Pascal Simon, University of Paris, Saclay

The study of magnetic impurities have received a recent resurgence of interest, fueled in part by possible sightings of topological superconductivity in chains of magnetic adatoms on superconducting substrates. I will first review and present recent results for a single classical magnetic impurity  embedded in a two dimension (2D) superconductor and show that the spatial extent of the Shiba bound state  is actually long-ranged compared to what was observed in 3D superconductors [1]. I will then argue that such a system can be viewed as the simplest heterostructure made of a conventional s-wave superconductor and a ferromagnet which breaks time-reversal symmetry. As such, odd-frequency pairing correlations, as first envisioned by   Berezinskii  more than forty years ago are generated around the impurity. I will show using measurements of the  local electron density of states by STM, how the superconducting odd-ω pairing function can be extracted [2].

I will then move on interpreting recent scanning tunneling spectroscopy measurements on a superconducting monolayer of lead (Pb) with nanoscale cobalt islands, which have revealed puzzling quasiparticle in-gap states [3]. We find that a vortex-like defect in the Rashba spin-orbit coupling binds a single Majorana zero-energy (mid-gap) state. Importantly, in contrast to the case of a superconducting vortex [4], our spin-orbit defect does not create a tower of in-gap excitation states. Our findings match the puzzling features observed in the experiment, particularly: (1) preservation of superconducting gap, and (2) short localization length of the zero-energy state compared to the superconductor coherence length [3].

References
[1] G. Ménard et al., Nature Physics 11, 1013 (2015).
[2] V. Perrin , F. L. N. Santos, G. Ménard, C. Brun, T. Cren, M. Civelli, P. Simon, Phys. Rev. Lett.125, 117003 (2020).

[3] G. C. Ménard et al., Nature Comm. 10, 1 (2019).

[4] C. Caroli, P.G. de Gennes, and J. Matricon, Physics Letters 9, 307(1964).