Theory of tunneling in superconductors

The theory developed by Feuchtwang for tunneling in normal metals is extended to superconductors. It is shown that for metal--insulator--superconductor and metal--insulator--thin-metal--superconductor junctions, our results agree with the standard Bardeen ''tunneling Hamiltonian'' approach. In the latter case, however, if there exists a potential barrier at the thin-metal--superconductor interface, then the standard approach is found to be incorrect. We give the proper expression for the tunneling current in this case