and Ag) within small-pore (SOD (sodalite), LTA (Linde type A (zeolite A)), GIS (gismondine), and ANA (analcime)) and medium-pore (MFI (ZSM-5)) zeolites. These protocols provide novel and diverse mechanism-based strategies for the design of catalysts with protected

  19. Finite temperature magnetic properties of small Fe chains and clusters on Pt(111)

    NASA Astrophysics Data System (ADS)

    Riemer, S.; Dorantes-Dávila, J.; Pastor, G. M.

    2016-04-01

    The magnetic properties of Fe chains and clusters on Pt(111) are investigated in the framework of a functional-integral theory of itinerant magnetism. The considered nanostructures show a ferromagnetic (FM) ground state with nearly saturated Fe local magnetic moments μFe0≃3.15 μB . In addition, small moments μPt0≃0.1 -0.3 μB are induced at the Pt substrate, which depend sensitively on the number of Fe atoms in their nearest-neighbor (NN) shell. The spin-fluctuation (SF) energies Δ Fl(ξ ) at the different atoms l are calculated as a function of the local exchange fields ξl, by using a real-space recursive expansion of the local Green's functions. Results for the temperature dependence of the average magnetization per atom μ¯N, local magnetic moments μl, and spin correlation functions γl k are derived. At the Fe atoms the dominant magnetic excitations are fluctuations of the local-moment orientations. The spin-flip energies Δ Fl(ξ ) in the deposited Fe clusters are found to be about 50 % smaller than in free-standing clusters of comparable size. This results in flatter SF-energy landscapes and in a weaker stability of the FM order at T >0 . The effective exchange interactions between the Fe local moments, which are derived from the electronic calculations, reveal competing FM and antiferromagnetic couplings at different distances. In contrast to Fe, the main spin excitations at the Pt atoms are fluctuations of the size of the induced local magnetic moments. The interplay between the different types of spin excitations and their effect on the temperature-dependent magnetic properties is discussed.

  20. Desmoplastic small round cell tumor with sphere-like clusters mimicking adenocarcinoma.

    PubMed

    Hattori, Yukinori; Yoshida, Akihiko; Sasaki, Naoshi; Shibuki, Yasuo; Tamura, Kenji; Tsuta, Koji

    2015-03-01

    Desmoplastic small round cell tumor (DSRCT) is a rare and aggressive neoplasm that predominantly affects young men. DSRCT often presents as multiple nodules on the serosal surface and is histologically categorized as a small round cell tumor. However, the cytological spectrum of DSRCT is not fully understood because of its rarity. Here, we report an unusual case of DSRCT that showed spheres of cells without stromal cores in pleural fluid cytology material, a finding that is typically associated with metastatic adenocarcinoma and mesothelioma. The specimen from a simultaneous needle biopsy showed the classic histology of DSRCT, comprising nests of small round cells set in desmoplasia. The diagnosis of DSRCT was further supported by immunohistochemical coexpression of cytokeratin and desmin, as well as Ewing sarcoma breakpoint region 1 gene rearrangement, which was determined by fluorescence in situ hybridization. The unusual cytological finding in this case illustrates a potential pitfall of the cytological diagnosis of pleural fluid or ascites. DSRCT should not be excluded from the differential diagnosis when sphere-like round cell clusters are observed in pleural or abdominal effusion, particularly in young male patients. PMID:24819999

  1. Small Launch Vehicle Design Approaches: Clustered Cores Compared with Multi-Stage Inline Concepts

    NASA Technical Reports Server (NTRS)

    Waters, Eric D.; Beers, Benjamin; Esther, Elizabeth; Philips, Alan; Threet, Grady E., Jr.

    2013-01-01

    In an effort to better define small launch vehicle design options two approaches were investigated from the small launch vehicle trade space. The primary focus was to evaluate a clustered common core design against a purpose built inline vehicle. Both designs focused on liquid oxygen (LOX) and rocket propellant grade kerosene (RP-1) stages with the terminal stage later evaluated as a LOX/methane (CH4) stage. A series of performance optimization runs were done in order to minimize gross liftoff weight (GLOW) including alternative thrust levels, delivery altitude for payload, vehicle length to diameter ratio, alternative engine feed systems, re-evaluation of mass growth allowances, passive versus active guidance systems, and rail and tower launch methods. Additionally manufacturability, cost, and operations also play a large role in the benefits and detriments for each design. Presented here is the Advanced Concepts Office's Earth to Orbit Launch Team methodology and high level discussion of the performance trades and trends of both small launch vehicle solutions along with design philosophies that shaped both concepts. Without putting forth a decree stating one approach is better than the other; this discussion is meant to educate the community at large and let the reader determine which architecture is truly the most economical; since each path has such a unique set of limitations and potential payoffs.

  2. The VMC survey - XV. The Small Magellanic Cloud-Bridge connection history as traced by their star cluster populations

    NASA Astrophysics Data System (ADS)

    Piatti, Andrés E.; de Grijs, Richard; Rubele, Stefano; Cioni, Maria-Rosa L.; Ripepi, Vincenzo; Kerber, Leandro

    2015-06-01

    We present results based on YJKs photometry of star clusters located in the outermost, eastern region of the Small Magellanic Cloud (SMC). We analysed a total of 51 catalogued clusters whose colour-magnitude diagrams (CMDs), having been cleaned from field-star contamination, were used to assess the clusters' reality and estimate ages of the genuine systems. Based on CMD analysis, 15 catalogued clusters were found to be possible non-genuine aggregates. We investigated the properties of 80 per cent of the catalogued clusters in this part of the SMC by enlarging our sample with previously obtained cluster ages, adopting a homogeneous scale for all. Their spatial distribution suggests that the oldest clusters, log(t yr-1) ≥ 9.6, are in general located at greater distances to the galaxy's centre than their younger counterparts - 9.0 ≤ log(t yr-1) ≤ 9.4 - while two excesses of clusters are seen at log(t yr-1) ˜ 9.2 and log(t yr-1) ˜ 9.7. We found a trail of younger clusters which follow the wing/bridge components. This long spatial sequence does not only harbour very young clusters, log(t yr-1) ˜ 7.3, but it also hosts some of intermediate ages, log(t yr-1) ˜ 9.1. The derived cluster and field-star formation frequencies as a function of age are different. The most surprising feature is an observed excess of clusters with ages of log(t yr-1) < 9.0, which could have been induced by interactions with the LMC.

  3. ChemMine tools: an online service for analyzing and clustering small molecules

    PubMed Central

    Backman, Tyler W. H.; Cao, Yiqun; Girke, Thomas

    2011-01-01

    ChemMine Tools is an online service for small molecule data analysis. It provides a web interface to a set of cheminformatics and data mining tools that are useful for various analysis routines performed in chemical genomics and drug discovery. The service also offers programmable access options via the R library ChemmineR. The primary functionalities of ChemMine Tools fall into five major application areas: data visualization, structure comparisons, similarity searching, compound clustering and prediction of chemical properties. First, users can upload compound data sets to the online Compound Workbench. Numerous utilities are provided for compound viewing, structure drawing and format interconversion. Second, pairwise structural similarities among compounds can be quantified. Third, interfaces to ultra-fast structure similarity search algorithms are available to efficiently mine the chemical space in the public domain. These include fingerprint and embedding/indexing algorithms. Fourth, the service includes a Clustering Toolbox that integrates cheminformatic algorithms with data mining utilities to enable systematic structure and activity based analyses of custom compound sets. Fifth, physicochemical property descriptors of custom compound sets can be calculated. These descriptors are important for assessing the bioactivity profile of compounds in silico and quantitative structure—activity relationship (QSAR) analyses. ChemMine Tools is available at: http://chemmine.ucr.edu. PMID:21576229

  4. Small polyanion recognition of a triazolium cyclodextrin click cluster in water.

    PubMed

    Le, Hoa Thi; Park, Seung Cheol; Kang, Chulhun; Lim, Choon Woo; Kim, Tae Woo

    2015-08-14

    In order to detect small polyanions (sPAs), which play important roles in many biological systems, a triazolium cyclodextrin click cluster (5, hexakis{6-(3-methyl-4-hydroxymethyl-1H-1,2,3-triazolium-1-yl)-6-deoxy}-α-cyclodextrin iodide) was synthesized and characterized. The competition binding to 5 occupied by 5-carboxyfluorescein of inositol-1,4,5-trisphosphate (IP3), phytic acid, adenosine triphosphate (ATP), ethylenediaminetetraacetic acid (EDTA), glucose, and glucose-6-phosphate was evaluated by UV/vis titration in HEPES (10 mM, pH 7.4) : methanol (1 : 1, v/v). We obtained the binding constants of IP3 and phytic acid to 5 (1.4 × 10(6) and 1.9 × 10(6) M(-1), respectively); however, the binding constants of ATP and EDTA were significantly lower (2.1 × 10(5) and 4.5 × 10(4) M(-1), respectively). Moreover, glucose and glucose-6-phosphate did not show any detectable binding. In addition, the sPA recognition of the triazolium cyclodextrin click cluster in water was confirmed by fluorescence titration. PMID:26140361

  5. Auditory disturbances promote temporal clustering of yawning and stretching in small groups of budgerigars (Melopsittacus undulatus).

