Titled: COLLECTIVE EXCITATIONS IN STRONGLY COUPLED SUPERCONDUCTORS AND CHARGED SUPERFLUIDS
Abstract: Collective excitations in a superfluid matter are one of key subjects in condensed matter physics. They represent a significant interest for both experiment and theory because they can determine the response spectra of condensed systems. The present talk is devoted tospectra of collective excitationsin superfluid Fermi gases[1-5].Here, we are particularly focused on collective modes ofcharged superfluids and superconductors[4-5]embracing the crossover regime of the coupling strength from the weak-coupling BCS pairing to the strong-coupling BEC regime. Several branches of collective excitations are a subject of the presentation. The gapless soundlike Anderson-Bogoliubov mode is well determinedin neutral superfluid systems such as cold atomic gases. In superconductors, this gapless mode is affected by the Coulomb interaction and pushed up to the plasma mode. In the long-wavelength limit, the plasma mode is gapped, and the size of the gap is the same both in the superfluid/superconducting and in the normal state. At nonzero temperatures, in a charged superfluid the other gaplessmode can appeardiscovered by Carlson and Goldman. In the BCS-BEC crossover, the temperature range of existence of this mode expandssignificantlybelow the transition temperature with respect to weak-coupling BCS superconductors.In the present work, the path integral methods for neutral and charged Fermi superfluid gases have been described within a unified formalism.The Gaussian fluctuation approximationisextendedin order to account for all branches of collective excitations existing in a charged superfluid, including plasma oscillations. The dispersion and dampingof collective excitations are determined in two ways: from the spectral functions and from the complex poles of the fluctuation propagator. We have foundresonant enhancement of the response provided by the pair-breaking modes when the plasma frequency lies near the pair-breaking continuum edge. This may facilitate the experimental observation of the pair-breaking modes.
References:
[1]H. Kurkjian, S.N. Klimin, J. Tempere, Y. Castin, Phys. Rev. Lett. 122, 093403 (2019).
[2]S.N. Klimin, J. Tempere, H. Kurkjian, Phys. Rev. A 100, 063634 (2019).
[3]S.N. Klimin, J. Tempere, H. Kurkjian, Phys. Rev. A 103, 043336 (2021).
[4]T. Repplinger, S. Klimin, M. Gélédan, J. Tempere, H. Kurkjian, e-print arXiv: 2201.11421 (2022).[5]S.N. Klimin, J. Tempere, T. Repplinger,H. Kurkjian, e-print arXiv: 2208.09757 (2022)

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