1,088 research outputs found
Casimir effect in the nonequilibrium steady-state of a quantum spin chain
We present a fully microscopics-based calculation of the Casimir effect in a
nonequilibrium system, namely an energy flux driven quantum XX chain. The force
between the walls (transverse-field impurities) is calculated in a
nonequilibrium steady state which is prepared by letting the system evolve from
an initial state with the two halves of the chain prepared at equilibrium at
different temperatures. The steady state emerging in the large-time limit is
homogeneous but carries an energy flux. The Casimir force in this
nonequilibrium state is calculated analytically in the limit when the
transverse fields are small. We find that the the Casimir force range is
reduced compared to the equilibrium case, and suggest that the reason for this
is the reduction of fluctuations in the flux carrying steady state.Comment: 11 page
Ginzburg - Landau Expansion in BCS - BEC Crossover Region of Disordered Attractive Hubbard Model
We have studied disorder effects on the coefficients of Ginzburg - Landau
(GL) expansion for attractive Hubbard model within the generalized DMFT+Sigma
approximation for the wide region of the values of attractive potential U -
from the weak-coupling limit, where superconductivity is described by BCS
model, towards the strong coupling, where superconducting transition is related
to Bose - Einstein condensation (BEC) of compact Cooper pairs. For the case of
semi-elliptic initial density of states disorder influence on the coefficients
A and B before the square and the fourth power of the order parameter is
universal for at all values of electronic correlations and is related only to
the widening of the initial conduction band (density of states) by disorder.
Similar universal behavior is valid for superconducting critical temperature
T_c (the generalized Anderson theorem) and specific heat discontinuity at the
transition. This universality is absent for the coefficient C before the
gradient term, which in accordance with the standard theory of "dirty"
superconductors is strongly suppressed by disorder in the weak-coupling region,
but can slightly grow in BCS - BEC crossover region, becoming almost
independent of disorder in the strong coupling region. This leads to rather
weak disorder dependence of the penetration depth and coherence length, as well
as the slope of the upper critical magnetic field at T_c, in BCS - BEC
crossover and strong coupling regions.Comment: 22 pages, 12 figures, as published in I.M. Lifshitz centenary issue
of Low Temperature Physic
Persistent currents in a bosonic mixture in the ring geometry
In this paper we analyze the possibility of persistent currents of a
two-species bosonic mixture in the one-dimensional ring geometry. We extend the
arguments used by Bloch to obtain a criterion for the stability of persistent
currents for the two-species system. If the mass ratio of the two species is a
rational number, persistent currents can be stable at multiples of a certain
total angular momenta. We show that the Bloch criterion can also be viewed as a
Landau criterion involving the elementary excitations of the system. Our
analysis reveals that persistent currents at higher angular momenta are more
stable for the two-species system than previously thought.Comment: 20 pages and 7 figure
Lifshitz Transition in the Two Dimensional Hubbard Model
Using large-scale dynamical cluster quantum Monte Carlo simulations, we study
the Lifshitz transition of the two dimensional Hubbard model with
next-nearest-neighbor hopping (), chemical potential and temperature as
control parameters. At , we identify a line of Lifshitz transition
points associated with a change of the Fermi surface topology at zero
temperature. In the overdoped region, the Fermi surface is complete and
electron-like; across the Lifshitz transition, the Fermi surface becomes
hole-like and develops a pseudogap. At (or very close to) the Lifshitz
transition points, a van Hove singularity in the density of states crosses the
Fermi level. The van Hove singularity occurs at finite doping due to
correlation effects, and becomes more singular when becomes more negative.
