The resistance is computed of an ${ \rm NI}_{ 1}{ \rm NI}_{ 2}{ \rm S}$ junction, where N = normal metal, S = superconductor, and ${ \rm I}_{ i}$ = 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 $l$, barrier separation $L$). It is found that the resistance at fixed $\Gamma_{ 2}$ shows a { \em minimum} as a function of $\Gamma_{ 1}$, when $\Gamma_{ 1}\approx\sqrt{ 2}\Gamma_{ 2}$, provided $l\gtrsim\Gamma_2 L$. 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. ***Submitted to Physica B.***
Categories: cond-mat