486 research outputs found
Measuring the equation of state of trapped ultracold bosonic systems in an optical lattice with in-situ density imaging
We analyze quantitatively how imaging techniques with single-site resolution
allow to measure thermodynamical properties that cannot be inferred from
time-of-light images for the trapped Bose-Hubbard model. If the normal state
extends over a sufficiently large range, the chemical potential and the
temperature can be extracted from a single shot, provided the sample is in
thermodynamic equilibrium. When the normal state is too narrow, temperature is
low but can still be extracted using the fluctuation-dissipation theorem over
the entire trap range as long as the local density approximation remains valid,
as was recently suggested by Qi Zhou and Tin-Lun Ho [arXiv:0908.3015]. However,
for typical present-day experiments, the number of samples needed is of the
order of 1000 in order to get the temperature at least accurate, but it
is possible to reduce the variance by 2 orders of magnitude if the
density-density correlation length is short, which is the case for the
Bose-Hubbard model. Our results provide further evidence that cold gases in an
optical lattices can be viewed as quantum analog computers.Comment: 8 pages, 10 figure
Vacancy supersolid of hard-core bosons on the square lattice
The ground state of hard-core bosons on the square lattice with nearest and
next-nearest neighbor repulsion is studied by Quantum Monte Carlo simulations.
A supersolid phase with vacancy condensation and 'star' diagonal ordering is
found for filling less than a quarter. At fillings above one quarter, a
supersolid phase exists between the star and the stripe crystal at
half-filling. No supersolid phase occurs above quarter-filling, if the ground
state at half-filling is either a checkerboard crystal or a superfluid. No
commensurate supersolid phase is observed.Comment: Replaced with published versio
Spin and charge dynamics of a quasi-one-dimensional antiferromagnetic metal
We use quantum Monte Carlo simulations to study a finite-temperature
dimensional-crossover-driven evolution of spin and charge dynamics in weakly
coupled Hubbard chains with a half-filled band. The low-temperature behavior of
the charge gap indicates a crossover between two distinct energy scales: a
high-energy one-dimensional (1D) Mott gap due to the umklapp process and a
low-energy gap which stems from long-range antiferromagnetic (AF) fluctuations.
Away from the 1D regime and at temperature scales above the charge gap, the
emergence of a zero-frequency Drude-like feature in the interchain optical
conductivity implies the onset of a higher-dimensional
metal. In this metallic phase, enhanced quasiparticle scattering off
finite-range AF fluctuations results in incoherent single-particle dynamics.
The coupling between spin and charge fluctuations is also seen in the spin
dynamical structure factor displaying damped spin
excitations (paramagnons) close to the AF wave-vector and
particle-hole continua near 1D momentum transfers spanning quasiparticles at
the Fermi surface. We relate our results to the charge deconfinement in
quasi-1D organic Bechgaard-Fabre salts.Comment: 14+ pages, 13 figures; new Fig. 5c, added Fig.
Influence of the trap shape on the superfluid-Mott transition in ultracold atomic gases
The coexistence of superfluid and Mott insulator, due to the quadratic
confinement potential in current optical lattice experiments, makes the
accurate detection of the superfluid-Mott transition difficult. Studying
alternative trapping potentials which are experimentally realizable and have a
flatter center, we find that the transition can be better resolved, but at the
cost of a more difficult tuning of the particle filling. When mapping out the
phase diagram using local probes and the local density approximation we find
that the smoother gradient of the parabolic trap is advantageous.Comment: 5 pages, 6 figure
Optimal Monte Carlo Updating
Based on Peskun's theorem it is shown that optimal transition matrices in
Markov chain Monte Carlo should have zero diagonal elements except for the
diagonal element corresponding to the largest weight. We will compare the
statistical efficiency of this sampler to existing algorithms, such as
heat-bath updating and the Metropolis algorithm. We provide numerical results
for the Potts model as an application in classical physics. As an application
in quantum physics we consider the spin 3/2 XY model and the Bose-Hubbard model
which have been simulated by the directed loop algorithm in the stochastic
series expansion framework.Comment: 6 pages, 5 figures, replaced with published versio
Complete classification of purely magnetic, non-rotating and non-accelerating perfect fluids
Recently the class of purely magnetic non-rotating dust spacetimes has been
shown to be empty (Wylleman, Class. Quant. Grav. 23, 2727). It turns out that
purely magnetic rotating dust models are subject to severe integrability
conditions as well. One of the consequences of the present paper is that also
rotating dust cannot be purely magnetic when it is of Petrov type D or when it
has a vanishing spatial gradient of the energy density. For purely magnetic and
non-rotating perfect fluids on the other hand, which have been fully classified
earlier for Petrov type D (Lozanovski, Class. Quant. Grav. 19, 6377), the fluid
is shown to be non-accelerating if and only if the spatial density gradient
vanishes. Under these conditions, a new and algebraically general solution is
found, which is unique up to a constant rescaling, which is spatially
homogeneous of Bianchi type , has degenerate shear and is of Petrov type
I( in the extended Arianrhod-McIntosh classification.
The metric and the equation of state are explicitly constructed and
properties of the model are briefly discussed. We finally situate it within the
class of normal geodesic flows with degenerate shear tensor.Comment: 12 pages; introduction partly rewritten, notation made more clear,
table of results adde
Silent universes with a cosmological constant
We study non-degenerate (Petrov type I) silent universes in the presence of a
non-vanishing cosmological constant L. In contrast to the L=0 case, for which
the orthogonally spatially homogeneous Bianchi type I metrics most likely are
the only admissible metrics, solutions are shown to exist when L is positive.
The general solution is presented for the case where one of the eigenvalues of
the expansion tensor is 0.Comment: 11 pages; several typos corrected which were still present in CGQ
version; minor change
Electromagnetic and Gravitational Invariants
The curvature invariants have been subject of recent interest in the context of the experimental detection of the gravitomagnetic field, namely due to the debate concerning the notions of "extrinsic" and "intrinsic" gravitomagnetism. In this work we explore the physical meaning of the curvature invariants, dissecting their relationship with the gravitomagnetic effects
Electromagnetic and Gravitational Invariants
The curvature invariants have been subject of recent interest in the context of the experimental detection of the gravitomagnetic field, namely due to the debate concerning the notions of "extrinsic" and "intrinsic" gravitomagnetism. In this work we explore the physical meaning of the curvature invariants, dissecting their relationship with the gravitomagnetic effects
Electromagnetic and Gravitational Invariants
The curvature invariants have been subject of recent interest in the context of the experimental detection of the gravitomagnetic field, namely due to the debate concerning the notions of "extrinsic" and "intrinsic" gravitomagnetism. In this work we explore the physical meaning of the curvature invariants, dissecting their relationship with the gravitomagnetic effects
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