ELECTRONIC-STRUCTURE OF SUPERCONDUCTING MULTILAYERS

The Bloch waves, the energy bands, and the local and global densities of states are computed for superconducting multilayers in which the Fe rmi energy in the superconducting (S) layers, epsilon(FS), may exceed the one in the normal (N) layers, epsilon(FN). Self-consistent pair po tentials and their equivalent square-well representations are consider ed. If the S-layer thickness exceeds several coherence lengths the den sities of states exhibit the subgap peak (besides the BCS peak) and th e Tomasch-McMillan-Anderson oscillations known from SNS junctions. The se features, due to off-diagonal (Andreev) scattering, decrease with i ncreasing ratio epsilon(FS)/epsilon(FN) because of the competition fro m diagonal scattering. The temperature dependence of the energetic pos ition of the subgap peak is weaker than that of the BCS peak. In multi layers with S-layer thickness of the order of magnitude of one coheren ce length and epsilon(FS)/epsilon(FN) much greater than 1, the local d ensity of states is BCS-like in the S layers and normal-metal-like in the N layers. This is due to electron localization in the S layers and consistent with recent scanning tunneling microscope conductance meas urements in YBa2Cu3O7.