Quasiparticle states in superconducting superlattices

The energy bands and the global density of states are computed forsuperconductor / normal-metal superlattices in the clean limit.Dispersion relations are derived for the general case of insulatinginterfaces, including the mismatch of Fermi velocities and effectiveband masses. We focus on the influence of finite interfacetransparency and compare our results with those for transparentsuperlattices and trilayers. Analogously to the rapidvariation on the atomic scale of the energy dispersion withlayer thicknesses in transparent superlattices, we find strongoscillations of the almost flat energy bands (transmissionresonances) in the case of finite transparency. In small-periodtransparent superlattices the BCS coherence peak disappears and asimilar subgap peak is formed due to the Andreev process. Withdecreasing interface transparency the characteristic double peakstructure in the global density of states develops towards a gaplessBCS-like result in the tunnel limit. This effect can beused as a reliable STM probe for interface transparency.