The resulting temperature dependence on the bare d-wave pairing susceptibility
close to the Lifshitz points is significantly different from that found in the
traditional van Hove scenarios. Such unambiguous numerical observation of the
Lifshitz transition at extends our understanding of the quantum
critical region in the phase diagram, and shines lights on future
investigations of the nature of the quantum critical point in the two
dimensional Hubbard model.Comment: 9 pages, 8 figures, accepted for publication in Physics Review
Casimir force between planes as a boundary finite size effect
The ground state energy of a boundary quantum field theory is derived in
planar geometry in D+1 dimensional spacetime. It provides a universal
expression for the Casimir energy which exhibits its dependence on the boundary
conditions via the reflection amplitudes of the low energy particle
excitations. We demonstrate the easy and straightforward applicability of the
general expression by analyzing the free scalar field with Robin boundary
condition and by rederiving the most important results available in the
literature for this geometry.Comment: 10 pages, 2 eps figures, LaTeX2e file. v2: A reference is added, some
minor modifications made to clarify the text. v3: 9 pages, 3 eps figures,
LaTeX2e file, revtex style. Paper throughly restructured and rewritten. Much
more details are given, but essential results and conclusions are unchanged.
Version accepted for publicatio
Casimir effect in the boundary state formalism
Casimir effect in the planar setting is described using the boundary state
formalism, for general partially reflecting boundaries. It is expressed in
terms of the low-energy degrees of freedom, which provides a large distance
expansion valid for general interacting field theories provided there is a
non-vanishing mass gap. The expansion is written in terms of the scattering
amplitudes, and needs no ultraviolet renormalization. We also discuss the case
when the quantum field has a nontrivial vacuum configuration.Comment: 11 pages. Proceedings contribution of talk given at the Workshop on
Quantum Field Theory under the Influence of External Conditions (QFEXT07),
University of Leipzig, September 16-21, 2007. To appear in J. Phys.
Phase Fluctuations near the Chiral Critical Point
The Helmholtz free energy density is parametrized as a function of
temperature and baryon density near the chiral critical point of QCD. The
parametrization incorporates the expected critical exponents and amplitudes. An
expansion away from equilibrium states is achieved with Landau theory. This is
used to calculate the probability that the system is found at a density other
than the equilibrium one. Such fluctuations are predicted to be very large in
heavy ion collisions.Comment: 7 pages, 8 figures, Winter Workshop on Nuclear Dynamics 201
Strongly enhanced effective mass in dilute two-dimensional electron systems: System-independent origin
We measure the effective mass in a dilute two-dimensional electron system in
(111)-silicon by analyzing temperature dependence of the Shubnikov-de Haas
oscillations in the low-temperature limit. A strong enhancement of the
effective mass with decreasing electron density is observed. The mass
renormalization as a function of the interaction parameter r_s is in good
agreement with that reported for (100)-silicon, which shows that the relative
mass enhancement is system- and disorder-independent being determined by
electron-electron interactions only.Comment: As publishe
A kinetic theory of diffusion in general relativity with cosmological scalar field
A new model to describe the dynamics of particles undergoing diffusion in
general relativity is proposed. The evolution of the particle system is
described by a Fokker-Planck equation without friction on the tangent bundle of
spacetime. It is shown that the energy-momentum tensor for this matter model is
not divergence-free, which makes it inconsistent to couple the Fokker-Planck
equation to the Einstein equations. This problem can be solved by postulating
the existence of additional matter fields in spacetime or by modifying the
Einstein equations. The case of a cosmological scalar field term added to the
left hand side of the Einstein equations is studied in some details. For the
simplest cosmological model, namely the flat Robertson-Walker spacetime, it is
shown that, depending on the initial value of the cosmological scalar field,
which can be identified with the present observed value of the cosmological
constant, either unlimited expansion or the formation of a singularity in
finite time will occur in the future. Future collapse into a singularity also
takes place for a suitable small but positive present value of the cosmological
constant, in contrast to the standard diffusion-free scenario.Comment: 17 pages, no figures. The present version corrects an erroneous
statement on the physical interpretation of the results made in the original
publicatio
Novel approach to a perfect lens
Within the framework of an exact analytical solution of Maxwell equations in
a space domain, it is shown that optical scheme based on a slab with negative
refractive index () (Veselago lens or Pendry lens) does not possess
focusing properties in the usual sense . In fact, the energy in such systems
does not go from object to its "image", but from object and its "image" to an
intersection point inside a metamaterial layer, or vice versa. A possibility of
applying this phenomenon to a creation of entangled states of two atoms is
discussed.Comment: 4 pages, 6 figure
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