    PubMed

    Miller, Michael L; Gallup, Andrew C; Vogel, Andrea R; Clark, Anne B

    2012-08-01

    Yawning may serve both social and nonsocial functions. When budgerigars (Melopsittacus undulatus) are briefly held, simulating capture by a predator, the temporal pattern of yawning changes. When this species is observed in a naturalistic setting (undisturbed flock), yawning and also stretching, a related behavior, are mildly contagious. On the basis of these findings, we hypothesized that a stressful event would be followed by the clustering of these behaviors in a group of birds, which may be facilitated both by a standard pattern of responding to a startling stressor and also contagion. In this study, we measured yawning and stretching in 4-bird groups following a nonspecific stressor (loud white noise) for a period of 1 hr, determining whether auditory disturbances alter the timing and frequency of these behaviors. Our results show that stretching, and to a lesser degree yawning, were nonrandomly clumped in time following the auditory disturbances, indicating that the temporal clustering is sensitive to, and enhanced by, environmental stressors while in small groups. No decrease in yawning such as found after handling stress was observed immediately after the loud noise but a similar increase in yawning 20 min after was observed. Future research is required to tease apart the roles of behavioral contagion and a time-setting effect following a startle in this species. This research is of interest because of the potential role that temporal clumping of yawning and stretching could play in both the collective detection of, and response to, local disturbances or predation threats. PMID:22268553

  6. Theoretical investigation on isomer formation probability and free energy of small C clusters

    NASA Astrophysics Data System (ADS)

    Lin, Zheng-Zhe

    2015-06-01

    Molecular dynamics simulations and free energy calculations are employed to investigate the evolution, formation probability, detailed balance, and isomerization rate of small C cluster isomer at 2500 K. For C10, the isomer formation probability predicted by free energy is in good agreement with molecular dynamics simulation. However, for C20, C30, and C36, the formation probabilities predicted by free energy are not in agreement with molecular dynamics simulations. Although the cluster systems are in equilibrium, detailed balance is not reached. Such results may be attributed to high transformation barriers between cage, bowl, and sheet isomers. In summary, for mesoscopic nanosystems the free energy criterion, which commonly holds for macroscopic systems in dynamic equilibrium, may not provide a good prediction for isomer formation probability. New theoretical criterion should be further investigated for predicting the isomer formation probability of a mesoscopic nanosystem. Project supported by the National Natural Science Foundation of China (Grant No. 11304239) and the Fundamental Research Funds for the Central Universities.

  7. Evaluation of Matrix9 silicon photomultiplier array for small-animal PET

    SciTech Connect

    Du, Junwei Schmall, Jeffrey P.; Yang, Yongfeng; Di, Kun; Roncali, Emilie; Mitchell, Gregory S.; Buckley, Steve; Jackson, Carl; Cherry, Simon R.

    2015-02-15

    Purpose: The MatrixSL-9-30035-OEM (Matrix9) from SensL is a large-area silicon photomultiplier (SiPM) photodetector module consisting of a 3 × 3 array of 4 × 4 element SiPM arrays (total of 144 SiPM pixels) and incorporates SensL’s front-end electronics board and coincidence board. Each SiPM pixel measures 3.16 × 3.16 mm{sup 2} and the total size of the detector head is 47.8 × 46.3 mm{sup 2}. Using 8 × 8 polished LSO/LYSO arrays (pitch 1.5 mm) the performance of this detector system (SiPM array and readout electronics) was evaluated with a view for its eventual use in small-animal positron emission tomography (PET). Methods: Measurements of noise, signal, signal-to-noise ratio, energy resolution, flood histogram quality, timing resolution, and array trigger error were obtained at different bias voltages (28.0–32.5 V in 0.5 V intervals) and at different temperatures (5 °C–25 °C in 5 °C degree steps) to find the optimal operating conditions. Results: The best measured signal-to-noise ratio and flood histogram quality for 511 keV gamma photons were obtained at a bias voltage of 30.0 V and a temperature of 5 °C. The energy resolution and timing resolution under these conditions were 14.2% ± 0.1% and 4.2 ± 0.1 ns, respectively. The flood histograms show that all the crystals in the 1.5 mm pitch LSO array can be clearly identified and that smaller crystal pitches can also be resolved. Flood histogram quality was also calculated using different center of gravity based positioning algorithms. Improved and more robust results were achieved using the local 9 pixels for positioning along with an energy offset calibration. To evaluate the front-end detector readout, and multiplexing efficiency, an array trigger error metric is introduced and measured at different lower energy thresholds. Using a lower energy threshold greater than 150 keV effectively eliminates any mispositioning between SiPM arrays. Conclusions: In summary, the Matrix9 detector system

  8. Full 3-D cluster-based iterative image reconstruction tool for a small animal PET camera

    NASA Astrophysics Data System (ADS)

    Valastyán, I.; Imrek, J.; Molnár, J.; Novák, D.; Balkay, L.; Emri, M.; Trón, L.; Bükki, T.; Kerek, A.

    2007-02-01

    Iterative reconstruction methods are commonly used to obtain images with high resolution and good signal-to-noise ratio in nuclear imaging. The aim of this work was to develop a scalable, fast, cluster based, fully 3-D iterative image reconstruction package for our small animal PET camera, the miniPET. The reconstruction package is developed to determine the 3-D radioactivity distribution from list mode type of data sets and it can also simulate noise-free projections of digital phantoms. We separated the system matrix generation and the fully 3-D iterative reconstruction process. As the detector geometry is fixed for a given camera, the system matrix describing this geometry is calculated only once and used for every image reconstruction, making the process much faster. The Poisson and the random noise sensitivity of the ML-EM iterative algorithm were studied for our small animal PET system with the help of the simulation and reconstruction tool. The reconstruction tool has also been tested with data collected by the miniPET from a line and a cylinder shaped phantom and also a rat.

  9. Modeling optical properties of silicon clusters by first principles: From a few atoms to large nanocrystals

    SciTech Connect

    Nurbawono, Argo; Liu, Shuanglong; Zhang, Chun

    2015-04-21

    Time dependent density functional tight binding (TDDFTB) method is implemented with sparse matrix techniques and improved parallelization algorithms. The method is employed to calculate the optical properties of various Si nanocrystals (NCs). The calculated light absorption spectra of small Si NCs from TDDFTB were found to be comparable with many body perturbation methods utilizing planewave basis sets. For large Si NCs (more than a thousand atoms) that are beyond the reach of conventional approaches, the TDDFTB method is able to produce reasonable results that are consistent with prior experiments. We also employed the method to study the effects of surface chemistry on the optical properties of large Si NCs. We learned that the optical properties of Si NCs can be manipulated with small molecule passivations such as methyl, hydroxyl, amino, and fluorine. In general, the shifts and profiles in the absorption spectra can be tuned with suitably chosen passivants.

  10. Quantitative Analysis of Clustered DNA Damages Induced by Silicon Beams of Different Kinetic Energy

    SciTech Connect

    Keszenman D. J.; Keszenman, D.J.; Bennett, P.V.; Sutherland, B.M.; Wilson, P.F.

    2013-05-14

    Humans may b exposed to highly energetic charged particle radiation as a result of medical treatments, occupational activitie or accidental events. In recent years, our increasing presence and burgeoning interest in space exploration beyond low Earth orbit has led to a large increase in the research of the biological effects ofcharged particle radiation typical of that encountered in the space radiation environment. The study of the effects of these types of radiation qualities in terms ofDNA damage induction and repair is fundamental to understand mechanisms both underlying their greater biological effectiveness as we)) as the short and long term risks of health effects such as carcinogenesis, degen rative diseases and premature aging. Charged particle radiation induces a variety of DNA alterations, notably bistranded clustered damages, defined as two or more closely-opposed strand break , oxidized bases or abasic sites within a few helical turns. The induction of such highly complex DNA damage enhances the probability of incorrect or incomplete repair and thus constitutes greater potential for genomic instability, cell death and transformation.

  11. Small nucleolar RNA clusters in trypanosomatid Leptomonas collosoma. Genome organization, expression studies, and the potential role of sequences present upstream from the first repeated cluster.

