Non-Majorana origin of anomalous current-phase relation and Josephson diode effect in BiSe/NbSe Josephson junctions.

Josephson junctions (JJs) are key to superconducting quantum technologies and the search for self-conjugate quasiparticles potentially useful for fault-tolerant quantum computing. In topological insulator (TI)-based JJs, measuring the current-phase relation (CPR) can reveal unconventional effects such as Majorana bound states (MBS) and nonreciprocal transport. However, reconstructing CPR as a function of magnetic field has not been attempted. Here, we present a platform for field-dependent CPR measurements in planar JJs made of NbSe and few-layer BiSe. When a flux quantum [Formula: see text] threads the junction, we observe anomalous peak-dip CPR structure and nonreciprocal supercurrent flow. We show that these arise from a nonuniform supercurrent distribution that also leads to a robust and tunable Josephson diode effect. Furthermore, despite numerous previous studies, we find no evidence of MBS. Our results establish magnetic field-dependent CPR as a powerful probe of TI-based superconducting devices and offer design strategies for nonreciprocal superconducting electronics.