Multiple Andreev reflection in single atom niobium junctions

TL;DR

This paper investigates the electronic transport properties of single atom niobium junctions, revealing dominant conduction channels and validating theoretical models with experimental data.

Contribution

It provides the first detailed analysis of multiple Andreev reflections in single atom niobium junctions, identifying conductance channels and their transmission probabilities.

Findings

Single channel dominates in tunneling junctions with large vacuum barriers.

Five conductance channels contribute in single Nb atom contacts.

Experimental data aligns with theoretical models without adjustable parameters.

Abstract

Single atom junctions between superconducting niobium leads are produced using the Mechanically Controllable Break Junction technique. The current-voltage characteristics of these junctions are analysed using an exact formulation for a superconducting quantum point contact. For tunnelling between two single atoms with a sufficiently large vacuum barrier, it is found that a single channel dominates the current, and that the current-voltage characteristic is described by the theory, without adjustable parameters. For a contact of a single Nb atom it is shown that five conductance channels contribute to the conductance, in agreement with the number expected based on the number of valence orbitals for this d-metal. For each of the channels the transmission probability is obtained from the fits and the limits of accuracy for these numbers are discussed.