Barrier-induced enhancement of Andreev reflection for minority-spin quasiparticles in ferromagnetic metal/insulator/superconductor ballistic junctions

The Andreev reflection probability in ferromagnetic metal/superconductor ballistic junctions depends on the spin orientation of the incident quasiparticle when a barrier of low or moderate strength is present at the interface. In contrast to the common conception that a barrier always reduces Andreev reflection probability, this probability is enhanced for quasiparticles in the minority spin band only, due to a reduction in the phase shift of the wave function. Because of the Andreev reflection probability enhancement, which is predicted to be below the superconducting gap near the gap edge, the total subgap conductance exhibits characteristic weak but measurable features (maxima).