Heat Transport in the Unitary Fermi Gas

The transport of heat provides a crucial window into the dynamical processes in a unitary Fermi gas. It can distinguish states of matter and offers powerful insights into the underlying microscopic mechanisms of transport. Employing radiofrequency spectroscopy as a local thermometer, we directly observe a striking signature of the superfluid phase transition in a unitary Fermi gas: while in a normal fluid, heat propagates diffusively, below the superfluid transition temperature heat propagates as a wave known as second sound. From the damping time of heat diffusion we obtain the thermal conductivity, while the speed and damping of second sound yield the superfluid density and the second sound diffusivity. The diffusivity displays a peak at the phase transition temperature, resembling the critical behavior found in liquid ⁴He. Our results inform theories of transport in strongly interacting fermionic matter, from nuclear matter to strongly correlated superconductors and quark-gluon plasma.