Plasmons in three-dimensional superconductors

T. Repplinger; S. Klimin; M. Gélédan; J. Tempere; H. Kurkjian

doi:10.1103/PhysRevB.107.014504

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Plasmons in three-dimensional superconductors

T. Repplinger, S. Klimin, M. Gélédan, J. Tempere, H. Kurkjian

DOI 10.1103/PhysRevB.107.014504 · Physical Review B

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Abstract

We study the plasma branch of an homogeneous three-dimensional electron gas in an s-wave superconducting state. Although a sum rule guarantees that the departure of the plasma branch always coincides with the plasma frequency ωp, the dispersion and lifetime of plasmons is strongly affected by the presence of the pair condensate, especially when ωp is close to the pair-breaking threshold 2Δ. When ωp is between 1.7Δ and 2Δ, the level repulsion is strong enough to give the plasma branch an anomalous, downward dispersion and a dispersion minimum strictly lower than ωp. Then for ωp>2Δ, plasmons damp out in pair-breaking excitations, acquiring a small damping rate at zero temperature, which we compute in a non-perturbative way. Finally, the density-density response function displays a resonance near 2Δ (not to be confused with the amplitude mode), which can beat with the main plasma resonance, and subsists for ωp large compared to Δ, thereby distinguishing charged from neutral condensates.