Physicists have synthesized single crystals of ytterbium diantimonide, creating a type I superconductor that spontaneously breaks time-reversal symmetry without an external magnetic field. Described in a study published in Physical Review Letters, the material forms unconventional spin triplets and may ultimately improve quantum computing designs.
The Synthesis of Ytterbium Diantimonide and its Zero-Resistance Threshold
Superconductors transport electricity without resistance and energy loss, but they traditionally separate into distinct categories based on how their electrons pair up and how they interact with magnetic fields. Conventional and unconventional superconductors entangle electrons in tandems called Cooper pairs, while type I and type II materials handle magnetic thresholds differently. Unconventional and type II varieties are typically viewed as the exotic workhorses that push physical boundaries. Researchers have now bridged these operational divides by synthesizing single crystals of a material called ytterbium diantimonide (YbSb2).
An international team of physicists led by the Indian Institute of Science Education and Research (IISER) Bhopal analyzed the material’s unique electrical and quantum qualities in findings published in Physical Review Letters. Using X-rays, the team verified the complex crystal structure and chemical purity of the YbSb2 single crystals. Measurements showed that its electrical resistance dropped to zero and it abruptly became superconducting at approximately -272 degrees Celsius (-457.6 degrees Fahrenheit), resting just above absolute zero.
Muon Spin Spectroscopy Reveals Internal Magnetic Behavior
Specific heat measurements of how electrons respond to thermal energy confirmed that YbSb2 exhibits type I superconducting properties alongside a fully gapped state. This energy barrier makes it harder to disentangle the paired electrons and knock the material out of its superconducting phase. To investigate the material’s internal magnetic behavior, the researchers utilized muon spin spectroscopy, a quantum probing technique that blasts a material with subatomic particles acting like miniature bar magnets.
When researchers zapped the material without applying an external magnetic field, tiny internal magnetic fields spontaneously appeared the moment the substance entered its superconducting phase. Because magnetic fields reverse direction when time is mathematically reversed, these spontaneous fields provided key evidence that YbSb2 breaks time-reversal symmetry on its own. Most physical laws operate equally whether time flows forward or backward, making this symmetry-breaking behavior a hallmark previously reserved for unconventional, type II superconductors.
Unconventional Electron Pairing Creates Net Magnetic Moment
The root of this spontaneous symmetry-breaking lies in how the material’s electrons pair up in a bulk superconducting state. Rather than organizing conventionally, the electrons form an unconventional spin triplet,
which stems from combinations of electron spins.
which is a Cooper pair but called a triplet due to the possible combinations of the electrons’ spins.
Researchers
This pairing is formally termed an internally antisymmetric non-unitary triplet (INT) state,
producing a net magnetic moment where the magnetic forces do not cancel out. Modeling based on fundamental physical qualities revealed that each paired electron originates from different energy orbitals. These combined properties could eventually allow YbSb2 to host gapless Majorana surface modes,
meaning the material’s surface may manifest quantum excitations acting as their own antiparticles while its bulk maintains unimpeded electron flow.
