Nonlinear transport through NS junctions due to imperfect Andreev reflection

We investigate a normal-metal-superconductor (point) contact in the limit where the number of conducting channels in the metallic wire is reduced to few channels. As the effective Fermi energy drops below the gap energy, a conducting band with a width twice the Fermi energy is formed. Depending on the mode of operation, the conduction band can be further squeezed, leading to various nonlinear effects in the current-voltage characteristics such as current saturation, a N-shaped negative differential resistance, bistability, and hysteresis. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 7, 572–577 (10 October 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.