    PubMed

    Liang, Xue-hai; Ochaion, Avivit; Xu, Yu-xin; Liu, Qing; Michaeli, Shulamit

    2004-02-13

    Trypanosomatid small nucleolar RNA (snoRNA) genes are clustered in the genome. snoRNAs are transcribed polycistronically and processed into mature RNAs. In this study, we characterized four snoRNA clusters in Leptomonas collosoma. All of the clusters analyzed carry both C/D and H/ACA RNAs. The H/ACA RNAs are composed of a single hairpin, a structure typical to trypanosome and archaea guide RNAs. Using deletion and mutational analysis of a tagged C/D snoRNA situated within the snoRNA cluster, we identified 10-nucleotide flanking sequences that are essential for processing snoRNA from its precursor. Chromosome walk was performed on a snoRNA cluster, and a sequence of 700 bp was identified between the first repeat and the upstream open reading frame. Cloning of this sequence in an episome vector enhanced the expression of a tagged snoRNA gene in an orientation-dependent manner. However, continuous transcript spanning of this region was detected in steady-state RNA, suggesting that snoRNA transcription also originates from an upstream-long polycistronic transcriptional unit. The 700-bp fragment may therefore represent an example of many more elements to be discovered that enhance transcription along the chromosome, especially when transcription from the upstream gene is reduced or when enhanced transcription is needed. PMID:14645367

  12. Effect of silicone oil on solid propellant combustion in small motors. [for rockets

    NASA Technical Reports Server (NTRS)

    Ramohalli, K.

    1980-01-01

    The feasibility of reducing troublesome nozzle blockage (by condensation deposits) in laboratory-scale solid rockets by addition of a silicone oil as a propellant ingredient was explored experimentally. An aluminized composite propellant and its counterpart with 1% silicone oil replacing part of the binder were fired in a 63.5 mm diameter, end-burning, all-metal burner. Pressure-time histories were recorded for all of the tests by a Taber gauge mounted at the downstream end of the chamber; temperature-time data at the nozzle throat were obtained in some of the runs by thermocouples having junctions positioned at the wall but insulated from the metal. Deposition of condensables on the nozzle walls causing a progressive increase in the chamber pressure with time was noted. The fraction of firings exhibiting practically no condensation was 59% with silicone and 32% without. On the average, temperature readings at the nozzle throat were higher with the silicone propellants. Although various phenomena may contribute to these findings, the results are not understood completely.

  13. The problem of the structure (state of helium) in small He{sub N}-CO clusters

    SciTech Connect

    Potapov, A. V. Panfilov, V. A.; Surin, L. A.; Dumesh, B. S.

    2010-11-15

    A second-order perturbation theory, developed for calculating the energy levels of the He-CO binary complex, is applied to small He{sub N}-CO clusters with N = 2-4, the helium atoms being considered as a single bound object. The interaction potential between the CO molecule and HeN is represented as a linear expansion in Legendre polynomials, in which the free rotation limit is chosen as the zero approximation and the angular dependence of the interaction is considered as a small perturbation. By fitting calculated rotational transitions to experimental values it was possible to determine the optimal parameters of the potential and to achieve good agreement (to within less than 1%) between calculated and experimental energy levels. As a result, the shape of the angular anisotropy of the interaction potential is obtained for various clusters. It turns out that the minimum of the potential energy is smoothly shifted from an angle between the axes of the CO molecule and the cluster of {theta} = 100{sup o} in He-CO to {theta} = 180{sup o} (the oxygen end) in He{sub 3}-CO and He{sub 4}-CO clusters. Under the assumption that the distribution of helium atoms with respect to the cluster axis is cylindrically symmetric, the structure of the cluster can be represented as a pyramid with the CO molecule at the vertex.

  14. Fingerprints of carbon, nitrogen, and silicon isotopes in small interstellar SiC grains from the Murchison meteorite

    NASA Technical Reports Server (NTRS)

    Hoppe, Peter; Geiss, Johannes; Buehler, Fritz; Neuenschwander, Juerg; Amari, Sachiko; Lewis, Roy S.

    1993-01-01

    We report ion microprobe determinations of the carbon, nitrogen, and silicon isotopic compositions of small SiC grains from the Murchison CM2 chondrite. Analyses were made on samples containing variable numbers of grains and on 14 individual grains. In some cases the multiple-grain sample compositions were probably dominated by only one or two grains. Total ranges observed are given. Only a few grains show values near the range limits. Both the total ranges of carbon and nitrogen isotopic compositions, and even the narrower ranges typical for the majority of the grains, are similar to those observed for larger SiC grains. Two rare components appear to be present in the smaller-size fraction, one characterized by C-12/C-13 about 12-16 and the other by very heavy nitrogen. The carbon and nitrogen isotopic compositions qualitatively may reflect hydrostatic H-burning via the CNO cycle and He-burning in red giants, as well as explosive H-burning in novae. The silicon isotopic compositions of most grains qualitatively show what is the signature of He-burning. The silicon isotopic composition of one grain, however, suggests a different process.

  15. Structure, Stability, and Mobility of Small Pd Clusters on the Stoichiometric and Defective TiO2 (110) Surfaces

    SciTech Connect

    Zhang, Jin; Alexandrova, Anastassia N.

    2011-11-07

    We report on the structure and adsorption properties of Pdn (n = 1–4) clusters supported on the rutile TiO2 (110) surfaces with the possible presence of a surface oxygen vacancy or a subsurface Ti-interstitial atom. As predicted by the density functional theory, small Pd clusters prefer to bind to the stoichiometric titania surface or at sites near subsurface Ti-interstitial atoms. The adsorption of Pd clusters changes the electronic structure of the underlying surface. For the surface with an oxygen vacancy, the charge localization and ferromagnetic spin states are found to be largely attenuated owing to the adsorption of Pd clusters. The potential energy surfaces of the Pd monomer on different types of surfaces are also reported. The process of sintering is then simulated via the Metropolis Monte Carlo method. The presence of oxygen vacancy likely leads to the dissociation of Pd clusters. On the stoichiometric surface or surface with Ti-interstitial atom, the Pd monomers tend to sinter into larger clusters, whereas the Pd dimer, trimer, and tetramer appear to be relatively stable below 600 K. This result agrees with the standard sintering model of transition metal clusters and experimental observations.

  16. Prediction of Structures and Atomization Energies of Small Silver Clusters, (Ag)n, n < 100

    SciTech Connect

    Chen, Mingyang; Dyer, Jason E.; Li, Keijing; Dixon, David A.

    2013-08-29

    Neutral silver clusters, Agn, were studied using density functional theory (DFT) followed by high level coupled cluster CCSD(T) calculations to determine the low energy isomers for each cluster size for small clusters. The normalized atomization energy, heats of formation, and average bond lengths were calculated for each of the different isomeric forms of the silver clusters. For n = 2–6, the preferred geometry is planar, and the larger n = 7–8 clusters prefer higher symmetry, three-dimensional geometries. The low spin state is predicted to be the ground state for every cluster size. A number of new low energy isomers for the heptamer and octamer were found. Additional larger Agn structures, n < 100, were initially optimized using a tree growth-hybrid genetic algorithm with an embedded atom method (EAM) potential. Finally, for n ≤ 20, DFT was used to optimize the geometries. DFT with benchmarked functionals were used to predict that the normalized atomization energies ((AE)s) for Agn start to converge slowly to the bulk at n = 55. The (AE) for Ag99 is predicted to be ~50 kcal/mol.

  17. Effect of intermodular connection on fast sparse synchronization in clustered small-world neural networks

    NASA Astrophysics Data System (ADS)

    Kim, Sang-Yoon; Lim, Woochang

    2015-11-01

    We consider a clustered network with small-world subnetworks of inhibitory fast spiking interneurons and investigate the effect of intermodular connection on the emergence of fast sparsely synchronized rhythms by varying both the intermodular coupling strength Jinter and the average number of intermodular links per interneuron Msyn(inter ). In contrast to the case of nonclustered networks, two kinds of sparsely synchronized states such as modular and global synchronization are found. For the case of modular sparse synchronization, the population behavior reveals the modular structure, because the intramodular dynamics of subnetworks make some mismatching. On the other hand, in the case of global sparse synchronization, the population behavior is globally identical, independently of the cluster structure, because the intramodular dynamics of subnetworks make perfect matching. We introduce a realistic cross-correlation modularity measure, representing the matching degree between the instantaneous subpopulation spike rates of the subnetworks, and examine whether the sparse synchronization is global or modular. Depending on its magnitude, the intermodular coupling strength Jinter seems to play "dual" roles for the pacing between spikes in each subnetwork. For large Jinter, due to strong inhibition it plays a destructive role to "spoil" the pacing between spikes, while for small Jinter it plays a constructive role to "favor" the pacing between spikes. Through competition between the constructive and the destructive roles of Jinter, there exists an intermediate optimal Jinter at which the pacing degree between spikes becomes maximal. In contrast, the average number of intermodular links per interneuron Msyn(inter ) seems to play a role just to favor the pacing between spikes. With increasing Msyn(inter ), the pacing degree between spikes increases monotonically thanks to the increase in the degree of effectiveness of global communication between spikes. Furthermore, we

