In the laboratory of the Indian institute IISER Bhopal, scientists synthesized crystals of ytterbium diantimonide YbSb₂ and discovered unexpected behavior: the material transitioned into a superconducting state at a temperature of about minus 272 degrees Celsius and, moreover, spontaneously generated tiny internal magnetic fields.
Conventional superconductors are divided into two classes by their response to a magnetic field. Type I abruptly loses superconductivity at a certain threshold, while type II allows magnetic vortices to penetrate. Until now, time-reversal symmetry breaking — when the laws of physics look different when time runs backward — was considered a hallmark only of unconventional type II superconductors.
In an experiment with muon spin spectroscopy without an external field, the instruments recorded the spontaneous appearance of magnetic fields exactly at the moment of the transition into the superconducting state. This is direct evidence of time-reversal symmetry breaking, because a magnetic field reverses direction when time is reversed.
Additional measurements of heat capacity and resistance confirmed that YbSb₂ remains a classical type I superconductor with a fully forbidden gap. The electrons in it form unconventional triplet Cooper pairs, whose spins do not cancel each other out and create their own magnetic moment.
Theoretical calculations showed that the material belongs to topological metals and can support gapless Majorana modes on its surface — quasiparticles that are their own antiparticles. Such states are resistant to external interference and promising for quantum computing.
The discovery shows that unusual quantum properties can hide even in "simple" type I superconductors, expanding the range of materials for future technologies.
