Successful PhD defense of Felix Küster

Single magnetic impurities embedded in a superconducting host material represent a microscopic interface between magnet and superconductor. Very locally around such atomic-scale magnets, electronic quasiparticle resonances emerge at sharply defined energies close to the Fermi level and governed by the impurity spin-to-superconductor coupling strength. These so-called Yu-Shiba-Rusinov states attract great interest in fundamental research and technology as the initial ingredient for application proposals, e.g. in quantum computers.

This thesis explores magnetic surface atoms on superconducting Nb(110) by scanning tunneling microscopy and spectroscopy with normal metallic, superconducting and magnetic tips. Theoretical first-principles and tight-binding models are employed to shed light on the complex phenomena observed in measurement data. An initial comparison of single adsorbates from different elements correlates spectroscopy in the superconducting and metallic phase as well as Josephson supercurrents to deduce well-defined trends. Chromium yields a particularly interesting candidate for the creation of custom tailored nanostructures. Precisely controlling their relative positions by atomic manipulation with the tip, two interacting Cr impurities are found to unfold an unprecedented flexibility to be tuned on demand, facilitated by indirect mediation through the anisotropic substrate structure. Artificially crafted spin chains are built to explore the predicted emergence of topological superconductivity, thereby unraveling general trends that effectively mimic sought-after Majorana zero modes. After evaluation of different approaches to spin-polarized tunneling, single atom tip apex functionalization is identified as a suitable tool to characterize the ordered spin textures under investigation. Finally, an outlook into two dimensional tailored spin nanostructures with several realized assemblies paves the way for fruitful future research on fundamental and device-based concepts.