  18. Small sample performance of bias-corrected sandwich estimators for cluster-randomized trials with binary outcomes.

    PubMed

    Li, Peng; Redden, David T

    2015-01-30

    The sandwich estimator in generalized estimating equations (GEE) approach underestimates the true variance in small samples and consequently results in inflated type I error rates in hypothesis testing. This fact limits the application of the GEE in cluster-randomized trials (CRTs) with few clusters. Under various CRT scenarios with correlated binary outcomes, we evaluate the small sample properties of the GEE Wald tests using bias-corrected sandwich estimators. Our results suggest that the GEE Wald z-test should be avoided in the analyses of CRTs with few clusters even when bias-corrected sandwich estimators are used. With t-distribution approximation, the Kauermann and Carroll (KC)-correction can keep the test size to nominal levels even when the number of clusters is as low as 10 and is robust to the moderate variation of the cluster sizes. However, in cases with large variations in cluster sizes, the Fay and Graubard (FG)-correction should be used instead. Furthermore, we derive a formula to calculate the power and minimum total number of clusters one needs using the t-test and KC-correction for the CRTs with binary outcomes. The power levels as predicted by the proposed formula agree well with the empirical powers from the simulations. The proposed methods are illustrated using real CRT data. We conclude that with appropriate control of type I error rates under small sample sizes, we recommend the use of GEE approach in CRTs with binary outcomes because of fewer assumptions and robustness to the misspecification of the covariance structure. PMID:25345738

  19. First-principles insights into interaction of Au with small Co clusters

    NASA Astrophysics Data System (ADS)

    Aghajani, Mahdieh; Javad Hashemifar, S.; Akbarzadeh, Hadi

    2014-08-01

    The effects of Au doping on structural, electronic, magnetic, and dynamical properties of the Con nano-clusters are investigated by using density functional computations and numeric atom-centered orbitals method. The Au doping that enhances stability of the planar structures of the Con clusters over their 3-dimensional structures is discussed. Doping with the nonmagnetic Au atom is found to decrease the spin moments of the Con clusters. The calculated binding energies and their various derivatives are used to investigate the structural stability of the doped clusters. The electronic energy levels of the clusters are accurately determined and discussed after applying the many body based GW correction. The vibrational and infrared spectra of the doped clusters are calculated to provide spectroscopic finger print of these clusters.

  20. Spatial clustering of Diabrotica virgifera virgifera and Agriotes ustulatus in small-scale maize fields without topographic relief drift

    Technology Transfer Automated Retrieval System (TEKTRAN)

    The soil living larvae of Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) and Agriotes ustulatus Schaller (Elateridae) can cause economic damage to maize roots. This study investigated the spatial clustering of both pests in four small-scale maize fields in southern Hungary, wher...

  1. A metal-organic framework constructed using a flexible tripodal ligand and tetranuclear copper cluster for sensing small molecules.

    PubMed

    Hou, Chaoyi; Bai, Yue-Ling; Bao, XiaoLi; Xu, Liangzhen; Lin, Rong-Guang; Zhu, Shourong; Fang, Jianhui; Xu, Jiaqiang

    2015-05-01

    A new porous metal-organic framework (MOF) {[Cu4(OH)2(tci)2(bpy)2]·11H2O} (1) based on a tetranuclear copper cluster with intracluster antiferromagnetic interactions was synthesized. Quartz crystal microbalance (QCM) sensor studies reveal sensitive and selective sensing for small molecules. PMID:25857286

  2. Kinetic-theory predictions of clustering instabilities in granular flows: beyond the small-Knudsen-number regime

    SciTech Connect

    Mitrano, Peter P.; Zenk, John R.; Benyahia, Sofiane; Galvin, Janine E.; Dahl, Steven R.; Hrenya, Christine M.

    2013-12-04

    In this work we quantitatively assess, via instabilities, a Navier–Stokes-order (small- Knudsen-number) continuum model based on the kinetic theory analogy and applied to inelastic spheres in a homogeneous cooling system. Dissipative collisions are known to give rise to instabilities, namely velocity vortices and particle clusters, for sufficiently large domains. We compare predictions for the critical length scales required for particle clustering obtained from transient simulations using the continuum model with molecular dynamics (MD) simulations. The agreement between continuum simulations and MD simulations is excellent, particularly given the presence of well-developed velocity vortices at the onset of clustering. More specifically, spatial mapping of the local velocity-field Knudsen numbers (Knu) at the time of cluster detection reveals Knu » 1 due to the presence of large velocity gradients associated with vortices. Although kinetic-theory-based continuum models are based on a small- Kn (i.e. small-gradient) assumption, our findings suggest that, similar to molecular gases, Navier–Stokes-order (small-Kn) theories are surprisingly accurate outside their expected range of validity.

  3. Silicon Carbide Clusters Found in the Canyon Diablo Meteorite: Implications of Cooling Histories for Group IAB Meteorites

    NASA Astrophysics Data System (ADS)

    Leung, I. S.; Winston, R.

    2009-12-01

    The Canyon Diablo Meteorite fell in the Arizona desert 50,000 years ago. Meteoritic irons tranported to humid areas often oxidize rapidly. One of our samples was a carbon nodule in a rusty Ni-Fe matrix. Another nodule we studied, retrieved by cutting open a fresh iron sample with a diamond-impregnated blade, was about 1 cm in diameter, with rdiating black veins. A hammer and carbide chisels were used to break up the nodules. Micron-sized grains in a Petri dish were hand-picked under a microscope.We found 8 individual silicon carbide (SiC) crystals which are either light blue, deep blue, light green, or deep green, and they are 80-120 microns in size. We also found 14 clusters of acicular or mosaic aggregates, 50-150 microns in size. A green mosaic contains more than 20 grains having black carbon rims. An X-ray study revealed that the individual crystals have well-ordered 3C, 6H, and 15R polytype structures. We interpret this as an indication of slow growth for a rather long period of time. On the other hand, the SiC aggregates seem to have nucleated rapidly in a chemically oversaturated environment, perhaps during a disturbance at a relatively recent time. Further work might help elucidate cooling, evolution and complex histories of IAB iron meteorites. It should be cautioned that if dissolution methods using strong acids to separate SiC would have destroyed the delicate aggregates, and disaggregated grains might have been classified as nanno-carbides, thus, an important aspect of history might have been obliterated.

  4. Stochastic dynamics of small ensembles of non-processive molecular motors: The parallel cluster model

    NASA Astrophysics Data System (ADS)

    Erdmann, Thorsten; Albert, Philipp J.; Schwarz, Ulrich S.

    2013-11-01

    Non-processive molecular motors have to work together in ensembles in order to generate appreciable levels of force or movement. In skeletal muscle, for example, hundreds of myosin II molecules cooperate in thick filaments. In non-muscle cells, by contrast, small groups with few tens of non-muscle myosin II motors contribute to essential cellular processes such as transport, shape changes, or mechanosensing. Here we introduce a detailed and analytically tractable model for this important situation. Using a three-state crossbridge model for the myosin II motor cycle and exploiting the assumptions of fast power stroke kinetics and equal load sharing between motors in equivalent states, we reduce the stochastic reaction network to a one-step master equation for the binding and unbinding dynamics (parallel cluster model) and derive the rules for ensemble movement. We find that for constant external load, ensemble dynamics is strongly shaped by the catch bond character of myosin II, which leads to an increase of the fraction of bound motors under load and thus to firm attachment even for small ensembles. This adaptation to load results in a concave force-velocity relation described by a Hill relation. For external load provided by a linear spring, myosin II ensembles dynamically adjust themselves towards an isometric state with constant average position and load. The dynamics of the ensembles is now determined mainly by the distribution of motors over the different kinds of bound states. For increasing stiffness of the external spring, there is a sharp transition beyond which myosin II can no longer perform the power stroke. Slow unbinding from the pre-power-stroke state protects the ensembles against detachment.

  5. Stochastic dynamics of small ensembles of non-processive molecular motors: The parallel cluster model

    SciTech Connect

    Erdmann, Thorsten; Albert, Philipp J.; Schwarz, Ulrich S.

    2013-11-07

    Non-processive molecular motors have to work together in ensembles in order to generate appreciable levels of force or movement. In skeletal muscle, for example, hundreds of myosin II molecules cooperate in thick filaments. In non-muscle cells, by contrast, small groups with few tens of non-muscle myosin II motors contribute to essential cellular processes such as transport, shape changes, or mechanosensing. Here we introduce a detailed and analytically tractable model for this important situation. Using a three-state crossbridge model for the myosin II motor cycle and exploiting the assumptions of fast power stroke kinetics and equal load sharing between motors in equivalent states, we reduce the stochastic reaction network to a one-step master equation for the binding and unbinding dynamics (parallel cluster model) and derive the rules for ensemble movement. We find that for constant external load, ensemble dynamics is strongly shaped by the catch bond character of myosin II, which leads to an increase of the fraction of bound motors under load and thus to firm attachment even for small ensembles. This adaptation to load results in a concave force-velocity relation described by a Hill relation. For external load provided by a linear spring, myosin II ensembles dynamically adjust themselves towards an isometric state with constant average position and load. The dynamics of the ensembles is now determined mainly by the distribution of motors over the different kinds of bound states. For increasing stiffness of the external spring, there is a sharp transition beyond which myosin II can no longer perform the power stroke. Slow unbinding from the pre-power-stroke state protects the ensembles against detachment.

