Researchers discovered the first type I superconductor that breaks time-reversal symmetry. Anshu Kataria and colleagues from the Indian Institute of Science Education and Research Bhopal studied YbSb₂ crystals. "Here, we report evidence of time-reversal symmetry breaking in the type I superconductor YbSb₂," the authors said. This finding opens new physics paths.
Superconductors are materials that conduct electricity with no resistance and expel magnetic fields when cooled to ultralow temperatures. And depending on their quantum structure, they can also showcase strange properties like magnetic levitation. But now things are getting a little weirder.
One of the most fascinating phenomena related to superconductors is the breaking of time-reversal symmetry. TRS is the principle that a physical system behaves the same way whether time runs forward or backward. Rare unconventional superconductors break this symmetry. When this happens, they spontaneously generate tiny internal magnetic fields when they enter their superconducting state.
Until recently, every superconductor known to do this was a type II material. But in a study published in the journal Physical Review Letters, researchers report the discovery of a type I superconductor that breaks time-reversal symmetry too.
A superconductor that breaks the rules
Anshu Kataria of the Indian Institute of Science Education and Research Bhopal in India and colleagues grew single crystals of YbSb₂, a material with an atomic layout found in both conventional and unconventional superconductors. They confirmed it was a type I superconductor by checking how it handled magnetic fields. Unlike type II superconductors, which let magnetic fields seep inside, type I materials push them out completely.
To see how YbSb₂ behaved, the researchers cooled the crystals to near absolute zero and implanted muons directly into the material. These subatomic particles act as highly sensitive probes of tiny magnetic fields inside it.
As the material cooled into its superconducting state, spontaneous internal magnetic fields appeared. This was evidence that this type I superconductor had broken time-reversal symmetry. If the symmetry were preserved, these magnetic fields would not appear spontaneously.
"Here, we report evidence of time-reversal symmetry breaking in the type I superconductor YbSb₂," wrote the study authors in their paper.
Are there more surprises in store?
The researchers propose that these spontaneous magnetic fields arise from an unconventional superconducting phase known as the internally antisymmetric nonunitary triplet (INT) state.
"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 possibility of topological superconductivity in a type I material opens up new avenues for fundamental physics. It suggests that exotic quantum states could emerge in a type of superconductor previously thought to preserve time-reversal symmetry.