Observation of Andreev Bound States at Spin-active Interfaces

We report on high-resolution differential conductance experiments on nanoscale superconductor-ferromagnet tunnel junctions with ultrathin oxide tunnel barriers. We observe subgap conductance features that are symmetric with respect to bias and shift according to the Zeeman energy with an applied magnetic field. These features can be explained by resonant transport via Andreev bound states induced by spin-active scattering at the interface. From the energy and Zeeman shift of the bound states, both the magnitude and sign of the spin-dependent interfacial phase shifts between spin-up and spin-down electrons can be determined. These results contribute to the microscopic insight into the triplet proximity effect at spin-active interfaces.