Andreev bound states in superconductor/ferromagnet point contact Andreev reflection spectra

TL;DR

This paper demonstrates how point contact Andreev reflection spectra can detect Andreev Bound States and quantify spin mixing angles at superconductor/ferromagnet interfaces, clarifying longstanding controversies and revealing consistent spin mixing behavior.

Contribution

It introduces a method to detect Andreev Bound States and measure spin mixing angles, advancing understanding of spin-dependent phenomena at superconductor-ferromagnet interfaces.

Findings

Detection of Andreev Bound States via spectra analysis.

Spin mixing angles are consistently narrow across samples.

The properties of ferromagnets influence spin mixing behavior.

Abstract

As charge carriers traverse a single superconductor ferromagnet interface they experience an additional spin-dependent phase angle which results in spin mixing and the formation of a bound state called the Andreev Bound State. This state is an essential component in the generation of long range spin triplet proximity induced superconductivity and yet the factors controlling the degree of spin mixing and the formation of the bound state remain elusive. Here we demonstrate that point contact Andreev reflection can be used to detect the bound state and extract the resulting spin mixing angle. By examining spectra taken from La1.15Sr1.85Mn2O7 single crystal - Pb junctions, together with a compilation of literature data on highly spin polarised systems, we show that the existence of the Andreev Bound State both resolves a number of long standing controversies in the Andreev literature as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly we find that for these high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.