Robust Spin Polarization of YSR States in spin-split superconductors

Yu-Shiba-Rusinov (YSR) states arise as sub-gap excitations of a magnetic impurity in a superconducting host. Magnetic impurities in such systems are often modelled as classical spins, such that the impurity is described as an external scattering potential for the quasiparticles of the superconductor. However, this description overlooks the quantum origin of the spin degree of freedom of magnetic atoms.

In this talk, I will describe a single-site approximation as a way to take into account the quantum nature of the impurity spin. Within this approximation, we study the spectral properties of the YSR excitations of a system of magnetic impurity in a spin-split superconductor, i.e. a superconductor in proximity to a ferromagnetic insulator at zero external magnetic field. The YSR excitations of this system exhibit a robust spin polarization that is protected from fluctuations and environmental noise by the exchange field of the ferromagnetic insulator, which can be as large as a few Tesla. Finally, we compare the results of the quantum approach to the classical approach, and demonstrate that taking into account quantum fluctuations of the impurity spin is crucial to correctly predict spin polarization of the YSR states. The studied system can potentially be realized in a tunnel junction connected to a quantum dot in proximity to a spin-split superconductor or in the STS experiments with a magnetic molecule on a surface of a superconductor/ferromagnetic insulator heterostructure.