Plate Nº 80 · recorded October 3, 2026
PhysicsReported finding
Cooper Pairs Survive Above Superconducting Transition in Uranium Ditelluride
Illinois physicists have directly confirmed that Cooper pairs in uranium ditelluride organize into pair density waves that persist above the critical temperature — a 20-year-old prediction finally observed.
By Nathan Brooks4 min read855 words
In brief
- Researchers observed pair density waves in uranium ditelluride persisting above the critical temperature, the first direct confirmation of a 2007 theoretical prediction.
- The superconducting phase in uranium ditelluride appears below 2 kelvins; the metal is a suspected triplet-pair superconductor like superfluid helium-3.
- The team needed purer crystals grown via a new molten flux method and a vector magnetic field scanning tunneling microscope to detect the fragile PDW modes; STM only probes surfaces, a stated limitation.
Physicists at the University of Illinois Urbana-Champaign's Grainger College of Engineering have captured the first direct evidence that Cooper pairs — the bound electron duos behind superconductivity — can organize into patterns that survive even after superconductivity itself switches off.
The finding, published in the Proceedings of the National Academy of Sciences, comes from experiments on uranium ditelluride, a metal that hid its superconducting talents until 2019. It confirms a prediction made roughly 20 years ago: that so-called pair density waves, or PDWs, can persist above the critical temperature at which a material loses its zero-resistance state.
"Pair density waves are the Cheshire Cat's grin of superconductivity," said Eduardo Fradkin, an Illinois Grainger Engineering physics professor and co-lead of the project. "They are the vestige that remains once the phase itself has disappeared."
How superconductivity works
In an ordinary metal, electrons scatter as they move, and that scattering is resistance. Cool certain metals far enough and something strange happens: electrons pair up and condense into a single low-energy quantum state, letting current flow without any loss at all.
Quantum mechanics normally forbids electrons from sharing a state. Electrons are fermions, a class of particles that cannot coexist identically. The workaround, explained in 1957 by Illinois physicists John Bardeen, Leon Cooper and Robert Schrieffer in what became BCS theory, is pairing. Two electrons bind through vibrations of the metal's atomic lattice, forming Cooper pairs. Because pairs act as bosons — a particle class with no such exclusion rule — they can all settle into the same quantum state and superconduct.
BCS theory explained everything until 1986, when "unconventional" superconductors appeared. Their structures violate the theory's assumptions, yet their electrons still form Cooper pairs and condense. How they do this remains an open research question.
Waves of pairs
Unconventional superconductors often host other exotic phases alongside superconductivity. One is the charge density wave, or CDW, in which electrons bunch into periodic patterns of high and low charge. In 2007, Fradkin and colleagues proposed something stranger: pair density waves, in which the Cooper pairs themselves arrange into nonuniform spatial patterns rather than spreading uniformly. Their theory predicted PDWs could exist above the critical temperature — Cooper pairs without a superconductor.
"We were out on a limb when we first suggested it," Fradkin said. "Although there have been experimental hints, there has been no direct confirmation of the phase's existence."
Confirmation is hard. "PDWs are tricky to analyze in real materials because they behave like conventional superconductors in some experiments and like CDWs in others," said Julian May-Mann, a former Illinois graduate student who worked on the theoretical analysis.
An odd metal
Uranium ditelluride was considered unremarkable until researchers found it superconducts below 2 kelvins. Further study suggested it may be a triplet-pair superconductor, in which paired electrons carry magnetic moments — a configuration confirmed in nature only in superfluid helium-3, studied by the late Illinois Nobel laureate Anthony Leggett. "I would not say that the question is completely settled, but the consensus is that uranium ditelluride is a triplet-pair superconductor," Fradkin said.
Vidya Madhavan, the experimental co-lead, noticed something odd in earlier scanning tunneling microscopy data: the metal's charge density waves could be destroyed by magnetic fields. That makes no sense for a CDW, which carries no magnetism. Her group brought the data to Fradkin's team, and both groups concluded a hidden pair density wave offered the best explanation.
Better crystals, clearer signal
PDWs are delicate and need highly regular crystals. Collaborators supplied purer samples grown with a new molten flux method, and the team examined them with a vector magnetic field scanning tunneling microscope, which can probe surfaces with magnetic fields in arbitrary directions — essential for anisotropic materials like uranium ditelluride.
"We couldn't see pair density waves in our earlier data because of material impurities that obscured our data," Madhavan said. "It would have been like trying to spot a light in a cloud of fog."
"By systematically varying both the magnitude and direction of the field, together with temperature, we could track how these modes evolved and build confidence that the behavior we observed was intrinsic," said Zhen Zhu, the postdoctoral researcher who carried out the experiments.
The observed modes responded to temperature and magnetic fields exactly as PDWs should — and crucially, some persisted above the critical temperature, after the superconducting phase vanished. "Any explanation of the experimental data that only relies on a CDW is at odds with these principles," May-Mann said, referring to the foundational rules governing how phases appear and disappear.
One caveat: STM imaging probes only surface effects. "It is possible for the interior of a material to behave differently than the surface, but these experimental results still give us a very strong hint at what's happening inside," Fradkin said. The team remains optimistic that the result opens new research directions into how superconductivity forms in unconventional materials.
Publication: Zhen Zhu et al., Evidence of intertwined pair density and charge density wave orders in UTe₂, Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2602117123
via Phys.org Physics (Source)
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