443 research outputs found
From Bloch model to the rate equations II: the case of almost degenerate energy levels
Bloch equations give a quantum description of the coupling between an atom
and a driving electric force. In this article, we address the asymptotics of
these equations for high frequency electric fields, in a weakly coupled regime.
We prove the convergence towards rate equations (i.e. linear Boltzmann
equations, describing the transitions between energy levels of the atom). We
give an explicit form for the transition rates. This has already been performed
in [BFCD03] in the case when the energy levels are fixed, and for different
classes of electric fields: quasi or almost periodic, KBM, or with continuous
spectrum. Here, we extend the study to the case when energy levels are possibly
almost degenerate. However, we need to restrict to quasiperiodic forcings. The
techniques used stem from manipulations on the density matrix and the averaging
theory for ordinary differential equations. Possibly perturbed small divisor
estimates play a key role in the analysis. In the case of a finite number of
energy levels, we also precisely analyze the initial time-layer in the rate
aquation, as well as the long-time convergence towards equilibrium. We give
hints and counterexamples in the infinite dimensional case
Colloidal brazil nut effect in sediments of binary charged suspensions
Equilibrium sedimentation density profiles of charged binary colloidal
suspensions are calculated by computer simulations and density functional
theory. For deionized samples, we predict a colloidal ``brazil nut'' effect:
heavy colloidal particles sediment on top of the lighter ones provided that
their mass per charge is smaller than that of the lighter ones. This effect is
verifiable in settling experiments.Comment: 4 pages, 4 figure
Coherent Control for a Two-level System Coupled to Phonons
The interband polarizations induced by two phase-locked pulses in a
semiconductor show strong interference effects depending on the time tau_1
separating the pulses. The four-wave mixing signal diffracted from a third
pulse delayed by tau is coherently controlled by tuning tau_1. The four-wave
mixing response is evaluated exactly for a two-level system coupled to a single
LO phonon. In the weak coupling regime it shows oscillations with the phonon
frequency which turn into sharp peaks at multiples of the phonon period for a
larger coupling strength. Destructive interferences between the two
phase-locked pulses produce a splitting of the phonon peaks into a doublet. For
fixed tau but varying tau_1 the signal shows rapid oscillations at the
interband-transition frequency, whose amplitude exhibits bursts at multiples of
the phonon period.Comment: 4 pages, 4 figures, RevTex, content change
Time evolution of a quantum many-body system: transition from integrability to ergodicity in thermodynamic limit
Numerical evidence is given for non-ergodic (non-mixing) behavior, exhibiting
ideal transport, of a simple non-integrable many-body quantum system in the
thermodynamic limit, namely kicked model of spinless fermions on a ring.
However, for sufficiently large kick parameters and we recover quantum
ergodicity, and normal transport, which can be described by random matrix
theory.Comment: 4 pages in RevTex (6 figures in PostScript included
Numerical Evidence of Luttinger and Fermi Liquid Behaviour in the 2D Hubbard Model
The two dimensional Hubbard model with a single spin-up electron interacting
with a finite density of spin-down electrons is studied using the quantum
Monte Carlotechnique, a new conjugate gradient method for the evaluation of
the Edwards wavefunction ansatz, and the standard second order perturbation
theory. We performed simulations up to 242 sites at reaching the zero
temperature properties with no ``fermion sign problem'' and found a
surprisingly good accuracy of the Edwards wavefunction ansatz at low density or
low doping. The conjugate gradient method was then applied to system up to 1922
sites and infinite for the Edwards state. Fermi liquid theory seems to
remain stable in 2D for all cases studied with the exception of the half
filling case where a ``Luttinger like behavior'' survives in the Hubbard model
, yielding a vanishing quasiparticle weight in the thermodynamic limit.Comment: 10 pages + 4 pictures, RevTex, SISSA 121/93/CM/M
Effect of Finite Impurity Mass on the Anderson Orthogonality Catastrophe in One Dimension
A one-dimensional tight-binding Hamiltonian describes the evolution of a
single impurity interacting locally with electrons. The impurity spectral
function has a power-law singularity
with the same exponent
that characterizes the logarithmic decay of the quasiparticle weight
with the number of electrons , . The exponent
is computed by (1) perturbation theory in the interaction strength and
(2) numerical evaluations with exact results for small systems and variational
results for larger systems. A nonanalytical behavior of is observed in
the limit of infinite impurity mass. For large interaction strength, the
exponent depends strongly on the mass of the impurity in contrast to the
perturbative result.Comment: 26 pages, RevTeX, 7 figures included, to be published in Phys. Rev.
