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Sep 23, 2013
09/13
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D. A. Drabold
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I discuss the properties of electron states in amorphous Si based on large scale calculations with realistic several thousand atom models. A relatively simple model for the localized to extended (Anderson) transition is reviewed. Then, the effect of thermal disorder on localized electron states is considered. It is found that under readily accessible conditions, localized (midgap or band tail) states and their conjugate energies may fluctuate dramatically. The possible importance of...
Source: http://arxiv.org/abs/cond-mat/9909455v1
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Sep 22, 2013
09/13
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MingLiang Zhang; D. A. Drabold
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For a group of charged particles obeying quantum mechanics interacting with an electromagnetic field, the charge and current density in a pure state of the system are expressed with the many-body wave function of the state. Using these as sources, the microscopic Maxwell equations can be written down for any given pure state of a many-body system. By employing semi-classical radiation theory with these sources, the microscopic Maxwell equations can be used to compute the strong radiation fields...
Source: http://arxiv.org/abs/1210.2888v2
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Sep 23, 2013
09/13
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Parthapratim Biswas; D. A. Drabold
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We discuss an inverse approach for atomistic modeling of glassy materials. The focus is on structural modeling and electronic properties of hydrogenated amorphous silicon and glassy GeSe2 alloy. The work is based upon a new approach "experimentally constrained molecular relaxation (ECMR)". Unlike conventional approaches (such as molecular dynamics (MD) and Monte Carlo simulations(MC), where a potential function is specified and the system evolves either deterministically (MD) or...
Source: http://arxiv.org/abs/0801.3685v1
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Sep 21, 2013
09/13
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Mingliang Zhang; D. A. Drabold
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From the change in kinetic energy induced by an external field, we discuss the applicable conditions for the Mott-Davis and Moseley-Lukes form of the Kubo-Greenwood formula (KGF) for the electrical conductivity which has been implemented in \textit{ab initio} codes. We show that the simplified KGF is suitable only for computing the AC conductivity at sufficiently high frequency and when the gradient of the carrier density is small.
Source: http://arxiv.org/abs/1011.4113v1
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Sep 21, 2013
09/13
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MingLiang Zhang; D. A. Drabold
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In this paper, we review and substantially develop the recently proposed "Microscopic Response Method", which has been devised to compute transport coefficients and especially associated temperature dependence in complex materials. The conductivity and Hall mobility of amorphous semiconductors and semiconducting polymers are systematically derived, and shown to be more practical than the Kubo formalism. The effect of a quantized lattice (phonons) on transport coefficients is fully...
Source: http://arxiv.org/abs/1008.1067v3
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Sep 22, 2013
09/13
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Y. Li; D. A. Drabold
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Recently, the prospects for amorphous phases of graphene (a-G) have been explored computationally. Initial models were flat, and contained odd-member rings, while maintaining three-fold coordination and sp2 bonding. Upon relaxation, puckering occurs, and may be traced to the existence of pentagons, in analogy with the situation for fullerenes. In this work, we systematically explore the inherent structures with energy close to the flat starting structure. As expected, the planar symmetry can be...
Source: http://arxiv.org/abs/1210.3553v1
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Jul 20, 2013
07/13
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N. Mousseau; D. A. Drabold
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Many properties of alloyed chalcogenide glasses can be closely correlated with the average coordination of these compounds. This is the case, for example, of the ultrasonic constants, dilatometric softening temperature and the vibrational densities of states. What is striking, however, is that these properties are nevertheless almost independent of the composition at given average coordination. Here, we report on some numerical verification of this experimental rule as applied to vibrational...
Source: http://arxiv.org/abs/cond-mat/9910032v1
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Sep 21, 2013
09/13
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Mingliang Zhang; D. A. Drabold
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Recently we have introduced the microscopic response method (MRM) to compute the conductivity and Hall mobility for complex system with topological and thermal disorder, which is more convenient than the Kubo formula. We prove that for a canonical ensemble the MRM leads to the same expression as the Kubo formula. When the gradient of carrier density is small, the MRM reduces to the widely used Kubo-Greenwood formula.
