Tunneling transport of unitary fermions across the superfluid transition

TL;DR

This study explores how a strongly interacting Fermi gas transitions from superfluid to normal phase, revealing nonlinear to Ohmic conduction change, dominant anomalous resistive currents, and enhanced transport due to sound mode tunneling.

Contribution

It provides new insights into tunneling transport mechanisms across the superfluid transition in unitary Fermi gases, highlighting the role of condensate-phonon coupling and sound mode tunneling.

Findings

Transition from nonlinear to Ohmic conduction at critical temperature

Dominant anomalous resistive current contribution near lowest temperatures

Enhanced conductance in the normal phase due to sound mode tunneling

Abstract

We investigate the transport of a Fermi gas with unitarity-limited interactions across the superfluid phase transition, probing its response to a direct current (dc) drive through a tunnel junction. As the superfluid critical temperature is crossed from below, we observe the evolution from a highly nonlinear to an Ohmic conduction characteristics, associated with the critical breakdown of the Josephson dc current induced by pair condensate depletion. Moreover, we reveal a large and dominant anomalous contribution to resistive currents, which reaches its maximum at the lowest attained temperature, fostered by the tunnel coupling between the condensate and phononic Bogoliubov-Anderson excitations. Increasing the temperature, while the zeroing of supercurrents marks the transition to the normal phase, the conductance drops considerably but remains much larger than that of a normal, uncorrelated Fermi gas tunneling through the same junction. We attribute such enhanced transport to incoherent tunneling of sound modes, which remain weakly damped in the collisional hydrodynamic fluid of unpaired fermions at unitarity.