Collective modes in Fulde-Ferrell-Larkin-Ovchinnikov superconductors: The role of long-range Coulomb interaction and signatures in density response
arxiv(2024)
Abstract
We theoretically investigate collective excitations in the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states of Pauli-limited superconducting
films. When the long-range Coulomb interaction is absent, excitation spectra
consist of two gapless and three gapped modes. The gapless modes are the
Nambu-Goldstone modes associated with the spontaneous breaking of the U(1) symmetry and the translational symmetry. The gapped modes include the
Higgs mode and the twofold degenerate modes that cause the oscillation of the
domain width and grayness of FFLO nodal planes. We find that the long-range
Coulomb interaction only gaps out the gapless phase mode through the
Anderson-Higgs mechanism, while the other modes remain unaffected. Furthermore,
the field evolution of the dispersion of the gapless elastic mode, the
Nambu-Goldstone mode associated with the translational symmetry breaking, is
associated with that of the bandwidth of the mid-gap Andreev bound states. We
demonstrate that the signature of the elastic mode can be detected by measuring
the density-density response function.
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