Source: http://arxiv.org/abs/1011.1527v1
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Sep 23, 2013
09/13
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Serge M. Nakhmanson; D. A. Drabold
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We use empirical molecular dynamics technique to study the low-energy vibrations in a large 4096 atom model for pure amorphous silicon and a set of models with voids of different size based on it. Numerical vibrational eigenvalues and eigenvectors for our models are obtained by exact diagonalization of their dynamical matrices. Our calculations show that localized low-energy vibrational excitations of rather complex structure are present in amorphous silicon models with voids. According to...
Source: http://arxiv.org/abs/cond-mat/9909457v1
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Sep 22, 2013
09/13
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Ming-Liang Zhang; D. A. Drabold
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Inspired by Holstein's work on small polaron hopping, the evolution equations of localized states and extended states in presence of atomic vibrations are derived for an amorphous semiconductor. The transition probabilities are obtained for four types of transitions: from one localized state to another localized state, from a localized state to an extended state, from an extended state to a localized state, and from one extended state to another extended state. At a temperature not too low, any...
Source: http://arxiv.org/abs/0906.1975v1
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Sep 20, 2013
09/13
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T. A. Abtew; D. A. Drabold
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We study the structural, dynamical and electronic properties of amorphous Si\sub{1-x}Ge\sub{x}:H alloys using first principles local basis molecular dynamics simulation. The network topology and defects in the amorphous network have been analyzed. Structural changes and an increase in number of defects have been found as the Ge atomic percentage increases from x=0.1 to x=0.5. The electronic density of states exhibits a decreasing band-gap and increased mid-gap and band-tail defect states as Ge...
Source: http://arxiv.org/abs/cond-mat/0607058v1
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Sep 22, 2013
09/13
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Ming-Liang Zhang; D. A. Drabold
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Starting from Holstein's work on small polaron hopping, the evolution equations for localized and extended states in the presence of atomic vibrations are systematically derived for an amorphous semiconductor. The transition probabilities are obtained for transitions between all combinations of localized and extended states. For any transition process involving a localized state, the activation energy is not simply the energy difference between the final and initial states; the reorganization...
Source: http://arxiv.org/abs/1004.0404v2
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Sep 24, 2013
09/13
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Ming-Liang Zhang; D. A. Drabold
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In this paper, we develop an approximate theory of the temperature coefficient of resistivity (TCR) and conductivity based upon the recently proposed Microscopic Response Method. By introducing suitable approximations for the lattice dynamics, localized and extended electronic states, we produce new explicit forms for the conductivity and TCR, which depend on easily accessible material parameters. The theory is in reasonable agreement with experiments on a-Si:H and a-Ge:H. A long-standing...
Source: http://arxiv.org/abs/1112.4723v2
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Sep 18, 2013
09/13
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T. A. Abtew; D. A. Drabold
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We use accurate first principles methods to study the network dynamics of hydrogenated amorphous silicon, including the motion of hydrogen. In addition to studies of atomic dynamics in the electronic ground state, we also adopt a simple procedure to track the H dynamics in light-excited states. Consistent with recent experiments and computer simulations, we find that dihydride structures are formed for dynamics in the light-excited states, and we give explicit examples of pathways to these...
Source: http://arxiv.org/abs/cond-mat/0606222v1
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Sep 20, 2013
09/13
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Serge M. Nakhmanson; D. A. Drabold
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We have performed an approximate ab initio calculation of vibrational properties of hydrogenated amorphous silicon (a-Si:H) using a molecular dynamics method. A 216 atom model for pure amorphous silicon (a-Si) has been employed as a starting point for our a-Si:H models with voids that were made by removing a cluster of silicon atoms out of the bulk and terminating the resulting dangling bonds with hydrogens. Our calculation shows that the presence of voids leads to localized low energy (30-50...
Source: http://arxiv.org/abs/cond-mat/9810215v1
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Sep 19, 2013
09/13
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S. M. Nakhmanson; D. A. Drabold
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Using empirical potential molecular dynamics we compute dynamical matrix eigenvalues and eigenvectors for a 4096 atom model of amorphous silicon and a set of models with voids of different size based on it. This information is then employed to study the localization properties of the low-energy vibrational states, calculate the specific heat C(T) and examine the low-temperature properties of our models usually attributed to the presence of tunneling states in amorphous silicon. The results of...
