Impurity and vortex-core states in superconducting spin-helical Dirac fermions

We study the bound states at an impurity and a vortex core in superconducting spin-helical Dirac fermions in two dimensions. The energy of the Yu-Shiba-Rusinov state, or the impurity bound state, approaches zero with a strongly spin-anisotropic coupling, both at the Dirac point and with large doping. The resulting zero energy states are doubly degenerate in contrast to the Majorana zero modes at vortices, and their spatial distributions and spin structures are also distinct. Such differences can be measured by tunneling experiments, which distinguishes impurity bound states from vortex-core states including Majorana zero modes.