Direct Observation of Heat Transport in Strongly Interacting Fermi Gases

The transport of heat is one of the most fundamental features of any material. It can distinguish states of matter and offers powerful insights into the underlying microscopic mechanisms of transport. In general, any measure sensitive to the energy distribution of the sample can be used as a thermometer, such as light emitted from a black body. As we demonstrate here in a strongly interacting Fermi gas, radio-frequency (rf) spectra of interacting atomic gases can serve as a direct in situ thermometer. The rf pulse transfers atoms in an energy-selective way into a non-interacting state, essentially providing a local map of quasi-particle excitations, in our case the number of broken fermion pairs. Using this technique for the unitary Fermi gas, we observe two distinctive modes for the transfer of heat: In a normal, non-superfluid gas heat propagates diffusively, while below the superfluid transition temperature heat propagates ballistically as second sound. From the damping time of heat diffusion we obtain the thermal conductivity, while the speed and damping of second sound yields the superfluid density and the second sound diffusivity. The response functions we measure are well represented by the two-fluid expressions by Hohenberg and Martin. The results inform theories of transport of strongly interacting fermionic matter, from strongly correlated superconductors to neutron stars and quark matter.