Implanted muons point to broken time-reversal symmetry in the type I superconductor YbSb2

The signal at issue is a magnetic field the sample makes by itself ^[2]. Superconductors normally expel magnetic fields once they are cooled far enough ^[1]. The rare unconventional ones that break time-reversal symmetry, the principle that a system behaves the same with time run forward or backward, instead generate tiny internal fields of their own as they turn superconducting ^[2]. If the symmetry held in YbSb2, no such field would appear ^[7].

The design needed two facts about the same crystals. Anshu Kataria's group at the Indian Institute of Science Education and Research Bhopal grew single crystals of the compound ^[4]. The first fact was the class. Type II superconductors let an applied field seep inside, type I materials push it out completely, and the team used that response to confirm YbSb2 is type I ^[5]. The second was the field itself. The researchers cooled the crystals to near absolute zero and implanted muons, subatomic particles that act as sensitive probes of small internal fields ^[6]. The fields showed up as the material cooled into its superconducting state ^[7]. That timing ties them to the superconductivity, because the crystal is compared with itself above and below its transition ^[7].

YbSb2 looked like an ordinary candidate on structure. Its atomic layout is found in both conventional and unconventional superconductors ^[4], so the arrangement of atoms could not have predicted which kind it would be.

On the type I question, I think the result is strong enough to end the old pattern, provided it reproduces. That pattern was empirical. Every superconductor known to break the symmetry was type II ^[3], and type I materials were thought to preserve it ^[13]. One solid counterexample is enough to end a rule built that way. The authors' abstract is measured. "Here, we report evidence of time-reversal symmetry breaking in the type I superconductor YbSb2," they wrote ^[8]. The paper's title in Physical Review Letters is firmer and calls it an observation ^[10].

The second half of the paper is a different grade of claim. The authors propose that the fields come from an internally antisymmetric nonunitary triplet, or INT, state ^[9]. "Calculations based on an effective low-energy model further suggest that this INT state may host gapless Majorana surface modes, pointing to the possibility of topological superconductivity in YbSb2," they wrote ^[12]. The muons measured a spontaneous field. The INT state is a proposed explanation for that field, and the Majorana modes are a model's prediction about the proposed state ^[9][12].

The thing this doesn't tell you is the size of the effect: the published summary does not report the transition temperature, the strength of the spontaneous field, or whether any probe besides muons has detected it. The preprint is posted on arXiv as 2601.07460 ^[11].