Impurity effects on vortex core states of topological s-wave superconductor

We study the impurity effects on bound states in a single vortex core of a topological s-wave superconductor (SC), which is a two-dimensional electron gas with Rashba spin-orbit coupling, Zeeman coupling and s-wave superconductivity. We calculate the Gor'kov Green's function in the presence of impurities in a way similar to Kopnin and Kravtsov (1976); We restrict the Hilbert space to that spanned by the eigenstates of the Bogoliubov-deGennes equation for a pure system, bounded near the vortex core. This topological s-wave SC has two types of vortices and each type is specified by the relative sign between the vorticity and Zeeman coupling. We find that the impurities affect bound states differently between the two types of vortices and bound states are more robust against impurities when vorticity and Zeeman coupling are opposite in signs.