Superconducting Quantum Interference at the Atomic Scale

TL;DR

This paper demonstrates atomic-scale control of supercurrent reversal in a Josephson junction via Yu-Shiba-Rusinov states using STM, revealing a quantum phase transition with high precision.

Contribution

It presents the first direct observation of a 0 to π transition in a YSR state at the atomic scale using STM, with phase-sensitive detection.

Findings

Detected supercurrent sign change via a second transport channel.

Observed the quantum phase transition with unprecedented resolution.

Provided atomic-scale characterization of the supercurrent reversal.

Abstract

A single spin in a Josephson junction can reverse the flow of the supercurrent. At mesoscopic length scales, such $\pi$-junctions are employed in various instances from finding the pairing symmetry to quantum computing. In Yu-Shiba-Rusinov (YSR) states, the atomic scale counterpart of a single spin in a superconducting tunnel junction, the supercurrent reversal so far has remained elusive. Using scanning tunneling microscopy (STM), we demonstrate such a 0 to $\pi$ transition of a Josephson junction through a YSR state as we continuously change the impurity-superconductor coupling. We detect the sign change in the critical current by exploiting a second transport channel as reference in analogy to a superconducting quantum interference device (SQUID), which provides the STM with the required phase sensitivity. The measured change in the Josephson current is a signature of the quantum phase transition and allows its characterization with unprecedented resolution.