3,715 research outputs found
Validity of single-channel model for a spin-orbit coupled atomic Fermi gas near Feshbach resonances
We theoretically investigate a Rashba spin-orbit coupled Fermi gas near
Feshbach resonances, by using mean-field theory and a two-channel model that
takes into account explicitly Feshbach molecules in the close channel. In the
absence of spin-orbit coupling, when the channel coupling between the
closed and open channels is strong, it is widely accepted that the two-channel
model is equivalent to a single-channel model that excludes Feshbach molecules.
This is the so-called broad resonance limit, which is well-satisfied by
ultracold atomic Fermi gases of Li atoms and K atoms in current
experiments. Here, with Rashba spin-orbit coupling we find that the condition
for equivalence becomes much more stringent. As a result, the single-channel
model may already be insufficient to describe properly an atomic Fermi gas of
K atoms at a moderate spin-orbit coupling. We determine a characteristic
channel coupling strength as a function of the spin-orbit coupling
strength, above which the single-channel and two-channel models are
approximately equivalent. We also find that for narrow resonance with small
channel coupling, the pairing gap and molecular fraction is strongly suppressed
by SO coupling. Our results can be readily tested in K atoms by using
optical molecular spectroscopy.Comment: 6 pages, 6 figure
Superfluid density and Berezinskii-Kosterlitz-Thouless transition of a spin-orbit coupled Fulde-Ferrell superfluid
We theoretically investigate the superfluid density and
Berezinskii-Kosterlitz-Thouless (BKT) transition of a two-dimensional Rashba
spin-orbit coupled atomic Fermi gas with both in-plane and out-of-plane Zeeman
fields. It was recently predicted that, by tuning the two Zeeman fields, the
system may exhibit different exotic Fulde-Ferrell (FF) superfluid phases,
including the gapped FF, gapless FF, gapless topological FF and gapped
topological FF states. Due to the FF paring, we show that the superfluid
density (tensor) of the system becomes anisotropic. When an in-plane Zeeman
field is applied along the \textit{x}-direction, the tensor component along the
\textit{y}-direction is generally larger than in most
parameter space. At zero temperature, there is always a discontinuity jump in
as the system evolves from a gapped FF into a gapless FF state. With
increasing temperature, such a jump is gradually washed out. The critical BKT
temperature has been calculated as functions of the spin-orbit coupling
strength, interatomic interaction strength, in-plane and out-of-plane Zeeman
fields. We predict that the novel FF superfluid phases have a significant
critical BKT temperature, typically at the order of , where
is the Fermi degenerate temperature. Therefore, their observation is within the
reach of current experimental techniques in cold-atom laboratories.Comment: 11 pages, 7 figure
Gapless topological Fulde-Ferrell superfluidity in spin-orbit coupled Fermi gases
Topological superfluids usually refer to a superfluid state which is gapped
in the bulk but metallic at the boundary. Here we report that a gapless,
topologically non-trivial superfluid with inhomogeneous Fulde-Ferrell pairing
order parameter can emerge in a two-dimensional spin-orbit coupled Fermi gas,
in the presence of both in-plane and out-of-plane Zeeman fields. The
Fulde-Ferrell pairing - induced by the spin-orbit coupling and in-plane Zeeman
field - is responsible for this gapless feature. This exotic superfluid has a
significant Berezinskii-Kosterlitz-Thouless (BKT) transition temperature and
has robust Majorana edge modes against disorder owing to its topological
nature.Comment: 5 pages, 5 figures; add the results on the critical BKT temperature
and superfluid density, as well as the discussion on the robustness of the
chiral edge states against disorde
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