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Résumé |
Understanding the interplay between individual magnetic impurities and superconductivity is crucial for
bottom-up construction of novel phases of matter, as well as to exploit the local response as a probing tool.
For decades, the description by Yu, Shiba and Rusinov (YSR) of single spins in a superconductor and its
extension to include quantum effects has proven highly successful: the pair-breaking potential of the spin
generates sub-gap bound states. I will first show how atomically-resolved shot noise can be used to reveal
the coherent and incoherent dynamics of such sub-gap bound states [1].
By tuning the energy of the sub-gap states through zero, the impurity screening by the superconductor
makes the ground state gain or lose an electron, signalling a parity breaking quantum phase transition. I will
present a set of scanning tunneling microscopy (STM) measurements that explicitly invalidate the classical
YSR paradigm, and propose an interpretation in terms of a multi-orbital Anderson impurity model [2]. In
particular, I show that in multi-orbital impurities, electronic correlations can conversely lead to a quantum
phase transition where the impurity mean occupation changes dramatically, without significant effect of the
screening by the superconductor. This finding implies that the YSR treatment is not always valid, and that
intra-atomic interactions, particularly Hund’s coupling that favours high-spin configurations, are an essential
ingredient for understanding the sub-gap states.
[1] U. Thupakula, V. Perrin, A. Palacio-Morales, L. Cario, M. Aprili, P. Simon, F. Massee, Phys. Rev. Lett. 128,
247001 (2022)
[2] M. Uldemolins, A. Mesaros, G. D. Gu, A. Palacio-Morales, M. Aprili, P. Simon, and F. Massee, “Interaction-
driven quantum phase transition of a single magnetic impurity in Fe(Se,Te)” 2023, arXiv:2310.06030. |
