1,127 research outputs found
Numerical studies of the vibrational isocoordinate rule in chalcogenide glasses
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 density of states.Comment: 7 pages, including 3 figure
Thermally-activated charge reversibility of gallium vacancies in GaAs
The dominant charge state for the Ga vacancy in GaAs has been the subject of
a long debate, with experiments proposing 1, 2 or 3 as the best
answer. We revisit this problem using {\it ab initio} calculations to compute
the effects of temperature on the Gibbs free energy of formation, and we find
that the thermal dependence of the Fermi level and of the ionization levels
lead to a reversal of the preferred charge state as the temperature increases.
Calculating the concentrations of gallium vacancies based on these results, we
reproduce two conflicting experimental measurements, showing that these can be
understood from a single set of coherent LDA results when thermal effects are
included.Comment: 4 pages, 4 figure
Self-vacancies in Gallium Arsenide: an ab initio calculation
We report here a reexamination of the static properties of vacancies in GaAs
by means of first-principles density-functional calculations using localized
basis sets. Our calculated formation energies yields results that are in good
agreement with recent experimental and {\it ab-initio} calculation and provide
a complete description of the relaxation geometry and energetic for various
charge state of vacancies from both sublattices. Gallium vacancies are stable
in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state
for intrinsic and n-type GaAs, confirming results from positron annihilation.
Interestingly, Arsenic vacancies show two successive negative-U transitions
making only +1, -1 and -3 charge states stable, while the intermediate defects
are metastable. The second transition (-/-3) brings a resonant bond relaxation
for V_As^-3 similar to the one identified for silicon and GaAs divacancies.Comment: 14 page
Gallium self-interstitial relaxation in Gallium Arsenide: an {ab initio} characterization
Ga interstitials in GaAs () are studied using the local-orbital
{ab-initio} code SIESTA in a supercell of {216+1} atoms. Starting from eight
different initial configurations, we find five metastable structures: the two
tetrahedral sites in addition to the 110-split,
111-split, and 100-split. Studying
the competition between various configuration and charges of , we find
that predominant gallium interstitials in GaAs are charged +1, neutral or at
most -1 depending on doping conditions and prefer to occupy the tetrahedral
configuration where it is surrounded by Ga atoms. Our results are in excellent
agreement with recent experimental results concerning the dominant charge of
, underlining the importance of finite size effects in the calculation
of defects.Comment: v1) 18 pages, 5 figures, submitted to PRB (Latex preprint version)
v2) 9 pages, 5 figures, reviewed version resubmitted to PRB (correction to
equation 1, some changes and reformulations, minor grammatical and typo
corrections, added reference
The Kinetic Activation-Relaxation Technique: A Powerful Off-lattice On-the-fly Kinetic Monte Carlo Algorithm
Many materials science phenomena, such as growth and self-organisation, are
dominated by activated diffusion processes and occur on timescales that are
well beyond the reach of standard-molecular dynamics simulations. Kinetic Monte
Carlo (KMC) schemes make it possible to overcome this limitation and achieve
experimental timescales. However, most KMC approaches proceed by discretizing
the problem in space in order to identify, from the outset, a fixed set of
barriers that are used throughout the simulations, limiting the range of
problems that can be addressed. Here, we propose a more flexible approach --
the kinetic activation-relaxation technique (k-ART) -- which lifts these
constraints. Our method is based on an off-lattice, self-learning, on-the-fly
identification and evaluation of activation barriers using ART and a
topological description of events. The validity and power of the method are
demonstrated through the study of vacancy diffusion in crystalline silicon.Comment: 5 pages, 4 figure
Structural, electronic, and dynamical properties of amorphous gallium arsenide: a comparison between two topological models
We present a detailed study of the effect of local chemical ordering on the
structural, electronic, and dynamical properties of amorphous gallium arsenide.
Using the recently-proposed ``activation-relaxation technique'' and empirical
potentials, we have constructed two 216-atom tetrahedral continuous random
networks with different topological properties, which were further relaxed
using tight-binding molecular dynamics. The first network corresponds to the
traditional, amorphous, Polk-type, network, randomly decorated with Ga and As
atoms. The second is an amorphous structure with a minimum of wrong (homopolar)
bonds, and therefore a minimum of odd-membered atomic rings, and thus
corresponds to the Connell-Temkin model. By comparing the structural,
electronic, and dynamical properties of these two models, we show that the
Connell-Temkin network is energetically favored over Polk, but that most
properties are little affected by the differences in topology. We conclude that
most indirect experimental evidence for the presence (or absence) of wrong
bonds is much weaker than previously believed and that only direct structural
measurements, i.e., of such quantities as partial radial distribution
functions, can provide quantitative information on these defects in a-GaAs.Comment: 10 pages, 7 ps figures with eps
Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique
A detailed description of the activation-relaxation technique (ART) is
presented. This method defines events in the configurational energy landscape
of disordered materials, such as a-Si, glasses and polymers, in a two-step
process: first, a configuration is activated from a local minimum to a nearby
saddle-point; next, the configuration is relaxed to a new minimum; this allows
for jumps over energy barriers much higher than what can be reached with
standard techniques. Such events can serve as basic steps in equilibrium and
kinetic Monte Carlo schemes.Comment: 7 pages, 2 postscript figure
Event-based relaxation of continuous disordered systems
A computational approach is presented to obtain energy-minimized structures
in glassy materials. This approach, the activation-relaxation technique (ART),
achieves its efficiency by focusing on significant changes in the microscopic
structure (events). The application of ART is illustrated with two examples:
the structure of amorphous silicon, and the structure of Ni80P20, a metallic
glass.Comment: 4 pages, revtex, epsf.sty, 3 figure
Nucleation and crystallization process of silicon using Stillinger-Weber potential
We study the homogeneous nucleation process in Stillinger-Weber silicon in
the NVT ensemble. A clear first-order transition from the liquid to crystal
phase is observed thermodynamically with kinetic and structural evidence of the
transformation. At 0.75 T_m, the critical cluster size is about 175 atoms. The
lifetime distribution of clusters as a function of the maximum size their reach
follows an inverse gaussian distribution as was predicted recently from the
classical theory of nucleation (CNT). However, while there is a qualitative
agreement with the CNT, the free energy curve obtained from the simulations
differs significantly from the theoretical predictions, suggesting that the
low-density liquid phase found recently could play a role in the nucleation
process.Comment: 21 page
Self-organization with equilibration: a model for the intermediate phase in rigidity percolation
Recent experimental results for covalent glasses suggest the existence of an
intermediate phase attributed to the self-organization of the glass network
resulting from the tendency to minimize its internal stress. However, the exact
nature of this experimentally measured phase remains unclear. We modify a
previously proposed model of self-organization by generating a uniform sampling
of stress-free networks. In our model, studied on a diluted triangular lattice,
an unusual intermediate phase appears, in which both rigid and floppy networks
have a chance to occur, a result also observed in a related model on a Bethe
lattice by Barre et al. [Phys. Rev. Lett. 94, 208701 (2005)]. Our results for
the bond-configurational entropy of self-organized networks, which turns out to
be only about 2% lower than that of random networks, suggest that a
self-organized intermediate phase could be common in systems near the rigidity
percolation threshold.Comment: 9 pages, 6 figure
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