6,631 research outputs found
Faint Field Galaxies Around Bright Stars - A New Strategy for Imaging at the Diffraction Limit
This paper presents a new strategy for observing faint galaxies with high
order natural guide star systems. We have imaged 5 high galactic latitude
fields within the isoplanatic patch of bright stars (8.5 < R < 10.3 mag). The
fields provide a rich set of faint field galaxies that are observable with a
natural guide star adaptive optics system on a large telescope. Due to the
small fields of many AO science cameras, these preliminary images are necessary
to identify candidate galaxies. We present the photometry and positions for 78
objects (at least 40 galaxies) near five bright stars, appropriate for
diffraction limited studies with the Keck and other AO systems on large
ground-based telescopes. The K band seeing conditions in each field were
excellent (0.4" - 0.7") allowing us to identify stars and estimate galaxy
sizes. We also simulate AO images of field galaxies to determine the
feasibility of infrared morphological studies at the diffraction limit. With
new high order AO systems coming on line with 8-10 meter class telescopes, we
believe these observations are invaluable in beginning to study faint galaxy
populations at the diffraction limit.Comment: 15 pages, Latex, 9 figures. Accepted for publication in P.A.S.
Finite temperature phase diagram of a polarized Fermi gas in an optical lattice
We present phase diagrams for a polarized Fermi gas in an optical lattice as
a function of temperature, polarization, and lattice filling factor. We
consider the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO), Sarma or breached pair
(BP), and BCS phases, and the normal state and phase separation. We show that
the FFLO phase appears in a considerable portion of the phase diagram. The
diagrams have two critical points of different nature. We show how various
phases leave clear signatures to momentum distributions of the atoms which can
be observed after time of flight expansion.Comment: Journal versio
Profiles of near-resonant population-imbalanced trapped Fermi gases
We investigate the density profiles of a partially polarized trapped Fermi
gas in the BCS-BEC crossover region using mean field theory within the local
density approximation. Within this approximation the gas is phase separated
into concentric shells. We describe how the structure of these shells depends
upon the polarization and the interaction strength. A Comparison with
experiments yields insight into the possibility of a polarized superfluid
phase.Comment: 4 pages, 5 Figures, Published versio
A unified picture of ferromagnetism, quasi-long range order and criticality in random field models
By applying the recently developed nonperturbative functional renormalization
group (FRG) approach, we study the interplay between ferromagnetism, quasi-long
range order (QLRO) and criticality in the -dimensional random field O(N)
model in the whole (, ) diagram. Even though the "dimensional reduction"
property breaks down below some critical line, the topology of the phase
diagram is found similar to that of the pure O(N) model, with however no
equivalent of the Kosterlitz-Thouless transition. In addition, we obtain that
QLRO, namely a topologically ordered "Bragg glass" phase, is absent in the
3--dimensional random field XY model. The nonperturbative results are
supplemented by a perturbative FRG analysis to two loops around .Comment: 4 pages, 4 figure
Shot-noise-driven escape in hysteretic Josephson junctions
We have measured the influence of shot noise on hysteretic Josephson
junctions initially in macroscopic quantum tunnelling (MQT) regime. Escape
threshold current into the resistive state decreases monotonically with
increasing average current through the scattering conductor, which is another
tunnel junction. Escape is predominantly determined by excitation due to the
wide-band shot noise. This process is equivalent to thermal activation (TA)
over the barrier at temperatures up to about four times above the critical
temperature of the superconductor. The presented TA model is in excellent
agreement with the experimental results
Pair formation and collapse in imbalanced Fermion populations with unequal masses
We present an exact Quantum Monte Carlo study of the effect of unequal masses
on pair formation in Fermionic systems with population imbalance loaded into
optical lattices. We have considered three forms of the attractive interaction
and find in all cases that the system is unstable and collapses as the mass
difference increases and that the ground state becomes an inhomogeneous
collapsed state. We also address the question of canonical vs grand canonical
ensemble and its role, if any, in stabilizing certain phases
Noise correlations of the ultra-cold Fermi gas in an optical lattice
In this paper we study the density noise correlations of the two component
Fermi gas in optical lattices. Three different type of phases, the BCS-state
(Bardeen, Cooper, and Schieffer), the FFLO-state (Fulde, Ferrel, Larkin, and
Ovchinnikov), and BP (breach pair) state, are considered. We show how these
states differ in their noise correlations. The noise correlations are
calculated not only at zero temperature, but also at non-zero temperatures
paying particular attention to how much the finite temperature effects might
complicate the detection of different phases. Since one-dimensional systems
have been shown to be very promising candidates to observe FFLO states, we
apply our results also to the computation of correlation signals in a
one-dimensional lattice. We find that the density noise correlations reveal
important information about the structure of the underlying order parameter as
well as about the quasiparticle dispersions.Comment: 25 pages, 11 figures. Some figures are updated and text has been
modifie
Relativistic BCS-BEC crossover in a boson-fermion model
We investigate the crossover from Bardeen-Cooper-Schrieffer (BCS) pairing to
a Bose-Einstein condensate (BEC) in a relativistic superfluid within a
boson-fermion model. The model includes, besides the fermions, separate bosonic
degrees of freedom, accounting for the bosonic nature of the Cooper pairs. The
crossover is realized by tuning the difference between the boson mass and boson
chemical potential as a free parameter. The model yields populations of
condensed and uncondensed bosons as well as gapped and ungapped fermions
throughout the crossover region for arbitrary temperatures. Moreover, we
observe the appearance of antiparticles for sufficiently large values of the
crossover parameter. As an application, we study pairing of fermions with
imbalanced populations. The model can potentially be applied to color
superconductivity in dense quark matter at strong couplings.Comment: ReVTex4, 19 pages, 10 figures; new chapter added about the case of
imbalanced fermion populations; minor modifications to main part; references
adde
Critical phenomena in a highly constrained classical spin system: Neel ordering from the Coulomb phase
Many classical, geometrically frustrated antiferromagnets have
macroscopically degenerate ground states. In a class of three-dimensional
systems, the set of degenerate ground states has power-law correlations and is
an example of a Coulomb phase. We investigate Neel ordering from such a Coulomb
phase, induced by weak additional interactions that lift the degeneracy. We
show that the critical point belongs to a universality class that is different
from the one for the equivalent transition out of the paramagnetic phase, and
that it is characterised by effective long-range interactions; alternatively,
ordering may be discontinuous. We suggest that a transition of this type may be
realised by applying uniaxial stress to a pyrochlore antiferromagnet.Comment: 4 pages, 3 figure
Generic Phase Diagram of Fermion Superfluids with Population Imbalance
It is shown by microscopic calculations for trapped imbalanced Fermi
superfluids that the gap function has always sign changes, i.e., the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state like, up to a critical imbalance
, beyond which normal state becomes stable, at temperature T=0. A phase
diagram is constructed in vs , where the BCS state without sign change
is stable only at . We reproduce the observed bimodality in the
density profile to identify its origin and evaluate as functions of
and the coupling strength. These dependencies match with the recent
experiments.Comment: 5 pages, 5 figures, replaced by the version to appear in PR
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