Time-Reversal Symmetry Breaking Found in Type I Superconductor
Researchers discovered spontaneous internal magnetic fields in the type I superconductor YbSb₂.
Scientists have discovered the first type I superconductor that breaks time-reversal symmetry, challenging prior assumptions about superconducting materials. The finding in YbSb₂ opens new possibilities for topological quantum states and fault-tolerant quantum computing.
Researchers observed spontaneous internal magnetic fields in YbSb₂ upon entering the superconducting state, indicating broken time-reversal symmetry. They cooled YbSb₂ crystals to near absolute zero to observe superconducting behavior. To detect the tiny internal magnetic fields, muons were implanted into the material to act as sensitive probes. Muon spin spectroscopy was used to probe these fields without applying an external magnetic field. In the superconducting phase, tiny internal magnetic fields spontaneously appeared. The appearance of internal magnetic fields that do not reverse with time provides the key evidence of broken time-reversal symmetry.
Anshu Kataria et al said: "Here, we report evidence of time-reversal symmetry breaking in the type I superconductor YbSb₂."
Theoretical interpretation points to an exotic superconducting phase
The spontaneous magnetism is attributed to an unconventional spin triplet state called the internally antisymmetric nonunitary triplet (INT) state, which hosts a net magnetic moment. The researchers propose that the spontaneous magnetic fields arise from this unconventional superconducting phase. In the bulk superconducting state, electrons in YbSb2 form unconventional spin triplet Cooper pairs. This spin triplet state is specifically an internally antisymmetric non-unitary triplet (INT) state.
The INT state has a net magnetic moment because magnetic forces do not cancel out. This net magnetic moment allows YbSb2 to break time-reversal symmetry without an external magnetic field. Modeling revealed that each electron in the Cooper pairs of YbSb2 comes from different energy orbitals.
Implications for topological superconductivity and quantum applications
Calculations based on an effective low-energy model suggest the INT state may host gapless Majorana surface modes. The possibility of topological superconductivity in YbSb₂ suggests exotic quantum states could emerge in type I superconductors previously thought to preserve time-reversal symmetry. Majorana modes are quantum excitations that act as their own antiparticles. Topological quantum materials like YbSb2 may better protect quantum information from heat and electromagnetic noise.
Researchers propose that the INT state in YbSb₂ may host gapless Majorana surface modes, which are exotic quantum excitations that act as their own antiparticles and could protect quantum information.
