First type-I superconductor found breaking time-reversal symmetry

Physicists have found the first type-I superconductor that breaks time-reversal symmetry, a hallmark of unconventional superconductivity previously seen only in type-II materials. The finding, reported in Physical Review Letters (137, 136002 (2026)), was highlighted in an APS Physics synopsis on 23 September and has drawn fresh press coverage this week.

The team, led by Anshu Kataria at the Indian Institute of Science Education and Research Bhopal, working with researchers from IIT Kanpur and the ISIS Neutron and Muon Source, grew single crystals of YbSb2, a compound whose crystal structure appears in both conventional and unconventional superconductors. They confirmed its type-I behaviour with complementary measurements of electrical resistivity, magnetisation and heat capacity.

Using two versions of muon spin spectroscopy, the researchers then detected the spontaneous emergence of tiny internal magnetic fields below the superconducting transition temperature — direct evidence that the material breaks time-reversal symmetry, meaning its behaviour would differ if time ran backwards. Such symmetry breaking is a hallmark of unconventional electron pairing.

First-principles calculations identified YbSb2 as a Z2 topological metal in which Cooper pairs form through interband “spin-triplet” pairing — electron pairs with parallel rather than antiparallel spins. Further analysis suggests the material could host gapless Majorana surface modes, a trait that could one day underpin fault-tolerant quantum computers. The authors say YbSb2 offers a new, cleaner platform for studying unconventional pairing, free of the exotic effects typical of type-II superconductors.

Sources: APS Physics synopsis (23 Sept 2026) https://physics.aps.org/articles/v19/s115 ; phys.org coverage https://phys.org/news/2026-10-scientists-superconductor-reversal-symmetry.html ; arXiv preprint https://arxiv.org/abs/2601.07460