Theory of superconductor-ferromagnet point contact spectra: the case of strong spin polarization

TL;DR

This paper develops a comprehensive theoretical model for superconductor-ferromagnet point contacts, emphasizing the significant role of interface potential shape and spin-mixing effects in creating spin-polarized Andreev bound states.

Contribution

It introduces a non-perturbative microscopic model for interface scattering that includes spin-filtering and spin-mixing, extending previous simplified approaches.

Findings

Spin-mixing effects can be large with realistic interface potentials.

Interface potential shape critically influences spin-mixing magnitude.

Realistic models predict spin-polarized sub-gap Andreev bound states.

Abstract

We study the impact of spin-active scattering on Andreev spectra of point contacts between superconductors(SCs) and strongly spin-polarized ferromagnets(FMs) using recently derived boundary conditions for the Quasiclassical Theory of Superconductivity. We describe the interface region by a microscopic model for the interface scattering matrix. Our model includes both spin-filtering and spin-mixing and is non-perturbative in both transmission and spin polarization. We emphasize the importance of spin-mixing caused by interface scattering, which has been shown to be crucial for the creation of exotic pairing correlations in such structures. We provide estimates for the possible magnitude of this effect in different scenarios and discuss its dependence on various physical parameters. Our main finding is that the shape of the interface potential has a tremendous impact on the magnitude of the spin-mixing effect. Thus, all previous calculations, being based on delta-function or box-shaped interface potentials, underestimate this effect gravely. As a consequence, we find that with realistic interface potentials the spin-mixing effect can easily be large enough to cause spin-polarized sub-gap Andreev bound states in SC/sFM point contacts. In addition, we show that our theory generalizes earlier models based on the Blonder-Tinkham-Klapwijk approach.