Spin-active interfaces and unconventional pairing in half-metal/superconductor junctions

We study the physical properties of a half-metallic ferromagnet∣superconductor (HM∣S) bilayer, allowing for an arbitrary bulk pairing symmetry of the superconductor and spin-dependent processes at the interface. In particular, we study how the possibility of unconventional pairing such as p- and d-wave and a spin-active interface influence the (i) conductance spectra, (ii) proximity effect, and (iii) local density of states of such a bilayer. Our calculation is done both analytically and numerically in the ballistic limit, using both a continuum and lattice model. It is found that the spin-dependent phase shifts occurring at the HM∣S interface seriously influence all of the aforementioned phenomena. We explain our results in terms of Andreev reflection in the presence of a spin-active interface, allowing for both spin-filtering and spin-mixing processes. We demonstrate how the surface bound states induced by the anisotropy of the superconducting order parameter at the HM∣S interface are highly sensitive to these spin-dependent processes. Our results can be directly tested experimentally using scanning tunnel microscope measurements and/or point-contact spectroscopy