Scientists discover first type I superconductor that breaks time-reversal symmetry

Scientists have uncovered the first type I superconductor that breaks time-reversal symmetry, a phenomenon previously only observed in type II superconductors. Superconductors are materials that conduct electricity without resistance and expel magnetic fields when cooled to extremely low temperatures. Time-reversal symmetry (TRS) is a principle stating that a physical system behaves the same whether time moves forward or backward.

When TRS is broken, the system spontaneously generates tiny internal magnetic fields during its superconducting state.

The research, published in Physical Review Letters, was conducted by Anshu Kataria and colleagues at the Indian Institute of Science Education and Research Bhopal, India. They grew single crystals of YbSb₂, a material found in both conventional and unconventional superconductors. By observing how YbSb₂ responded to magnetic fields, they confirmed it was a type I superconductor, which repels magnetic fields completely.

Further investigation involved cooling the crystals to near absolute zero and implanting muons into the material to detect any internal magnetic fields. As the material transitioned into its superconducting state, spontaneous internal magnetic fields emerged, indicating that YbSb₂ had broken time-reversal symmetry.

The authors propose that these spontaneous magnetic fields arise from an unconventional superconducting phase called the internally antisymmetric nonunitary triplet (INT) state. Calculations based on an effective low-energy model suggest this INT state might host gapless Majorana surface modes, hinting at the potential for topological superconductivity in YbSb₂.

This discovery of a type I superconductor with time-reversal symmetry breaking opens up new avenues for exploring fundamental physics and exotic quantum states in materials previously thought to preserve TRS.