Source: http://arxiv.org/abs/cond-mat/9911246v1
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Sep 18, 2013
09/13
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M. -L. Zhang; D. A. Drabold
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By viewing a velocity gradient in a fluid as an internal disturbance and treating it as a constraint on the wave function of a system, a linear evolution equation for the wave function is obtained from the Lagrange multiplier method. It allows us to define the microscopic response to a velocity gradient in a pure state. Taking a spatial coarse-graining average over this microscopic response and averaging it over admissible initial states, we achieve the observed macroscopic response and...
Source: http://arxiv.org/abs/1211.2362v1
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Sep 23, 2013
09/13
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M. -L. Zhang; D. A. Drabold
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eye 55
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We introduce a theoretical framework for computing transport coefficients for complex materials. As a first example, we resolve long-standing inconsistencies between experiment and theory pertaining to the conductivity and Hall mobility for amorphous silicon and show that the Hall sign anomaly is a consequence of localized states. Next, we compute the AC conductivity of amorphous polyanaline. The formalism is applicable to complex materials involving defects and band-tail states originating...
Source: http://arxiv.org/abs/1006.3800v2
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Sep 23, 2013
09/13
by
Ming-Liang Zhang; D. A. Drabold
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We discuss the foundations and extend the range of applicability of the widely used Kubo-Greenwood formula (KGF) for the electronic conductivity. The conductivity is derived from the current density, and only the probability amplitude rather than the transition probability is used. It is shown that the contribution to the conductivity from degenerate states in a low or zero frequency external electric field and the contribution from states near resonance with a finite frequency external field...
Source: http://arxiv.org/abs/0904.0212v1
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56
Sep 23, 2013
09/13
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Yue Pan; Mingliang Zhang; D. A. Drabold
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In this paper, we study several structural models of amorphous silicon, and discuss structural and electronic features common to all. We note spatial correlations between short bonds, and similar correlations between long bonds. Such effects persist under a first principles relaxation of the system and at finite temperature. Next we explore the nature of the band tail states and find the states to possess a filamentary structure. We detail correlations between local geometry and the band tails.
Source: http://arxiv.org/abs/0710.0175v1
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Sep 18, 2013
09/13
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B. Prasai; G. Chen; D. A. Drabold
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We employed ab-initio molecular dynamics to directly simulate the effects of Ag alloying ($\sim5%$ Ag concentration) on the phase change properties of Ge$_{2}$Sb$_{2}$Te$_{5}$. The short range order is preserved, whereas a slight improvement in the chemical order is observed. A slight decrease in the fraction of tetrahedral Ge (sp$^{3}$ bonding) is reflected in the reduction of the optical band gap and in the increased dielectric constant. Simulations of the amorphous to crystalline phase...
Source: http://arxiv.org/abs/1211.3383v1
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Sep 22, 2013
09/13
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I. Chaudhuri; F. Inam; D. A. Drabold
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We present a microscopic picture of silver dynamics in GeSe$_{3}$:Ag glass obtained from {\it ab initio} simulation. The dynamics of Ag is explored at two temperatures, 300K and 700K. In the relaxed network, Ag occupies bond centers between suitably separated host sites. At 700K, Ag motion proceeds via a trapping-release dynamics, between "super traps" or cages consisting of multiple bond-center sites in a small volume. Our work offers a first principles identification of trapping...
Source: http://arxiv.org/abs/0811.1982v1
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Jun 28, 2018
06/18
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Kiran Prasai; Parthapratim Biswas; D. A. Drabold
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Materials with optimized band gap are needed in many specialized applications. In this work, we demonstrate that Hellmann-Feynman forces associated with the gap states can be used to find atomic coordinates with a desired electronic density of states. Using tight-binding models, we show that this approach can be used to arrive at electronically designed models of amorphous silicon and carbon. We provide a simple recipe to include a priori electronic information in the formation of computer...
Topics: Condensed Matter, Materials Science
Source: http://arxiv.org/abs/1506.04403
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Sep 22, 2013
09/13
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Bin Cai; Binay Prasai; D. A. Drabold
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In this chapter, by using ab-initio molecular dynamics, we introduce the latest simulation results on two materials for flash memory devices: Ge2Sb2Te5 and Ge-Se-Cu-Ag. This chapter is a review of our previous work including some of our published figures and text in Cai et al. (2010) and Prasai & Drabold (2011) and also includes several new results.
