Linear response of a superfluid Fermi gas inside its pair-breaking continuum

TL;DR

This paper investigates the collective excitations in a superfluid Fermi gas, revealing a robust Higgs mode resonance within the pair-breaking continuum observable across various response functions and temperatures.

Contribution

It demonstrates that the Higgs mode resonance persists inside the pair-breaking continuum at all wavevectors and temperatures, a feature previously underappreciated in superfluid Fermi gases.

Findings

Resonance appears in multiple response functions across all wavevectors.

The Higgs mode persists up to near the critical temperature T_c.

The resonance coexists with other collective modes like Anderson-Bogoliubov and phononic modes.

Abstract

We study the signatures of the collective modes of a superfluid Fermi gas in its linear response functions for the order-parameter and density fluctuations in the Random Phase Approximation (RPA). We show that a resonance associated to the Popov-Andrianov (or sometimes "Higgs") mode is visible inside the pair-breaking continuum at all values of the wavevector $q$, not only in the (order-parameter) modulus-modulus response function but also in the modulus-density and density-density responses. At nonzero temperature, the resonance survives in the presence of thermally broken pairs even until the vicinity of the critical temperature $T_c$, and coexists with both the Anderson-Bogoliubov modes at temperatures comparable to the gap $\Delta$ and with the low-velocity phononic mode predicted by RPA near $T_c$. The existence of a Popov-Andrianov-"Higgs" resonance is thus a robust, generic feature of the high-energy phenomenology of pair-condensed Fermi gases, and should be accessible to state-of-the-art cold atom experiments.