27,894 research outputs found
Theoretical and numerical analysis for the quasi-continuum approximation of a material particle model
10.1090/S0025-5718-02-01456-4Mathematics of Computation72242657-67
Breathers in oscillator chains with Hertzian interactions
We prove nonexistence of breathers (spatially localized and time-periodic
oscillations) for a class of Fermi-Pasta-Ulam lattices representing an
uncompressed chain of beads interacting via Hertz's contact forces. We then
consider the setting in which an additional on-site potential is present,
motivated by the Newton's cradle under the effect of gravity. Using both direct
numerical computations and a simplified asymptotic model of the oscillator
chain, the so-called discrete p-Schr\"odinger (DpS) equation, we show the
existence of discrete breathers and study their spectral properties and
mobility. Due to the fully nonlinear character of Hertzian interactions,
breathers are found to be much more localized than in classical nonlinear
lattices and their motion occurs with less dispersion. In addition, we study
numerically the excitation of a traveling breather after an impact at one end
of a semi-infinite chain. This case is well described by the DpS equation when
local oscillations are faster than binary collisions, a situation occuring e.g.
in chains of stiff cantilevers decorated by spherical beads. When a hard
anharmonic part is added to the local potential, a new type of traveling
breather emerges, showing spontaneous direction-reversing in a spatially
homogeneous system. Finally, the interaction of a moving breather with a point
defect is also considered in the cradle system. Almost total breather
reflections are observed at sufficiently high defect sizes, suggesting
potential applications of such systems as shock wave reflectors
Phase Fluctuations and Pseudogap Phenomena
This article reviews the current status of precursor superconducting phase
fluctuations as a possible mechanism for pseudogap formation in
high-temperature superconductors. In particular we compare this approach which
relies on the two-dimensional nature of the superconductivity to the often used
-matrix approach. Starting from simple pairing Hamiltonians we present a
broad pedagogical introduction to the BCS-Bose crossover problem. The finite
temperature extension of these models naturally leads to a discussion of the
Berezinskii-Kosterlitz-Thouless superconducting transition and the related
phase diagram including the effects of quantum phase fluctuations and
impurities. We stress the differences between simple Bose-BCS crossover
theories and the current approach where one can have a large pseudogap region
even at high carrier density where the Fermi surface is well-defined. The
Green's function and its associated spectral function, which explicitly show
non-Fermi liquid behaviour, is constructed in the presence of vortices. Finally
different mechanisms including quasi-particle-vortex and vortex-vortex
interactions for the filling of the gap above are considered.Comment: 129 pages, Elsart, 28 EPS figures; Physics Reports, in press. Authors
related information under
"http://nonlin.bitp.kiev.ua/~sharapov/superconductivity.html
Shear viscosity over entropy density ratio with extended quasi-particles
We consider an effective field theory description of beyond-quasi-particle
excitations aiming to associate the transport properties of the system with the
spectral density of states. Tuning various properties of the many-particle
correlations, we investigate how the robust microscopic features are translated
into the macroscopic observables like shear viscosity and entropy density. The
liquid-gas crossover is analysed using several examples. A thermal constraint
on the fluidity measure, the ratio of shear viscosity to entropy density, is
discussed.Comment: 13 pages, 7 figures, uses revtex4-1; published versio
Polarons in semiconductor quantum-dots and their role in the quantum kinetics of carrier relaxation
While time-dependent perturbation theory shows inefficient carrier-phonon
scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic
description of carrier-phonon interaction predicts fast carrier capture and
relaxation. The considered processes do not fulfill energy conservation in
terms of free-carrier energies because polar coupling of localized quantum-dot
states strongly modifies this picture.Comment: 6 pages, 6 figures, accepted for publication in Phys.Rev.
On the 3-particle scattering continuum in quasi one dimensional integer spin Heisenberg magnets
We analyse the three-particle scattering continuum in quasi one dimensional
integer spin Heisenberg antiferromagnets within a low-energy effective field
theory framework. We exactly determine the zero temperature dynamical structure
factor in the O(3) nonlinear sigma model and in Tsvelik's Majorana fermion
theory. We study the effects of interchain coupling in a Random Phase
Approximation. We discuss the application of our results to recent
neutron-scattering experiments on the Haldane-gap material .Comment: 8 pages of revtex, 5 figures, small changes, to appear in PR
- …