444 research outputs found
First and second sound in cylindrically trapped gases
We investigate the propagation of density and temperature waves in a
cylindrically trapped gas with radial harmonic confinement. Starting from
two-fluid hydrodynamic theory we derive effective 1D equations for the chemical
potential and the temperature which explicitly account for the effects of
viscosity and thermal conductivity. Differently from quantum fluids confined by
rigid walls, the harmonic confinement allows for the propagation of both first
and second sound in the long wave length limit. We provide quantitative
predictions for the two sound velocities of a superfluid Fermi gas at
unitarity. For shorter wave-lengths we discover a new surprising class of
excitations continuously spread over a finite interval of frequencies. This
results in a non-dissipative damping in the response function which is
analytically calculated in the limiting case of a classical ideal gas.Comment: 4 pages, 2 figures. Published version in Phys. Rev. Let
Dynamical instability and dispersion management of an attractive condensate in an optical lattice
We investigate the stability of an attractive Bose-Einstein condensate in a
moving 1D optical lattice in the presence of transverse confinement. By means
of a Bogoliubov linear stability analysis we find that the system is
dynamically unstable for low quasimomenta and becomes stable near the band
edge, in a specular fashion with respect to the repulsive case. For low
interactions the instability occurs via long wavelength excitations that are
not sufficient for spoiling the condensate coherence, producing instead an
oscillating density pattern both in real and momentum space. This behaviour is
illustrated by simulations for the expansion of the condensate in a moving
lattice.Comment: 5 pages, 4 figure
Equilibrium and dynamics of a trapped superfluid Fermi gas with unequal masses
Interacting Fermi gases with equal populations but unequal masses are
investigated at zero temperature using local density approximation and the
hydrodynamic theory of superfluids in the presence of harmonic trapping.
We derive the conditions of energetic stability of the superfluid
configuration with respect to phase separation and the frequencies of the
collective oscillations in terms of the mass ratio and the trapping frequencies
of the two components. We discuss the behavior of the gas after the trapping
potential of a single component is switched off and show that, near a Feshbach
resonance, the released component can still remain trapped due to many-body
interaction effects. Explicit predictions are presented for a mixture of Li
and K with resonant interaction.Comment: 4 pages, 2 figure
Collective Excitations of a "Gravitationally" Self-Bound Bose Gas
We investigate the collective excitations of an atomic Bose-Einstein
condensate in the self-binding regime produced by electromagnetically induced
``gravity'' (1/r attraction). Analytical expressions for the frequencies of the
monopole and quadrupole modes are obtained at zero temperature, using the
sum-rule approach, and compared with the exact results available in the
Thomas-Fermi limit. The low-energy dynamics of such condensates is shown to be
dominated by the effective ``plasma'' frequency. An analog of the Jeans
gravitational instability is analyzed.Comment: 4 pages, 1 eps figur
Equation of state and collective frequencies of a trapped Fermi gas along the BEC-unitarity crossover
We show that the study of the collective oscillations in a harmonic trap
provides a very sensitive test of the equation of state of a Fermi gas near a
Feshbach resonance. Using a scaling approach, whose high accuracy is proven by
comparison with exact hydrodynamic solutions, the frequencies of the lowest
compressional modes are calculated at T=0 in terms of a dimensionless parameter
characterizing the equation of state. The predictions for the collective
frequencies, obtained from the equations of state of mean field BCS theory and
of recent Monte-Carlo calculations, are discussed in detail.Comment: 4 pages, 3 figure
Umklapp collisions and center of mass oscillation of a trapped Fermi gas
Starting from the the Boltzmann equation, we study the center of mass
oscillation of a harmonically trapped normal Fermi gas in the presence of a
one-dimensional periodic potential. We show that for values of the the Fermi
energy above the first Bloch band the center of mass motion is strongly damped
in the collisional regime due to umklapp processes. This should be contrasted
with the behaviour of a superfluid where one instead expects the occurrence of
persistent Josephson-like oscillations.Comment: 11 pages, 3 figures, corrected typo
Effects of Disorder in a Dilute Bose Gas
We discuss the effects of a weak random external potential on the properties
of the dilute Bose gas at zero temperature. The results recently obtained by
Huang and Meng for the depletion of the condensate and of the superfluid
density are recovered. Results for the shift of the velocity of sound as well
as for its damping due to collisions with the external field are presented. The
damping of phonons is calculated also for dense superfluids. (submitted to
Phys.Rev.B)Comment: 21 pages, Plain Tex, 2 figures available upon request, preprint UTF
31
Scissors mode and superfluidity of a trapped Bose-Einstein condensed gas
We investigate the oscillation of a dilute atomic gas generated by a sudden
rotation of the confining trap (scissors mode). This oscillation reveals the
effects of superfluidity exhibited by a Bose-Einstein condensate. The scissors
mode is investigated also in a classical gas above T_c in various collisional
regimes. The crucial difference with respect to the superfluid case arises from
the occurence of low frequency components, which are responsible for the rigid
value of the moment of inertia. Different experimental procedures to excite the
scissors mode are discussed.Comment: 4 pages, 3 figure
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