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

summary

Video file (mp4)

The gist

The gist: Time-reversal symmetry breaking in spontaneously emerging Josephson junctions without intentionally constructed weak links is realized in ultra-pure single-crystal microstructures of

In short

Researchers found spontaneous time-reversal symmetry breaking in single-crystal Sr2RuO4 without external contacts by studying Josephson junctions at superconducting domain walls. This revealed exotic transport like a supercurrent diode effect and current-switchable bistable states, suggesting a twocomponent order parameter and providing evidence for specific pairing symmetries.

Key concepts

Spontaneous Time-Reversal Symmetry Breaking (TRSB)
This is when a superconductor naturally breaks time-reversal symmetry without external influence. In Sr2RuO4, this happens at domain walls where different superconducting states meet, leading to non-reciprocal transport properties that are key to exotic phenomena.
Superconducting Domain Wall (DW) Junctions
These are junctions that spontaneously form at the boundary between different superconducting domains within the crystal. They act as microscopic devices where the coupling between overlapping degenerate ground states allows for unique Josephson effects, unlike conventional weak links.
Twocomponent Order Parameter
The paper suggests that Sr2RuO4 has a complex superconducting state described by two components. These components spontaneously segregate into different domains, and the interaction between these spatially separated domains is what drives the observed TRSB and exotic transport features.

Terminology used across episodes

This episode discusses

The paper

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

Remko Fermin, *Kaveh Lahabi

Huygens-Kamerlingh Onnes Laboratory, Leiden University · MESA+ Institute for Nanotechnology, University of Twente · Toyota Riken Kyoto University Research Center (TRiKUC)

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.

DOI: 10.1038/s42005-026-02926-8

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: We're looking at the paper: "Time-reversal symmetry breaking and inherent Josephson diodes in a multi-component order parameter superconductor".

Mira: Here's the gist of the paper. Time-reversal symmetry breaking in spontaneously emerging Josephson junctions without intentionally constructed weak links is realized in ultra-pure single-crystal microstructures of Sr2RuO4, revealing exotic transport phenomena like a supercurrent diode effect and current-switchable bistable states.

Kai: Spontaneous Time-Reversal Symmetry Breaking (TRSB) Time-reversal symmetry breaking in superconductors is the hallmark of exotic macroscopic quantum states with additional degrees of freedom.

Mira: A superconductor can exhibit spontaneous TRSB for different reasons, such as chiral states, spin-polarized triplet states, and loop supercurrents.

Kai: Strontium ruthenate (Sr2RuO4) stands out as one of the most well-documented cases of TRSB.

Paper summary: Kai: Looking at the "Time-reversal symmetry breaking and inherent Josephson diodes in a multi-component order parameter superconductor" paper, we see they've shown that time-reversal symmetry breaking can emerge spontaneously in single crystals of Sr2RuO4 right at domain walls <ref:2504.19361#pg1>.

Mira: They argue this is due to a multi-component order parameter structure, and this structure naturally breaks into spatially segregated domains that stabilize below the critical temperature Abstract. This isn't something you have to engineer with external contacts or proximity effects.

Lev: The implication for us in quantum error correction research is that we need to account for these intrinsic domain structures when designing any device architecture, because they can lead to inherent switching properties like the bistable critical current states <ref:2504.19361#pg2>.

Kai: The paper points toward developing symmetry breaking Josephson devices, like diodes and phase batteries, as a direct application of this finding Abstract. It shows that these domain wall based devices are a platform for studying fundamental symmetry breaking in multi-component superconductors.

Mira: Ultimately, it suggests that the pairing in the Eg irreducible representation of D4h might be what causes these mobile domains and is favored by the experimental evidence presented here <ref:2504.19361#pg3>.

Lev: So, for those of us building real hardware, we need to look at how a bias current couples to these superconducting domain walls, because that's where all this functional transport happens Abstract.

Conclusion: Kai: So, this paper is looking at how time-reversal symmetry breaking can show up spontaneously in single crystals of Sr2RuO4 right where Josephson junctions form naturally at domain walls.

Mira: And it’s about how that spontaneous breaking happens because the material has a multi-component order parameter structure that naturally breaks into spatially segregated domains below the critical temperature.

Lev: From an engineering side, that means we can't just expect a uniform superconducting state; you get these inherent domain walls, and they dictate how current flows across them.

Kai: The authors are focusing on how these domain wall junctions behave, showing things like a supercurrent diode effect and even those fractional Shapiro steps which aren't seen in normal cases.

Mira: They’re connecting the dots by suggesting this microscopic behavior corroborates earlier bulk probes that hinted at time-reversal symmetry breaking in Sr2RuO4.

Lev: So, for us building real hardware, it means we have a platform for functional Josephson devices like diodes or phase batteries right there on these domain walls.

Kai: The authors are pointing toward a twocomponent order parameter that splits into domains that spontaneously stabilize at temperatures below Tc, which they link to a pairing symmetry in the Eg irreducible representation of D4h.

Mira: That’s the core idea—that the structure of the pairing itself dictates how these domains form and interact spatially.

Lev: If this is true, then manipulating an in-plane magnetic field could be a way to tune those domain wall junctions, potentially switching between different current states as you saw in their setup.

Kai: What this really means for us is that superconducting domain walls are a platform we can use to study fundamental symmetry breaking mechanisms without needing external proximity effects or engineered contacts.

Mira: It gives us a new way to probe the pairing symmetry of Sr2RuO4, which has been a big debate in the field for years.

Lev: So, now we need to think about how we actually stabilize these domain structures when you try to run them on real hardware with actual noise and defects.

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