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

Abstract

We investigate the superfluid phase transition and single-particle excitations in the BCS (BareenCooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas with mass imbalance. In our recent paper [R. Hanai, et. al., Phys. Rev. A 88, 053621 (2013)we showed that an extended T -matrix approximation (ETMA) can overcome the serious problem known in the ordinary (non-self-consistent) T -matrix approximation that it unphysically gives double-valued superfluid phase transition temperature T c in the presence of mass imbalance. However, at the same time, the ETMA was also found to give the vanishing T c in the weak-coupling and highly mass-imbalanced case. In this paper, we inspect the correctness of this ETMA result, using the self-consistent T -matrix approximation (SCTMA). We show that the vanishing T c is an artifact of the ETMA, coming from an internal inconsistency of this theory. The superfluid phase transition actually always occurs, irrespective of the ratio of mass imbalance. We also apply the SCTMA to the pseudogap problem in a mass-imbalanced Fermi gas. We show that pairing fluctuations induce different pseudogap phenomena between the the light component and heavy component. We also point out that a 6 Li-40 K mixture is a useful system for the realization of a hetero pairing state, as well as for the study of component-dependent pseudogap phenomena.

Beyond-mean-field description of a trapped unitary Fermi gas with mass and population imbalance

“…[28], together with a screening correction to the interaction. More recently, beyond-mean-field t-matrix approaches were employed to obtain a mass-imbalanced phase diagram with equal populations, first with partial [29,30] and later with full self-consistency [31,32]. In this context, it is important to remark that t-matrix approaches are ab initio and do not make use of phenomenological parameters.…”

Kovtun–Son–Starinets Conjecture and Effects of Mass Imbalance in the Normal State of an Ultracold Fermi Gas in the BCS–BEC Crossover Region

“…To evaluate the ratio η/s in the BCS-BEC crossover region, we consistently include strong-coupling corrections to the shear viscosity η and the entropy density s, within the framework of the self-consistent T -matrix approximation (SCTMA). 16,17,29,51,52) We then clarify how the ratio η/s behaves in the phase diagram of a mass-imbalanced Fermi gas with respect to the temperature and the interaction strength. We also identify where the minimum of η/s is obtained in this phase diagram, as well as the lower bound (η/s) l.b.…”

“…2, we explain how to consistently evaluate η and s in the normal state of a mass-imbalanced Fermi gas, within the framework of SCTMA. 29,49,51,52) In Sec. 3, we show our numerical results on, not only η/s, but also η and s, as well as effects of mass imbalance, in the BCS-BEC crossover region.…”

Thermal transitions, pseudogap behavior, and BCS-BEC crossover in Fermi-Fermi mixtures

“…Imbalance in Fermi superfluids can be realized through, (i) population imbalance or (ii) mass imbalance. While the former has been investigated in detail both experimentally [43][44][45][46][47] and theoretically [48][49][50][51][52], studies carried out on unequal mass Fermi-Fermi mixtures are relatively few [38,[53][54][55][56][57][58][59]. Experimentally a mass imbalanced Fermi-Fermi mixture is achievable in a 6 Li − 40 K mixture.…”

“…It was shown that while for a mass balanced system, instability towards a supersolid phase accompanied by a Lifsitz point is observed only at weak interactions, the imbalance in mass promotes this behavior and makes it observable even at unitarity. Among the other techniques Tmatrix and extended T-matrix approaches [38,57,58] are utilized to determine the thermal scales of the mass imbalanced mixture, both in terms of its thermodynamic as well as quasiparticle behavior. It was observed that unlike the balanced Fermi gas the Fermi-Fermi mixture with imbalance in mass consists of more than one pseudogap scales [38,57].…”