  6. Small-Scale Interstellar Structure Toward the Open Cluster CHI Persei-Fuse II

    NASA Technical Reports Server (NTRS)

    Sonneborn, George (Technical Monitor); Friedman, Scott

    2003-01-01

    The purpose of this study was to measure the physical conditions of gas along sight lines toward 6 stars in the core Chi Persei open cluster. These sight lines traverse gas in both the Orion and Perseus spiral arms of the Galaxy, at distances of 500 and 2000 pc, respectively. The stars have angular separations ranging from 45 to 280 arcsec; 60 arcsec corresponds to linear distances of 0.15 and 0.6 pc in the two arms. Thus, abundance variations in these observations would constitute evidence for small-scale variations in the properties of the interstellar medium. Ground-based Na I observations at high resolution (approx. 15 km/sec) toward 172 stars (including the 6 in this study) in the double open cluster h and Chi Persei have revealed complex spatial variation. These variations are especially evident in the gas at velocities of -40 and -55 km/sec, corresponding to the Perseus spiral arm. 21 cm observations of HI emission using the Low Resolution DRAO Survey, with a 12-arcmin beam, also show variations. Averaging the Na I apparent optical depth profiles of neighboring sight lines in order to mimic such a beam size reduces the variation, as compared to the individual Na I measurements, but still show variations larger than seen in the 21 cm profiles. Na I is not the dominant ionization state of Na in the interstellar medium. Thus, it is possible that the variations seen really trace physical structures in the interstellar medium, or they may simply result from variations in the radiation field seen by the gas, or be due to some other environmental circumstance. To distinguish among these possibilities in the present study we obtained FUSE spectra toward the 6 targets in order to measure the molecular hydrogen absorption profiles along these sight lines. The higher J states of H2 are populated by the ambient W radiation field, and thus can provide insight into the environment affecting the gas. If both the high and low J states reveal absorption line profiles with

  7. Emergence of Small-World Anatomical Networks in Self-Organizing Clustered Neuronal Cultures

    PubMed Central

    de Santos-Sierra, Daniel; Sendiña-Nadal, Irene; Leyva, Inmaculada; Almendral, Juan A.; Anava, Sarit; Ayali, Amir; Papo, David; Boccaletti, Stefano

    2014-01-01

    In vitro primary cultures of dissociated invertebrate neurons from locust ganglia are used to experimentally investigate the morphological evolution of assemblies of living neurons, as they self-organize from collections of separated cells into elaborated, clustered, networks. At all the different stages of the culture's development, identification of neurons' and neurites' location by means of a dedicated software allows to ultimately extract an adjacency matrix from each image of the culture. In turn, a systematic statistical analysis of a group of topological observables grants us the possibility of quantifying and tracking the progression of the main network's characteristics during the self-organization process of the culture. Our results point to the existence of a particular state corresponding to a small-world network configuration, in which several relevant graph's micro- and meso-scale properties emerge. Finally, we identify the main physical processes ruling the culture's morphological transformations, and embed them into a simplified growth model qualitatively reproducing the overall set of experimental observations. PMID:24489675

  8. [Dynamic study of small metallic clusters]; Estudio Dinamico de Pequenos Agregados Metalicos

    SciTech Connect

    Lopez, M.J.; Jellinek, J.

    1995-12-31

    We present a brief introduction to computer simulation techniques (particularly to classical molecular dynamics) and their application to the study of the thermodynamic properties of a material system. The basic concepts are illustrated in the study of structural and energetic properties such as the liquid-solid transition and the fragmentation of small clusters of nickel. [Espanol] Presentamos una breve introducci{acute o}n de las t{acute e}cnicas de simulaci{acute o}n por ordenador (en particular de la Din{acute a}mica Molecular cl{acute a}sica) y de su aplicaci{acute o}n al estudio de las propiedades termodin{acute a}micas de un sistema material. Los conceptos b{acute a}sicos se ilustran en el estudio de las propieades estructurales y energ{acute e}ticas, as{acute i} como de la transici{acute o}n de fase s{acute o}lido-l{acute i}quido y de las fragmentaciones de peque{tilde n}os agregados de n{acute i}quel.

  9. Clustering on very small scales from a large, complete sample of confirmed quasar pairs

    NASA Astrophysics Data System (ADS)

    Eftekharzadeh, Sarah; Myers, Adam D.; Djorgovski, Stanislav G.; Graham, Matthew J.; Hennawi, Joseph F.; Mahabal, Ashish A.; Richards, Gordon T.

    2016-06-01

    We present by far the largest sample of spectroscopically confirmed binaryquasars with proper transverse separations of 17.0 ≤ Rprop ≤ 36.6 h-1 kpc. Our sample, whichis an order-of-magnitude larger than previous samples, is selected from Sloan Digital Sky Survey (SDSS) imaging over an area corresponding to the SDSS 6th data release (DR6). Our quasars are targeted using a Kernel Density Estimation technique (KDE), and confirmed using long-slit spectroscopy on a range of facilities.Our most complete sub-sample of 44 binary quasars with g<20.85, extends across angular scales of 2.9" < Δθ < 6.3", and is targeted from a parent sample that would be equivalent to a full spectroscopic survey of nearly 300,000 quasars.We determine the projected correlation function of quasars (\\bar Wp) over proper transverse scales of 17.0 ≤ Rprop ≤ 36.6 h-1 kpc, and also in 4 bins of scale within this complete range.To investigate the redshift evolution of quasar clustering on small scales, we make the first self-consistent measurement of the projected quasar correlation function in 4 bins of redshift over 0.4 ≤ z ≤ 2.3.

  10. Ab initio study of small Au nY 2 ( n=1-4) clusters

    NASA Astrophysics Data System (ADS)

    Jian-Jun, Guo; Ji-Xian, Yang; Dong, Die

    2008-11-01

    The geometries of the lowest-lying isomers of Au nY 2 ( n=1-4) clusters are determined systematically via the density functional method B3LYP with relativistic effective core potentials (RECP) and LANL2DZ basis set. Several low-lying isomers were determined, and many of them in electronic configurations with a high-spin multiplicity. The gold-yttrium interaction is strong enough to modify the known growth pattern of bare gold clusters. The stability trend of Y-doped Au n clusters is compared to that of pure Au n clusters. The results show that the inclusion of two Y atoms in the clusters improves the cluster stability, and indicate higher stability as the structures grow in size. The present calculations are useful to understand the enhanced catalytic activity and selectivity gained by using yttrium-doped gold catalyst.

  11. The small iron-sulfur protein from the ORP operon binds a [2Fe-2S] cluster.

    PubMed

    Maiti, Biplab K; Moura, Isabel; Moura, José J G; Pauleta, Sofia R

    2016-09-01

    A linear cluster formulated as [S2MoS2CuS2MoS2](3-), a unique heterometallic cluster found in biological systems, was identified in a small monomeric protein (named as Orange Protein). The gene coding for this protein is part of an operon mainly present in strict anaerobic bacteria, which is composed (in its core) by genes coding for the Orange Protein and two ATPase proposed to contain Fe-S clusters. In Desulfovibrio desulfuricans G20, there is an ORF, Dde_3197 that encodes a small protein containing several cysteine residues in its primary sequence. The heterologously produced Dde_3197 aggregates mostly in inclusion bodies and was isolated by unfolding with a chaotropic agent and refolding by dialysis. The refolded protein contained sub-stoichiometric amounts of iron atoms/protein (0.5±0.2), but after reconstitution with iron and sulfide, high iron load contents were detected (1.8±0.1 or 3.4±0.2) using 2- and 4-fold iron excess. The visible absorption spectral features of the iron-sulfur clusters in refolded and reconstituted Dde_3197 are similar and resemble the ones of [2Fe-2S] cluster containing proteins. The refolded and reconstituted [2Fe-2S] Dde_3197 are EPR silent, but after reduction with dithionite, a rhombic signal is observed with gmax=2.00, gmed=1.95 and gmin=1.92, consistent with a one-electron reduction of a [2Fe-2S](2+) cluster into a [2Fe-2S](1+) state, with an electron spin of S=½. The data suggests that Dde_3197 can harbor one or two [2Fe-2S] clusters, one being stable and the other labile, with quite identical spectroscopic properties, but stable to oxygen. PMID:27240719

  12. Density of resonant states and a manifestation of photonic band structure in small clusters of spherical particles

    NASA Astrophysics Data System (ADS)

    Yamilov, Alexey; Cao, Hui

    2003-08-01

    We introduce a numerical recipe for calculating the density of the resonant states of the clusters of dielectric spheres. Using truncated multipole expansions (generalized multisphere Mie solution) we obtain the scattering matrix of the problem. By introducing an infinitesimal absorption in the spheres we express the dwell time of the electromagnetic wave in terms of the elements of the scattering matrix. Using the parameters in recent light localization experiments [Phys. Rev. Lett. 87, 153901 (2001)], we demonstrate that the density of the resonant states, related to the dwell time, shows the formation of the photonic band structure in small clusters of dielectric spheres as the small as five particles. Density of resonant states of a cluster of 32 spheres exhibits a well defined structure similar to the density of electromagnetic states of the infinite photonic crystal. Our results suggest that, due to the formation of small ordered clusters, a significant modification of the density of electromagnetic states can occur in a random collection of monodisperse spheres.

