Theory of Andreev Bound States in S-F-S Junctions and S-F Proximity Devices

TL;DR

This paper reviews Andreev bound states in superconductor-ferromagnet hybrid structures, highlighting how spin-dependent effects influence their spectra and linking wave-function and Green-function theories through coherence functions.

Contribution

It introduces a unified framework for understanding Andreev bound states in S-F systems using coherence functions, bridging different theoretical approaches.

Findings

Spin-dependent scattering phase shifts affect Andreev spectra.

Triplet rotation influences Andreev point contact spectra.

Coherence functions connect wave-function and Green-function theories.

Abstract

Andreev bound states are an expression of quantum coherence between particles and holes in hybrid structures composed of superconducting and non-superconducting metallic parts. Their spectrum carries important information on the nature of the pairing, and determines the current in Josephson devices. Here I give a short review on Andreev bound states in systems involving superconductors and ferromagnets with strong spin-polarization. I show how the processes of spin-dependent scattering phase shifts and of triplet rotation influence Andreev point contact spectra, and provide a general framework for non-local Andreev phenomena in such structures in terms of coherence functions. Finally, I demonstrate how the concept of coherence functions cross-links wave-function and Green-function based theories, by showing that coherence functions fulfilling the equations of motion for quasiclassical Green functions can be used to derive a set of generalised Andreev equations.