542 research outputs found
Atom-molecule theory of broad Feshbach resonances
We derive the atom-molecule theory for an atomic gas near a broad Feshbach
resonance, where the energy dependence of the atom-molecule coupling becomes
crucial for understanding experimental results. We show how our many-body
theory incorporates the two-atom physics exactly. In particular, we calculate
the magnetic moment of a two-component gas of ^{6}Li atoms for a wide range of
magnetic fields near the broad Feshbach resonance at about 834 Gauss. We find
excellent agreement with the experiment of Jochim et al. [Phys. Rev. Lett. 91,
240402 (2003)].Comment: 4 pages, 2 figure
Growth and Collapse of a Bose Condensate with Attractive Interactions
We consider the dynamics of a quantum degenerate trapped gas of Li-7 atoms.
Because the atoms have a negative s-wave scattering length, a Bose condensate
of Li-7 becomes mechanically unstable when the number of condensate atoms
approaches a maximum value. We calculate the dynamics of the collapse that
occurs when the unstable point is reached. In addition, we use the quantum
Boltzmann equation to investigate the nonequilibrium kinetics of the atomic
distribution during and after evaporative cooling. The condensate is found to
undergo many cycles of growth and collapse before a stationary state is
reached.Comment: Four pages of ReVTeX with four postscript figure
Spin drag in an ultracold Fermi gas on the verge of a ferromagnetic instability
Recent experiments [Jo et al., Science 325, 1521 (2009)] have presented
evidence of ferromagnetic correlations in a two-component ultracold Fermi gas
with strong repulsive interactions. Motivated by these experiments we consider
spin drag, i.e., frictional drag due to scattering of particles with opposite
spin, in such systems. We show that when the ferromagnetic state is approached
from the normal side, the spin drag relaxation rate is strongly enhanced near
the critical point. We also determine the temperature dependence of the spin
diffusion constant. In a trapped gas the spin drag relaxation rate determines
the damping of the spin dipole mode, which therefore provides a precursor
signal of the ferromagnetic phase transition that may be used to experimentally
determine the proximity to the ferromagnetic phase.Comment: 4 pages, 3 fig
Spin-Seebeck effect in a strongly interacting Fermi gas
We study the spin-Seebeck effect in a strongly interacting, two-component
Fermi gas and propose an experiment to measure this effect by relatively
displacing spin up and spin down atomic clouds in a trap using spin-dependent
temperature gradients. We compute the spin-Seebeck coefficient and related
spin-heat transport coefficients as functions of temperature and interaction
strength. We find that when the inter-spin scattering length becomes larger
than the Fermi wavelength, the spin-Seebeck coefficient changes sign as a
function of temperature, and hence so does the direction of the
spin-separation. We compute this zero-crossing temperature as a function of
interaction strength and in particular in the unitary limit for the inter-spin
scattering
Crossover temperature of Bose-Einstein condensation in an atomic Fermi gas
We show that in an atomic Fermi gas near a Feshbach resonance the crossover
between a Bose-Einstein condensate of diatomic molecules and a Bose-Einstein
condensate of Cooper pairs occurs at positive detuning, i.e., when the
molecular energy level lies in the two-atom continuum. We determine the
crossover temperature as a function of the applied magnetic field and find
excellent agreement with the experiment of Regal et al. [Phys. Rev. Lett. 92,
040403 (2004)] that has recently observed this crossover temperature.Comment: 4 pages, 2 figure
Low energy monopole Modes of a Trapped atomic Fermi Gas
We consider the low energy collective monopole modes of a trapped weakly
interacting atomic Fermi gas in the collisionless regime. The spectrum is
calculated for varying coupling strength and chemical potential. Using an
effective Hamiltonian, we derive analytical results that agree well with
numerical calculations in various regimes. The onset of superfluidity is shown
to lead to effects such as the vanishing of the energy required to create a
Cooper molecule at a critical coupling strength and to the emergence of pair
vibration excitations. Our analysis suggests ways to experimentally detect the
presence of the superfluid phase in trapped atomic Fermi gases.Comment: 5 pages & 1 figure. Accepted for Phys. Rev. Let
Twin peaks in rf spectra of Fermi gases at unitarity
We calculate the radio-frequency spectrum of balanced and imbalanced
ultracold Fermi gases in the normal phase at unitarity.
For the homogeneous case the spectrum of both the majority and minority
components always has a single peak even in the pseudogap regime.
We furthermore show how the double-peak structures observed in recent
experiments arise due to the inhomogeneity of the trapped gas.
The main experimental features observed above the critical temperature in the
recent experiment of Schunck et al. [Science 316, 867, (2007)] are recovered
with no fitting parameters.Comment: v3: version accepted for publication as a Rapid Communication in PRA.
With respect to v2, minor changes in the text and in the inset of Fig.
Deformation of a Trapped Fermi Gas with Unequal Spin Populations
The real-space densities of a polarized strongly-interacting two-component
Fermi gas of Li atoms reveal two low temperature regimes, both with a
fully-paired core. At the lowest temperatures, the unpolarized core deforms
with increasing polarization. Sharp boundaries between the core and the excess
unpaired atoms are consistent with a phase separation driven by a first-order
phase transition. In contrast, at higher temperatures the core does not deform
but remains unpolarized up to a critical polarization. The boundaries are not
sharp in this case, indicating a partially-polarized shell between the core and
the unpaired atoms. The temperature dependence is consistent with a tricritical
point in the phase diagram.Comment: Accepted for publication in Physical Review Letter
Bright soliton trains of trapped Bose-Einstein condensates
We variationally determine the dynamics of bright soliton trains composed of
harmonically trapped Bose-Einstein condensates with attractive interatomic
interactions. In particular, we obtain the interaction potential between two
solitons. We also discuss the formation of soliton trains due to the quantum
mechanical phase fluctuations of a one-dimensional condensate.Comment: 4 pages, 2 figures, submitted to PR
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