Yu-Shiba-Rusinov States in Single Atoms and Molecules on Superconductors

Katharina Franke (Freie Universität Berlin)

Magnetic adatoms and molecules on a superconducting substrate act as a pair-breaking scattering potential for the Cooper pairs and induce Yu-Shiba-Rusinov (YSR) states inside the superconducting energy gap. These states can be probed on the atomic scale by low-temperature scanning tunneling spectroscopy.
Here, we show that individual Manganese (Mn) atoms give rise to a distinct number of YSR states, depending on the crystal field imposed by the adsorption site. The spatial extent of these states directly reflects their origin as the singly occupied d-states [1]. When the atoms are brought into sufficiently close distance, we detect the formation of symmetric and anti-symmetric combinations of the corresponding YSR wave functions [2].
In contrast to the atomic adsorbates, molecules bear more flexibility and the possibility to tune their strength of the magnetic exchange scattering potential 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 allows to study the quantum phase transition between a screened and unscreened spin [3].

References:
[1] M. Ruby, Y. Peng, F. von Oppen, B. W. Heinrich, K. J. Franke, Phys. Rev. Lett. 117, 186801 (2016).
[2] M. Ruby, B. W. Heinrich, Y. Peng, F. von Oppen, K. J. Franke, Phys. Rev. Lett. 120, 156803 (2018).
[3] L. Farinacci, G. Ahmadi, G. Reecht, M. Ruby, N. Bogdanoff, O. Peters, B. W. Heinrich, F. von Oppen, K. J. Franke, Phys. Rev. Lett. 121, 196803 (2018).