Observation of pseudogap behaviour in a strongly interacting Fermi gas

Ultracold atomic Fermi gases present an opportunity to study strongly interacting fermionic systems in a controlled and uncomplicated setting. The ability to tune attractive interactions has led to the discovery of superfluidity in these systems with an extremely high transition temperature with respect to the Fermi temperature near T/TF=0.2. This superfluidity is the electrically neutral analogue of superconductivity; however, superfluidity in atomic Fermi gases occurs in the limit ofstrong interactions and defies a conventional Bardeen–Cooper– Schrieffer (BCS) description. For these strong interactions, it is predicted that the onset of pairing and superfluidity can occur at different temperatures. Thus, for a range of temperatures, a pseudogap region may exist, in which the system retains some of the characteristics of the superfluid phase—such as a BCS-like dispersion and a partially gapped density of states—but does not exhibit superfluidity. By making two independent measurements—the direct observation of pair condensation in momentum space and a measurement of the single-particle spectral function using an analogue to photoemission spectroscopy6—we directly probe the pseudogap phase. Our measurements reveal a BCS-like dispersion with back-bending near the Fermiwavevector kF, which persists well above the transition temperature for pair condensation.