Thermography of the superfluid transition in a strongly interacting Fermi gas

TL;DR

This paper demonstrates a novel thermography technique for directly imaging heat transport in a strongly interacting Fermi gas, revealing the superfluid transition through changes in heat propagation modes.

Contribution

It introduces a spatially resolved thermometry method using radiofrequency spectroscopy to observe superfluid transition and heat transport properties in a strongly interacting Fermi gas.

Findings

Direct imaging of the superfluid transition as a switch from thermal diffusion to second sound.

Observation of a peak in second sound diffusivity at the transition.

Full characterization of heat and density response in the Fermi gas.

Abstract

Heat transport is a fundamental property of all physical systems and can serve as a fingerprint identifying different states of matter. In a normal liquid a hot spot diffuses while in a superfluid heat propagates as a wave called second sound. Despite its importance for understanding quantum materials, direct imaging of heat transport is challenging, and one usually resorts to detecting secondary effects, such as changes in density or pressure. Here we establish thermography of a strongly interacting atomic Fermi gas, a paradigmatic system whose properties relate to strongly correlated electrons, nuclear matter and neutron stars. Just as the color of a glowing metal reveals its temperature, the radiofrequency spectrum of the interacting Fermi gas provides spatially resolved thermometry with sub-nanokelvin resolution. The superfluid phase transition is directly observed as the sudden change from thermal diffusion to second sound propagation, and is accompanied by a peak in the second sound diffusivity. The method yields the full heat and density response of the strongly interacting Fermi gas, and therefore all defining properties of Landau's two-fluid hydrodynamics. Our measurements serve as a benchmark for theories of transport in strongly interacting fermionic matter.