6,338 research outputs found
Comment on ``Phase and Phase Diffusion of a Split Bose-Einstein Condensate''
Recently Javanainen and Wilkens [Phys. Rev. Lett. 78, 4675 (1997)] have
analysed an experiment in which an interacting Bose condensate, after being
allowed to form in a single potential well, is "cut" by splitting the well
adiabatically with a very high potential barrier, and estimate the rate at
which, following the cut, the two halves of the condensate lose the "memory" of
their relative phase. We argue that, by neglecting the effect of interactions
in the initial state before the separation, they have overestimated the rate of
phase randomization by a numerical factor which grows with the interaction
strength and with the slowness of the separation process.Comment: 2 pages, no figures, to appear in Phys. Rev. Let
Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey
We present the results of a search for cool white dwarfs in the United
Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area
Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky
Survey (SDSS) to identify cool hydrogen-rich white dwarf candidates by their
neutral optical colors and blue near-infrared colors, as well as faint Reduced
Proper Motion magnitudes. Optical spectroscopy was obtained at Gemini
Observatory, and showed the majority of the candidates to be newly identified
cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants.
Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with
effective temperature T_eff ~ 6000 K. The current followup of 1400 deg2 of sky
has produced thirteen new white dwarfs. Model fits to the photometry show that
seven of the newly identified white dwarfs have 4120 K <= T_eff <= 4480 K, and
cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km/s <= v_tan <= 85 km/s
and are likely to be thick disk 10-11 Gyr-old objects. The other half of the
sample has 4610 K <= T_eff <= 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr,
and 60 km/s <= v_tan <= 100 km/s. These are either thin disk remnants with
unusually high velocities, or lower-mass remnants of thick disk or halo late-F
or G stars.Comment: To appear in ApJ, accepted April 18 2011. 34 pages include 11 Figures
and 5 Table
Spin 1/2 Fermions in the Unitary Regime: A Superfluid of a New Type
We have studied, in a fully non-perturbative calculation, a dilute system of
spin 1/2 interacting fermions, characterized by an infinite scattering length
at finite temperatures. Various thermodynamic properties and the condensate
fraction were calculated and we have also determined the critical temperature
for the superfluid-normal phase transition in this regime. The thermodynamic
behavior appears as a rather surprising and unexpected melange of fermionic and
bosonic features. The thermal response of a spin 1/2 fermion at the BCS-BEC
crossover should be classified as that of a new type of superfluid.Comment: 4 pages, 1 figure, published versio
On the specific heat of a fermionic atomic cloud in the unitary regime
In the unitary regime, when the scattering amplitude greatly exceeds in
magnitude the average inter-particle separation, and below the critical
temperature thermal properties of an atomic fermionic cloud are governed by the
collective modes, specifically the Bogoliubov-Anderson sound modes. The
specific heat of an atomic cloud in a elongated trap in particular has a rather
compex temperature dependence, which changes from an exponential behavior at
very low temperatures (), to for
and then continuosly to at temperatures just below the critical temperature, when the surface
modes play a dominant role. Only the low () and high () temperature
power laws are well defined. For the intermediate temperatures one can
introduce at most a gradually increasing with temperature exponent.Comment: 4 page
Criteria of off-diagonal long-range order in Bose and Fermi systems based on the Lee-Yang cluster expansion method
The quantum-statistical cluster expansion method of Lee and Yang is extended
to investigate off-diagonal long-range order (ODLRO) in one- and
multi-component mixtures of bosons or fermions. Our formulation is applicable
to both a uniform system and a trapped system without local-density
approximation and allows systematic expansions of one- and multi-particle
reduced density matrices in terms of cluster functions which are defined for
the same system with Boltzmann statistics. Each term in this expansion can be
associated with a Lee-Yang graph. We elucidate a physical meaning of each
Lee-Yang graph; in particular, for a mixture of ultracold atoms and bound
dimers, an infinite sum of the ladder-type Lee-Yang 0-graphs is shown to lead
to Bose-Einstein condensation of dimers below the critical temperature. In the
case of Bose statistics, an infinite series of Lee-Yang 1-graphs is shown to
converge and gives the criteria of ODLRO at the one-particle level.
