Time-reversal symmetry breaking and inherent Josephson diodes in a multi-component order parameter superconductor

Abstract

Spontaneous time-reversal symmetry breaking is a defining feature of unconventional superconductivity and a prerequisite of the technologically relevant superconducting diode effect. However, its realization remains rare and exclusive to proximity-engineered weak links. Here we examine an alternative pathway to spontaneous time-reversal symmetry breaking by examining junctions that appear in single-crystal microstructures of the unconventional superconductor Sr${}_2$RuO${}_4$, lacking external superconducting proximity. We interpret the Josephson coupling as superconducting domains of a multi-component order parameter based on a plethora of anomalous effects - including a field-free Josephson diode effect, fractional Shapiro steps, magnetic interference patterns deviating from the conventional flux quantum periodicity, and bias current-programmable bistable states. These microscopic devices corroborate earlier bulk probes of time-reversal symmetry breaking, giving a renewed impulse to the long-debated pairing symmetry of Sr${}_2$RuO${}_4$. Our results highlight superconducting domain walls as a platform for functional Josephson devices and the study of fundamental symmetry breaking.