Universal Sound Diffusion in a Strongly Interacting Fermi Gas

TL;DR

This study reveals a universal quantum limit of sound diffusivity in a strongly interacting Fermi gas, showing a decrease with temperature and reaching a universal value at the superfluid transition, impacting theories of fermion transport.

Contribution

It demonstrates the existence of a quantum-limited diffusivity in a homogeneous Fermi gas, linking sound propagation to fundamental quantum bounds and broad physical systems.

Findings

Sound diffusivity decreases with lowering temperature in the normal state.

At the superfluid transition, diffusivity reaches a universal value proportional to h/m.

Results provide insights into fermion transport in various strongly interacting systems.

Abstract

Transport of strongly interacting fermions governs modern materials -- from the high-$T_c$ cuprates to bilayer graphene --, but also nuclear fission, the merging of neutron stars and the expansion of the early universe. Here we observe a universal quantum limit of diffusivity in a homogeneous, strongly interacting Fermi gas of atoms by studying sound propagation and its attenuation via the coupled transport of momentum and heat. In the normal state, the sound diffusivity ${D}$ monotonically decreases upon lowering the temperature $T$, in contrast to the diverging behavior of weakly interacting Fermi liquids. As the superfluid transition temperature is crossed, ${D}$ attains a universal value set by the ratio of Planck's constant ${h}$ and the particle mass ${m}$. This finding of quantum limited sound diffusivity informs theories of fermion transport, with relevance for hydrodynamic flow of electrons, neutrons and quarks.