Current-voltage characteristics of SIS structures with localized states in the material of the barrier layer

Elastic resonant tunneling through a single localized state in an insulating layer (I-layer) situated in the constriction zone between two thick superconducting electrodes is investigated theoretically, and the current-voltage characteristic (IVC) of the structure is calculated. The accompanying analysis leads to the prediction that an appreciable current can flow through the structure, not at \eV\ = 2 Delta (Delta is the modulus of the order parameter of the superconducting electrodes) as in the case of an ordinary SIS junction, but at \eV\ greater than or equal to Delta, and also that the IVC can acquire segments of negative differential resistance in the case of tunneling through a single localized state. Averaging of the IVC over an ensemble of localized states distributed uniformly throughout the volume of the I-layer and with respect to the energy near the chemical potential mu in the limit Gamma 0/Delta much greater than 1 (Gamma(0) is the half-width of the resonance line of the localized state) produces a smaller excess current than in a junction of the SNS type. It is shown that the IVC's exhibit a transition from an excess current to a deficit current as To decreases in the high-voltage range. (C) 1998 American Institute of Physics. [S1063-7761(98)02408-1].