Transport Properties in the Unitary Fermi Gas

Understanding transport in strongly correlated fermion systems is among the grand challenges of many-body physics. Atomic Fermi gases near a Feshbach resonance serve as a paradigmatic form of fermionic matter, with strong connections to high-temperature superconductors, nuclear matter, and neutron stars. We here study the transport of density and heat in a resonant, two-component Fermi mixture of 6Li in a uniform box potential by resonantly exciting low-frequency modes. We spatially resolve both the local density and temperature of the balanced spin mixture, allowing us to measure both the density-density and entropy-density response and to observe the onset of "second sound" below the superfluid transition. By imbalancing the spin mixture, we observe a strong increase in damping of first sound as the gas turns normal beyond the Clogston-Chandrasekhar limit of superfluidity. Our measurements elucidate the interplay of spin, density, and heat transport in strongly interacting Fermi systems, and may in the future serve as a marker for phase transitions into exotic states of fermionic matter.