IPB colloquium by Rok Zitko

 IPB colloquium will be held on Monday, 17 October 2016 at 14:00 in “Dr. Dragan Popović” library reading room of the Institute of Physics Belgrade. The talk entitled

"Magnetic nanostructures in contact with superconductors: from Shiba to Majorana states"

will be given by Dr. Rok Žitko (Institute "Jozef Stefan", Ljubljana, Slovenia)

Abstract of the talk:

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. These are known in different research communities either as the Yu-Shiba-Rusinov states or the Andreev bound states. They are observable as spectroscopically sharp resonances in the tunneling spectra, located well inside the superconducting gap for sufficiently strong exchange coupling. In more general cases (such as high impurity spin, magnetic anisotropy, coupling to vibrational modes, presence of multiple screening channels or multiple impurities) the spectra of the sub-gap excitations become complex with multiple excitations that reveal the strongly correlated nature of the problem. When multiple impurities form a long ferromagnetically ordered chain and provided that there is sufficiently strong spin-orbit coupling, the system is predicted to enter a topologically nontrivial state characterized by fractional Majorana modes localized on the chain ends. The signatures of such zero-energy modes have now been observed in several experiments.