Spin polarization versus lifetime effects at point contacts between superconducting niobium and normal metals

TL;DR

This study investigates the challenges in distinguishing spin polarization effects from finite lifetime effects in point-contact spectroscopy between superconducting niobium and various metals, proposing Fermi surface mismatch as a solution.

Contribution

The paper provides experimental evidence supporting the use of Fermi surface mismatch to resolve polarization-lifetime ambiguity in Nb-metal contacts.

Findings

Nb-normal metal interfaces have up to 80% transparency.

Spin polarization at Nb-metal interfaces is small or negligible.

Results support Fermi surface mismatch as a key factor in analysis.

Abstract

Point-contact Andreev reflection spectroscopy is used to measure the spin polarization of metals but analysis of the spectra has encountered a number of serious challenges, one of which is the difficulty to distinguish the effects of spin polarization from those of the finite lifetime of Cooper pairs. We have recently confirmed the polarization-lifetime ambiguity for Nb-Co and Nb-Cu contacts and suggested to use Fermi surface mismatch, the normal reflection due to the difference of Fermi wave vectors of the two electrodes, to solve this dilemma. Here we present further experiments on contacts between superconducting Nb and the ferromagnets Fe and Ni as well as the noble metals Ag and Pt that support our previous results. Our data indicate that the Nb - normal metal interfaces have a transparency of up to about 80 per cent and a small, if not negligible, spin polarization.