Source: http://arxiv.org/abs/1103.6051v1
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5.0
Jun 29, 2018
06/18
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Anup Pandey; Heath Scherich; D. A. Drabold
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Density functional theory (DFT) calculations are carried out to study the structure and electronic structure of amorphous zinc oxide (a-ZnO). The models were prepared by the "melt-quench" method. The models are chemically ordered with some coordination defects. The effect of trivalent dopants in the structure and electronic properties of a-ZnO is investigated. Models of a-X_0.375 Z_0.625 O (X=Al, Ga and In) were also prepared by the "melt- quench" method. The trivalent...
Topics: Materials Science, Condensed Matter
Source: http://arxiv.org/abs/1610.00156
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Sep 24, 2013
09/13
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Parthapratim Biswas; De Nyago Tafen; D. A. Drabold
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An ideal atomistic model of a disordered material should contradict no experiments,and should also be consistent with accurate force fields (either {\it ab initio}or empirical). We make significant progress toward jointly satisfying {\it both} of these criteria using a hybrid reverse Monte Carlo approach in conjunction with approximate first principles molecular dynamics. We illustrate the method by studying the complex binary glassy material g-GeSe$_2$. By constraining the model to agree with...
Source: http://arxiv.org/abs/cond-mat/0412342v1
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Sep 18, 2013
09/13
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Parthapratim Biswas; Raymond Atta-Fynn; D. A. Drabold
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An implementation of the Reverse Monte Carlo algorithm is presented for the study of amorphous tetrahedral semiconductors. By taking into account a number of constraints that describe the tetrahedral bonding geometry along with the radial distribution function, we construct a model of amorphous silicon using the reverse monte carlo technique. Starting from a completely random configuration, we generate a model of amorphous silicon containing 500 atoms closely reproducing the experimental static...
Source: http://arxiv.org/abs/cond-mat/0401205v1
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Sep 18, 2013
09/13
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T. A. Abtew; M. Zhang; D. A. Drabold
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We present an ab initio calculation of the DC conductivity of amorphous silicon and hydrogenated amorphous silicon. The Kubo-Greenwood formula is used to obtain the DC conductivity, by thermal averaging over extended dynamical simulation. Its application to disordered solids is discussed. The conductivity is computed for a wide range of temperatures and doping is explored in a naive way by shifting the Fermi level. We observed the Meyer-Neldel rule for the electrical conductivity with E_MNR =...
Source: http://arxiv.org/abs/0705.2384v3
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203
Sep 18, 2013
09/13
by
Raymond Atta-Fynn; Parthapratim Biswas; D. A. Drabold
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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 a detailed study on an interesting system: amorphous silicon. We compute first principles deformation potentials (by computing the sensitivity of specific electronic eigenstates to individual classical normal modes of vibration). We also probe thermal...
Source: http://arxiv.org/abs/cond-mat/0401066v1
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Sep 20, 2013
09/13
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De Nyago Tafen; D. A. Drabold; M. Mitkova
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In this paper, we present models of Ge-Se glasses heavily doped with Ag obtained from {\it ab initio} simulation and study the dynamics of the network with an emphasis on the motion of Ag$^+$ ions. The models are analyzed with partial pair correlation functions, static structure factors and novel wavelet techniques. The electronic properties are characterized by the electronic density of states and analysis of specific electronic eigenstates. As Ag content increases, the optical band gap...
Source: http://arxiv.org/abs/cond-mat/0506471v1
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Sep 18, 2013
09/13
by
T. A. Abtew; F. Inam; D. A. Drabold
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We report first principles ab initio density functional calculations of hydrogen dynam- ics in hydrogenated amorphous silicon. Thermal motion of the host Si atoms drives H diffusion, as we demonstrate by direct simulation and explain with simple models. Si-Si bond centers and Si ring centers are local energy minima as expected. We also describe a new mechanism for break- ing Si-H bonds to release free atomic H into the network: a fluctuation bond center detachment (FBCD) assisted diffusion. H...
Source: http://arxiv.org/abs/cond-mat/0611375v2
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Sep 18, 2013
09/13
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S. N. Taraskin; D. A. Drabold; S. R. Elliott
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Analytic results for the asymptotic decay of the electron density matrix in insulators have been obtained in all three dimensions ($D=1 - 3$) for a tight-binding model defined on a simple cubic lattice. The anisotropic decay length is shown to be dependent on the energy parameters of the model. The existence of the power-law prefactor, $+AFw-propto r^{-D/2}$, is demonstrated.
