Low-temperature transport through a quantum dot between two superconductor leads

By means of infinite-U noncrossing approximation we investigate the transport properties of a quantum dot coupled to two BCS superconductors. The nonequilibrium density of states and differential conductance are carried out in the regime where gap energy Δ is much larger than the Kondo temperature T_K. In the equilibrium density of states two sharp peaks around the gap bounds are observed. Application of a finite voltage bias leads these peaks to split, leaving suppressed peaks near the edges of energy gap of each lead. Further, in the nonlinear differential conductance there are three peaks, one around zero bias, another two at biases ±2Δ. With decreasing temperature, the differential conductances at biases ±2Δ anomalously increase, while the linear conductance descends.