5,841 research outputs found
Surface-enhanced pair transfer in quadrupole states of neutron-rich Sn isotopes
We investigate the neutron pair transfer modes associated with the low-lying
quadrupole states in neutron-rich Sn isotopes by means of the quasiparticle
random phase approximation based on the Skyrme-Hartree-Fock-Bogoliubov mean
field model. The transition strength of the quadrupole pair-addition mode
feeding the state is enhanced in the Sn isotopes with . The
transition density of the pair-addition mode has a large spatial extension in
the exterior of nucleus, reaching far to fm. The quadrupole
pair-addition mode reflects sensitively a possible increase of the effective
pairing interaction strength in the surface and exterior regions of
neutron-rich nuclei.Comment: 14 page
Calculation of NMR Properties of Solitons in Superfluid 3He-A
Superfluid 3He-A has domain-wall-like structures, which are called solitons.
We calculate numerically the structure of a splay soliton. We study the effect
of solitons on the nuclear-magnetic-resonance spectrum by calculating the
frequency shifts and the amplitudes of the soliton peaks for both longitudinal
and transverse oscillations of magnetization. The effect of dissipation caused
by normal-superfluid conversion and spin diffusion is calculated. The
calculations are in good agreement with experiments, except a problem in the
transverse resonance frequency of the splay soliton or in magnetic-field
dependence of reduced resonance frequencies.Comment: 15 pages, 10 figures, updated to the published versio
Two-band superfluidity from the BCS to the BEC limit
We analyze the evolution of two-band superfluidity from the weak coupling
Bardeen-Cooper-Schrieffer (BCS) to the strong coupling Bose-Einstein
condensation (BEC) limit. When the interband interaction is tuned from negative
to positive values, a quantum phase transition occurs from a 0-phase to a
-phase state, depending on the relative phase of two order parameters.
Furthermore, population imbalances between the two bands can be created by
tuning the intraband or interband interactions. We also find two undamped low
energy collective excitations corresponding to in-phase and out-of-phase modes.
Lastly, we derive the coupled Ginzburg-Landau equations, and show that they
reduce to coupled Gross-Pitaevskii equations for two types of bosons in the BEC
limit.Comment: 4 pages and 3 figure
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
Superfluidity of Dirac Fermions in a Tunable Honeycomb Lattice: Cooper Pairing, Collective Modes, and Critical Currents
Motivated by recent experiments on atomic Dirac fermions in a tunable
honeycomb optical lattice, we study the attractive Hubbard model of
superfluidity in the anisotropic honeycomb lattice. At weak-coupling, we find
that the maximum mean field pairing transition temperature, as a function of
density and interaction strength, occurs for the case with isotropic hopping
amplitudes. In this isotropic case, we go beyond mean field theory and study
collective fluctuations, treating both pairing and density fluctuations for
interaction strengths ranging from weak to strong coupling. We find evidence
for a sharp sound mode, together with a well-defined Leggett mode over a wide
region of the phase diagram. We also calculate the superfluid order parameter
and collective modes in the presence of nonzero superfluid flow. The
flow-induced softening of these collective modes leads to dynamical
instabilities involving stripe-like density modulations as well as a
Leggett-mode instability associated with the natural sublattice symmetry
breaking charge-ordered state on the honeycomb lattice. The latter provides a
non-trivial test for the experimental realization of the one-band Hubbard
model. We delineate regimes of the phase diagram where the critical current is
limited by depairing or by such collective instabilities, and discuss
experimental implications of our results.Comment: 20 pages, 12 figures. v3: published versio
BCS-to-BEC crossover from the exact BCS solution
The BCS-to-BEC crossover, as well as the nature of Cooper pairs, in a
superconducting and Fermi superfluid medium is studied from the exact ground
state wavefunction of the reduced BCS Hamiltonian. As the strength of the
interaction increases, the ground state continuously evolves from a
mixed-system of quasifree fermions and pair resonances (BCS), to pair
resonances and quasibound molecules (pseudogap), and finally to a system of
quasibound molecules (BEC). A single unified scenario arises where the
Cooper-pair wavefunction has a unique functional form. Several exact analytic
expressions, such as the binding energy and condensate fraction, are derived.
We compare our results with recent experiments in ultracold atomic Fermi gases.Comment: 5 pages, 4 figures. Revised version with one figure adde
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
BCS-BEC crossover of collective excitations in two-band superfluids
We use the functional integral approach to study low energy collective
excitations in a continuum model of neutral two-band superfluids at T=0 for all
couplings with a separable pairing interaction. In the long wavelength and low
frequency limit, we recover Leggett's analytical results in weak coupling (BCS)
for s-wave pairing, and further obtain analytical results in strong coupling
(BEC) for both two and three dimensional systems. We also analyse numerically
the behavior of the out-of-phase {\it exciton} (finite frequency) mode and the
in-phase {\it phonon} (Goldstone) mode from weak to strong coupling limits,
including the crossover region. In principle, the evolution of Goldstone and
finite frequency modes from weak to strong coupling may be accessible
experimentally in the superfluid phase of neutral Fermi atomic gases, and could
serve as a test of the validity of the theoretical analysis and approximations
proposed here.Comment: 14 pages, 9 figures. Submitted to PR
Fermions at unitarity and Haldane Exclusion Statistics
We consider a gas of neutral fermionic atoms at ultra-low temperatures, with
the attractive interaction tuned to Feshbach resonance. We calculate, the
variation of the chemical potential and the energy per particle as a function
of temperature by assuming the system to be an ideal gas obeying the Haldane-Wu
fractional exclusion statistics. Our results for the untrapped gas compare
favourably with the recently published Monte Carlo calculations of two groups.
For a harmonically trapped gas, the results agree with experiment, and also
with other published work.Comment: 4 pages, 1 postscript figur
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