Fermi-Level Pinning of Yu-Shiba-Rusinov States in Weak Magnetic Field

As is well known, magnetic impurities adsorbed on superconductors, e.g. of the s-wave type, can introduce a bound gap-state (Yu-Shiba-Rusinov resonance). We here investigate within a minimal model how the impurity moment arranges with respect to a weak homogeneous external magnetic field employing a fully self-consistent mean-field treatment. Our investigation reveals a critical window of impurity-to-substrate coupling constants, J. The width of the critical region, δ J, scales like δ J ~ B/Δ, where B is the magnitude of the external field and Δ is the bulk order parameter. While tuning J through the window, the energy of the Yu-Shiba-Rusinov (YSR) resonance is pinned to the Fermi energy εF and the impurity moment rotates in a continuous fashion. We emphasize the pivotal role of self-consistency: In treatments ignoring self-consistency, the critical window adopts zero width, δ J=0; consequently, there is no pinning of the YSR-resonance to εF, the impurity orientation jumps and therefore this orientation cannot be controlled continuously by fine-tuning the coupling J. In this sense, our study highlights the significance of self-consistency for understanding intricate magnetic interactions between superconductive materials and Shiba chains.