  13. The VMC Survey - XXI. New star cluster candidates discovered from infrared photometry in the Small Magellanic Cloud

    NASA Astrophysics Data System (ADS)

    Piatti, Andrés E.; Ivanov, Valentin D.; Rubele, Stefano; Marconi, Marcella; Ripepi, Vincenzo; Cioni, Maria-Rosa L.; Oliveira, Joana M.; Bekki, Kenji

    2016-07-01

    We report the first search for new star clusters performed using the VISTA near-infrared YJKs Magellanic Clouds survey (VMC) data sets. We chose a pilot field of ˜0.4 deg2 located in the South-west of the Small Magelllanic Cloud bar, where the star field is among the densest and highest reddened region in the galaxy. In order to devise an appropriate automatic procedure we made use of dimensions and stellar densities observed in the VMC data sets of the known clusters in this area. We executed different kernel density estimations over a sample of more than 358 000 stars with magnitudes measured in the three YJKs filters. We analysed the new cluster candidates whose colour-magnitude diagrams (CMDs), cleaned from field star contamination, were used to assess the clusters' reality and estimate reddenings and ages of the genuine systems. As a result 38 objects (≈ a 55 per cent increase in the known star clusters located in the surveyed field) of 0.15-0.40 arcmin (2.6-7.0 pc) in radius resulted to have near-infrared CMD features which resemble those of star clusters of young to moderate intermediate age (log(t yr-1) ˜7.5-9.0). Most of the new star cluster candidates are hardly recognizable in optical images without the help of a sound star field decontaminated CMD analysis. For highly reddened star cluster candidates (E(B - V) ≥ 0.6 mag) the VMC data sets were necessary in order to recognize them.

  14. The VMC Survey - XXI. New star cluster candidates discovered from infrared photometry in the Small Magellanic Cloud

    NASA Astrophysics Data System (ADS)

    Piatti, Andrés E.; Ivanov, Valentin D.; Rubele, Stefano; Marconi, Marcella; Ripepi, Vincenzo; Cioni, Maria-Rosa L.; Oliveira, Joana M.; Bekki, Kenji

    2016-07-01

    We report the first search for new star clusters performed using the VISTA near-infrared YJKs Magellanic Clouds survey (VMC) data sets. We chose a pilot field of ~ 0.4 deg^2 located in the South-West of the Small Magelllanic Cloud (SMC) bar, where the star field is among the densest and highest reddened region in the galaxy. In order to devise an appropriate automatic procedure we made use of dimensions and stellar densities observed in the VMC data sets of the known clusters in this area. We executed different kernel density estimations over a sample of more than 358000 stars with magnitudes measured in the three YJKs filters. We analysed the new cluster candidates whose colour-magnitude diagrams (CMDs), cleaned from field star contamination, were used to assess the clusters' reality and estimate reddenings and ages of the genuine systems. As a result 38 objects (~ a 55% increase in the known star clusters located in the surveyed field) of 0.15 - 0.40 arcmin (2.6 - 7.0 pc) in radius resulted to have near-infrared CMD features which resemble those of star clusters of young to moderate intermediate age (log(t /yr ) ~ 7.5-9.0). Most of the new star cluster candidates are hardly recognizable in optical images without the help of a sound star field decontaminated CMD analysis. For highly reddened star cluster candidates (E(B - V ) >= 0.6 mag) the VMC data sets were necessary in order to recognize them.

  15. The VMC Survey - XXI. New star cluster candidates discovered from infrared photometry in the Small Magellanic Cloud

    NASA Astrophysics Data System (ADS)

    Piatti, Andrés E.; Ivanov, Valentin D.; Rubele, Stefano; Marconi, Marcella; Ripepi, Vincenzo; Cioni, Maria-Rosa L.; Oliveira, Joana M.; Bekki, Kenji

    2016-04-01

    We report the first search for new star clusters performed using the VISTA near-infrared YJKs Magellanic Clouds survey (VMC) data sets. We chose a pilot field of ˜ 0.4 deg2 located in the South-West of the Small Magelllanic Cloud (SMC) bar, where the star field is among the densest and highest reddened region in the galaxy. In order to devise an appropriate automatic procedure we made use of dimensions and stellar densities observed in the VMC data sets of the known clusters in this area. We executed different kernel density estimations over a sample of more than 358000 stars with magnitudes measured in the three YJKs filters. We analysed the new cluster candidates whose colour-magnitude diagrams (CMDs), cleaned from field star contamination, were used to assess the clusters' reality and estimate reddenings and ages of the genuine systems. As a result 38 objects (≈ a 55% increase in the known star clusters located in the surveyed field) of 0.15 - 0.40 arcmin (2.6 - 7.0 pc) in radius resulted to have near-infrared CMD features which resemble those of star clusters of young to moderate intermediate age (log(t yr-1) ˜ 7.5-9.0). Most of the new star cluster candidates are hardly recognizable in optical images without the help of a sound star field decontaminated CMD analysis. For highly reddened star cluster candidates (E(B - V) ≥ 0.6 mag) the VMC data sets were necessary in order to recognize them.

  16. Reactivity of oxygen deficient cerium oxide clusters with small gaseous molecules.

    PubMed

    Nagata, Toshiaki; Miyajima, Ken; Hardy, Robert Allan; Metha, Gregory F; Mafuné, Fumitaka

    2015-06-01

    Oxygen deficient cerium oxide cluster ions, Ce(n)O(m)(+) (n = 2-10, m = 1-2n) were prepared in the gas phase by laser ablation of a cerium oxide rod. The reactivity of the cluster ions was investigated using mass spectrometry, finding that oxygen deficient clusters are able to extract oxygen atoms from CO, CO2, NO, N2O, and O2 in the gas phase. The oxygen transfer reaction is explained in terms of the energy balance between the bond dissociation energy of an oxygen containing molecule and the oxygen affinity of the oxygen-deficient cerium oxide clusters, which is supported by DFT calculations. The reverse reaction, i.e., formation of the oxygen deficient cluster ions from the stoichiometric ones was also examined. It was found that intensive heating of the stoichiometric clusters results in formation of oxygen deficient clusters via Ce(n)O(2n)(+) → Ce(n)O(2n-2)(+) + O2, which was found to occur at different temperatures depending on cluster size, n. PMID:25965076

  17. PRESENT-DAY MASS FUNCTION OF SIX SMALL MAGELLANIC CLOUD INTERMEDIATE-AGE AND OLD STAR CLUSTERS

    SciTech Connect

    Glatt, Katharina; Grebel, Eva K.; Jordi, Katrin; Gallagher, John S. III; Harbeck, Daniel; Da Costa, Gary; Clementini, Gisella; Tosi, Monica; Nota, Antonella; Sabbi, Elena; Sirianni, Marco

    2011-08-15

    We determined the present-day mass functions (PDMFs) of the five intermediate-age star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, and Lindsay 38 and the old star cluster NGC 121 in the Small Magellanic Cloud (SMC) based on observations with the Hubble Space Telescope Advanced Camera for Surveys. The global PDMFs are well matched by Salpeter-like power laws from their main-sequence turnoffs to {approx}0.6 M{sub sun} with a power-law exponent {alpha} ranging from 1.51 {+-} 0.11 (Lindsay 1) to 2.29 {+-} 0.15 (NGC 339). We derive total stellar masses of {approx}10{sup 5} M{sub sun}, except for Lindsay 38, whose mass is of the order of {approx}10{sup 4} M{sub sun}. Differences between the PDMFs most likely reflect the varying stages of dynamical evolution of the clusters. These SMC clusters do not follow the {alpha} versus concentration parameter c correlation as found for Galactic globular clusters of similar mass. This might be an age effect or due to their location in a galaxy where bulge and disk crossings do not play a role. No correlation is found between {alpha} and the cluster core and tidal radii (r{sub c} and r{sub t} , respectively), the half-light radii r{sub h} , age, central surface brightness, metallicity, and galactocentric radius r{sub gc}. All six clusters mass-segregated to different degrees. The two clusters Lindsay 1 and Kron 3 barely show signs for mass segregation, but have low-mass star deficient global PDMFs and might be the remnants of star clusters whose outer parts were stripped. A trend exists between the degree of mass segregation and the ratio age/relaxation time t{sub r,h}, which indicates the stage of dynamical evolution for a cluster. Our data thus suggest that the SMC clusters in the present sample had a range of initial densities and presumably different amounts of mass loss that led to different rates of dynamical evolution. The clusters' positions in the r{sub h,m}/r{sub t} versus r{sub 0}/r{sub h,m} plane imply that all of the

  18. Velocity bias from the small-scale clustering of SDSS-III BOSS galaxies

    NASA Astrophysics Data System (ADS)

    Guo, Hong; Zheng, Zheng; Zehavi, Idit; Dawson, Kyle; Skibba, Ramin A.; Tinker, Jeremy L.; Weinberg, David H.; White, Martin; Schneider, Donald P.

