Mateo Uldemonlins: Quasiparticle focusing of YSR states in gapped superconductors | PYSQT

Seminar by Mateo Uldemonlins, organized by the PYSQT (PhDs and Young Scientists Quantum Technologies) network on the 31-10-2023.

Quasiparticle focusing of Yu-Shiba-Rusinov states in gapped superconductors -

A magnetic impurity on a conventional superconductor is one of the most simple
pair-breaking defects that induces in-gap Yu-Shiba-Rusinov (YSR) bound states.
Predicted soon after the publication of the BCS theory for superconductivity, they now
enjoy a renewed wave of interest thanks to the advancements in atomic
functionalization and local imaging techniques. These developments have motivated
significant theoretical and experimental efforts in designing complex structures of YSR
states in real space to engineer topological superconductivity and the long-sought
topologically protected qubits. The possibilities of engineering collective impurity
states (e.g., YSR chains, lattices, etc.) hosting exotic states of matter are crucially
influenced by the intricate spatial structure of YSR, which blends together information
about substrate bandstructure, pairing function, and impurity coupling.

In this talk, I will first introduce the paradigmatic model used to describe a magnetic
impurity on a two-dimensional superconductor and discuss the main aspects of YSR
physics. Secondly, I will present our recent work on the quasiparticle effect of YSR
states, where we use a generalized saddle-point approximation to unveil a simple
analytical relationship between the spatial structure of the YSR bound states and the
momentum-space anisotropy of the Fermi surface, namely the angular dependent
Fermi velocity and curvature of the Fermi surface. I will show that our analytical
approximation is quantitatively accurate against tight-binding calculations in lattice
models. I will also discuss its application to describe the anisotropy of YSR states
observed in scanning tunneling spectroscopy experiments of magnetic impurities on
NbSe2.

https://doi.org/10.1103/PhysRevB.105....