Illinois Team Sees Electron Pairs Persisting Above Superconducting Temperature
Scanning tunneling microscopy of uranium ditelluride gives the first direct evidence of pair density waves that outlast superconductivity itself, physicists report in PNAS.
Physicists at the University of Illinois Urbana-Champaign have found electron pairs that survive in a material after it stops being a superconductor, the first direct evidence of a phenomenon theorists predicted but nobody had observed.
The work, published in the Proceedings of the National Academy of Sciences, was led by physics professors Eduardo Fradkin and Vidya Madhavan, with postdoctoral researcher Zhen Zhu and former graduate student Julian May-Mann, who did the theoretical analysis. They studied uranium ditelluride, a material that is a superconductor at very low temperatures and has drawn wide interest because it may carry current in an unusual way.
A superconductor carries electricity with no resistance. The standard explanation, known as BCS theory, says electrons bind into Cooper pairs when the material cools below a critical temperature, and the pairs move together without losing energy. In that picture, the pairs appear when superconductivity does and vanish when it ends.
The Illinois team saw something different. Using a scanning tunneling microscope that can apply a magnetic field in any direction, they mapped the electronic structure of high-quality uranium ditelluride crystals grown by a molten flux method. They changed the strength and direction of the field and the temperature, and tracked how the superconducting modes changed. They found pair density waves, patterns in which the density of Cooper pairs rises and falls in a regular way across the material, and these patterns remained above the critical temperature.
Fradkin described the effect as "the Cheshire Cat's grin of superconductivity," a faint pattern that remains after the main phase has disappeared. The finding suggests that electrons can begin pairing before the material becomes a superconductor, which means pairing and superconductivity are not the same thing.
That matters because physicists still lack a full account of high-temperature superconductors, the materials that could carry power without loss at temperatures much easier to reach than the near absolute zero required today. A theory of how electrons pair, and what stops pairs from becoming a superconducting state, is central to designing better ones. Pair density waves have been proposed as an ingredient in several families of these materials, and a clean observation in one of them gives theorists something concrete to test.
The authors caution that the measurements only probe the surface. The interior of the crystal could behave differently, and other techniques will have to confirm the result. The team also studied one material, so it is not yet clear how general the behavior is.
Next steps include looking for the same signature in other unconventional superconductors and measuring how the pair density waves change as conditions vary. If the effect turns out to be common, it could change how researchers think about the road to superconductivity, treating the onset as a sequence of steps in which pairs form first and only later lock together into a single quantum state that carries current without resistance.
