RESONANT-TUNNELING IN NORMAL METAL-SUPERCONDUCTOR JUNCTIONS - LOW-VOLTAGE CONDUCTANCE FEATURES AND EXCESS CURRENT

We have studied the conductance of normal metal-superconductor junctio ns with a potential barrier on the interface containing localized elec tron states. The probabilities of the Andreev reflection and the quasi particle transmission are calculated using a one-dimensional model wit h a single localized state. The conductance of the junctions with many localized states described by a uniform distribution of the bound sta te energies around the Fermi level is calculated in the low-temperatur e region. It is shown that the Andreev reflection processes assisted b y the resonant tunneling result in new features of the conductance, su ch as a zero-bias maximum, a low-energy gaplike structure, and a two-g ap behavior of the conductance at applied voltages near the value corr esponding to the energy gap of the superconductor. The current-voltage characteristics for high applied voltages reveal both an excess curre nt and current deficit, depending on the parameters of the localized s tates and the transparency of the tunnel barrier.