Würzburg ToCoTronics Colloquium

The combination of s-type superconductors with three-dimensional topological insulators creates a promising platform for fault-tolerant topological quantum circuits based on Majorana braiding. The backbone of the braiding mechanism is a three-terminal Josephson junction. To implement this concept, a deeper understanding of the underlying mechanisms in topological insulator nanoribbon networks equipped with superconducting electrodes is required. The samples for our experiments are fabricated by a combination of selective area growth of the topological insulator and shadow mask evaporation of the superconductor. As a first step, we investigate nanoribbon kinks and T-junctions of the topological insulator. In kink structures, a π-periodic change of the conductance is observed as a function of the angle of an in-plane magnetic field. We attribute this phenomenon to an orbital effect leading to an alignment or misalignment of the phase-coherent states on the lower and upper surfaces of the kink branches depending on the magnetic field orientation. The experimental results are supported by a theoretical analysis based on a surface Rashba-Dirac model and tight-binding simulations. As a next step, the transport properties of topological insulator-based three-terminal Josephson junctions are mapped and the cross-coupling of the junctions is analyzed. Under the application of an out-of-plane magnetic field, a multi-terminal geometry induced diode effect is observed.