Andreev-reflection spectroscopy with superconducting indium - a case study

TL;DR

This study uses Andreev reflection spectroscopy with superconducting indium to analyze interface properties with various normal metals, revealing consistent interface transparency and diffusive transport characteristics across different contacts.

Contribution

It applies a modified BTK theory to interpret Andreev reflection data, demonstrating that interfaces are highly transparent and predominantly diffusive, with minimal tunneling barriers.

Findings

Z-parameter around 0.5 indicating high interface transparency

Interfaces have negligible dielectric tunneling barriers

Contacts are mainly in the diffusive transport regime

Abstract

We have investigated Andreev reflection at interfaces between superconducting indium (T_c = 3.4 K) and several normal conducting non-magnetic metals (palladium, platinum, and silver) down to T = 0.1 K as well as zinc (T_c = 0.87 K) in its normal state at T = 2.5 K. We analysed the point-contact spectra with the modified one-dimensional BTK theory valid for ballistic transport. It includes Dynes' quasi-particle lifetime as fitting parameter Gamma in addition to superconducting energy gap 2Delta and strength Z of the interface barrier. For contact areas from less than 1 nm^2 to 10000 nm^2 the BTK Z-parameter was close to 0.5, corresponding to transmission coefficients of about 80%, independent of the normal metal. The Z-parameter varies by less than +/-0.1 around its average value, indicating that the interfaces have a negligible dielectric tunneling barrier. Also Fermi surface mismatch does not account for the observed Z. The extracted value Z approx 0.5 can be explained by assuming that practically all of our point contacts are in the diffusive regime.