Single Channel Josephson Effect in a High Transmission Atomic Contact

TL;DR

This paper investigates the Josephson effect in atomic-scale contacts using STM, revealing deviations from traditional models at high transmission and demonstrating the importance of full current-phase relations and multiple Cooper pair tunneling.

Contribution

It introduces a comprehensive model of the Josephson effect in atomic contacts considering high transmission and Coulomb blockade, showing deviations from the Ambegaokar-Baratoff relation.

Findings

Significant deviations from AB relation at high transmission.

Excellent agreement between model and experimental data.

Multiple Cooper pair tunneling observed at high transmission.

Abstract

The Josephson effect in scanning tunneling microscopy (STM) is an excellent tool to probe the properties of the superconducting order parameter on a local scale through the Ambegaokar-Baratoff (AB) relation. Using single atomic contacts created by means of atom manipulation, we demonstrate that in the extreme case of a single transport channel through the atomic junction modifications of the current-phase relation lead to significant deviations from the linear AB formula relating the critical current to the involved gap parameters. Using the full current-phase relation for arbitrary channel transmission, we model the Josephson effect in the dynamical Coulomb blockade regime because the charging energy of the junction capacitance cannot be neglected. We find excellent agreement with the experimental data. Projecting the current-phase relation onto the charge transfer operator shows that at high transmission multiple Cooper pair tunneling may occur. These deviations become non-negligible in Josephson-STM, for example, when scanning across single adatoms.