Observation of pseudogap behaviour in a strongly interacting Fermi gas

Abstract

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

Heteropairing and component-dependent pseudogap phenomena in an ultracold Fermi gas with different species with different masses

“…In this regard, we note that each component may feel different harmonic potential in a real trapped Fermi gas, leading to a local population (spin) imbalance [28,36]. In addition, the photoemissiontype experiment developed by the JILA group [53,54], which is a powerful technique to experimentally examine single-particle properties of an ultracold Fermi gas, has no spatial resolution, so that we need to treat an observed photoemission spectrum as a spatially averaged one in a trap. Thus, to deal with these realistic situations, the extension of our work to include effects of a harmonic trap is necessary.…”

“…Once one of them is realized, one could clarify its superfluid properties, maximally using the high tunability of Fermi gases [51] and various experimental techniques [52][53][54][55][56][57][58]. Since an ultracold Fermi gas is expected as a useful quantum simulator for strongly interacting Fermi systems, this challenge would also be important on the viewpoint of this application.…”

“…In our recent paper [42], we showed that the ordinary (non-self-consistent) T -matrix approximation (TMA) [70], which has been extensively used to successfully clarify various interesting BCS-BEC crossover physics in the mass-balanced case [53,54,[71][72][73][74][75][76], breaks down in the presence of mass imbalance, to unphysically give double-valued T c in the crossover region, as shown in Fig. 1.…”

Pseudogap temperature and effects of a harmonic trap in the BCS-BEC crossover regime of an ultracold Fermi gas

“…However, because of the complexity of this system, other scenarios have been also proposed, such as antiferromagnetic spin fluctuations [24,25], and a hidden order [26]. In contrast, in cold Fermi gases, because of the simplicity of the system, the origin can be uniquely identified as strong pairing fluctuations [27][28][29][30][31][32][33][34][35][36][37]. Thus, this system would be very useful for the study of the preformed-pair scenario.…”

Virial expansion for a strongly correlated Fermi gas with imbalanced spin populations

“…The third virial coefficient of strongly interacting Bose gases has been determined [21]. The virial expansion of singleparticle spectral function [18] or dynamic structure factor [19] has also been developed to understand, respectively, the relevant measurements of momentum-resolved rf spectroscopy [22,23] or Bragg spectroscopy [24].…”