Magnetic bound states in 2D superconductors

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Exchange coupling of magnetic adsorbates on superconductors induces Yu-Shiba-Rusinov (YSR) states. These decay rapidly on a three-dimensional superconductor. In contrast, a two-dimensional superconductor enhances the decay length and, thus, enhances coupling of YSR states. The splitting and shift of coupled YSR states provide signatures of hybridization and magnetic interactions. In certain parameter regimes, extended structures are expected to host topological edge states. To overcome the limited range of interactions on bulk three-dimensional superconductors, we will grow and characterize heterostructures based on two-dimensional superconductors. We will profit from the expertise in MBE growth of one of the partners, while the other partners will use high-resolution scanning tunneling microscopy to track the evolution of the YSR states from single molecular adsorbates to two-dimensional islands on monolayers of NbSe2 and graphene on Rhenium. The flexibility in the choice of molecular adsorbate provides a large parameter space for different magnetic moments, different exchange coupling to the surface, as well as substrate-mediated intermolecular coupling strength. This will allow us to explore a large range of the magnetic phase diagram and search for topological states. Additionally, we will grow heterostructures of atomically thin layers of NbSe2 and magnetic transition-metal dichalcogenides. Here, the choice of the magnetic layer allows us to tune the exchange coupling strength to the substrate. We will also investigate the influence of the island size on the possibility of topological edge states and eventually investigate the interaction of adjacent islands. Our systematic analysis of YSR states of magnetic metal-organic and inorganic layers on two-dimensional superconductors will enable fundamental insights into exchange coupling to the substrate as well as magnetic textures of the adlayers. Our insights may help in the search for topological states in heterostructures of two-dimensional superconductors.