QDev Seminar: Rok Zitko

Faculty of Mathematics and Physics, University of Ljubjana

The physics of sub-gap states induced by quantum impurities

Recent advances in the fabrication of nanometer scale hybrid semiconductor-superconductor devices as well as in the scanning tunneling spectroscopy of adsorbate covered surfaces of superconductors have made possible very detailed experimental studies of the old problem of paramagnetic impurities in a superconducting host using local probes with very high energy and spatial resolution. At the same time, improved theoretical tools have been devised to reliably and accurately calculate the excitation spectra of the corresponding quantum impurity problems with gapped continuum electrons. These developments have enabled very stringent tests between experiment and theory. The results demonstrate the importance to describe the magnetic impurities as quantum objects with non-trivial internal dynamics due to the coupling to their environment.

I will discuss the physics of bound states induced by the exchange interaction between the magnetic impurities and the Bogoliubov quasiparticles in superconductors, focusing on the effects of strong correlations. Particularly interesting behaviour is found for cases of high-spin impurities in the presence of magnetic anisotropy, and when nearby impurities are coupled though exchange interaction. I will also briefly comment about non-Fermi-liquid features found in the two-channel Kondo model with superconducting channels.