Superconducting quantum interference at the atomic scale
Sujoy Karan, Haonan Huang, Ciprian Padurariu, Björn Kubala, Andreas Theiler, Annica M. Black‐Schaffer, Gonzalo Morrás, A. Levy Yeyati, Juan Carlos Cuevas, Joachim Ankerhold, Klaus Kern, Christian R. Ast
Abstract
Abstract A single spin in a Josephson junction can reverse the flow of the supercurrent by changing the sign of the superconducting phase difference across it. At mesoscopic length scales, these π-junctions are employed in various applications, such as finding the pairing symmetry of the underlying superconductor, as well as quantum computing. At the atomic scale, the counterpart of a single spin in a superconducting tunnel junction is known as a Yu–Shiba–Rusinov state. Observation of the supercurrent reversal in that setting has so far remained elusive. Here we demonstrate such a 0 to π transition of a Josephson junction through a Yu–Shiba–Rusinov 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, which provides our scanning tunnelling microscope 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 high resolution.