Bound states in superconductors revealed by Josephson tunneling

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The local competition of the superconducting ground state with a nanoscale exchange or Coulomb potential opens an extremely rich field of physics, including Shiba or novel Majorana-type states. The latter are discussed as promising examples for the realization of fault-tolerant quantum computing. By unifying the experimental approaches from our two groups, this joint proposal aims at resolving fundamental questions about the local interaction of magnetism and superconductivity. In particular, we are seeking for signatures of topological states in one- and two-dimensional magnetic nanostructures on type I superconductors. The Berlin group has strong experience in Scanning Tunneling Microscopy/Spectroscopy (STM/STS) of bound states around magnetic nanostructures on superconducting substrates. The Grenoble group's expertise in quantum nano-electronics and related transport measurements is very complementary. This will enable the two groups to jointly implement the technique of Scanning Josephson Tunneling Microscopy and thereby reveal the variations in the superconducting order parameter at the atomic scale around single magnetic adatoms, few-atom clusters, nanowires and magnetic molecules. We will further couple microwave fields into the Scanning Tunneling Microscope, allowing for simultaneous measurements of Shapiro steps and high-resolution STM/STS. From the combined knowledge of the local density of states, the Josephson critical current and the microwave response, we will seek for evidence of topological states in superconductors.