Vortex Lines in Topological Insulator-Superconductor Heterostructures

3D topological insulator/s-wave superconductor heterostructures have been predicted as candidate systems for the observation of Majorana fermions in the presence of superconducting vortices. In these systems, Majorana fermions are expected to form at the interface between the topological insulator and the superconductor while the bulk plays no role. Yet the bulk of a 3D topological insulator penetrated by a magnetic flux is not inert and can gap the surface vortex modes destroying their Majorana nature. In this work, we demonstrate the circumstances under which only the surface physics is important and when the bulk physics plays an important role in the location and energy of the Majorana modes.