Tunneling spectroscopy for ferromagnet/superconductor junctions

Abstract

In tunneling spectroscopy studies of ferromagnet/superconductor (F/S) junctions, the effects of spin polarization, Fermi wave-vector mismatch (FWM) between the F and S regions, and interfacial resistance play a crucial role. We study the low bias conductance spectrum of these junctions, governed by Andreev reflection at the F/S interface. We present results for a range of values of the relevant parameters and find that a rich variety of features appears, depending on pairing state and other conditions. We show that in the presence of FWM, spin polarization can enhance Andreev reflection and give rise to a zero-bias conductance peak for an s-wave superconductor. This implies that the extraction of spin polarization from measurements in F/S structures requires careful analysis. We consider both d- and s-wave superconductors as well as mixed states of the $d+is$ form and find that mixed states can have a different signature.