    2015-01-01

    We present the measurements and modelling of the projected and redshift-space clustering of CMASS galaxies in the Sloan Digital Sky Survey-III Baryon Oscillation Spectroscopic Survey Data Release 11. For a volume-limited luminous red galaxy sample in the redshift range of 0.48 < z < 0.55, we perform halo occupation distribution modelling of the small- and intermediate-scale (0.1-60 h-1 Mpc) projected and redshift-space two-point correlation functions, with an accurate model built on high-resolution N-body simulations. To interpret the measured redshift-space distortions, the distribution of galaxy velocities must differ from that of the dark matter inside haloes of ˜1013-1014 h-1 M⊙, i.e. the data require the existence of galaxy velocity bias. Most notably, central galaxies on average are not at rest with respect to the core of their host haloes, but rather move around it with a 1D velocity dispersion of 0.22^{+0.03}_{-0.04} times that of the dark matter, implying a spatial offset from the centre at the level of ≲1 per cent of the halo virial radius. The luminous satellite galaxies move more slowly than the dark matter, with velocities 0.86^{+0.08}_{-0.03} times those of the dark matter, which suggests that the velocity and spatial distributions of these satellites cannot both be unbiased. The constraints mainly arise from the Fingers-of-God effect at non-linear scales and the smoothing to the Kaiser effect in the translinear regime; the robustness of the results is demonstrated by a variety of tests. We discuss the implications of the existence of galaxy velocity bias for investigations of galaxy formation and cosmology.

  19. Solvation of cyclopentadienyl and substituted cyclopentadienyl radicals in small clusters. I. Nonpolar solvents

    NASA Astrophysics Data System (ADS)

    Fernandez, J. A.; Yao, J.; Bernstein, E. R.

    1999-03-01

    Cyclopentadienyl (cpd), methylcpd (mcpd), fluorocpd (Fcpd), and cyanocpd (CNcpd) are generated photolytically, cooled in a supersonic expansion, and clustered with nonpolar solvents. The solvents employed are Ar, N2, CH4, CF4, and C2F6. These radicals and their clusters are studied by a number of laser spectroscopic techniques: Fluorescence excitation (FE), hole burning (HB), and mass resolved excitation (MRE) spectroscopies, and excited state lifetime studies. The radical D1←D0 transition is observed for these systems: The radical to cluster spectroscopic shifts for the clusters are quite large, typically 4 to 5 times those found for stable aromatic species and other radicals. Calculations of cluster structure are carried out for these systems using parameterized potential energy functions. Cluster geometries are similar for all clusters with the solvent placed over the cpd ring and the center-of-mass of the solvent displaced toward the substituent. The calculated cluster spectroscopic shifts are in reasonable agreement with the observed ones for N2 and CF4 with all radicals, but not for C2F6 with the radicals. The Xcpd/Ar data are sacrificed to generate excited state potential parameters for these systems. CH4 is suggested to react with all but the CNcpd radical and may begin to react even with CNcpd. van der Waals vibrations are calculated for these clusters in the harmonic approximation for both D1 and D0 electronic states; calculated van der Waals vibrational energies are employed to assign major cluster vibronic features in the observed spectra.

  20. Quantum model of a solid-state spin qubit: Ni cluster on a silicon surface by the generalized spin Hamiltonian and X-ray absorption spectroscopy investigations

    NASA Astrophysics Data System (ADS)

    Farberovich, Oleg V.; Mazalova, Victoria L.; Soldatov, Alexander V.

    2015-11-01

    We present here the quantum model of a Ni solid-state electron spin qubit on a silicon surface with the use of a density-functional scheme for the calculation of the exchange integrals in the non-collinear spin configurations in the generalized spin Hamiltonian (GSH) with the anisotropic exchange coupling parameters linking the nickel ions with a silicon substrate. In this model the interaction of a spin qubit with substrate is considered in GSH at the calculation of exchange integrals Jij of the nanosystem Ni7-Si in the one-electron approach taking into account chemical bonds of all Si-atoms of a substrate (environment) with atoms of the Ni7-cluster. The energy pattern was found from the effective GSH Hamiltonian acting in the restricted spin space of the Ni ions by the application of the irreducible tensor operators (ITO) technique. In this paper we offer the model of the quantum solid-state N-spin qubit based on the studying of the spin structure and the spin-dynamics simulations of the 3d-metal Ni clusters on the silicon surface. The solution of the problem of the entanglement between spin states in the N-spin systems is becoming more interesting when considering clusters or molecules with a spectral gap in their density of states. For quantifying the distribution of the entanglement between the individual spin eigenvalues (modes) in the spin structure of the N-spin system we use the density of entanglement (DOE). In this study we have developed and used the advanced high-precision numerical techniques to accurately assess the details of the decoherence process governing the dynamics of the N-spin qubits interacting with a silicon surface. We have studied the Rabi oscillations to evaluate the N-spin qubits system as a function of the time and the magnetic field. We have observed the stabilized Rabi oscillations and have stabilized the quantum dynamical qubit state and Rabi driving after a fixed time (0.327 μs). The comparison of the energy pattern with the

  1. The Cologne Carbon Cluster Experiment: ro-vibrational spectroscopy on C 8 and other small carbon clusters

    NASA Astrophysics Data System (ADS)

    Neubauer-Guenther, P.; Giesen, T. F.; Berndt, U.; Fuchs, G.; Winnewisser, G.

    2003-02-01

    We report on our ongoing efforts in obtaining the IR-spectra of the linear carbon cluster molecules C n with n=8-13. So far C 8, C 9, C 10, and C 13 have been recorded at Cologne. With the exception of C 8 all assignments have been secured. For C 8 a tentative assignment could be derived with the bandcenter of the σu antisymmetric stretching mode located at ν0=2067.9779 cm -1 and a preliminary rotational constant in the vibrational ground state of B″=0.02068 cm -1. The measured signal to noise ratio of the ro-vibrational band is fairly weak and thus the lower J ro-vibrational transitions can not be assigned with certainty. As a consequence the band center remains uncertain by 4 J or 0.17 cm -1. For a more reliable assignment the sensitivity of the system has to be increased by at least one order of magnitude. The envisaged sensitivity increase of our experiment will be discussed along with the intention to perform terahertz observations of the low energetic bending ro-vibrational spectra. These sub-mm wave measurements will be carried out simultaneously with the IR measurements.

  2. Structure, energetic and phase transition of small nickel-palladium heterogeneous clusters

    NASA Astrophysics Data System (ADS)

    Hewage, Jinasena W.

    2014-06-01

    Molecular dynamics simulation (MD) with Sutton-Chen potential for palladium-palladium, nickel-nickel and palladium-nickel interactions has been used to generate the minimum energy structures and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of Ni n Pd(13- n) for n ≤ 13. Thirteen particle icosahedral clusters of neat palladium and nickel atoms were first reproduced accordingly with the results in literature. Then in the palladium icosahedra, each palladium atom has been successively replaced by nickel atom. Calculation is repeated for both palladium-centered and nickel-centered clusters. It is found that the nickel-centered clusters are more stable than the palladium-centered clusters and cohesive energy increases along the palladium end to nickel end. Phase transition of each cluster from one end-species to the other end-species is studied by means of caloric curve, root mean square bond fluctuation and heat capacity. Trend in variation of melting temperature is opposite to the energy trend. Palladium-centered cluster shows a premelting at low temperature due to the solid-solid structural transition. Species-centric order parameters developed by Hewage and Amar is used to understand the dynamic behavior in the solid-solid transition of palladium-centered cluster to more stable nickel-centered cluster (premelting). This species-centric order parameter calculation further confirmed the stability of nickel-centered species over those of palladium-centered species and solid-solid structural transition at low temperature.

  3. Plasmon-Enhanced Multi-Ionization of Small Metal Clusters in Strong Femtosecond Laser Fields

    NASA Astrophysics Data System (ADS)

    Köller, L.; Schumacher, M.; Köhn, J.; Teuber, S.; Tiggesbäumker, J.; Meiwes-Broer, K. H.

