Remnant superfluid collective phase oscillations in the normal state of systems with resonant pairing

TL;DR

This paper investigates how superfluid-like collective oscillations persist in the normal state of systems with resonant pairing, revealing remnants of superfluidity above the transition temperature.

Contribution

It introduces the concept of remnant superfluid collective oscillations in the normal state of resonantly paired systems, extending understanding of superfluid signatures beyond the superfluid phase.

Findings

Sound wave-like excitations persist above T_c for certain wave vectors.

The region of superfluid-like excitations expands as temperature approaches T_c.

Modes become damped near zero momentum above T_c.

Abstract

The signature of superfluidity in bosonic systems is a sound wave-like spectrum of the single particle excitations which in the case of strong interactions is roughly temperature independent. In fermionic systems, where fermion pairing arises as a resonance phenomenon between free fermions and paired fermionic states (examples are: the atomic gases of lithium or potassium controlled by a Feshbach resonance, polaronic systems in the intermediary coupling regime, d-wave hole pairing in the strongly correlated Hubbard system), remnants of such superfluid characteristics are expected to be visible in the normal state. The single particle excitations maintain there a sound wave like structure for wave vectors above a certain q_{min}(T) where they practically coincide there with the spectrum of the superfluid phase for T<T_{c}. Upon approaching the transition from above this region in q-space extends down to small momenta, except for a narrow region around q=0 where such modes change into damped free particle