5,507 research outputs found
Dynamical correlations near dislocation jamming
Dislocation assemblies exhibit a jamming or yielding transition at a critical
external shear stress value . Nevertheless the nature of this
transition has not been ascertained. Here we study the heterogeneous and
collective nature of dislocation dynamics within a crystal plasticity model
close to , by considering the first-passage properties of the
dislocation dynamics. As the transition is approached in the moving phase, the
first passage time distribution exhibits scaling, and a related peak {\it
dynamical} susceptibility diverges as , with . We relate this scaling
to an avalanche description of the dynamics. While the static structural
correlations are found to be independent of the external stress, we identify a
diverging dynamical correlation length in the direction perpendicular
to the dislocation glide motion.Comment: 4 pages, 5 figure
Exchange-correlation potentials for inhomogeneous electron systems in two dimensions from exact diagonalization: comparison with the local-spin-density approximation
We consider electronic exchange and correlation effects in density-functional
calculations of two-dimensional systems. Starting from wave function
calculations of total energies and electron densities of inhomogeneous model
systems, we derive corresponding exchange-correlation potentials and energies.
We compare these with predictions of the local-spin-density approximation and
discuss its accuracy. Our data will be useful as reference data in testing,
comparing and parametrizing exchange and correlation functionals for
two-dimensional electronic systems.Comment: Submitted to Physical Review B on January 3, 2012. Second revised
version submitted on April 13, 201
Quantum Circuits for General Multiqubit Gates
We consider a generic elementary gate sequence which is needed to implement a
general quantum gate acting on n qubits -- a unitary transformation with 4^n
degrees of freedom. For synthesizing the gate sequence, a method based on the
so-called cosine-sine matrix decomposition is presented. The result is optimal
in the number of elementary one-qubit gates, 4^n, and scales more favorably
than the previously reported decompositions requiring 4^n-2^n+1 controlled NOT
gates.Comment: 4 pages, 3 figure
From Brittle to Ductile Fracture in Disordered Materials
We introduce a lattice model able to describe damage and yielding in
heterogeneous materials ranging from brittle to ductile ones. Ductile fracture
surfaces, obtained when the system breaks once the strain is completely
localized, are shown to correspond to minimum energy surfaces. The similarity
of the resulting fracture paths to the limits of brittle fracture or minimum
energy surfaces is quantified. The model exhibits a smooth transition from
brittleness to ductility. The dynamics of yielding exhibits avalanches with a
power-law distribution
Optimization and plasticity in disordered media
We study the plastic yielding of disordered media using the perfectly plastic
random fuse model. The yield surfaces are shown to be different from those
obtained minimizing the sum of the local yield thresholds, i.e. the so-called
minimum 'energy' surfaces. As a result, the global yield stress is lower than
expected from naive optimization and the difference persists as the sample size
increases. At variance with minimum energy surfaces, height-height fluctuations
of yield surfaces exhibit multiscaling. We provide a theoretical argument that
explains how this behavior arises from the very different nature of the
optimization problem in both cases.Comment: Accepted for publication in Physical Review Letter
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