Collective excitations of a degenerate gas at the BEC-BCS crossover

TL;DR

This paper investigates collective excitations in a fermionic gas across the BEC-BCS crossover, revealing expected axial mode behavior and surprising radial mode shifts, including signs of a superfluid to collisionless phase transition.

Contribution

It provides experimental measurements of collective modes in a fermionic gas across the BEC-BCS crossover, highlighting novel features in the radial mode and potential phase transition signatures.

Findings

Axial compression mode matches theoretical predictions near Feshbach resonance.

Radial compression mode exhibits negative frequency shifts with increased coupling.

Abrupt change in excitation frequency suggests a superfluid to collisionless phase transition.

Abstract

We study collective excitation modes of a fermionic gas of $^6$Li atoms in the BEC-BCS crossover regime. While measurements of the axial compression mode in the cigar-shaped trap close to a Feshbach resonance confirm theoretical expectations, the radial compression mode shows surprising features. In the strongly interacting molecular BEC regime we observe a negative frequency shift with increasing coupling strength. In the regime of a strongly interacting Fermi gas, an abrupt change in the collective excitation frequency occurs, which may be a signature for a transition from a superfluid to a collisionless phase.