We conducted the first thermodynamic experiment on an optically-trapped Fermi gas of atoms in the regime of strong interparticle interactions [Science magazine, online publication]. In addition to measuring the heat capacity of this unique physical system, our study represents the first attempt at direct thermometry in the strongly interacting regime.  Making use of novel energy input and temperature measurement techniques, we observe a transition in the heat capacity which theory interprets as the onset of a high temperature superfluid state.

Our precision study of the heat capacity is made possible by a novel energy input technique. The optical trap is briefly turned off, and the atomic cloud is allowed to expand. When the trap is turned on again, the cloud has grown in size, and this increased size is converted to additional energy when the gas returns to equilibrium. By varying the duration of time that the trap is off, we can precisely control the energy input to the gas. A measurement of the heat capacity of the gas then involves adding various amounts of energy to the system and measuring the resulting temperature.

Measuring the temperature of a strongly-interacting Fermi gas is no small task.  Unlike thermometry for noninteracting systems, there is no universally agreed-upon technique for measuring temperature in the strongly-interacting regime. However, in collaboration with a theoretical group at the University of Chicago headed by Kathryn Levin, we have suggested a pragmatic temperature measurement scheme inspired by the close resemblance of strongly-interacting clouds of atoms to noninteracting clouds.  While our temperature measurement technique may prove to be only approximately correct, it is the first attempt at direct thermometry in the strongly-interacting regime. The use of this thermometry technique allows for quantitative comparison between theory and experimental results. We find that our heat capacity measurements in the strongly interacting regime are in excellent agreement with predictions made by the Chicago theoretical group. Furthermore, their theory interprets our observation of a transition in the heat capacity of a strongly-interacting Fermi gas as evidence for a superfluid phase transition.

Remarkably, the Chicago group's theoretical formalism was originally developed to explain the behavior of high temperature superconductors. While dilute Fermi gases and high temperature superconductors might seem like very disparate systems, the underlying physics may be quite similar.  In fact, the good agreement between experiment and theory in our study of the heat capacity lends some support to the idea that particles in a strongly-interacting Fermi gas can undergo pairing at temperatures higher than the superfluid transition temperature. This process may be analogous to the transition from a normal to a superconducting state in a high temperature superconductor.

These results have been highlighted in a Duke University press release and Nature magazine's research highlights.