Current flow in topological insulator Josephson junctions due to imperfections

Recent experiments on planar superconductor-topological insulator-superconductor (S-TI-S) junctions, e.g., in the Corbino geometry, have reported low-temperature nonzero Josephson currents in states with integer fluxoid (flux) induced in the junction by a perpendicular magnetic field. This effect was discussed in connection with Majorana zero modes localized in Josephson vortices of such junctions. Here, we provide an explanation for this phenomenon, attributing it to imperfections. We focus on the “atomic” limit in which the low-energy bound states of different vortices do not overlap. In this limit, we can associate the nonvanishing critical current with the irregularities, e.g., in the junction's width. The low-temperature contribution to the current is provided by the bound states with low but nonzero energy. We also propose clear experimental tests based on microwave spectroscopy, revealing distinctive selection rules for vortex transitions.