Quantifying the quantum nature of high spin YSR excitations in transverse magnetic field

TL;DR

This study uses low-temperature scanning tunneling microscopy to analyze high-spin Yu-Shiba-Rusinov states of manganese phthalocyanine molecules on lead, revealing their quantum behavior in magnetic fields.

Contribution

It provides detailed characterization and modeling of YSR states in high-spin molecules, highlighting the effects of magnetic anisotropy and exchange interactions.

Findings

Two types of YSR excitations depending on adsorption geometry.

One molecular type behaves as an isolated spin, the other as a coupled spin system.

Insights into the quantum phase evolution of YSR states in magnetic fields.

Abstract

Excitations of individual and coupled spins on superconductors provide a platform to study quantum spin impurity models as well as a pathway toward realizing topological quantum computing. Here, we characterize, using ultra-low temperature scanning tunneling microscopy/spectroscopy, the Yu-Shiba-Rusinov (YSR) states of individual manganese phthalocyanine molecules with high spin character on the surface of an ultra-thin lead film in variable transverse magnetic field. We observe two types of YSR excitations, depending on the adsorption geometry of the molecule. Using a zero-bandwidth model, we detail the role of the magnetic anisotropy, spin-spin exchange, and Kondo exchange. We illustrate that one molecular type can be treated as an individual spin akin to an isolated spin on the metal center, whereas the other molecular type invokes a coupled spin system represented by a spin on the center and the ligand. Using the field-dependent evolution of the YSR excitations and comparisons to modeling, we describe the quantum phase of each of the molecules. These results provide an insight into the quantum nature of YSR excitations in magnetic field, and a platform to study spin impurity models on superconductors in magnetic field.