Signatures of finite momentum Cooper pairing in the Fraunhofer pattern of superconductor-Bi₂Se₃ Josephson junctions

Unconventional superconductivity is predicted to arise at the intersection of magnetism and superconductivity. One such form of unconventional superconductivity has been proposed to occur when Cooper pairs acquire a finite momentum shift due to a magnetic exchange coupling. 3D topological insulators, which have strong spin-orbit coupling and surface states that consist of nondegenerate Dirac cones, are a prime candidate for observing a finite momentum shift. We study superconducting (S)-Bi₂Se₃-S Josephson junctions subjected to out-of-plane and in-plane magnetic fields by looking at the evolution of the Fraunhofer patterns. The Fraunhofer pattern evolves from a conventional pattern to an anomalous one as an in-plane field is applied, which is suggestive of a phase modulation in the superconducting order parameter. The phase accumulation in the junction due to in-plane and out-of-plane fields is simulated based on a finite momentum pair model, and the simulations match well with the observed transport signal.