Spin current in ferromagnet/insulator/superconductor junctions

TL;DR

This paper develops a scattering theory for spin-polarized tunneling in ferromagnet/insulator/d-wave superconductor junctions, revealing how exchange fields modify Andreev reflection and lead to split conductance peaks.

Contribution

It introduces a comprehensive theoretical framework for spin and charge conductance in ferromagnet-insulator-superconductor junctions, accounting for exchange fields and barrier effects.

Findings

Exchange fields modify Andreev reflection properties.

Zero-bias conductance peak splits due to exchange interaction.

Surface bound states do not contribute to spin current.

Abstract

A theory of spin polarized tunneling spectroscopy based on a scattering theory is given for tunneling junctions between ferromagnets and d-wave superconductors. The spin filtering effect of an exchange field in the insulator is also treated. We clarify that the properties of the Andreev reflection are largely modified due to a presence of an exchange field in the ferromagnets, and consequently the Andreev reflected quasiparticle shows an evanescent-wave behavior depending on the injection angle of the quasiparticle. Conductance formulas for the spin current as well as the charge current are given as a function of the applied voltage and the spin-polarization in the ferromagnet for arbitrary barrier heights. It is shown that the surface bound states do not contribute to the spin current and that the zero-bias conductance peak expected for a d-wave superconductor splits into two peaks under the influence of the exchange interaction in the insulator.