Two-dimensional spin-imbalanced Fermi gases at nonzero temperature: Phase separation of a noncondensate

Abstract

We study a trapped two-dimensional spin-imbalanced Fermi gas over a range of temperatures. In the moderate temperature regime, associated with current experiments, we find reasonable semiquantitative agreement with the measured density profiles as functions of varying spin imbalance and interaction strength. Our calculations show that, in contrast to the three-dimensional case, the phase separation which appears as a spin balanced core, can be associated with non-condensed fermion pairs. We present predictions at lower temperatures where a quasi-condensate will first appear, based on the pair momentum distribution and following the protocols of Jochim and collaborators. While these profiles also indicate phase separation, they exhibit distinctive features which may aid in identifying the condensation regime.Ultracold Fermi gases are a valuable resource for learning about highly correlated superfluids. Their utility comes from their tunability [1] which allows the dimensionality, band structure, interaction strength, and spin imbalance to be freely varied. With these various parameters one can, in principle, simulate a number of important condensed matter systems ranging from preformed pair and related effects in the high T c cuprates [2-4] to intrinsic topological superfluids [5][6][7] and other exotic pairing states.In this paper we focus on recent experiments [8,9] of two-dimensional (2D) spin-imbalanced Fermi gases. These address the interesting conflict between the tendency towards enhanced pairing (which is associated with two dimensionality [10]), and spin imbalance which acts to greatly undermine pairing. These imbalance effects are believed [11] to have related effects in studies of color superconductivity and quark-gluon plasmas. In condensed matter systems, lower dimensional imbalanced superfluids are thought to be ideal for observing more exotic phases, such as the elusive LOFF state [12], or algebraic order [13,14].The approach we use has been rather successful in addressing 2D low temperature quasi-condensation [15] in balanced gases. In this paper we present predictions for future very low temperature experiments on spinimbalanced gases. Importantly, our calculations, which find no true long range order, are consistent with the Mermin-Wagner theorem [16]. Following the experiments of Jochim and collaborators [17,18], we show how quasicondensation is reflected in the pair-momentum distribution which has a strong peak at low momentum. This peak, which we study throughout the crossover from BCS to BEC, disappears somewhat abruptly at a fixed temperature, T qc , which denotes quasi-condensation. We find T qc varies only weakly with the polarization.A central finding in this paper is that 2D spinimbalanced systems in a trap exhibit a new form of phase separation involving non-condensed pairs appearing primarily in the center region of the trap. This is to be contrasted with 3D gases [19,20] where the phase separation is associated with a true condensate. In the 2D polarized case, because almost all the ...

Dimensional crossover in ultracold Fermi gases from Functional Renormalisation

“…Another interesting aspect would be the study of spinand mass-imbalanced Fermi gases within the dimensional crossover, since here the influence of mismatching Fermi surfaces and stronger fluctuations in lower dimensions might result in competing effects concerning pairing [79][80][81][82][83]. This may shed further physical insight, for example in the search for high temperature superconductors.…”

Exotic superfluidity and pairing phenomena in atomic Fermi gases in mixed dimensions

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