Spin defects in gapped systems

We investigate the physics of a single quantum spin impurity in strongly correlated systems like superconductors, where the Kondo effect coexists with subgap states, such as Yu-Shiba-Rusinov (YSR) and zero-energy modes. Understanding the interplay of the impurity with these stable subgap states (which have been experimentally observed in various setups) holds the promise of incorporating them in applications in quantum computing as good candidates for qubits due to their resilience against various perturbations.

Previously, spin impurities have been studied in gapless conformal field theories (CFTs), where their behavior is described as the flow from ultraviolet (UV) to infrared (IR) regimes between two boundary conformal field theories (BCFTs). In our theoretical work, we explore how this behavior changes when bulk perturbations introduce a mass gap. By constructing integrable models of 1D superconductors with magnetic defects, we show the gradual destruction of Kondo clouds by bulk interactions, transitioning from many-body Kondo screening to single-particle screening, and eventually reaching a regime where the impurity is unscreened, akin to ferromagnetic coupling.