Source: http://arxiv.org/abs/cond-mat/0110473v3
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Jun 29, 2018
06/18
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Anup Pandey; Parthapratim Biswas; Bishal Bhattarai; D. A. Drabold
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We apply a new method "force enhanced atomic refinement" (FEAR) to create a computer model of amorphous silicon (a-Si), based upon the highly precise X-ray diffraction experiments of Laaziri et al. The logic underlying our calculation is to estimate the structure of a real sample a-Si using experimental data and chemical information included in a non-biased way, starting from random coordinates. The model is in close agreement with experiment and also sits at a suitable minimum energy...
Topics: Materials Science, Condensed Matter
Source: http://arxiv.org/abs/1610.00065
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Sep 20, 2013
09/13
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Y. Pan; F. Inam; M. Zhang; D. A. Drabold
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Exponential band edges have been observed in a variety of materials, both crystalline and amorphous. In this paper, we infer the structural origins of these tails in amorphous and defective crystalline Si by direct calculation with current {\it ab initio} methods. We find that exponential tails appear in relaxed models of diamond with suitable point defects. In amorphous silicon (a-Si), we find that structural filaments of short bonds and long bonds exist in the network, and that the tail...
Source: http://arxiv.org/abs/0802.1292v1
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Sep 22, 2013
09/13
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M. -L. Zhang; Y. Pan; F. Inam; D. A. Drabold
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It is argued that topological disorder in amorphous solids can be described by local strains related to local reference crystals and local rotations. An intuitive localization criterion is formulated from this point of view. The Inverse Participation Ratio and the location of mobility edges in band tails is directly related to the character of the disorder potential in amorphous solid, the coordination number, the transition integral and the nodes of wave functions of the corresponding...
Source: http://arxiv.org/abs/0805.2575v2
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Sep 18, 2013
09/13
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F. Inam; G. Chen; D. N. Tafen; D. A. Drabold
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Raman and calorimetric studies on Ge_xSe_{1-x} glasses have provided evidence for the existence of the intermediate phase (IP) in chalcogenide and other glasses. Here, we present X-Ray Absorption Near Edge Structure (XANES) measurements on germanium selenide glasses in the IP composition range, and detect an electronic signature of the IP. Ab initio molecular dynamics (MD) based models of these glasses are discussed, and an atomistic picture of the IP, based upon the models and available...
Source: http://arxiv.org/abs/0806.1039v1
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Sep 23, 2013
09/13
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Raymond Atta-Fynn; Parthapratim Biswas; Pablo Ordejon; D. A. Drabold
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We investigate the electronic structure of gap and band tail states in amorphous silicon. Starting with two 216-atom models of amorphous silicon with defect concentration close to the experiments, we systematically study the dependence of electron localization on basis set, density functional and spin polarization using the first principles density functional code Siesta. We briefly compare three different schemes for characterizing localization: information entropy, inverse participation ratio...
Source: http://arxiv.org/abs/cond-mat/0312333v1
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Sep 24, 2013
09/13
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K. Bandyopadhyay; A. K. Bhattacharya; Parthapratim Biswas; D. A. Drabold
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We present a technique for entropy optimization to calculate a distribution from its moments. The technique is based upon maximizing a discretized form of the Shannon entropy functional by mapping the problem onto a dual space where an optimal solution can be constructed iteratively. We demonstrate the performance and stability of our algorithm with several tests on numerically difficult functions. We then consider an electronic structure application, the electronic density of states of...
Source: http://arxiv.org/abs/cond-mat/0412717v1
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Sep 18, 2013
09/13
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Parthapratim Biswas; Raymond Atta-Fynn; S. Chakraborty; D. A. Drabold
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Ideal models of complex materials must satisfy all available information about the system. Generally, this information consists of experimental data, information implicit to sophisticated interatomic interactions and potentially other {\it a priori} information. By jointly imposing first-principles or tight-binding information in conjunction with experimental data, we have developed a method: Experimentally Constrained Molecular Relaxation (ECMR) that uses {\it all} of the information...
