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

Abstract

We present Josephson junctions with the metallic chiral helimagnet Cr$_{1/3}$NbS$_{2}$ as the weak link that exhibit magnetic diffraction patterns with asymmetry in both applied magnetic field and the critical (switching) current. The nonreciprocity between positive and negative critical currents (the Josephson diode effect) reaches efficiencies of $|\eta|>20\%$, where $\eta \equiv (I_{c+}-|I_{c-}|)/(I_{c+}+|I_{c-}|)$, and the effect persists even at zero applied field. As a chiral helimagnet, Cr$_{1/3}$NbS$_{2}$ lacks inversion symmetry and time reversal symmetry, providing a platform to explore the interplay of superconductivity with both symmetries broken. We propose that pinned Abrikosov vortices are a primary mechanism for the asymmetric magnetic-field response, while the nonzero spin chirality of Cr$_{1/3}$NbS$_{2}$ gives rise to the diode effect. Simulations of magnetic diffraction patterns with vortices show offsets from zero field consistent with observations, while simulations of chiral spin structures with out-of-plane canting show a diode effect.