Higgs oscillations in a unitary Fermi superfluid

TL;DR

This paper reports the direct observation of amplitude oscillations in a unitary Fermi superfluid following an interaction quench, revealing new insights into collective excitations and their temperature dependence.

Contribution

The study provides the first direct measurement of amplitude oscillations in a fermionic superfluid using Bragg spectroscopy, advancing understanding of symmetry-breaking dynamics.

Findings

Amplitude oscillations occur at twice the gap frequency.

Oscillations decay faster than BCS theory predicts.

Response magnitude strongly depends on temperature.

Abstract

Symmetry-breaking phase transitions are central to our understanding of states of matter. When a continuous symmetry is spontaneously broken, new excitations appear that are tied to fluctuations of the order parameter. In superconductors and fermionic superfluids, the phase and amplitude can fluctuate independently, giving rise to two distinct collective branches. However, amplitude fluctuations are difficult to both generate and measure, as they do not couple directly to the density of fermions and have only been observed indirectly to date. Here, we excite amplitude oscillations in an atomic Fermi gas with resonant interactions by an interaction quench. Exploiting the sensitivity of Bragg spectroscopy to the amplitude of the order parameter, we measure the time-resolved response of the atom cloud, directly revealing amplitude oscillations at twice the frequency of the gap. The magnitude of the oscillatory response shows a strong temperature dependence, and the oscillations appear to decay faster than predicted by time-dependent BCS theory applied to our experimental setup.