Excitation properties and effects of mass imbalance in the BCS-BEC crossover regime of an ultracold Fermi gas

Abstract

We investigate single-particle properties of a mass-imbalanced Fermi gas in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. In the presence of mass imbalance, we point out that the ordinary T -matrix approximation, which has been extensively used to clarify various BCS-BEC crossover physics in the mass-balanced case, unphysically gives a double-valued solution in terms of the superfluid phase transition temperature T c in the crossover region. To overcome this serious problem, we include higher order strong-coupling corrections beyond the T -matrix level. Using this extended T -matrix theory, we calculate single-particle excitations in the normal state above T c . The so-called pseudogap phenomena originating from pairing fluctuations are shown to be different between the light-mass component and heavy-mass component, which becomes more remarkable at higher temperatures. Since Fermi condensates with hetero-Cooper pairs have recently been discussed in various fields, such as exciton (polariton) condensates as well as color superconductivity, our results would be useful for the further development of Fermi superfluid physics, beyond the conventional superfluid state with homo-Cooper pairs.

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

“…Thus, the BCS-BEC crossover phenomenon, which has been already observed in 6 Li and 40 K Fermi gases, is also expected in a 6 Li-40 K mixture (m L /m H = 0.15). We emphasize that the ETMA gives the different prediction that this Fermi-Fermi mixture does not exhibit the superfluid phase transition in the BCS regime [42].…”

“…3. In this sense, the ETMA employed in our previous paper [42] has an internal inconsistency. That is, while the dressed Green's function is used in the fermion loop in Fig.…”

“…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.…”

“…However, apart from the recovery of the expected single-valued T c (see Fig 1) Fermi gas [42]. We take m L /m H = 0.9, where m L (m H ) is a mass of a light (heavy) atom.…”

“…[42], we overcame this difficulty by employing an extended T -matrix approximation (ETMA) [77,78], which involves higher order pairing fluctuations beyond the TMA.…”