Collective modes in layered superconductors

We calculate the collective-mode spectrum for a layered superconductor structure. We find that for wave vectors directed close to the direction of the superlattice axis, the plasmon mode remains below the superconducting gap edge (i.e., the Anderson-Higgs mechanism does not give rise to a massive collective mode as it does in the bulk three-dimensional system). We also find a clear signature of superconductivity in the form of a unique line splitting in the collective-mode spectrum as the plasmon crosses the superconducting gap. Experimental implications are discussed.