Then came the harder question: does it preserve time-reversal symmetry? To find out, the team cooled the crystals to near absolute zero and implanted muons directly into the material. Muons are subatomic particles that act as extremely sensitive probes of the tiny magnetic fields hidden inside a sample. As the material crossed into its superconducting state, spontaneous internal magnetic fields appeared — a signal that would be absent if time-reversal symmetry were intact.

"Here, we report evidence of time-reversal symmetry breaking in the type I superconductor YbSb₂," the authors wrote in their paper.

A hint of something topological

The researchers propose that these spontaneous magnetic fields come from an unconventional superconducting phase called the internally antisymmetric nonunitary triplet (INT) state. In plain terms, this is a configuration in which pairs of electrons that carry the superconducting current organize in an unusual way, producing internal magnetism.

The theoretical side of the work points to an even more intriguing possibility. "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," the authors write. Majorana modes are exotic particle-like excitations that can exist on the surface of certain materials, and they interest physicists because of their potential role in fault-tolerant quantum computing. The word "possibility" deserves emphasis here: the Majorana modes emerge from model calculations, not from direct observation, and confirming them experimentally would require separate measurements.

Why it matters — and what remains uncertain

If the finding holds up, it shows that exotic quantum states can appear in a class of superconductors long assumed to preserve time-reversal symmetry. That opens new avenues for fundamental physics, since type I materials were not previously considered candidates for topological superconductivity — a phase of matter whose properties depend on global quantum features rather than local structure.

Some caution is warranted. The evidence rests on muon measurements on one material, and the theoretical interpretation, however suggestive, relies on an effective low-energy model rather than a full microscopic description. Independent replication and further experiments will be needed to establish the INT state and the proposed Majorana surface modes.

Still, the result is a reminder that the boundary between "conventional" and "unconventional" superconductivity is blurrier than textbooks suggest. YbSb₂ sits right on that boundary, and it may not be alone.