Physicists at the Indian Institute of Science Education and Research Bhopal have discovered the first known type-I superconductor that breaks time-reversal symmetry. The finding, reported in *Physical Review Letters* and highlighted in an APS *Physics* synopsis on September 23, 2026, upends long-standing assumptions about the "simplest" class of superconductors.
A rule-breaking type-I superconductor
In physics, time-reversal symmetry means the laws of nature should look the same if time runs backward. When a superconductor breaks this symmetry on its own, it signals something exotic about how its electrons pair up — a hallmark of "unconventional" superconductivity. Until now, every known case had been found in type-II superconductors. Type-I materials, which completely expel magnetic fields, were considered the textbook-simple class where such phenomena had no place.
Led by Prof. Ravi Prakash Singh, the team — with Anshu Kataria as first author — grew single crystals of YbSb₂, a material whose atomic layout is shared by both conventional and unconventional superconductors. Using electrical resistivity, magnetization, and heat-capacity measurements, they first confirmed it was genuinely a type-I superconductor. Then they cooled the crystals to near absolute zero and probed them with muon spin spectroscopy (μSR), an ultra-sensitive magnetic probe. As the material entered its superconducting state, tiny internal magnetic fields spontaneously appeared — with no external field applied. That is direct evidence of time-reversal symmetry breaking. If the symmetry were preserved, these fields would not arise on their own.
Transverse-field μSR measurements confirmed a fully gapped type-I superconducting state. First-principles calculations identified YbSb₂ as a ℤ₂ topological metal with a Dirac nodal line near the Fermi level — an electronic structure that sets the stage for exotic superconducting states.
Symmetry analysis within the Ginzburg–Landau framework points to an internally antisymmetric nonunitary triplet (INT) state as the most probable superconducting ground state, meaning the electrons pair up in an unconventional spin-triplet configuration. Effective low-energy model calculations further suggest this state may host gapless Majorana surface modes — making YbSb₂ a candidate topological superconductor. Type-I superconductivity, triplet pairing, and nontrivial topology coexisting in one material is exceptionally rare.
A clean new platform for unconventional physics
The discovery matters because researchers studying unconventional pairing have so far been confined to type-II superconductors, where vortices, strong correlations, and other exotic effects can muddy the signal. YbSb₂ is structurally simpler and behaviorally cleaner, offering a fresh platform for studying the links between superconductivity, magnetism, and topology. The authors note the material could support Majorana zero modes — among the most sought-after ingredients for fault-tolerant quantum computers.
The work involved collaborators from the Indian Institute of Technology Kanpur and the ISIS Neutron and Muon Source in the UK, with additional contributions from the University of Warwick. The paper was posted to arXiv in January 2026 and published after peer review.
For readers, the takeaway is broader than one crystal: physics "common sense" gets rewritten every few years. The superconductor considered the simplest of them all turns out to host some of the strangest electron pairing known. The materials for the next generation of quantum hardware may well be hiding in unremarkable-looking crystals like this one.