Oxygen vacancy diffusion in alumina: Newatomistic simulation methods applied to an old problem

Understanding diffusion in alumina is along-standing challenge in ceramic science. The present article applies a novel combination of metadynamics and kinetic Monte Carlo simulation approaches to the investigation of oxygen vacancy diffusion in alumina. Three classes of diffusive jumps with different activation energies were identified, the resulting diffusion coefficient being best fitted by an Arrhenius equation having apre-exponential factor of 7.88 x 10*-2 m*2s*-1 and anactivation energy of 510.83 kJ mol*1. This activation energy is very close to values for the most pure aluminas studied experimentally (activation energy 531 kJ mol*-1). The good agreement indicates that the dominating atomic-scale diffusion mechanism in alumina is vacancy diffusion.

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Original source : http://infoscience.epfl.ch/record/141270...

Quantum Adiabatic Algorithms, Small Gaps, and Different Paths. (arXiv:0909.4766v1 [quant-ph])

We construct a set of instances of 3SAT which are not solved efficiently
using the simplest quantum adiabatic algorithm. These instances are obtained by
picking random clauses all consistent with two disparate planted solutions and
then penalizing one of them with a single additional clause. We argue that by
randomly modifying the beginning Hamiltonian, one obtains (with substantial
probability) an adiabatic path that removes this difficulty. This suggests that
the quantum adiabatic algorithm should in general be run on each instance with
many different random paths leading to the problem Hamiltonian. We do not know
whether this trick will help for a random instance of 3SAT (as opposed to an
instance from the particular set we consider), especially if the instance has
an exponential number of disparate assignments that violate few clauses. We use
a continuous imaginary time Quantum Monte Carlo algorithm in a novel way to
numerically investigate the ground state as well as the first excited state of
our system. Our arguments are supplemented by Quantum Monte Carlo data from
simulations with up to 150 spins.

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Original source : http://arxiv.org/abs/0909.4766...