Observation of a Zero-Field Josephson Diode Effect in a Helimagnet Josephson Junction

TL;DR

This paper reports the discovery of a zero-field Josephson diode effect in Cr$_{1/3}$NbS$_{2}$, a chiral helimagnet, revealing non-reciprocal superconducting behavior linked to vortex pinning and spin chirality.

Contribution

It demonstrates the existence of a zero-field Josephson diode effect in a helimagnet Josephson junction and proposes mechanisms involving vortices and spin chirality.

Findings

Josephson diode effect observed with up to 20% efficiency.

Asymmetry persists even at zero magnetic field.

Simulations support vortex and chiral spin structure mechanisms.

Abstract

Cr$_{1/3}$NbS$_{2}$ is a transition metal dichalcogenide that is also a chiral helimagnet, and so lacks inversion symmetry and has non-zero Berry curvature in position and momentum space. It is well known that the combination of broken time-reversal symmetry and broken inversion symmetry can generate non-reciprocal phenomena, but the interplay between these kinds of systems and superconductivity is not well known. We present Josephson junctions fabricated from Cr$_{1/3}$NbS$_{2}$ that give magnetic diffraction patterns with asymmetry in both the magnetic field and the critical current. The non-reciprocity in positive critical current and negative critical current, generally called the Josephson diode effect, has an efficiency of up to $\eta=20\%$ in some parts of the magnetic diffraction pattern and persists even at zero applied field. We propose that pinned Abrikosov vortices are a main mechanism for the asymmetric magnetic field response in this system, and that the non-zero spin chirality of the Cr$_{1/3}$NbS$_{2}$ causes the diode effect. Simulations of magnetic diffraction patterns from Josephson junctions with vortices present show offsets from zero-field consistent with observations, while simulations of chiral spin structures with an out-of-plane canting show a diode effect.