EFFECTS OF COLLECTIVE MODES ON PAIR TUNNELING INTO SUPERCONDUCTORS

The effect of collective modes inside superconductors on N-S (normal-metal-superconductor) tunneling and S-S (superconductor-superconductor) tunneling is investigated. For the N-S case, we calculate the electron-pair tunneling current in the regime where the Andreev-scattering effect can be observed. The resulting current has a nonlinear contribution from the pair tunneling into collective modes inside the superconductor as well as the usual linear contribution due to the tunneling into the condensate (i.e., Andreev-scattering effect). We also consider S-S tunneling between a bulk superconductor and a superconducting thin film at zero and finite temperature. The electron-pair tunneling is found to be a sensitive probe to study the dynamical properties of the superconducting condensate, in particular in two dimensions and/or near the superconductor-insulator transition.