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Universal sound diffusion in a strongly interacting Fermi gas

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Science  04 Dec 2020:
Vol. 370, Issue 6521, pp. 1222-1226
DOI: 10.1126/science.aaz5756

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Watching sound die out

A gas of strongly interacting fermionic atoms can serve as a model for systems with densities and energies spanning many orders of magnitude. This universality of physics comes about thanks to a property known as scale invariance. Patel et al. exploited this concept to draw universal conclusions about the attenuation of sound in such systems by studying a homogeneous gas of lithium-6 atoms at very low temperatures (see the Perspective by Schaefer). They found that below the superfluid transition, the sound diffusivity behaved not unlike what has been observed in helium-4, a fluid of strongly interacting bosons.

Science, this issue p. 1222; see also p. 1162

Abstract

Transport of strongly interacting fermions is crucial for the properties of modern materials, 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 atomic Fermi gas by studying sound propagation and its attenuation through the coupled transport of momentum and heat. In the normal state, the sound diffusivity D monotonically decreases upon lowering the temperature, in contrast to the diverging behavior of weakly interacting Fermi liquids. Below the superfluid transition temperature, D attains a universal value set by the ratio of Planck’s constant and the particle mass. Our findings inform theories of fermion transport, with relevance for hydrodynamic flow of electrons, neutrons, and quarks.

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