Source: http://arxiv.org/abs/0707.4012v1
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Sep 21, 2013
09/13
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D. A. Drabold; T. A. Abtew; F. Inam; Y. Pan
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In this paper we discuss the application of current it ab initio computer simulation techniques to hydrogenated amorphous silicon (a-Si:H). We begin by discussing thermal fluctuation in the number of coordination defects in the material, and its temperature dependence. We connect this to the ``fluctuating bond center detachment" mechanism for liberating H bonded to Si atoms. Next, from extended thermal MD simulation, we illustrate various mechanisms of H motion. The dynamics of the lattice...
Source: http://arxiv.org/abs/0709.1655v1
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Sep 23, 2013
09/13
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Parthapratim Biswas; De Nyago Tafen; Raymond Atta-Fynn; D. A. Drabold
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We propose a novel approach to model amorphous materials using a first principles density functional method while simultaneously enforcing agreement with selected experimental data. We illustrate our method with applications to amorphous silicon and glassy GeSe$_2$. The structural, vibrational and electronic properties of the models are found to be in agreement with experimental results. The method is general and can be extended to other complex materials.
Source: http://arxiv.org/abs/cond-mat/0408003v1
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Sep 18, 2013
09/13
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D. R. Alfonso; C. Noguez; D. A. Drabold; S. E. Ulloa
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The relaxed geometries and electronic properties of the hydrogenated phases of the Si(111)-7$\times$7 surface are studied using first-principles molecular dynamics. A monohydride phase, with one H per dangling bond adsorbed on the bare surface is found to be energetically favorable. Another phase where 43 hydrogens saturate the dangling bonds created by the removal of the adatoms from the clean surface is found to be nearly equivalent energetically. Experimental STM and differential reflectance...
Source: http://arxiv.org/abs/cond-mat/9510165v1
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Sep 21, 2013
09/13
by
Y. Li; F. Inam; A. Kumar; M. F. Thorpe; D. A. Drabold
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Ordered graphene has been extensively studied. In this paper we undertake a first density functional study of it topologically disordered analogues of graphene, in the form of a random network, consisting predominantly of hexagonal rings, but also including pentagons and heptagons. After some preliminaries with crystalline material, we relax various random network models and find that the presence of carbon pentagons induce local curvature, thus breaking the initial planar symmetry, in some...
Source: http://arxiv.org/abs/1104.1958v1
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Sep 17, 2013
09/13
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Matthew J Cliffe; Martin T. Dove; D. A. Drabold; Andrew L. Goodwin
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We show that the information gained in spectroscopic experiments regarding the number and distribution of atomic environments can be used as a valuable constraint in the refinement of the atomic-scale structures of nanostructured or amorphous materials from pair distribution function (PDF) data. We illustrate the effectiveness of this approach for three paradigmatic disordered systems: molecular C60, a-Si, and a-SiO2 . Much improved atomistic models are attained in each case without any...
Source: http://arxiv.org/abs/0912.1971v2
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Sep 17, 2013
09/13
by
S. N. Taraskin; P. A. Fry; Xiadong Zhang; D. A. Drabold; S. R. Elliott
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Analytical results for the asymptotic spatial decay of the density matrix $+AFw-rho({+AFw-bf r},{+AFw-bf r^+AFw-prime})$ in the tight-binding model of the two-dimensional metal are presented. In various dimensions D, it is found analytically and numerically that the density matrix decays with distance according to the power law, $+AFw-rho({+AFw-bf r},{+AFw-bf r^+AFw-prime}) +AFw-propto |{+AFw-bf r}-{+AFw-bf r^+AFw-prime}|^{-(D+-1)/2}$.
Source: http://arxiv.org/abs/cond-mat/0207443v2
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Sep 21, 2013
09/13
by
G. Fabricius; E. Artacho; D. Sanchez-Portal; P. Ordejon; D. A. Drabold; J. M. Soler
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We simulate the liquid silicon surface with first-principles molecular dynamics in a slab geometry. We find that the atom-density profile presents a pronounced layering, similar to those observed in low-temperature liquid metals like Ga and Hg. The depth-dependent pair correlation function shows that the effect originates from directional bonding of Si atoms at the surface, and propagates into the bulk. The layering has no major effects in the electronic and dynamical properties of the system,...
Source: http://arxiv.org/abs/cond-mat/9907380v1
