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

TL;DR

This paper explores the behavior of the twist mode in normal and superfluid trapped Fermi gases, revealing how superfluidity and temperature affect the mode's strength, collectivity, and damping.

Contribution

It provides a microscopic analysis of the twist mode in Fermi gases, highlighting the impact of superfluidity and temperature on the mode's characteristics.

Findings

Normal phase exhibits a narrow peak near trapping frequency

Superfluid phase shows damping and fragmentation of the mode

Strong pairing regime suppresses the twist mode entirely

Abstract

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.