I will argue that the formation of Yu-Shiba-Rusinov excitations in proximitized thin metal films is largely mediated by a type of Andreev-bound state named after de Gennes and Saint-James. This is shown by studying an experimentally motivated model and computing the overlap of the wavefunctions of these two subgap states. We find that the overlap stays close to unity even as the system moves away from weak coupling across the parity-changing quantum phase transition. Based on this observation, we introduce a single-site model of the bound state coupled to a quantum spin. The adequacy of the single-site description is assessed by reintroducing the coupling to the continuum as a weak perturbation and studying its scaling flow using Anderson’s poor man’s scaling [1].
Next, I will describe how, combining scanning tunneling spectroscopy with a theoretical model that builds upon the single-site description investigated above, we have mapped the excitation spectrum of an Fe-porphyrin molecule on the Au/V(100) proximitized surface into a manifold of entangled Yu-Shiba-Rusinov and spin excitations [2]. Pair excitations emerge in the tunneling spectra as peaks outside the spectral gap only in the strong coupling regime, where the presence of a bound quasiparticle in the ground state ensures the even fermion parity of the excitation. Our results unravel the quantum nature of magnetic impurities on superconductors and demonstrate that pair excitations unequivocally reveal the parity of the ground state [2].
[1] “Theory of a single magnetic impurity on a thin metal film in proximity to a superconductor”, Jon Ortuzar, Jose Ignacio Pascual, F. Sebastian Bergeret, and MAC, Phys.Rev.B 108, 024511 (2023).
[2] “Cooper Pair Excitation Mediated by a Molecular Quantum Spin on a Superconducting Proximitized Gold Film”. Stefano Trivini, Jon Ortuzar, Katerina Vaxevani, Jingchen Li, F. Sebastian Bergeret, MAC, and Jose Ignacio Pascual, Phys.Rev.Lett.130, 136004 (2023).
