An implementation of the Reverse Monte Carlo algorithm is presented for study amorphous tetrahedral semiconductors. By taking into account a number constraints that describe bonding geometry along with radial distribution function, we construct model silicon using reverse monte carlo technique. Starting from completely random configuration, generate containing 500 atoms closely reproducing experimental static structure factor and bond angle in improved agreement electronic properties....

Abstract The general and practical inversion of diffraction data–producing a computer model correctly representing the material explored–is an important unsolved problem for disordered materials. Such modeling should proceed by using our full knowledge base, both from experiment theory. In this paper, we describe robust method to jointly exploit power ab initio atomistic simulation along with information carried data. is applied two very different systems: amorphous silicon compositions...

We present a technique for entropy optimization to calculate distribution from its moments. The is based upon maximizing discretized form of the Shannon functional by mapping problem onto dual space where an optimal solution can be constructed iteratively. demonstrate performance and stability our algorithm with several tests on numerically difficult functions. then consider electronic structure application, density states amorphous silica, study convergence Fermi level increasing number

An ideal atomistic model of a disordered material should contradict no experiments,and also be consistent with accurate force fields (either {\it ab initio}or empirical). We make significant progress toward jointly satisfying both} these criteria using hybrid reverse Monte Carlo approach in conjunction approximate first principles molecular dynamics. illustrate the method by studying complex binary glassy g-GeSe$_2$. By constraining to agree partial structure factors and initio} simulation,...

We introduce a structural modeling technique, called force-enhanced atomic refinement (FEAR). The technique incorporates interatomic forces in reverse Monte Carlo (RMC) simulations for by fitting experimental diffraction data using the conventional RMC algorithm, and minimizes total energy from an potential. illustrate usefulness of approach studying $a\text{\ensuremath{-}}{\text{SiO}}_{2}$ $a\text{\ensuremath{-}}\text{Si}$. electronic properties FEAR models agree well with neutron x-ray...

From density-functional calculations, we show that localized states stemming from defects or topological disorder exhibit an anomalously large electron--phonon coupling. We provide a simple analysis to explain the observation and perform detailed study on interesting system: amorphous silicon. compute first-principles deformation potentials (by computing sensitivity of specific electronic eigenstates individual classical normal modes vibration). also probe thermal fluctuations in eigenvalues...

The coupling between lattice vibrations and electrons is one of the central concepts condensed matter physics. subject has been deeply studied for crystalline materials, but far less so amorphous glassy which are among most important applications. In this paper, we explore electron-lattice using current tools a first-principles computer simulation. We choose three materials to illustrate phenomena: silicon (a-Si), selenium (a-Se) gallium nitride (a-GaN). each case, show that there strong...

An 800-atom model of nano-porous carbon obtained from an<italic>ab initio</italic>method. The topology is warped/wrapped amorphous graphene.

Recently it has been shown that binary mixtures of equal-sized fine granular materials exhibit spontaneous separation under vertical vibration in the presence air [Science 295, 1877 (2002)]. Here we describe a model this behavior based on soft-sphere molecular dynamics coupled to motion surrounding air. It exhibits many features observed experimentally including almost complete components into well defined regions with extremely sharp boundaries. The basic mechanism is robust and insensitive...

In this paper, we review a host of methods used to model amorphous materials. We particularly describe which impose constraints on the models ensure that final meets priori requirements (on structure, topology, chemical order, etc). particular, work based quench from melt simulations, 'decorate and relax' method, is shown be reliable scheme for forming certain binary glasses. A 'building block' approach also suggested yields pleading GeSe(1.5). report nature vulcanization in an Se network...

We apply a method called ``force-enhanced atomic refinement'' (FEAR) to create computer model of amorphous silicon $(a\text{-Si})$ based upon the highly precise x-ray diffraction experiments Laaziri et al. [Phys. Rev. Lett. 82, 3460 (1999)]. The logic underlying our calculation is estimate structure real sample $a\text{-Si}$ using experimental data and chemical information included in nonbiased way, starting from random coordinates. close agreement with experiment also sits at suitable...

The structural origin of the first sharp diffraction peak (FSDP) in amorphous silica is studied by analyzing chemical and radial ordering silicon (Si) oxygen (O) atoms binary networks. study shows that order involving Si--O O--O pairs play a major role formation FSDP silica. This supplemented small contributions arising from relatively weak Si--Si correlations Fourier space. A shell-by-shell analysis between Si--Si, Si--O, network reveals position intensity are largely determined atomic pair...

