624 research outputs found
Bosons Confined in Optical Lattices: the Numerical Renormalization Group revisited
A Bose-Hubbard model, describing bosons in a harmonic trap with a
superimposed optical lattice, is studied using a fast and accurate variational
technique (MF+NRG): the Gutzwiller mean-field (MF) ansatz is combined with a
Numerical Renormalization Group (NRG) procedure in order to improve on both.
Results are presented for one, two and three dimensions, with particular
attention to the experimentally accessible momentum distribution and possible
satellite peaks in this distribution. In one dimension, a comparison is made
with exact results obtained using Stochastich Series Expansion.Comment: 10 pages, 15 figure
The SHARE survey: presentation and first results for the French edition
The SHARE survey (Survey on Health Ageing and Retirement in Europe) is an international and multidisciplinary operation launched in 2002, led by a European network coordinated by the MEA of the University of Mannheim. Its ambition is to become an instrument of reference for interdisciplinary research on ageing. A test of this survey on approximately 12000 households took place in 10 European countries in 2004. This first wave already allows the realization of comparative work on participating countries, either descriptive or microeconometric. Data have been made available to researchers in spring 2005, after publication of a volume of first results. This article presents the survey and gives an outline of its potentialities, using some first descriptive results for France.Ageing, health, pensions
Finite-temperature effects on the superfluid Bose-Einstein condensation of confined ultracold atoms in three-dimensional optical lattices
We discuss the finite-temperature phase diagram in the three-dimensional
Bose-Hubbard (BH) model in the strong correlation regime, relevant for
Bose-Einstein condensates in optical lattices, by employing a quantum rotor
approach. In systems with strong on site repulsive interactions, the rotor U(1)
phase variable dual to the local boson density emerges as an important
collective field. After establishing the connection between the rotor
construction and the the on--site interaction in the BH model the robust
effective action formalism is developed which allows us to study the superfluid
phase transition in various temperature--interaction regimes
Quantitative Determination of Temperature in the Approach to Magnetic Order of Ultracold Fermions in an Optical Lattice
We perform a quantitative simulation of the repulsive Fermi-Hubbard model using an ultracold gas trapped in an optical lattice. The entropy of the system is determined by comparing accurate measurements of the equilibrium double occupancy with theoretical calculations over a wide range of parameters. We demonstrate the applicability of both high-temperature series and dynamical mean-field theory to obtain quantitative agreement with the experimental data. The reliability of the entropy determination is confirmed by a comprehensive analysis of all systematic errors. In the center of the Mott insulating cloud we obtain an entropy per atom as low as 0.77k(B) which is about twice as large as the entropy at the Neel transition. The corresponding temperature depends on the atom number and for small fillings reaches values on the order of the tunneling energy
Revealing the Superfluid Lambda Transition in the Universal Thermodynamics of a Unitary Fermi Gas
We have observed the superfluid phase transition in a strongly interacting
Fermi gas via high-precision measurements of the local compressibility, density
and pressure down to near-zero entropy. Our data completely determine the
universal thermodynamics of strongly interacting fermions without any fit or
external thermometer. The onset of superfluidity is observed in the
compressibility, the chemical potential, the entropy, and the heat capacity. In
particular, the heat capacity displays a characteristic lambda-like feature at
the critical temperature of . This is the first clear
thermodynamic signature of the superfluid transition in a spin-balanced atomic
Fermi gas. Our measurements provide a benchmark for many-body theories on
strongly interacting fermions, relevant for problems ranging from
high-temperature superconductivity to the equation of state of neutron stars.Comment: 11 pages, 8 figure
Novel Mechanism of Supersolid of Ultracold Polar Molecules in Optical Lattices
We study the checkerboard supersolid of the hard-core Bose-Hubbard model with
the dipole-dipole interaction. This supersolid is different from all other
supersolids found in lattice models in the sense that superflow paths through
which interstitials or vacancies can hop freely are absent in the crystal. By
focusing on repulsive interactions between interstitials, we reveal that the
long-range tail of the dipole-dipole interaction have the role of increasing
the energy cost of domain wall formations. This effect produces the supersolid
by the second-order hopping process of defects. We also perform exact quantum
Monte Carlo simulations and observe a novel double peak structure in the
momentum distribution of bosons, which is a clear evidence for supersolid. This
can be measured by the time-of-flight experiment in optical lattice systems
Experimental evidence for an intermediate phase in the multiferroic YMnO3
We have studied YMnO by high-temperature synchrotron X-ray powder
diffraction, and have carried out differential thermal analysis and dilatometry
on a single crystal sample. These experiments show two phase transitions at
about 1100K and 1350K, respectively. This demonstrates the existence of an
intermediate phase between the room temperature ferroelectric and the high
temperature centrosymmetric phase. This study identifies for the first time the
different high-temperature phase transitions in YMnO.Comment: 10 pages 5 figures. New version, Additional data, Journal of Physics:
Condensed Matter, in Pres
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