9,924 research outputs found
Time-Dependent Density-Functional Theory for Superfluids
A density-functional theory is established for inhomogeneous superfluids at
finite temperature, subject to time-dependent external fields in isothermal
conditions. After outlining parallelisms between a neutral superfluid and a
charged superconductor, Hohenberg-Kohn-Sham-type theorems are proved for
gauge-invariant densities and a set of Bogolubov-Popov equations including
exchange and correlation is set up. Earlier results applying in the linear
response regime are recovered.Comment: 12 pages. Europhysics Letters, in pres
Sound propagation in elongated superfluid fermion clouds
We use hydrodynamic equations to study sound propagation in a superfluid
Fermi gas inside a strongly elongated cigar-shaped trap, with main attention to
the transition from the BCS to the unitary regime. We treat first the role of
the radial density profile in the quasi-onedimensional limit and then evaluate
numerically the effect of the axial confinement in a configuration in which a
hole is present in the gas density at the center of the trap. We find that in a
strongly elongated trap the speed of sound in both the BCS and the unitary
regime differs by a factor sqrt{3/5} from that in a homogeneous
three-dimensional superfluid. The predictions of the theory could be tested by
measurements of sound-wave propagation in a set-up such as that exploited by
M.R. Andrews et al. [Phys. Rev. Lett. 79, 553 (1997)] for an atomic
Bose-Einstein condensate
Collisional oscillations of trapped boson-fermion mixtures approaching collapse
We study the collective modes of a confined gaseous cloud of bosons and
fermions with mutual attractive interactions at zero temperature. The cloud
consists of a Bose-Einstein condensate and a spin-polarized Fermi gas inside a
spherical harmonic trap and the coupling between the two species is varied by
increasing either the magnitude of the interspecies s-wave scattering length or
the number of bosons. The mode frequencies are obtained in the collisional
regime by solving the equations of generalized hydrodynamics and are compared
with the spectra calculated in the collisionless regime within a random-phase
approximation. We find that, as the mixture is driven towards the collapse
instability, the frequencies of the modes of fermionic origin show a blue shift
which can become very significant for large numbers of bosons. Instead the
modes of bosonic origin show a softening, which becomes most pronounced in the
very proximity of collapse. Explicit illustrations of these trends are given
for the monopolar spectra, but similar trends are found for the dipolar and
quadrupolar spectra except for the surface (n=0) modes which are essentially
unaffected by the interactions.Comment: 9 pages, 5 figures, revtex
Nonequilibrium transport through magnetic vibrating molecules
We calculate the nonequilibrium conductance through a molecule or a quantum
dot in which the occupation of the relevant electronic level is coupled with
intensity to a phonon mode, and also to two conducting leads. The
system is described by the Anderson-Holstein Hamiltonian. We solve the problem
using the Keldysh formalism and the non-crossing approximation (NCA) for both,
the electron-electron and the electron-phonon interactions. We obtain a
moderate decrease of the Kondo temperature with for fixed
renormalized energy of the localized level . The meaning and value
of are discussed. The spectral density of localized electrons
shows in addition to the Kondo peak of width , satellites of this peak
shifted by multiples of the phonon frequency . The nonequilibrium
conductance as a function of bias voltage at small temperatures, also
displays peaks at multiples of in addition to the central dominant
Kondo peak near .Comment: 11 pages, 13 figures, accepted in Phys. Rev.
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