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

Abstract

We study the mass imbalanced Fermi-Fermi mixture within the framework of a two-dimensional lattice fermion model. Based on the thermodynamic and species dependent quasiparticle behavior we map out the finite temperature phase diagram of this system and show that unlike the balanced Fermi superfluid there are now two different pseudogap regimes as PG-I and PG-II. While within the PG-I regime both the fermionic species are pseudogapped, PG-II corresponds to the regime where pseudogap feature survives only in the light species. We believe that the single particle spectral features that we discuss in this paper are observable through the species resolved radio frequency spectroscopy and momentum resolved photo emission spectroscopy measurements on systems such as, 6 Li -40 K mixture. We further investigate the interplay between the population and mass imbalances and report that at a fixed population imbalance the BCS-BEC crossover in a Fermi-Fermi mixture would require a critical interaction (U c ), for the realization of the uniform superfluid state. The effect of imbalance in mass on the exotic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid phase has been probed in detail in terms of the thermodynamic and quasiparticle behavior of this phase. It has been observed that in spite of the s-wave symmetry of the pairing field a nodal superfluid gap is realized in the LO regime. Our results on the various thermal scales and regimes are expected to serve as benchmarks for the experimental observations on 6 Li -40 K mixture.

Imbalanced Fermi systems and exotic superconducting phases

“…At η = 1 corresponding to zero imbalance in mass, both the pseudogap scales collapses to one. The phase diagram shows that, while akin to the population imbalanced system the T c is strongly suppressed by imbalance in mass, the short range pair correlations survive over a large temperature regime of T ∼ 2T c [124].…”

“…Evidently, all the numerical approaches have different set of merits and caveats, and consequently different regimes of applicability. We now discuss the numerical implementation of the SPA technique in some detail [122,123,124,125,126]. The merits of this technique rests on its applicability, in principle to systems of arbitrary dimensions and lattice sizes, with moderate computation cost and its access to real frequency properties of the system.…”

“…This imposes constraint on the lattice size that can be accessed within reasonable computation time. In order to access larger system sizes a cluster based update scheme could be utilized wherein instead of diagonalizing for the entire lattice at each attempted update, a smaller cluster of size N c = L c ×L c surrounding the update site is diagonalized [123,124,126]. This reduces the cost per update to ∼ O(N 3 c ) and to ∼ N × O(N 3 c ) per lattice sweep, which scales linearly with the system size.…”

“…The only beyond mean field study of mass imbalanced Fermi-Fermi mixture in two-dimensions, at and above the ground state was carried out using the SPA [124]. Within the framework of attractive Hubbard model the imbalance in mass was taken into account through imbalance in the hopping amplitude of the two species as, t H /t L = m L /m H = η.…”

Spin–orbit coupling adjusting topological superfluid of mass-imbalanced Fermi gas

“…[5,10,16] Most investigation of ultracold Fermi gas with SOC focus on the mass-balanced cases. [17][18][19][20][21][22][23][24][25][26][27][28] Differently, the twospecies (mass-imbalanced) Fermi-Fermi degenerate mixture would show more special properties, [29][30][31][32][33][34][35][36][37][38][39][40] It is found that Feshbach resonance can occur in the 6 Li- 40 K mixture, [29] and this result plays an indispensable role in exploring the multibody physics of strongly interacting regions and opens up a way for people to study mixed Fermi materials. Recently, the mixture of 161 Dy- 40 K offers an ideal platform with mass imbalance and strong interaction, [32] and it can effectively inhibit the elastic loss near the resonance.…”