Magnetotransport experiments on side-contacted topological insulator three-terminal Josephson junctions

Multiterminal Josephson junctions have recently attracted substantial attention. The ability to study Andreev bound states in a multi-phase parameter space opens up exciting avenues for generating topologically non-trivial phases in superconducting devices. A particularly interesting case is the trijunction based on a topological insulator (TI), which is predicted to host Majorana zero modes for suitable phase configurations (Fu and Kane, PRL 2008). Here we turn to the platform of the side-contacted TI trijunction and present experimental evidence for a multiterminal Josephson effect. We study the transport properties and find typical features of a two-dimensional critical current contour (CCC) as well as bias regimes in which only a single junction remains dissipationless. We pay particular attention to these features and study the effects of magnetic fields on single junctions within the trijunction, where, depending on the field direction, the supercurrents show Fraunhofer patterns, SQUID oscillations, or a monotonic decay. We find good agreement with semiclassical simulations for a single TI junction that take into account the top and bottom surface states. We observe pronounced non-convex shape deformations of the CCC under perpendicular magnetic fields that reflect the flux buildup in the loop formed by our trijunctions and provide a simple model sufficient to explain the observed effect. Our results provide the foundations for future studies of Majorana zero modes on this platform.