Spin-polarized currents and impurity-induced bound states in mesoscopic s-wave superconducting loops

The spin current is investigated in mesoscopic s-wave superconducting loops with spin-orbit interaction by solving the Bogoliubov-de Gennes equations self-consistently. Due to the breaking of spin-reversal symmetry, the spin-polarized currents with opposite chirality flowing near the inner and outer edges of the sample can be observed. A universal state bound to the introduced magnetic and nonmagnetic impurities appears in the strong scattering limit. Differently from the case of a nonmagnetic impurity, midgap bound states arise for the magnetic case, and the energy level can cross the Fermi level at some weak impurity strength. Consequently, the spin current jumps at the crossing point and shows novel features with impurity strength. Further, the current evolution depends sensitively on the located site of a single impurity, and more zero-energy modes can show up for an appropriate relative position between many magnetic impurities due to the quantum interference effect. Copyright (C) EPLA, 2015