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Atomic Configurations (atomic + configuration)
Selected AbstractsUnified quantum molecular dynamics method based on centroid molecular dynamics and semiempirical molecular orbital theoryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2003Yasuhito Ohta Abstract We have developed a unified quantum molecular dynamics method, in which the centroid molecular dynamics including the quantum effect of atomic nuclei is unified with the semiempirical molecular orbital calculation for the atomic configuration generated in the molecular dynamics. This unified method has successfully been applied to a couple of diatomic molecular systems for examining its performance. The method has yielded excellent agreement with exact quantum results and taken sufficiently short CPU time for practical use. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [source] A comparative investigation of the damage build-up in GaN and Si during rare earth ion implantationPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2008Florence Gloux Abstract The medium range implantation of rare earth ions at room temperature in GaN layers leads to the formation of point defect clusters, basal and prismatic stacking faults from the lowest fluence. When a threshold fluence of about 3 × 1015 at/cm2 is reached, a highly disordered ,nanocrystalline layer' (NL) is observed to form at the surface. This layer is made of a mixture of misoriented nanocrystallites and voids. Beyond this NL, I1, I2 and E basal stacking faults (BSFs) have been identified, as well as in GaN implanted at lower fluences than the threshold. Prismatic stacking faults (PSFs) with Drum atomic configuration connect the I1 BSFs. A similar investigation of the damage in Eu implanted Si shows a completely different behaviour; in this case, from the relatively low fluence 1 × 1014 at/cm2, amorphization starts in patches at the projected range and extends very rapidly towards the surface and the bulk, to form a uniform amorphous layer already at 2 × 1014 at/cm2. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Cooling-field dependence of exchange bias and asymmetric reversal modes in a nanoparticles system with ferromagnetic core and antiferromagnetic matrix morphologyPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2009Yong Hu Abstract Interest in exchange bias (EB) in magnetic nanoparticles has increased in the past few years by virtue of its potential for application in fields such as ultrahigh-density magnetic recording. A modified Monte Carlo Metropolis method is performed to simulate the effect of cooling field on EB and asymmetric reversal modes of a granular system of ferromagnetic (FM) nanoparticles embedded in an antiferromagnetic (AFM) matrix, based on three-dimensional classical Heisenberg model. The results show that the EB first decreases slightly due to the energy barriers in the antiferromagnet, while the coercivity and vertical magnetization shift increase with the increase of cooling field, finally, they all level off as the cooling field is strong enough. Whereas the cooling- and measuring-field angular dependence of asymmetric reversal modes reveal asymmetric reversal mechanism and interesting rotation process of FM spins, confirming the existence of the net magnetization on the surface of AFM matrix. The reason may be due to the energy competition and geometric frustration of system. However, the strong interfacial coupling may change the intrinsic atomic configuration of antiferromagnet to influence the EB and reversal modes. [source] Antiferromagnetic-to-ferromagnetic transition induced by aluminum in the Ti(Fe1,xAlx)2 intermetallic compounds: a 57Fe Mössbauer spectroscopy studyPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2007N. N. Delyagin Abstract Mössbauer spectroscopy has been used to investigate the hyperfine field distributions and the local spin configurations of Fe atoms in the magnetically ordered Ti(Fe1,xAlx)2 compounds. The evolution of the spin arrangement of the Fe atoms was observed over the concentration range 0 , x , 0.25 at 5 K. The Al impurity suppresses the ferromagnetic exchange in the Fe(6h) layers and simultaneously provides the conditions for a transition of the Fe(2a) atoms, which are nonmagnetic in the TiFe2 compound, to the states with nonzero magnetic moment. The magnetic behavior of the Fe(2a) atoms is the key factor governing the anti- to ferromagnetic transition in the Ti(Fe1,xAlx)2 compounds. The magnetic ordering temperatures of the Ti(Fe1,xAlx)2 compounds were determined. In addition, the parameters of the electric-field gradient in TiFe2 as well as the relationship between the value of the magnetic moment of the Fe atom and the type of atomic configuration were discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photo-induced improvement of radiative efficiency and structural changes in GaAsN alloysPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2006H. Yaguchi Abstract We have investigated the excitation power density and nitrogen concentration dependence of the changes in the radiative efficiency of GaAsN alloys to examine the mechanism of the photo-induced improvement of radiative efficiency. With increasing excitation power density, the radiative efficiency increased more rapidly. The measure of the improvement Iafter/Ibefore superlinearly increased with increasing nitrogen concentration x up to ,1%. This suggests that the nonradiative recombination centers eliminated by photoexcitation are not defects formed by a single nitrogen atom but complexes formed by gathering of several nitrogen atoms. Micro Raman study revealed that the GaAs-like LO mode phonon peak intensity increased with photoexcitation time in a similar way to the increase in the radiative efficiency. Considering that this phenomenon is in a time scale of several seconds, the photo-induced structural changes correspond not to long range inter-diffusion but to local changes in atomic configuration which lead to the decrease in the density of nonradiative recombination centers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] TEM Study of Precipitates in Melt-Spun Mg-Zn-Y-La Alloys,ADVANCED ENGINEERING MATERIALS, Issue 6 2005J. Rao Recently, nano-crystalline MgZn1Y2 alloys exhibiting excellent tensile yield strength elongation ratio have been developed as a consequence of preliminary investigations of alloy compositions using melt-spun ribbons. Their results show that the novel long-period hexagonal atomic configurations in the Mg-Ln-Zn alloys as well as the atomic scale enrichment of Ln and Zn elements into the misfit atomic array sites indicates the possibility that a high-strength Mg-based alloy is developed by the control of the novel structure and the atomic scale segregation mode. [source] Estimation of the absolute internal-rotation entropy of molecules with two torsional degrees of freedom from stochastic simulationsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2005Eva