Switching Current Distributions in Superconductor--Topological Insulator--Superconductor Junctions

It has been proposed that localized Majorana fermion (MF) modes can exist in lateral Josephson junctions with a 3D-topological insulator barrier at locations at which the phase difference across the junction is an odd multiple of $\pi $. These states enter the junctions bound to the nodes of the Josephson vortices as a perpendicular magnetic field is increased. Each mode contributes a local 4$\pi $-periodic sin($\varphi $/2)-component to the junction's current-phase relation, adding to the usual sin($\varphi )$ dependence. The sign of this new term encodes the parity of the Majorana pair. As a way to detect these states and measure their parity, we study the distribution of switching currents in Nb-Bi2Se3-Nb junctions fabricated on thin Bi2Se3 films in which the superconductivity is induced by a pair of closely spaced Nb electrodes. We expect that such measurements will be sensitive to the parity of the MFs, yielding a splitting of the distribution. Preliminary measurements of the critical current distributions show the onset of unusual features when the magnetic field is increased which we are analyzing to determine if they may arise from Majorana fermions in the junctions.