A Ghostly Superconducting State Persists Above the Transition
Researchers at the University of Illinois Urbana-Champaign have found direct evidence that pair density waves can survive in uranium ditelluride after ordinary superconductivity disappears.
Superconductivity may disappear without taking all of its structure with it. Physicists at the University of Illinois Urbana-Champaign have found evidence that electron pairs in uranium ditelluride continue arranging themselves in rippling, spatially varying patterns after the material has left its superconducting state — a phenomenon known as a pair density wave, or PDW.
The result is the first direct evidence for a theoretical state predicted roughly two decades ago. Measurements of uranium ditelluride found that these paired electrons persist in an organized pattern even after superconductivity vanishes, suggesting that the ingredients of the superconducting phase can remain present after the material no longer behaves as a conventional superconductor.
A pair density wave is not simply a collection of Cooper pairs moving together. The pairs themselves form a patterned arrangement whose density changes across the material, producing a kind of electronic ripple. The new measurements indicate that this pattern can exist above the superconducting critical temperature, rather than being only a feature of the fully established phase.
That distinction matters because a material’s transition into superconductivity is usually treated as the point at which paired electrons begin acting collectively without resistance. The uranium ditelluride results suggest the pairing process can begin earlier: Cooper pairs may organize into a hidden structure first, with the larger superconducting state emerging only later.
The research, published in the Proceedings of the National Academy of Sciences, therefore separates two ideas that are often treated as one — the existence of electron pairs and the emergence of full superconducting order. The PDW remains in the material’s ordinary phase after superconductivity has disappeared, providing a direct view of a state that had previously existed mainly as a prediction.
Eduardo Fradkin, one of the project’s co-leads, described pair density waves as “the Cheshire Cat’s grin of superconductivity”: the visible trace of a state whose main presence has gone. That image is unusually precise. The superconducting phase is absent, but its paired-electron structure has not entirely left the material, leaving behind an organized pattern that could help explain how the transition begins.