    1999-05-01

    The multiply charging process of platinum cluster ions under intense field conditions show a strong dependence on the width of the femtosecond laser pulse. Increasing the pulse width from 140 to 600 fs while keeping the energy per pulse constant leads to an increase in the highest observed charge state z* of the ejected atoms from z* = 13 to z* = 20. This increased charging efficiency is explained by the evolution of the plasmon energy of the metal cluster upon the change in electron density during the Coulomb explosion process. Thus the time dependence of the charging of a cluster in an intense light field has been observed in real time.

  4. Theoretical studies of the global minima and polarizabilities of small lithium clusters

    NASA Astrophysics Data System (ADS)

    Hu, Han-Shi; Zhao, Ya-Fan; Hammond, Jeff R.; Bylaska, Eric J.; Aprà, Edoardo; van Dam, Hubertus J. J.; Li, Jun; Govind, Niranjan; Kowalski, Karol

    2016-01-01

    Lithium clusters Lin (n = 1-20) have been investigated with density functional theory (DFT) and coupled-cluster (CC) methods. The global minima are located via an improved basin-hopping algorithm. Simulated polarizabilities are in good agreement with the measured data generally. The simulated polarizabilities for Li6, Li12 and Li19 are in reasonable agreement when thermal effects are included, except the Li3 cluster. A linear correlation for the inverse relationship between the CCSD calculated polarizabilities and ionization potential (IP) has been reported to have the linear coefficient of 0.996, which further strengthens our simulations.

  5. Exploration of the mechanism of the hydrolysis of chlorine nitrate in small water clusters using electronic structure methods

    SciTech Connect

    McNamara, J.P.; Hillier, I.H.

    1999-09-09

    High-level electronic structure calculations have been used to study the mechanism of hydrolysis of chlorine nitrate in neutral water clusters containing three to eight solvating water molecules. The calculations clarify some of the current uncertainties in the hydrolysis mechanism. As the size of the water cluster is increased, ClONO{sub 2} shows increasing ionization along the O{sub 2}NO-Cl bond consistent with the proposed predissociation in which the electrophilicity of the chlorine atom is enhanced, thus making it more susceptible to nucleophilic attack from a surface water molecule. A species akin to the experimentally observed intermediate, H{sub 2}OCl{sup +} is found to be stable in a cluster containing eight water molecules. The hydrolysis products, ionized nitric (H{sub 3}O{sup +}/NO{sub 3}{sup {minus}}) and molecular hypochlorous (HOCl) acids, are found to be stable in two different types of structures, containing six and eight water molecules. For the water cluster containing six water molecules, which has a structure related to ordinary hexagonal ice, ClONO{sub 2} is hydrolyzed to yield H{sub 3}O{sup +}/NO{sub 3}{sup {minus}}/HOCl, with essentially no barrier. The calculations thus predict that hydrolysis of ClONO{sub 2} on PSC ice aerosols can proceed spontaneously in small neutral water clusters.

  6. Investigating the borderline between a young star cluster and a small stellar association: a test case with Bochum 1

    NASA Astrophysics Data System (ADS)

    Bica, E.; Bonatto, C.; Dutra, C. M.

    2008-10-01

    Context: Usually, a loose stellar distribution can be classified as an OB stellar group, an association, or a young open cluster. We compare data with the typical OB association Vul OB1. Aims: Here, we discuss the nature of Bochum 1, a typical example of an object affected by the above classification problem. Methods: Field-decontaminated 2MASS photometry is used to analyse colour-magnitude diagrams (CMDs) and stellar radial density profiles (RDPs) of the structures present in the region of Bochum 1. Results: The field-decontaminated CMD of Bochum 1 shows main sequence (MS) and pre-main sequence (PMS) stars. We report two new small angular-size, compact young clusters and one embedded cluster in the area of Bochum 1. Vul OB1 harbours the young open cluster NGC 6823 and the very compact embedded cluster Cr 404. The Vul OB1 association includes the H II region Sh2-86, and its stellar content is younger (≈3 Myr) than that of Bochum 1 (≈9 Myr), which shows no gas emission. Bochum 1 harbours one of the newly found compact clusters as its core. The RDP of Bochum 1 is irregular and cannot be fitted by a King-like profile, which suggests significant erosion or dispersion of stars from a primordial cluster. Similarly to Bochum 1, the decontaminated CMD of NGC 6823 presents conspicuous MS and PMS sequences. Taken separately, RDPs of MS and PMS stars follow a King-like profile. The core shows an excess density of MS stars that mimics the profile of a post-core collapse cluster. At such a young age, it can be explained by an excess of stars formed in the prominent core. Conclusions: The present study suggests that Bochum 1 is a star cluster fossil remain that might be dynamically evolving into an OB association. Bochum 1 may be a missing link connecting early star cluster dissolution with the formation of low-mass OB associations.

  7. A full-configuration-interaction nuclear orbital approach and application for small doped He clusters

    SciTech Connect

    Lara-Castells, M. P. de Aguirre, N. F. Delgado-Barrio, G. Villarreal, P.; Mitrushchenkov, A. O.

    2015-01-22

    An efficient full-configuration-interaction 'nuclear orbital' treatment was developed as a benchmark quantum-chemistry-like method to calculate, ground and excited, fermionic 'solvent' wave-functions and applied to {sup 3}He{sub N} clusters with atomic or molecular impurities [J. Chem. Phys. (Communication) 125, 221101 (2006)]. The main difficulty in handling doped {sup 3}He{sub N} clusters lies in the Fermi-Dirac nuclear statistics, the wide amplitudes of the He-dopant and He-He motions, and the hard-core He-He interaction at short distances. This paper overviews the theoretical approach and its recent applications to energetic, structural and spectroscopic aspects of different dopant-{sup 3}He{sub N} clusters. Preliminary results by using the latest version of the FCI-NO computational implementation, to bosonic Cl{sub 2}(X)-({sup 4}He){sub N} clusters, are also shown.

  8. Hartree-Fock calculation of the differential photoionization cross sections of small Li clusters

    NASA Astrophysics Data System (ADS)

    Galitskiy, S. A.; Artemyev, A. N.; Jänkälä, K.; Lagutin, B. M.; Demekhin, Ph. V.

    2015-01-01

    Cross sections and angular distribution parameters for the single-photon ionization of all electron orbitals of Li2-8 are systematically computed in a broad interval of the photoelectron kinetic energies for the energetically most stable geometry of each cluster. Calculations of the partial photoelectron continuum waves in clusters are carried out by the single center method within the Hartree-Fock approximation. We study photoionization cross sections per one electron and analyze in some details general trends in the photoionization of inner and outer shells with respect to the size and geometry of a cluster. The present differential cross sections computed for Li2 are in a good agreement with the available theoretical data, whereas those computed for Li3-8 clusters can be considered as theoretical predictions.

  9. Hartree-Fock calculation of the differential photoionization cross sections of small Li clusters.

    PubMed

    Galitskiy, S A; Artemyev, A N; Jänkälä, K; Lagutin, B M; Demekhin, Ph V

    2015-01-21

    Cross sections and angular distribution parameters for the single-photon ionization of all electron orbitals of Li2-8 are systematically computed in a broad interval of the photoelectron kinetic energies for the energetically most stable geometry of each cluster. Calculations of the partial photoelectron continuum waves in clusters are carried out by the single center method within the Hartree-Fock approximation. We study photoionization cross sections per one electron and analyze in some details general trends in the photoionization of inner and outer shells with respect to the size and geometry of a cluster. The present differential cross sections computed for Li2 are in a good agreement with the available theoretical data, whereas those computed for Li3-8 clusters can be considered as theoretical predictions. PMID:25612708

  10. A full-configuration-interaction nuclear orbital approach and application for small doped He clusters

    NASA Astrophysics Data System (ADS)

    de Lara-Castells, M. P.; Aguirre, N. F.; Delgado-Barrio, G.; Villarreal, P.; Mitrushchenkov, A. O.

    2015-01-01

    An efficient full-configuration-interaction "nuclear orbital" treatment was developed as a benchmark quantum-chemistry-like method to calculate, ground and excited, fermionic "solvent" wave-functions and applied to 3HeN clusters with atomic or molecular impurities [J. Chem. Phys. (Communication) 125, 221101 (2006)]. The main difficulty in handling doped 3HeN clusters lies in the Fermi-Dirac nuclear statistics, the wide amplitudes of the He-dopant and He-He motions, and the hard-core He-He interaction at short distances. This paper overviews the theoretical approach and its recent applications to energetic, structural and spectroscopic aspects of different dopant-3HeN clusters. Preliminary results by using the latest version of the FCI-NO computational implementation, to bosonic Cl2(X)-(4He )N clusters, are also shown.