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Robust Fulde-Ferrell-Larkin-Ovchinikov superfluidity in laser-assisted bilayer Fermi gases

Qing Sun, Liang‐Liang Wang, Gediminas Juzeliūnas, An-Chun Ji

Published Jan 8, 2018• 0 citations
OpenAlex

Abstract

Inspired by a recent observation of the supersolidity for bosonic atoms in a laser-assisted superlattice potential [J. Li, et. al., Nature 543, 91 (2017)], we consider an attractive two-component degenerate Fermi gas in a bilayer structure with two atomic hyperfine ground states involved. We find that the synthetic spin-orbit coupling (SOC) induced by the laser-assisted interlayer tunneling has a dramatical effect both on the two-body and many-body physics for atomic fermions. A bound molecular state appears, and the molecular dispersion can contain two minima at finite momenta even for vanishing intra- and inter-layer detunings. This is responsible for the pairing instability of the underlying attractive Fermi gas. As a result, the ground state of the system is found to be a Fulde-Ferrell-Larkin-Ovchinikov (FFLO) superfluid with a spontaneous density-modulation of the order parameter. Due to the specific pairing mechanism in this system, the FFLO state is robust and appears in a wide parameter regime with the characteristic pairing momentum on the order of the SOC strength. The unique features of the two-body and many-body state can be observed in current experiments with ultracold atoms.

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Understanding novel pairings in attractive degenerate Fermi gases is crucial for exploring rich superfluid physics. In this report, we reveal unconventional pairings induced by spin-orbit coupling (SOC) in a one-dimensional optical lattice, using a state-of-the-art density-matrix renormalization group method. When both bands are partially occupied, we find a strong competition between the interband Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and intraband Bardeen-Cooper-Schrieffer (BCS) pairings. In particular, for the weak and moderate SOC strengths, these two pairings can coexist, giving rise to a new phase called the FFLO-BCS phase, which exhibits a unique three-peak structure in pairing momentum distribution. For the strong SOC strength, the intraband BCS pairing always dominates in the whole parameter regime, including the half filling. We figure out the whole phase diagrams as functions of filling factor, SOC strength, and Zeeman field. Our results are qualitatively different from recent mean-field predictions. Finally, we address that our predictions could be observed in a weaker trapped potential.
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The Larkin-Ovchinnikov (LO) state which combines the superfluidity and spatial periodicity of pairing order parameters and exhibits supersolid properties has been attracting intense attention in both condensed-matter physics and ultracold atoms. Conventionally, realization of the LO state from an intrinsic sss-wave interacting system necessitates breaking the time-reversal (TR) and sometimes spatial-inversion (SI) symmetries. Here we report a prediction that the LO state can be realized in a TR- and SI-symmetric system representing a bilayer Fermi gas subjected to a laser-assisted interlayer tunneling. We show that the intralayer sss-wave atomic interaction acts effectively like a ppp-wave interaction in the pseudospin space. This provides distinctive pairing effects in the present system with pseudspin spin-orbit coupling, and leads to a spontaneous density modulation of the pairing order predicted in a very broad parameter regime. Unlike the conventional schemes, our results do not rely on the spin imbalance or external Zeeman fields, showing a highly feasible way to observe the long-sought-after LO superfluid phase using the laser-assisted bilayer Fermi gases.
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We investigate the pairing physics of a three-component spin-orbit coupled Fermi gas in two spatial dimensions. The three atomic hyperfine states of the system are coupled by the recently realized synthetic spin-orbit coupling (SOC), which mixes different hyperfine states into helicity branches in a momentum-dependent manner. As a consequence, the interplay of spin-orbit coupling and the hyperfine-state-dependent interactions leads to the emergence of Fulde-Ferrell (FF) pairing states with finite center-of-mass momenta even in the absence of the Fermi-surface asymmetry that is usually mandatory to stabilize an SOC-induced FF state. We show that, for different combinations of spin-dependent interactions, the ground state of the system can either be the conventional Bardeen-Cooper-Schrieffer pairing state with zero center-of-mass momentum or be the FF pairing states. Of particular interest here is the existence of a three-component FF pairing state in which every two out of the three components form FF pairing. We map out the phase diagram of the system and characterize the properties of the three-component FF state, such as the order parameters, the gapless contours, and the momentum distributions. Based on these results, we discuss possible experimental detection schemes for the interesting pairing states in the system.