Non-Reciprocal Current-Phase Relation and Superconducting Diode Effect in Topological-Insulator-Based Josephson Junctions

TL;DR

This paper introduces a method for precise measurement of the current-phase relation in topological insulator-based Josephson junctions, revealing non-reciprocal supercurrent behavior and a tunable Josephson diode effect linked to edge supercurrent profiles.

Contribution

The study provides the first detailed field-dependent CPR measurements in NbSe2/Bi2Se3 JJs, identifying edge-amplified supercurrent profiles as the origin of non-reciprocal effects, challenging previous MBS-based explanations.

Findings

Observation of anomalous peak-dip CPR behavior at flux quantum

Identification of edge-amplified supercurrent as the cause of non-reciprocity

Demonstration of a tunable Josephson diode effect

Abstract

Josephson junctions (JJ) are essential for superconducting quantum technologies and searches of self-conjugate quasiparticles, pivotal for fault-tolerant quantum computing. Measuring the current-phase relation (CPR) in JJ based on topological insulators (TI) can provide critical insights into unconventional phenomena in these systems, such as the presence of Majorana bound states (MBS) and the nature of non-reciprocal transport. However, reconstructing CPR as a function of magnetic field in such JJs has remained experimentally challenging. Here, we introduce a platform for precise CPR measurements in planar JJs composed of NbSe$_2$ and few layer thick Bi$_2$Se$_3$ (TI) as a function of magnetic field. When a single flux quantum $\Phi_\mathrm{0}$ threads the junction, we observe anomalous peak-dip-shaped CPR behaviour and non-reciprocal supercurrent flow. We demonstrate that these anomalies stem from the edge-amplified sloped supercurrent profile rather than MBS signatures often invoked to explain puzzles emerging near $\Phi_\mathrm{0}$ in TI-based JJ. Furthermore, we show that such a supercurrent profile gives rise to a previously overlooked, robust and tunable Josephson diode effect. These findings establish field-dependent CPR measurements as a critical tool for exploring topological superconducting devices and offer new design principles for non-reciprocal superconducting electronics.