In-gap excitations due to defects in topological superconductor with spin-orbit coupling*

Recent microscopy experiments on superconducting monolayer of lead(Pb) grown over clusters of cobalt atoms have raised urgent questions about in-gap electronic states in presence of strong spin-orbit coupling and magnetism.
Using analytics and numerics we find that a topological defect in Rashba spin-orbit coupling (in contrast to a vortex in superconducting pairing) fully explains puzzling features of this experiment: 1) Two different zero-modes, one point-like at center of cluster and the other ring-shaped around it; 2) The zero mode pair is protected by a large energy gap; 3) The localization lengthscale inside the cluster is far smaller than superconducting coherence length , but comparable to it outside. The theory predicts that this is a Majorana pair, with remarkable isolation in energy thanks to the defect in strong spin-orbit coupling. We discuss the role of magnetic textures in our theoretical scenario.