983 research outputs found
Identification of relaxation and diffusion mechanisms in amorphous silicon
The dynamics of amorphous silicon at low temperatures can be characterized by
a sequence of discrete activated events, through which the topological network
is locally reorganized. Using the activation-relaxation technique, we create
more than 8000 events, providing an extensive database of relaxation and
diffusion mechanisms. The generic properties of these events - size, number of
atoms involved, activation energy, etc. - are discussed and found to be
compatible with experimental data. We introduce a complete and unique
classification of defects based on their topological properties and apply it to
study of events involving only four-fold coordinated atoms. For these events,
we identify and present in detail three dominant mechanisms.Comment: 4 pages, three figures, submitted to PR
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
Binary continuous random networks
Many properties of disordered materials can be understood by looking at
idealized structural models, in which the strain is as small as is possible in
the absence of long-range order. For covalent amorphous semiconductors and
glasses, such an idealized structural model, the continuous-random network, was
introduced 70 years ago by Zachariasen. In this model, each atom is placed in a
crystal-like local environment, with perfect coordination and chemical
ordering, yet longer-range order is nonexistent. Defects, such as missing or
added bonds, or chemical mismatches, however, are not accounted for. In this
paper we explore under which conditions the idealized CRN model without defects
captures the properties of the material, and under which conditions defects are
an inherent part of the idealized model. We find that the density of defects in
tetrahedral networks does not vary smoothly with variations in the interaction
strengths, but jumps from close-to-zero to a finite density. Consequently, in
certain materials, defects do not play a role except for being thermodynamical
excitations, whereas in others they are a fundamental ingredient of the ideal
structure.Comment: Article in honor of Mike Thorpe's 60th birthday (to appear in J.
Phys: Cond Matt.
Activated sampling in complex materials at finite temperature: the properly-obeying-probability activation-relaxation technique
While the dynamics of many complex systems is dominated by activated events,
there are very few simulation methods that take advantage of this fact. Most of
these procedures are restricted to relatively simple systems or, as with the
activation-relaxation technique (ART), sample the conformation space
efficiently at the cost of a correct thermodynamical description. We present
here an extension of ART, the properly-obeying-probability ART (POP-ART), that
obeys detailed balance and samples correctly the thermodynamic ensemble.
Testing POP-ART on two model systems, a vacancy and an interstitial in
crystalline silicon, we show that this method recovers the proper
thermodynamical weights associated with the various accessible states and is
significantly faster than MD in the diffusion of a vacancy below 700 K.Comment: 10 pages, 3 figure
Efficient tight-binding Monte Carlo structural sampling of complex materials
While recent work towards the development of tight-binding and ab-initio
algorithms has focused on molecular dynamics, Monte Carlo methods can often
lead to better results with relatively little effort. We present here a
multi-step Monte Carlo algorithm that makes use of the possibility of quickly
evaluating local energies. For the thermalization of a 1000-atom configuration
of {\it a}-Si, this algorithm gains about an order of magnitude in speed over
standard molecular dynamics. The algorithm can easily be ported to a wide range
of materials and can be dynamically optimized for a maximum efficiency.Comment: 5 pages including 3 postscript figure
Energy landscape of relaxed amorphous silicon
We analyze the structure of the energy landscape of a well-relaxed 1000-atom
model of amorphous silicon using the activation-relaxation technique (ART
nouveau). Generating more than 40,000 events starting from a single minimum, we
find that activated mechanisms are local in nature, that they are distributed
uniformly throughout the model and that the activation energy is limited by the
cost of breaking one bond, independently of the complexity of the mechanism.
The overall shape of the activation-energy-barrier distribution is also
insensitive to the exact details of the configuration, indicating that
well-relaxed configurations see essentially the same environment. These results
underscore the localized nature of relaxation in this material.Comment: 8 pages, 12 figure
Importance de la photorespiration dans le bilan photosynthétique au cours de la croissance foliaire
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
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