18,071 research outputs found

### A self-consistent theory of atomic Fermi gases with a Feshbach resonance at the superfluid transition

A self-consistent theory is derived to describe the BCS-BEC crossover for a
strongly interacting Fermi gas with a Feshbach resonance. In the theory the
fluctuation of the dressed molecules, consisting of both preformed Cooper-pairs
and ``bare'' Feshbach molecules, has been included within a self-consistent
$T$-matrix approximation, beyond the Nozi\`{e}res and Schmitt-Rink strategy
considered by Ohashi and Griffin. The resulting self-consistent equations are
solved numerically to investigate the normal state properties of the crossover
at various resonance widths. It is found that the superfluid transition
temperature $T_c$ increases monotonically at all widths as the effective
interaction between atoms becomes more attractive. Furthermore, a residue
factor $Z_m$ of the molecule's Green function and a complex effective mass have
been determined, to characterize the fraction and lifetime of Feshbach
molecules at $T_c$. Our many-body calculations of $Z_m$ agree qualitatively
well with the recent measurments on the gas of $^6$Li atoms near the broad
resonance at 834 Gauss. The crossover from narrow to broad resonances has also
been studied.Comment: 6 papes, 6 figure

### Topological superfluid in one-dimensional spin-orbit coupled atomic Fermi gases

ARC Centre of Excellence for Quantum-Atom Optics, Centre for Atom Optics and
Ultrafast Spectroscopy, Swinburne University of Technology, Melbourne 3122,
AustraliaComment: 7 pages, 8 figures; submitted to Physical Review

### Collective mode evidence of high-spin bosonization in a trapped one-dimensional atomic Fermi gas with tunable spin

We calculate the frequency of collective modes of a one-dimensional
repulsively interacting Fermi gas with high-spin symmetry confined in harmonic
traps at zero temperature. This is a system realizable with fermionic
alkaline-earth-metal atoms such as $^{173}$Yb, which displays an exact
SU($\kappa$) spin symmetry with $\kappa\geqslant2$ and behaves like a spinless
interacting Bose gas in the limit of infinite spin components
$\kappa\rightarrow\infty$, namely high-spin bosonization. We solve the
homogeneous equation of state of the high-spin Fermi system by using Bethe
ansatz technique and obtain the density distribution in harmonic traps based on
local density approximation. The frequency of collective modes is calculated by
exactly solving the zero-temperature hydrodynamic equation. In the limit of
large number of spin-components, we show that the mode frequency of the system
approaches to that of a one-dimensional spinless interacting Bose gas, as a
result of high-spin bosonization. Our prediction of collective modes is in
excellent agreement with a very recent measurement for a Fermi gas of
$^{173}$Yb atoms with tunable spin confined in a two-dimensional tight optical
lattice.Comment: 11 pages, 8 figure

### First and second sound in a two-dimensional dilute Bose gas across the Berezinskii-Kosterlitz-Thouless transition

We theoretically investigate first and second sound of a two-dimensional (2D)
atomic Bose gas in harmonic traps by solving Landau's two-fluid hydrodynamic
equations. For an isotropic trap, we find that first and second sound modes
become degenerate at certain temperatures and exhibit typical avoided crossings
in mode frequencies. At these temperatures, second sound has significant
density fluctuation due to its hybridization with first sound and has a
divergent mode frequency towards the Berezinskii-Kosterlitz-Thouless (BKT)
transition. For a highly anisotropic trap, we derive the simplified
one-dimensional hydrodynamic equations and discuss the sound-wave propagation
along the weakly confined direction. Due to the universal jump of the
superfluid density inherent to the BKT transition, we show that the first sound
velocity exhibits a kink across the transition. Our predictions can be readily
examined in current experimental setups for 2D dilute Bose gases.Comment: 5 pages, 4 figure

### Topological Fulde-Ferrell superfluid in spin-orbit coupled atomic Fermi gases

We theoretically predict a new topological matter - topological inhomogeneous
Fulde-Ferrell superfluid - in one-dimensional atomic Fermi gases with equal
Rashba and Dresselhaus spin-orbit coupling near s-wave Feshbach resonances. The
realization of such a spin-orbit coupled Fermi system has already been
demonstrated recently by using a two-photon Raman process and the extra
one-dimensional confinement is easy to achieve using a tight two-dimensional
optical lattice. The topological Fulde-Ferrell superfluid phase is
characterized by a nonzero center-of-mass momentum and a non-trivial Berry
phase. By tuning the Rabi frequency and the detuning of Raman laser beams, we
show that such an exotic topological phase occupies a significant part of
parameter space and therefore it could be easily observed experimentally, by
using, for example, momentum-resolved and spatially resolved radio-frequency
spectroscopy.Comment: 5 pages, 4 figure

### Critical temperature of a Rashba spin-orbit coupled Bose gas in harmonic traps

We investigate theoretically Bose-Einstein condensation of an ideal, trapped
Bose gas in the presence of Rashba spin-orbit coupling. Analytic results for
the critical temperature and condensate fraction are derived, based on a
semi-classical approximation to the single-particle energy spectrum and density
of states, and are compared with exact results obtained by explicitly summing
discrete energy levels for small number of particles. We find a significant
decrease of the critical temperature and of the condensate fraction due to a
finite spin-orbit coupling. For large coupling strength and finite number of
particles $N$, the critical temperature scales as $N^{2/5}$ and $N^{2/3}$ in
three and two dimensions, respectively, contrasted to the predictions of
$N^{1/3}$ and $N^{1/2}$ in the absence of spin-orbit coupling. Finite size
corrections in three dimensions are also discussed.Comment: 9 pages and 8 figures; published version in Physical Review

### Inhomogeneous Fulde-Ferrell superfluidity in spin-orbit coupled atomic Fermi gases

Inhomogeneous superfluidity lies at the heart of many intriguing phenomena in
quantum physics. It is believed to play a central role in unconventional
organic or heavy-fermion superconductors, chiral quark matter, and neutron star
glitches. However, so far even the simplest form of inhomogeneous
superfluidity, the Fulde-Ferrell (FF) pairing state with a single
centre-of-mass momentum, is not conclusively observed due to the intrinsic
complexibility of any realistic Fermi systems in nature. Here we theoretically
predict that the controlled setting of ultracold fermionic atoms with synthetic
spin-orbit coupling induced by a two-photon Raman process, demonstrated
recently in cold-atom laboratories, provides a promising route to realize the
long-sought FF superfluidity. At experimentally accessible low temperatures
(i.e., $0.05T_{F}$, where $T_{F}$ is the Fermi temperature), the FF superfluid
state dominates the phase diagram, in sharp contrast to the conventional case
without spin-orbit coupling. We show that the finite centre-of-mass momentum
carried by Cooper pairs is directly measurable via momentum-resolved
radio-frequency spectroscopy. Our work opens the way to direct observation and
characterization of inhomogeneous superfluidity.Comment: 5 pages and 4 figures; Please see also arXiv:1211.1831 by V. B.
Shenoy for relevant discussion

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