81 research outputs found
Hidden structure in amorphous solids
Recent theoretical studies of amorphous silicon [Y. Pan et al. Phys. Rev.
Lett. 100 206403 (2008)] have revealed subtle but significant structural
correlations in network topology: the tendency for short (long) bonds to be
spatially correlated with other short (long) bonds). These structures were
linked to the electronic band tails in the optical gap. In this paper, we
further examine these issues for amorphous silicon, and demonstrate that
analogous correlations exist in amorphous SiO2, and in the organic molecule,
b-carotene. We conclude with a discussion of the origin of the effects and its
possible generality
Experimentally Constrained Molecular Relaxation: The Case of Glassy GeSe2
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. By constraining the model to agree with partial structure factors
and {\it ab initio} simulation, we obtain a 647-atom model in close agreement
with experiment, including the first sharp diffraction peak in the static
structure factor. We compute the electronic state densities and compare to
photoelectron spectroscopies. The approach is general and flexible.Comment: 6 pages, 4 figure
Direct ab initio MD simulation of silver ion diffusion in chalcogenide glasses
In this paper, we present new models of germanium selenide chalcogenide
glasses heavily doped with silver. The models were readily obtained with ab
initio molecular dynamics and their structure agrees closely with diffraction
measurements. Thermal molecular dynamics simulation reveals the dynamics of Ag+
ions and the existence of trapping centers as conjectured in other theory work.
We show that first principles simulation is a powerful tool to reveal the
motion of ions in glass.Comment: 3 pages, 3 figures, submitted to Phys. Stat. Sol. {b} Rapid Research
Letter
Inclusion of Experimental Information in First Principles Modeling of Materials
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. 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.Comment: 11 pages, 8 PostScript figures, submitted to J. Phys.: Condens.
Matter in honor of Mike Thorpe's 60th birthda
Network structure and dynamics of hydrogenated amorphous silicon
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 is then linked to the electrons, and we
point out that the squared electron-lattice coupling (and the thermally-induced
mean square variation in electron energy eigenvalues) is robustly proportional
to the localization of the conjugate state, if localization is measured with
inverse participation ratio. Finally we discuss the Staebler-Wronski effect
using these methods, and argue that a sophisticated local heating picture
(based upon reasonable calculations of the electron-lattice coupling and
molecular dynamic simulation) explains significant aspects of the phenomenon.Comment: 10 pages, 5 figures, accepted in J. Non. Cryst. So
Ab-initio study of the stability and electronic properties of wurtzite and zinc-blende BeS nanowires
In this work we study the structural stability and electronic properties of
the Beryllium sulphide nanowires (NWs) in both zinc blende (ZB) and wurtzite
(WZ) phases with triangle and hexagonal cross section, using first principle
calculations within plane-wave pseudopotential method. A phenomenological model
is used to explain the role of dangling bonds in the stability of the NWs. In
contrast to the bulk phase, ZB-NWs with diameter less than 133.3 (angstrom) are
found to be less favorable over WZ-NWs, in which the surface dangling bonds
(DBs) on the NW facets play an important role to stabilize the NWs.
Furthermore, both ZB and WZ NWs are predicted to be semiconductor and the
values of the band gaps are dependent on the surface DBs as well as the size
and shape of NWs. Finally, we performed atom projected density-of states
(PDOSs) analysis by calculating the localized density of states on the surface
atoms, as well as on the core and edge atoms.Comment: 9 Pages, 6 Figure
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