24,982 research outputs found
Quantum phase transitions in fully connected spin models: an entanglement perspective
We consider a set of fully connected spins models that display first- or
second-order transitions and for which we compute the ground-state entanglement
in the thermodynamical limit. We analyze several entanglement measures
(concurrence, R\'enyi entropy, and negativity), and show that, in general,
discontinuous transitions lead to a jump of these quantities at the transition
point. Interestingly, we also find examples where this is not the case.Comment: 9 pages, 7 figures, published versio
Atmospheric turbulence in phase-referenced and wide-field interferometric images: Application to the SKA
Phase referencing is a standard calibration procedure in radio
interferometry. It allows to detect weak sources by using quasi-simultaneous
observations of closeby sources acting as calibrators. Therefore, it is assumed
that, for each antenna, the optical paths of the signals from both sources are
similar. However, atmospheric turbulence may introduce strong differences in
the optical paths of the signals and affect, or even waste, phase referencing
for cases of relatively large calibrator-to-target separations and/or bad
weather. The situation is similar in wide-field observations, since the random
deformations of the images, mostly caused by atmospheric turbulence, have
essentially the same origin as the random astrometric variations of
phase-referenced sources with respect to the phase center of their calibrators.
In this paper, we present the results of a Monte Carlo study of the astrometric
precision and sensitivity of an interferometric array (a realization of the
Square Kilometre Array, SKA) in phase-referenced and wide-field observations.
These simulations can be extrapolated to other arrays by applying the
corresponding corrections. We consider several effects from the turbulent
atmosphere (i.e., ionosphere and wet component of the troposphere) and also
from the antenna receivers. We study the changes in dynamic range and
astrometric precision as a function of observing frequency, source separation,
and strength of the turbulence. We find that, for frequencies between 1 and 10
GHz, it is possible to obtain images with high fidelity, although the
atmosphere strongly limits the sensitivity of the instrument compared to the
case with no atmosphere. Outside this frequency window, the dynamic range of
the images and the accuracy of the source positions decrease. [...] (Incomplete
abstract. Please read manuscript.)Comment: 9 pages, 11 figures. Accepted for publication in A&A
On the energy saved by interlayer interactions in the superconducting state of cuprates
A Ginzburg-Landau-like functional is proposed reproducing the main low-energy
features of various possible high-Tc superconducting mechanisms involving
energy savings due to interlayer interactions. The functional may be used to
relate these savings to experimental quantities. Two examples are given,
involving the mean-field specific heat jump at Tc and the superconducting
fluctuations above Tc. Comparison with existing data suggests, e.g., that the
increase of Tc due to the so-called interlayer tunneling (ILT) mechanism of
interlayer kinetic-energy savings is negligible in optimally-doped Bi-2212.Comment: 12 pages, no figures. Version history: 21-aug-2003, first version
(available on http://arxiv.org/abs/cond-mat/0308423v1); 15-jan-2004, update
to match Europhys. Lett. publication (minor grammar changes, updates in
bibliography - e.g., refs. 5 and 26
Mixed-state dynamics in one-dimensional quantum lattice systems: a time-dependent superoperator renormalization algorithm
We present an algorithm to study mixed-state dynamics in one-dimensional
quantum lattice systems. The algorithm can be used, e.g., to construct thermal
states or to simulate real time evolutions given by a generic master equation.
Its two main ingredients are (i) a superoperator renormalization scheme to
efficiently describe the state of the system and (ii) the time evolving block
decimation (TEBD) technique to efficiently update the state during a time
evolution. The computational cost of a simulation increases significantly with
the amount of correlations between subsystems but it otherwise depends only
linearly in the system size. We present simulations involving quantum spins and
fermions in one spatial dimension.Comment: See also F. Verstraete et al. cond-mat/040642
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