Magnetic impurities in conventional superconductors induce a pair-breaking potential, which leads to bound states inside the superconducting energy gap. These states are called Yu-Shiba-Rusinov (YSR) states, and can be probed by scanning tunneling spectroscopy at the atomic scale. The energy of these states depends on the strength of both exchange and potential scattering. The individual YSR states can be regarded as the building blocks for topological superconductivity in adatom chains on conventional superconductors.
Here, we explore different strategies to tune the energy of the YSR states. In the first case, we tune the strength of the magnetic exchange scattering to the Cooper pairs. Upon tip approach we are able to continuously vary the energy of YSR states induced by Fe-porphin molecules on Pb(111) across the Fermi energy. This model system further allows to study the quantum phase transition between a screened and unscreened spin .
In the second case, we make use of the charge-density wave (CDW) of NbSe2, which coexists with superconductivity, to tune the energy of YSR states of individual Fe atoms. All atoms are placed in the same atomic adsorption site, but at different positions with respect to the CDW. The YSR states exhibit different energies and different oscillatory patterns. We ascribe the shift in energies to the variation of the density of states as well as to changes in the potential scattering strength . These results are important for designing topological nanostructures.
 E. Liebhaber, S. Acero Gonzalez, R. Baba, G. Reecht. B. W. Heinrich, S. Rohlf, K. Rossnagel, F. von Oppen, K. J. Franke, Nano Lett. 20, 339 (2020).