Yu-Shiba-Rusinov Excitations in Superconducting Hetero-structures

Yu-Shiba-Rusinov Excitations in Superconducting Hetero-structures - Featured

Title: Yu-Shiba-Rusinov Excitations in Superconducting Hetero-structures
When: Friday, May 24, 2024, 12:00
Place: Department of Theoretical Condensed Matter Physics, Faculty of Sciences, Module 5, Seminar Room (5th Floor)
Speaker: Miguel A. Cazalilla, Donostia International Physics Center & Ikerbasque, Spain.

Yu-Shiba-Rusinov (YSR) excitations of magnetic impurities in superconductors have been attracting a great deal of attention in recent years in connection to topological quantum computing. In this seminar, I will describe our recent results concerning the properties of such excitation for magnetic impurities in various types of superconductor hetero-structures. In the first half of this seminar, I will argue that the formation of YSR excitations for magnetic impurities in thin normal metal films proximitized by a conventional superconductor 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 Nambu spinors 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 adapt 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, combining scanning tunneling spectroscopy with a theoretical model that builds upon the single-site description, the excitation spectrum of a magnetic Fe-porphyrin molecule on the Au/V(100) proximitized surface has been mapped into a manifold of entangled YSR 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]. In the second half of the seminar, I will describe the properties of YSR excitations of a single magnetic impurity in a spin-split superconductor (SS), namely a superconductor thin films in proximity to a ferromagnet or ferromagnetic insulator. We have recently developed a detailed model to describe this system [4] and found that the spin splitting in the superconducting host leads to a robust spin polarization of the YSR excitations [3,4]. This is in part caused by the energy splitting of the doublet of lowest lying states in the even fermion parity sector being, which leads e.g. to the existence of two YSR peaks in the tunneling spectrum of a strongly coupled magnetic impurity. Using Wilson’s numerical renormalization-group (NRG) we have computed the energy splitting and the tunneling spectrum. In the weak coupling regime, the energy splitting has been also computed using perturbation theory and, in the strong coupling limit, it can understood using a simple two-site model of the SS. In between these two limits, the energy splitting can be written in terms of the ratio of the impurity Kondo temperature and the mean superconducting gap using a universal scaling function, which we have obtained using NRG. Our analysis shows the splitting is a measurable consequence of both the magnetic interactions of Ruderman-Kittel-Kasuya-Yosida (RKKY)-type between the impurity and the SS and the (odd-frequency) triplet correlations in the SS.


  1. Jon Ortuzar, Jose Ignacio Pascual, F. Sebastian Bergeret, and MAC, Phys.Rev.B 108, 024511 (2023).
  2. Stefano Trivini, Jon Ortuzar, Katerina Vaxevani, Jingchen Li, F. Sebastian Bergeret, MAC, and Jose Ignacio Pascual, Phys. Rev. Lett.130, 136004 (2023).
  3. A. Skurativska, J. Ortuzar, D. Bercioux, F. S. Bergeret, and MAC, Phys. Rev. B 07, 224507 (2023).
  4. C.-H. Huang, A. Skurativska, F. S. Bergeret, and MAC, report arXiv:2402.07184 (2024).
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