Researchers report that a surprising diode-like behavior in the kagome superconductor CsV3Sb5 appears only after the crystal surface is altered or its charge order is intact. The effect, called a field-free superconducting diode effect (SDE), means the material carries supercurrent more easily in one direction than the other without an applied magnetic field. The team finds no intrinsic SDE in pristine devices, but a large nonreciprocal supercurrent appears after surface oxidation or asymmetric etching.

The group tested thin devices made from CsV3Sb5 and compared pristine samples to ones with deliberate surface changes. They also applied small magnetic fields and made samples with titanium (Ti) doping to weaken the material’s charge-density-wave (CDW) order — a pattern of electronic charge that repeats across the crystal. The SDE showed up only in samples with surface modification and disappeared when long-range CDW order was suppressed by Ti doping.

At a basic level, a field-free SDE requires two broken symmetries. One is inversion symmetry, meaning the material does not look the same if you flip spatial directions. The other is time-reversal symmetry (TRS), meaning the physics changes if the direction of time is reversed. Bulk CsV3Sb5 is centrosymmetric — it has inversion symmetry — so it should not show a field-free SDE unless inversion symmetry is broken at the surface or locally. The experiments suggest that surface damage or etching breaks inversion symmetry at the device edge and that an internal TRS-breaking field is present as well.

The observed SDE is not steady. Its direction and size change randomly from one current sweep to the next. The authors interpret this stochastic behavior as evidence for metastable domains that break time-reversal symmetry in different ways. Small out-of-plane magnetic fields can pick a preferred domain and stabilize the diode response, consistent with the idea that the field selects among otherwise random TRS-breaking configurations.