Parth B. Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard J. Fletcher, Julian Struck, Martin W. Zwierlein
Science 370, 1222-1226 (2020)
MIT News: Physicists capture the sound of a “perfect” fluid
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Transport of strongly interacting fermions governs modern materials — from the high-Tc 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.