Transport Properties of One-Dimensional Hubbard Models
We present results for the zero and finite temperature Drude weight D(T) and
for the Meissner fraction of the attractive and the repulsive Hubbard model, as
well as for the model with next nearest neighbor repulsion. They are based on
Quantum Monte Carlo studies and on the Bethe ansatz. We show that the Drude
weight is well defined as an extrapolation on the imaginary frequency axis,
even for finite temperature. The temperature, filling, and system size
dependence of D is obtained. We find counterexamples to a conjectured
connection of dissipationless transport and integrability of lattice models.Comment: 10 pages, 14 figures. Published versio
The relative influences of disorder and of frustration on the glassy dynamics in magnetic systems
The magnetisation relaxations of three different types of geometrically
frustrated magnetic systems have been studied with the same experimental
procedures as previously used in spin glasses. The materials investigated are
YMoO (pyrochlore system), SrCrGaO (piled
pairs of Kagom\'e layers) and (HO)Fe(SO)(OH) (jarosite
compound). Despite a very small amount of disorder, all the samples exhibit
many characteristic features of spin glass dynamics below a freezing
temperature , much smaller than their Curie-Weiss temperature .
The ageing properties of their thermoremanent magnetization can be well
accounted for by the same scaling law as in spin glasses, and the values of the
scaling exponents are very close. The effects of temperature variations during
ageing have been specifically investigated. In the pyrochlore and the
bi-Kagom\'e compounds, a decrease of temperature after some waiting period at a
certain temperature re-initializes ageing and the evolution at the new
temperature is the same as if the system were just quenched from above .
However, as the temperature is raised back to , the sample recovers the
state it had previously reached at that temperature. These features are known
in spin glasses as rejuvenation and memory effects. They are clear signatures
of the spin glass dynamics. In the Kagom\'e compound, there is also some
rejuvenation and memory, but much larger temperature changes are needed to
observe the effects. In that sense, the behaviour of this compound is
quantitatively different from that of spin glasses.Comment: latex VersionCorrigee4.tex, 4 files, 3 figures, 5 pages (Proceedings
of the International Conference on Highly Frustrated Magnetism (HFM2003),
August 26-30, 2003, Institut Laue Langevin (ILL), Grenoble, France
Exact calculation of spectral properties of a particle interacting with a one dimensional fermionic system
Using the Bethe ansatz analysis as was reformulated by Edwards, we calculate
the spectral properties of a particle interacting with a bath of fermions in
one dimension for the case of equal particle-fermion masses. These are directly
related to singularities apparent in optical experiments in one dimensional
systems. The orthogonality catastrophe for the case of an infinite particle
mass survives in the limit of equal masses. We find that the exponent
of the quasiparticle weight, is different for the two
cases, and proportional to their respective phaseshifts at the Fermi surface;
we present a simple physical argument for this difference. We also show that
these exponents describe the low energy behavior of the spectral function, for
repulsive as well as attractive interaction.Comment: 22 pages + 1 postscript figure, REVTE
Integrability and coherence of hopping between 1D correlated electrons systems
We present numerical evidence that the hopping of electrons between chains
described by the model is coherent in the integrable cases ( and
) and essentially incoherent otherwise. This effect is {\it not} related
to the value of the exponent , (which is restricted to the interval
[0,1/8] when ), and we propose that enhanced coherence is
characteristic of integrable systems.Comment: 9 pages, LateX, 4 figures in uuencoded format, submitted to Phys.
Rev. Let
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