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arXiv:1901.00967 · Physical Review A

Enhanced Fulde-Ferrell-Larkin-Ovchinnikov and Sarma superfluid states near an orbital Feshbach resonance

Dongyang Yu, Wei Zhang, Wu‐Ming Liu

Published Nov 13, 2019• 3 citations
arXivHTMLPDF

Abstract

We investigate the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and Sarma superfluid states in alkaline-earth-like $^{173}\mathrm{Yb}$ atomic gases near an orbital Feshbach resonance at zero temperature with population imbalances in both the open and closed channels. We find that, in uniform space, both the Fulde-Ferrell and Sarma states are greatly enhanced by the spin-exchange interaction on the Bardeen-Cooper-Schrieffer side of the Feshbach resonance. While trapped in a harmonic potential, a cloud of long-lived $^{173}\mathrm{Yb}$ atomic gas with small fraction of electronically excited-state population can stabilize not only the Sarma states with both the one and two Fermi surfaces but also the Fulde-Ferrell state, and leave detectable structures in the distribution of polarizations of different bands. As the degenerate $^{173}\mathrm{Yb}$ cloud is readily available and the signatures predicted can be easily detected in in situ and/or time-of-flight images, our findings are helpful to realize and detect the long-sought FFLO and Sarma states in experiments.

DOI
10.1103/physreva.100.053612

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We investigate the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and Sarma superfluid states in alkaline-earth-like 173Yb^{173}\mathrm{Yb}173Yb atomic gases near an orbital Feshbach resonance at zero temperature with population imbalances in both the open and closed channels. We find that, in uniform space, both the Fulde-Ferrell and Sarma states are greatly enhanced by the spin-exchange interaction on the Bardeen-Cooper-Schrieffer side of the Feshbach resonance. While trapped in a harmonic potential, a cloud of long-lived 173Yb^{173}\mathrm{Yb}173Yb atomic gas with small fraction of electronically excited-state population can stabilize not only the Sarma states with both the one and two Fermi surfaces but also the Fulde-Ferrell state, and leave detectable structures in the distribution of polarizations of different bands. As the degenerate 173Yb^{173}\mathrm{Yb}173Yb cloud is readily available and the signatures predicted can be easily detected in in situ and/or time-of-flight images, our findings are helpful to realize and detect the long-sought FFLO and Sarma states in experiments.
We investigate the nature of superfluid pairing in a strongly interacting Fermi gas near orbital Feshbach resonances with spin-population imbalance in three dimensions, which can be well described by a two-band or two-channel model. We show that a Sarma superfluid with gapless single-particle excitations is favored in the closed channel at large imbalance. It is thermodynamically stable against the formation of an inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov superfluid and features a well-defined Goldstone-Anderson-Bogoliubov phonon mode and a massive Leggett mode as collective excitations at low momentum. At large momentum, the Leggett mode disappears and the phonon mode becomes damped at zero temperature, due to the coupling to the particle-hole excitations. We discuss possible experimental observation of a strongly interacting Sarma superfluid with ultracold alkaline-earth-metal Fermi gases.
We theoretically explore a promising route to achieve the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in a spin-imbalanced ultracold Fermi gas. In the current stage of cold atom physics, search for this exotic Fermi superfluid is facing two serious difficulties: One is the desperate destruction of the FFLO long-range order by FFLO pairing fluctuations, which precludes entering the phase through a second-order transition, even in three dimension. The other is the fierce competition with the phase separation into the BCS (Bardeen-Cooper-Schrieffer) state and the spin-polarized normal state. By including strong FFLO pairing fluctuations within the framework of the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink, we show that the anisotropy of Fermi surface introduced by an optical lattice makes the FFLO state stable against the paring fluctuations. This stabilized FFLO state is also found to be able to overcome the competition with the phase separation under a certain condition. Since the realization of unconventional Fermi superfluids is one of the most exciting challenges in cold atom physics, our results would contribute to the further development of this field.
We show that the combination of spin-orbit coupling and in-plane Zeeman field in a two-dimensional degenerate Fermi gas can lead to a larger parameter region for Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases than that using spin-imbalanced Fermi gases. The resulting FFLO superfluids are also more stable due to the enhanced energy difference between FFLO and conventional Bardeen-Cooper-Schrieffer (BCS) excited states. We clarify the crucial role of the symmetry of Fermi surface on the formation of finite momentum pairing. The phase diagram for FFLO superfluids is obtained in the BCS-BEC crossover region and possible experimental observations of FFLO phases are discussed.
The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase was first predicted in two-dimensional superconductors about 50 years ago, but so far unambiguous experimental evidence is still lacking. The recently experimentally realized spin-imbalanced Fermi gases may potentially unveil this elusive state, but they require very stringent experimental conditions. In this Rapid Communication, we show that FFLO phases may be observed even in a three-dimensional (3D) degenerate Fermi gas with spin-orbit coupling and an in-plane Zeeman field. The FFLO phase is driven by the interplay between the asymmetry of the Fermi surface and the superfluid order, instead of the interplay between magnetic and superconducting order in solid materials. The predicted FFLO phase exists in a giant parameter region, possesses a stable long-range superfluid order due to the 3D geometry, and can be observed with an experimentally already achieved temperature ($T\ensuremath{\sim}0.05{E}_{F}$), thus opening a fascinating avenue for exploring FFLO physics.
We study the effects of synthetic spin-orbit coupling on the pairing physics in quasi-one-dimensional ultracold Fermi gases of alkaline-earth-metal-like atoms near an orbital Feshbach resonance (OFR). The interplay between spin-orbit coupling and pairing interactions near the OFR leads to an interesting topological Fulde-Ferrell state, where the nontrivial topology of the state is solely encoded in the closed channel with a topologically trivial Fulde-Ferrell pairing in the open channel. We confirm the topological property of the system by characterizing the Zak phase and the edge states. The topological Fulde-Ferrell state can be identified by the momentum-space density distribution obtained from time-of-flight images.
We review the concepts and the present state of theoretical studies of spin-imbalanced superfluidity, in particular the elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in the context of ultracold quantum gases. The comprehensive presentation of the theoretical basis for the FFLO state that we provide is useful also for research on the interplay between magnetism and superconductivity in other physical systems. We focus on settings that have been predicted to be favourable for the FFLO state, such as optical lattices in various dimensions and spin-orbit coupled systems. These are also the most likely systems for near-future experimental observation of the FFLO state. Theoretical bounds, such as Bloch's and Luttinger's theorems, and experimentally important limitations, such as finite-size effects and trapping potentials, are considered. In addition, we provide a comprehensive review of the various ideas presented for the observation of the FFLO state. We conclude our review with an analysis of the open questions related to the FFLO state, such as its stability, superfluid density, collective modes and extending the FFLO superfluid concept to new types of lattice systems.