5,549 research outputs found
Low-temperature transport in Heisenberg chains
A technique to determine accurately transport properties of integrable and
non-integrable quantum-spin chains at finite temperatures by Quantum
Monte-Carlo is presented. The reduction of the Drude weight by interactions in
the integrable gapless regime is evaluated. Evidence for the absence of a Drude
weight in the gapless regime of a non-integrable system with longer-ranged
interactions is presented. We estimate the effect of the non-integrability on
the transport properties and compare with recent experiments on one-dimensional
quantum-spin chains.Comment: accepted for publication (PRL
Can quantum regression theorem be reconciled with quantum fluctuation dissipation theorem ?
In the attempt to derive the regression theorem from the fluctuation
dissipation theorem several authors claim the violation of the former theorem
in the quantum case. Here we pose the question: does it exists a quantum
fluctuation dissipation theorem (QFDT) in its conventional interpretation? It
is shown that the relation usually called as the QFDT is the condition of
detailed macroscopic energetic balance. Following this interpretation the
existing conflict between the two theorems in the quantum case is removed.Comment: 13 pages, 3rd Int. Conf. on Unsolved Problems on Noise (will be
published in Proceedings
Conductivity of quantum-spin chains: A Quantum Monte Carlo approach
We discuss zero-frequency transport properties of various spin-1/2 chains. We
show, that a careful analysis of Quantum Monte-Carlo (QMC) data on the
imaginary axis allows to distinguish between intrinsic ballistic and diffusive
transport. We determine the Drude weight, current-relaxation life-time and the
mean-free path for integrable and a non-integrable quantum-spin chain. We
discuss, in addition, some phenomenological relations between various
transport-coefficients and thermal response functions
Geometric Transitions, Flops and Non-Kahler Manifolds: I
We construct a duality cycle which provides a complete supergravity
description of geometric transitions in type II theories via a flop in
M-theory. This cycle connects the different supergravity descriptions before
and after the geometric transitions. Our construction reproduces many of the
known phenomena studied earlier in the literature and allows us to describe
some new and interesting aspects in a simple and elegant fashion. A precise
supergravity description of new torsional manifolds that appear on the type IIA
side with branes and fluxes and the corresponding geometric transition are
obtained. A local description of new G_2 manifolds that are circle fibrations
over non-Kahler manifolds is presented.Comment: Harvmac, 79 pages, 1 .eps figure; v4: Text further expanded with
additional results, typos corrected and references adde
Cloaking and anamorphism for light and mass diffusion
We first review classical results on cloaking and mirage effects for
electromagnetic waves. We then show that transformation optics allows the
masking of objects or produces mirages in diffusive regimes. In order to
achieve this, we consider the equation for diffusive photon density in
transformed coordinates, which is valid for diffusive light in scattering
media. More precisely, generalizing transformations for star domains introduced
in [Diatta and Guenneau, J. Opt. 13, 024012, 2011] for matter waves, we
numerically demonstrate that infinite conducting objects of different shapes
scatter diffusive light in exactly the same way. We also propose a design of
external light-diffusion cloak with spatially varying sign-shifting parameters
that hides a finite size scatterer outside the cloak. We next analyse
non-physical parameter in the transformed Fick's equation derived in [Guenneau
and Puvirajesinghe, R. Soc. Interface 10, 20130106, 2013], and propose to use a
non-linear transform that overcomes this problem. We finally investigate other
form invariant transformed diffusion-like equations in the time domain, and
touch upon conformal mappings and non-Euclidean cloaking applied to diffusion
processes.Comment: 42 pages, Latex, 14 figures. V2: Major changes : some formulas
corrected, some extra cases added, overall length extended from 21 pages (V1)
to 42 pages (present version V2). The last version will appear at Journal of
Optic
Spin current swapping and Hanle spin Hall effect in the two dimensional electron gas
We analyze the effect known as "spin current swapping" (SCS) due to
electron-impurity scattering in a uniform spin-polarized two-dimensional
electron gas. In this effect a primary spin current (lower index for
spatial direction, upper index for spin direction) generates a secondary spin
current if , or , with , if . Contrary
to naive expectation, the homogeneous spin current associated with the uniform
drift of the spin polarization in the electron gas does not generate a swapped
spin current by the SCS mechanism. Nevertheless, a swapped spin current will be
generated, if a magnetic field is present, by a completely different mechanism,
namely, the precession of the spin Hall spin current in the magnetic field. We
refer to this second mechanism as Hanle spin Hall effect, and we notice that it
can be observed in an experiment in which a homogeneous drift current is passed
through a uniformly magnetized electron gas. In contrast to this, we show that
an unambiguous observation of SCS requires inhomogeneous spin currents, such as
those that are associated with spin diffusion in a metal, and no magnetic
field. An experimental setup for the observation of the SCS is therefore
proposed.Comment: 8 pages, 5 figure
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