arXiv:cond-mat/0005098 · Physical Review A

Scissors mode in a superfluid Fermi gas

Anna Minguzzi, Mario P. Tosi

Published Jan 16, 2001• 39 citations

Abstract

We evaluate the frequencies of scissors modes for density and concentration fluctuations in a vapor of fermionic atoms placed in two hyperfine levels inside a spherical harmonic trap. Both the superfluid and normal states are considered, with inclusion of the interactions at the random-phase level. Two main results are obtained: (i) the transition to the superfluid state is signaled by the disappearance of soft transverse modes of the normal fluid in the collisionless regime; and (ii) the eigenfrequency of the density fluctuations in the superfluid coincides with that of the normal fluid in the collisional regime. The latter property is related to the opening of the gap in the single-pair spectrum.

DOI: 10.1103/physreva.63.023609

Related papers

Scissors modes of two-component degenerate gases: Bose-Bose and Bose-Fermi mixtures

Mirta Rodríguez, P. Pedri, Päivi Törmä +1 more

We investigate the scissors modes in binary mixtures of degenerate dilute quantum gases for both Bose-Bose and Bose-Fermi mixtures. For the latter we consider both the superfluid and normal hydrodynamic and collisionless regimes. We analyze the dependence of the frequencies of the scissors modes and their character as a function of the Bose-Fermi coupling and the trap geometry. We show that the scissors mode can reveal a clear trace of the hydrodynamic behavior of the Fermi gas.

arXiv:cond-mat/0310498Physical Review AFeb 26, 200412 citations

Microscopic description of the twist mode in normal and superfluid trapped Fermi gases

M. Grasso, Michael Urban, X. Viñas

We investigate the ``twist'' mode (rotation of the upper against the lower hemisphere) of a dilute atomic Fermi gas in a spherical trap. The normal and superfluid phases are considered. The linear response to this external perturbation is calculated within the microscopic Hartree-Fock-Bogoliubov approach. In the normal phase the excitation spectrum is concentrated in a rather narrow peak very close to the trapping frequency. In the superfluid phase the strength starts to be damped and fragmented and the collectivity of the mode is progressively lost when the temperature decreases. In the weak-pairing regime some reminiscence of the collective motion still exists, whereas in the strong-pairing regime the twist mode is completely washed out. The disappearance of the twist mode in the strong-pairing regime with decreasing temperature is interpreted in the framework of the two-fluid model.

arXiv:cond-mat/0407475Physical Review AJan 19, 20052 citations

Low Energy Collective Modes of a Superfluid Trapped Atomic Fermi Gas

Georg M. Bruun, Ben R. Mottelson

We consider the low energy collective mode spectrum of a superfluid Fermi gas in a spherical trap in the collisionless regime. Using a self-consistent random-phase approximation, the effects of superfluidity on modes of dipole and quadrupole symmetries are systematically examined. The spectrum is calculated for varying pairing strength and temperature, and we identify several spectral features such as the emergence of Goldstone modes that can be used to detect the onset of superfluidity. Our analysis is relevant for present experiments aimed at observing a superfluid phase transition in trapped Fermi gases.

arXiv:cond-mat/0107628Physical Review LettersDec 11, 200170 citations

Equilibrium State And Excitations In Trapped Fermi Vapours

Anna Minguzzi, M. P. Tosi

In summary, in this Chapter we have given a brief overview on the dynamical properties of a trapped Fermi gas, both in the collisionless and in the collisional regime. While in a one-component system interactions are negligible (and nevertheless the system presents a rich spectrum), in the two-component case they can influence in a relevant way the equilibrium state and the dispersion relation. We have described mainly the case of a normal fluid, but we have pointed out that the long-wavelength behavior of a superfluid can be described in terms of the properties of the normal fluid in the collisional regime.

Kluwer Academic Publishers eBooksDec 10, 20050 citations

Radial quadrupole and scissors modes in trapped Fermi gases across the BCS phase transition

Michael Urban

The excitation spectra of the radial quadrupole and scissors modes of ultracold Fermi gases in elongated traps are studied across the BCS superfluid-normal phase transition in the framework of a transport theory for quasiparticles. In the limit of zero temperature, this theory reproduces the results of superfluid hydrodynamics, while in the opposite limit, above the critical temperature, it reduces to the collisionless Vlasov equation. In the intermediate temperature range, the excitation spectra have two or three broad peaks, respectively, which are roughly situated at hydrodynamic and collisionless frequencies, and whose strength is shifted from the hydrodynamic to the collisionless modes with increasing temperature. By fitting the time-dependent quadrupole deformation with a damped oscillation of a single frequency, we can understand the ``jump'' of the frequency of the radial quadrupole mode as a function of interaction strength which has recently been reported by the Innsbruck group.

arXiv:0808.3719Physical Review ANov 13, 200814 citations

Density profiles and collective excitations of a trapped two component Fermi vapour

Maria Laura Amoruso, I. Meccoli, Anna Minguzzi +1 more

We discuss the ground state and the small-amplitude excitations of a degenerate vapour of fermionic atoms placed in two hyperfine states inside a spherical harmonic trap. An equations-of-motion approach is set up to discuss the hydrodynamic dissipation processes from the interactions between the two components of the fluid beyond mean-field theory and to emphasize analogies with spin dynamics and spin diffusion in a homogeneous Fermi liquid. The conditions for the establishment of a collisional regime via scattering against cold-atom impurities are analyzed. The equilibrium density profiles are then calculated for a two-component vapour of 40K atoms: they are little modified by the interactions for presently relevant values of the system parameters, but spatial separation of the two components will spontaneously arise as the number of atoms in the trap is increased. The eigenmodes of collective oscillation in both the total particle number density and the concentration density are evaluated analytically in the special case of a symmetric two-component vapour in the collisional regime. The dispersion relation of the surface modes for the total particle density reduces in this case to that of a one-component Fermi vapour, whereas the frequencies of all other modes are shifted by the interactions.

arXiv:cond-mat/9909102Sep 7, 199912 citations