REFLECTIONLESS TUNNELING THROUGH A DOUBLE-BARRIER NS JUNCTION

The resistance is computed of an NI1NI2S junction, where N is the normal metal, S the superconductor, and I-i the insulator or tunnel barrier (transmission probability per mode Gamma(i)). The ballistic case is considered, as well as the case that the region between the two barriers contains disorder (mean free path I, barrier separation L). It is found that the resistance at fixed Gamma(2) shows a minimum as a function of Gamma(1), when Gamma(1) approximate to root 2 Gamma(2), provided l greater than or similar to Gamma(2)L. The minimum is explained in terms of the appearance of transmission eigenvalues close to one, analogous to the ''reflectionless tunneling'' through a NIS junction with a disordered normal region. The theory is supported by numerical simulations.