Darian Abstract A method of statistical estimation is applied to the problem of evaluating the absolute entropy of internal rotation in a molecule with two torsional degrees of freedom. The configurational part of the entropy is obtained as that of the joint probability density of an arbitrary form represented by a two-dimensional Fourier series, the coefficients of which are statistically estimated using a sample of the torsional angles of the molecule obtained by a stochastic simulation. The internal rotors in the molecule are assumed to be attached to a common frame, and their reduced moments of inertia are initially calculated as functions of the two torsional angles, but averaged over all the remaining internal degrees of freedom using the stochastic-simulation sample of the atomic configurations of the molecule. The torsional-angle dependence of the reduced moments of inertia can be also averaged out, and the absolute internal-rotation entropy of the molecule is obtained in a good approximation as the sum of the configurational entropy and a kinetic contribution fully determined by the averaged reduced moments of inertia. The method is illustrated using Monte Carlo simulations of isomers of stilbene and halogenated derivatives of propane. The two torsional angles in cis -stilbene are found to be much more strongly correlated than those in trans -stilbene, while the degree of the angular correlation in propane increases strongly on substitution of hydrogen atoms with chlorine. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 651,660, 2005 [source] Statistical thermodynamics of internal rotation in a hindering potential of mean force obtained from computer simulationsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2003Vladimir Hnizdo Abstract A method of statistical estimation is applied to the problem of one-dimensional internal rotation in a hindering potential of mean force. The hindering potential, which may have a completely general shape, is expanded in a Fourier series, the coefficients of which are estimated by fitting an appropriate statistical,mechanical distribution to the random variable of internal rotation angle. The function of reduced moment of inertia of an internal rotation is averaged over the thermodynamic ensemble of atomic configurations of the molecule obtained in stochastic simulations. When quantum effects are not important, an accurate estimate of the absolute internal rotation entropy of a molecule with a single rotatable bond is obtained. When there is more than one rotatable bond, the "marginal" statistical,mechanical properties corresponding to a given internal rotational degree of freedom are educed. The method is illustrated using Monte Carlo simulations of two public health relevant halocarbon molecules, each having a single internal-rotation degree of freedom, and a molecular dynamics simulation of an immunologically relevant polypeptide, in which several dihedral angles are analyzed. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1172,1183, 2003 [source] Quantitative evaluation of the atomic structure of defects and composition fluctuations at the nanometer scale inside InGaN/GaN heterostructuresPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2004P. Ruterana Abstract Investigation is carried out by high-resolution electron microscopy on threading dislocations using data treatments with procedures that allow the extraction of the most likely atomic configurations. We also report In composition fluctuations inside InGaN/GaN quantum wells by coupling HRTEM, image simulation and Finite Element Modelling (FEM) of the thin foil relaxation. The results show that the indium content may be close to x = 1.0 in the clusters and this is much higher that was previously suggested by 2D FEM modelling. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Structures of stable and metastable Ge2Bi2Te5, an intermetallic compound in a GeTe,Bi2Te3 pseudobinary systemACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2007Toshiyuki Matsunaga Ge2Bi2Te5 in the GeTe,Bi2Te3 pseudobinary system has two single-crystalline phases: a metastable phase with an NaCl-type structure and a stable phase with a nine-layer trigonal structure. In the metastable phase, the structure consists, in the hexagonal notation, of infinitely alternating stacks of Te and Ge/Bi layers at equal intervals along the c axis. On the other hand, in the stable phase those two layers are stacked alternately nine times to form an NaCl block. The blocks are then piled to construct a nine-layered trigonal structure with cubic close-packed stacking. Both ends of each block are covered with Te layers, contrary to the infinite alternation of Ge/Bi and Te layers in the structure of the metastable phase. The Ge/Bi layers in the metastable phase contain as much as 20,at.,% vacancies; on the other hand, those in the stable phase are filled with atoms. These two crystalline phases in Ge2Bi2Te5 have identical atomic configurations to the two corresponding phases found in Ge2Sb2Te5. [source] X-ray study and structure simulation of amorphous tungsten oxideACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2002L. A. Lugovskaya In this work, X-ray studies of the amorphous oxide films obtained by thermal evaporation of WO3 powder in a vacuum and by anodic oxidation were carried out. X-ray diffraction patterns were obtained in the symmetric reflection geometry on a DRON-4 diffractometer (Mo K, radiation, LiF monochromator) in automatic mode. Molecular dynamics simulation of amorphous tungsten oxide atomic configurations has been carried out in the micro-canonical ensemble (NVE) for N,=,208 atoms and N,=,624 atoms, in a cubic cell, using pairwise Born,Mayer interaction potentials and periodic boundary conditions. One of the purposes of the present work is to analyze the influence of the parameters and the cutoff of the interaction potentials on the interatomic distances. The values obtained in the molecular dynamics simulation for the pair functions D(r) are compared with the experimental data for amorphous oxides in order to choose the most convenient aforesaid values. The values of the average interatomic distances and the coordination numbers obtained by both methods are also compared. The result shows that the tungsten subsystem can be well reproduced using the potential cutoff radius of about 4,Å, but the oxygen subsystem can be well reproduced when the cutoff of the potential for the W,O pairs is equal to 2.8,Å. The configuration built during the molecular dynamics experiment consists of distorted octahedra. These octahedra form chains, as in the WO3 phases of type ReO3, and hexagonal rings, of the same type as in the WO3(1/3)H2O phase, when we extract (1/3)O at every WO3 unit. The pair function D(r) and scattering intensity I(s) distribution curves calculated for simulation configurations show a satisfactory agreement with experiment. [source] | |||||||||||||