Irreversible entropy transport enhanced by fermionic superfluidity

TL;DR

This paper investigates how entropy and particle flows behave between two fermionic superfluids, revealing nonlinear dynamics and enhanced entropy transport due to superfluidity, challenging traditional hydrodynamic predictions.

Contribution

It demonstrates large entropy flows in fermionic superfluids, introduces a phenomenological model for nonlinear entropy dynamics, and shows superfluidity can increase entropy transport speed.

Findings

Entropy transported per particle exceeds superfluid hydrodynamics predictions.

Entropy transport speed increases with superfluidity.

Channel geometry affects diffusive and advective entropy timescales.

Abstract

The nature of particle and entropy flow between two superfluids is often understood in terms of reversible flow carried by an entropy-free, macroscopic wavefunction. While this wavefunction is responsible for many intriguing properties of superfluids and superconductors, its interplay with excitations in non-equilibrium situations is less understood. Here, we observe large concurrent flows of both particles and entropy through a ballistic channel connecting two strongly interacting fermionic superfluids. Both currents respond nonlinearly to chemical potential and temperature biases. We find that the entropy transported per particle is much larger than the prediction of superfluid hydrodynamics in the linear regime and largely independent of changes in the channel's geometry. In contrast, the timescales of advective and diffusive entropy transport vary significantly with the channel geometry. In our setting, superfluidity counterintuitively increases the speed of entropy transport. Moreover, we develop a phenomenological model describing the nonlinear dynamics within the framework of generalised gradient dynamics. Our approach for measuring entropy currents may help elucidate mechanisms of heat transfer in superfluids and superconducting devices.