Non-Majorana origin of anomalous current-phase relation and Josephson diode effect in Bi ₂ Se ₃ /NbSe ₂ Josephson junctions

Abstract

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 2 and few-layer Bi 2 Se 3 . When a flux quantum <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">Φ</mml:mi> <mml:mn>0</mml:mn> </mml:msub> </mml:mrow> </mml:math> 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.