We investigate the electronic structure of gap and band tail states in amorphous silicon. Starting with two 216-atom models silicon defect concentration close to experiments, we systematically study dependence electron localization on basis set, density functional, spin polarization using first-principles density-functional code SIESTA. briefly compare three different schemes for characterizing localization: information entropy, inverse participation ratio, spatial variance. Our results show...

We have extended our experimentally constrained molecular relaxation technique [P. Biswas et al., Phys. Rev. B 71, 54204 (2005)] to hydrogenated amorphous silicon: a 540-atom model with 7.4% hydrogen and 611-atom 22% were constructed. Starting from random configuration, using physically relevant constraints, ab initio interactions, the experimental static structure factor, we construct realistic models of silicon. Our confirm presence high-frequency localized band in vibrational density...

The survival probability P(c,t) of a random walk t steps with static traps at concentration c is studied in two and three dimensions by an efficient Monte Carlo method based on mapping onto polymer model. On the basis theoretical work Donsker Varadhan [Commun. Pure Appl. Math. 28, 525 (1975); 32, 721 (1979)] Rosenstock [J. Phys. (N.Y.) 11, 487 (1970)] one expects data collapse for -ln[P(c,t)]/ln(t) plotted vs square root [lambda t]/ln(t) [with lambda = -ln(1-c)], dimensions,...

Ideal models of complex materials must satisfy all available information about the system. Generally, this consists experimental data, implicit to sophisticated interatomic interactions and potentially other a priori information. By jointly imposing first-principles or tight-binding in conjunction with we have developed method: experimentally constrained molecular relaxation (ECMR) that uses available. We apply method model medium range order amorphous silicon using fluctuation electron...

Abstract Materials with optimized band gap are needed in many specialized applications. In this work, we demonstrate that Hellmann-Feynman forces associated the states can be used to find atomic coordinates yield desired electronic density of states. Using tight-binding models, show approach may arrive at electronically designed models amorphous silicon and carbon. We provide a simple recipe include priori information formation computer materials prove have profound structural consequences....

Despite the current focus on newer materials, silicon-based solar cells are still being developed. The authors study amorphous/crystalline interfaces for based $a$-Si:H/$c$-Si heterojunctions. Toward this end, they perform large-scale $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ simulations of thousands atoms to reveal medium-range order in amorphous hydrated silicon. They find a highly...

A study of the formation voids and molecular hydrogen in hydrogenated amorphous silicon is presented based upon a hybrid approach that involves inversion experimental nuclear magnetic resonance data conjunction with ab initio total-energy relaxations an augmented solution space. The novelty this appear naturally model networks unlike conventional approaches, where are created artificially by removing atoms from networks. Two representative models 16 18 at. % studied work. result shows...

We present a dynamical approach to generate defect-free continuous-random-network (CRN) models of hydrogenated amorphous silicon ($a$-Si:H). Using the atomic coordination number as collective variable and few configurational constraints, we have shown that classical metadynamics can be used construct CRN $a$-Si with arbitrary concentrations dangling-bond defects. These defective networks been subsequently produce high-quality $a$-Si:H using ab initio total-energy calculations hydrogen (H)...

The structure of the first sharp diffraction peak (FSDP) amorphous silicon (${\it a}$-Si) near 2 Angstrom$^{-1}$ is addressed with particular emphasis on position, intensity, and width curve. By studying a number continuous random network (CRN) models ${\it a}$-Si, it shown that position intensity FSDP are primarily determined by radial atomic correlations in length scale 15 Angstroms. A shell-by-shell analysis contribution from different shells reveals key contributions to originate second...

Deletion mutation of the RNA 5' leader sequence simian immunodeficiency virus (SIV) was used to localize packaging signal. sequences upstream major splice donor (SD) site produced a phenotype most consistent with defect when analysed by both RNase protection assay and RT-PCR. Sequences downstream SD were deleted varying effects but did not affect packaging: large deletion had little effect on function, whereas nested profound replication characterized reduced protein production. Secondary...

We propose a novel approach to model amorphous materials using first principles density functional method while simultaneously enforcing agreement with selected experimental data. illustrate our applications silicon and glassy GeSe$_2$. The structural, vibrational electronic properties of the models are found be in results. is general can extended other complex materials.

Conducting bridge random access memory materials have special promise for FLASH memory, other applications beside, and also potential continued miniaturization. They are electronic of unique flexibility. Here, we offer new models Cu‐doped alumina, reveal qualitative differences in the behavior transition metal ions chalcogenide oxide hosts, showing that Cu clusters an amorphous alumina host, contrast with chalcogenides which atoms do not cluster. We further elucidate processes electron...