A uranium crystal the size of a grain of rice was cooled to one-fiftieth of a degree above absolute zero. At such temperatures, heat is not transported chaotically but strictly according to the rules of the quantum world. The scientists directed heat through the sample and watched how it spread along different crystal axes, like checking which way water flows faster across the folds of paper.
In a conventional superconductor, electrons pair up and move without friction, but in UTe₂ the pairs are arranged differently: their spins point in the same direction, like compass needles aligned by a magnetic field. Measurements showed that along one of the crystal axes, the energy gap, the threshold that must be overcome to break a pair, does not disappear completely but becomes very small. This is like a door left ajar: you can't get through, but you can feel the draft.
These data indicate that the superconducting state of UTe₂ is fully gapped, but with points where the gap nearly vanishes. The minimum gap is only one-tenth of the characteristic gap. Such a structure rules out mixing of different pairing types and provides a key to understanding the topology of this material — how quantum states are protected from disturbances.
UTe₂ is one of the few known superconductors where electron pairs have the same spin direction, as if all pairs were dancing in the same rhythm rather than out of step.