Observation of pseudogap behavior in a strongly interacting Fermi gas

TL;DR

This paper reports the direct observation of a pseudogap phase in a strongly interacting ultracold Fermi gas, revealing a BCS-like dispersion that persists above the superfluid transition temperature, challenging traditional theories.

Contribution

It provides the first direct experimental evidence of a pseudogap phase in a strongly interacting Fermi gas using momentum space and spectral function measurements.

Findings

Observation of a BCS-like dispersion above T_c

Persistence of a pseudogap region at high temperatures

Direct measurement of pair condensation and spectral function

Abstract

Ultracold atomic Fermi gases present an opportunity to study strongly interacting Fermi 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, near T/T_F = 0.2. This superfluidity is the electrically neutral analog of superconductivity; however, superfluidity in atomic Fermi gases occurs in the limit of strong interactions and defies a conventional BCS description. For these strong interactions, it is predicted that the onset of pairing and superfluidity can occur at different temperatures. This gives rise to a pseudogap region where, for a range of temperatures, 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 analog to photoemission spectroscopy, we directly probe the pseudogap phase. Our measurements reveal a BCS-like dispersion with back-bending near the Fermi wave vector k_F that persists well above the transition temperature for pair condensation.