Applications to a dilute Bose system of hard spheres are also made. In the case
of Fermi statistics, an infinite series of Lee-Yang 2-graphs is shown to
converge and gives the criteria of ODLRO at the two-particle level.
Applications to a two-component Fermi gas in the tightly bound limit are also
made.Comment: 21 pages, 10 figure
Density-matrix functionals for pairing in mesoscopic superconductors
A functional theory based on single-particle occupation numbers is developed
for pairing. This functional, that generalizes the BCS approach, directly
incorporates corrections due to particle number conservation. The functional is
benchmarked with the pairing Hamiltonian and reproduces perfectly the energy
for any particle number and coupling.Comment: 4 pages, 4 figures, revised versio
The Vortex State in a Strongly Coupled Dilute Atomic Fermionic Superfluid
We show that in a dilute Fermionic superfluid, when the Fermions interact
with an infinite scattering length, a vortex state is characterized by a strong
density depletion along the vortex core. This feature can make a direct
visualization of vortices in Fermionic superfluids possible.Comment: 4 pages, 3 figures, published version, some small changes and new and
updated reference
Landau damping of Bogoliubov excitations in optical lattices at finite temperature
We study the damping of Bogoliubov excitations in an optical lattice at
finite temperatures. For simplicity, we consider a Bose-Hubbard tight-binding
model and limit our analysis to the lowest excitation band. We use the Popov
approximation to calculate the temperature dependence of the number of
condensate atoms in each lattice well. We calculate the Landau
damping of a Bogoliubov excitation in an optical lattice due to coupling to a
thermal cloud of excitations. While most of the paper concentrates on 1D
optical lattices, we also briefly present results for 2D and 3D lattices. For
energy conservation to be satisfied, we find that the excitations in the
collision process must exhibit anomalous dispersion ({\it i.e.} the excitation
energy must bend upward at low momentum), as also exhibited by phonons in
superfluid . This leads to the sudden disappearance of all damping
processes in -dimensional simple cubic optical lattice when , where is the on-site interaction, and is the hopping matrix
element. Beliaev damping in a 1D optical lattice is briefly discussed.Comment: 28 pages, 9 figure
The StoreGate: a Data Model for the Atlas Software Architecture
The Atlas collaboration at CERN has adopted the Gaudi software architecture
which belongs to the blackboard family: data objects produced by knowledge
sources (e.g. reconstruction modules) are posted to a common in-memory data
base from where other modules can access them and produce new data objects. The
StoreGate has been designed, based on the Atlas requirements and the experience
of other HENP systems such as Babar, CDF, CLEO, D0 and LHCB, to identify in a
simple and efficient fashion (collections of) data objects based on their type
and/or the modules which posted them to the Transient Data Store (the
blackboard). The developer also has the freedom to use her preferred key class
to uniquely identify a data object according to any other criterion. Besides
this core functionality, the StoreGate provides the developers with a powerful
interface to handle in a coherent fashion persistable references, object
lifetimes, memory management and access control policy for the data objects in
the Store. It also provides a Handle/Proxy mechanism to define and hide the
cache fault mechanism: upon request, a missing Data Object can be transparently
created and added to the Transient Store presumably retrieving it from a
persistent data-base, or even reconstructing it on demand.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 4 pages, LaTeX, MOJT00
Pionic BEC--BCS crossover at finite isospin chemical potential
We study the character change of the pionic condensation at finite isospin
chemical potential \mu_\mathrm{I} by adopting the linear sigma model as a
non-local interaction between quarks. At low |\mu_\mathrm{I}| the condensation
is purely bosonic, then the Cooper pairing around the Fermi surface grows
gradually as |\mu_\mathrm{I}| increases. This q-\bar q pairing is weakly
coupled in comparison with the case of the q-q pairing that leads to color
superconductivity.Comment: 17 pages, 3 figures, typos in eq.(6) and refs.[37] and [41] are
corrected, published in Phys. Rev.
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