Abstract
The Leggett collective excitations for a two-band Fermi gas with s-wave pairing and Josephson interband coupling in the BCS-BEC crossover at finite temperatures are investigated within the Gaussian pair fluctuation approach. Eigenfrequencies and damping factors for Leggett modes are determined in a nonperturbative way, using the analytic continuation of the fluctuation propagator through a branch cut in the complex frequency plane, as in Kurkjian et al (2019 Phys. Rev. Lett. 122 093403). The treatment is performed beyond the low-energy expansion, which is necessary when the collective excitation energy reaches the pair-breaking continuum edge. The results are applied in particular to cold atomic gases at the orbital Feshbach resonance and in a regime far from BEC, which can be relevant for future experiments.
Highlights
The collective excitations predicted by Leggett [1] are specific for multiband neutral or charged superfluids
In our preceding works [19, 20], we have determined for a one-band system the frequency and the damping factor for phononic and pair-breaking collective excitations in a nonperturbative way using the analytic continuation for the Gaussian fluctuation propagator
This method has been straightforwardly extended for two-band Fermi superfluids, which reveal Leggett collective excitations absent in one-band systems
Summary
Leggett collective excitations in a two-band Fermi superfluid at finite temperatures. The Leggett collective excitations for a two-band Fermi gas with s-wave pairing and Josephson licence. Interband coupling in the BCS-BEC crossover at finite temperatures are investigated within the. Eigenfrequencies and damping factors for Leggett modes are attribution to the determined in a nonperturbative way, using the analytic continuation of the fluctuation propagator author(s) and the title of the work, journal citation through a branch cut in the complex frequency plane, as in Kurkjian et al The treatment is performed beyond the low-energy expansion, which is necessary when the collective excitation energy reaches the pair-breaking continuum edge. The results are applied in particular to cold atomic gases at the orbital Feshbach resonance and in a regime far from BEC, which can be relevant for future experiments
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