First Type I Superconductor to Break Time-Reversal Symmetry Found in YbSb₂
Muons implanted in ytterbium diantimonide crystals revealed spontaneous magnetic fields inside a superconductor that was thought incapable of producing them, hinting at exotic topological behavior.
Physicists have found the first type I superconductor that breaks time-reversal symmetry, a property that had only ever been seen in the other main family of superconductors. The material is YbSb₂, a compound of ytterbium and antimony, and the finding was published in Physical Review Letters.
Superconductors come in two broad kinds. Type I materials, including simple metals like lead and mercury, expel magnetic fields completely when they cool below their critical temperature. Type II materials let fields penetrate in small tubes called vortices. Spontaneous breaking of time-reversal symmetry, meaning the superconducting state looks different when you run the clock backward, had been observed only in type II materials until now.
The team, led by Anshu Kataria and colleagues at the Indian Institute of Science Education and Research Bhopal, grew single crystals of YbSb₂ and cooled them to near absolute zero. They then used muon spin relaxation, a technique that implants muons, short-lived subatomic particles, into the sample. Muons act as extremely sensitive probes of tiny magnetic fields inside a material.
In zero-field measurements, the researchers saw spontaneous internal magnetic fields appear just below the superconducting transition temperature, without any magnetic field applied from outside. That is the signature of broken time-reversal symmetry. In separate transverse-field measurements, the same crystals behaved as a fully gapped type I superconductor, which confirmed the classification.
The authors propose that the fields arise from an unconventional pairing state called the internally antisymmetric nonunitary triplet, or INT, state. In ordinary superconductors, electrons pair up with opposite spins. In a triplet state, the pairs carry spin, and in a nonunitary version the arrangement itself can generate a net magnetic moment, which would produce the fields the muons detected.
The theory work goes further. Calculations based on an effective low-energy model suggest the INT state may host gapless Majorana surface modes, according to the paper. Majorana modes are quasiparticles that act as their own antiparticles, and they are a leading candidate for building quantum bits that resist errors. If YbSb₂ really is a topological superconductor, that would be unexpected for a type I material.
Caution is warranted. The evidence for broken time-reversal symmetry comes from muon measurements, and the topological interpretation rests on modeling. Confirming Majorana modes would require direct surface measurements, such as scanning tunneling spectroscopy, which would be the natural next step.
Still, the result widens the map of where exotic superconductivity can occur. Researchers had mostly looked for it in type II compounds. YbSb₂ suggests that simple, clean type I materials deserve a second look, and that the boundary between the two families is less rigid than textbooks imply.
