Plate Nº 61 · recorded October 13, 2026
PhysicsReported finding
Cooper pairs survive superconductivity's death in uranium ditelluride
Physicists observed pair density waves in uranium ditelluride persisting above 2 kelvins, showing Cooper pairs can form before superconductivity begins.
By Marcus Bennett4 min read738 words
In brief
- Researchers directly observed pair density waves in uranium ditelluride surviving above the superconducting critical temperature, first reported October 7, 2026.
- Uranium ditelluride becomes superconducting below 2 kelvins, a property discovered only in 2019.
- Fradkin and colleagues predicted pair density waves in 2007; this study, in PNAS, provides the first direct confirmation of the phase persisting above the transition.
- The observation relied on cleaner crystals from a new molten flux growth method and a vector magnetic field scanning tunneling microscope.
- STM measures only the material's surface, so the bulk behavior remains indirectly inferred.
Physicists have directly observed paired electrons surviving in uranium ditelluride after superconductivity itself has vanished. The finding, published in the Proceedings of the National Academy of Sciences on October 7, 2026, marks the first direct evidence that pair density waves — rippling patterns of electron pairs — can persist in a material's ordinary phase above the superconducting transition.
"Pair density waves are the Cheshire Cat's grin of superconductivity," said Eduardo Fradkin, a physics professor at the University of Illinois Urbana-Champaign's Grainger College of Engineering and a project co-lead. "They are the vestige that remains once the phase itself has disappeared."
The result implies that Cooper pairs — the coupled electrons that carry superconducting currents — can form before a material becomes superconducting, not at the moment of the phase transition, as standard theory predicts.
What exactly did the team measure?
Superconductivity appears when a metal cooled below a critical temperature conducts electricity with zero resistance. In uranium ditelluride, that happens below 2 kelvins, a property only discovered in 2019.
Vidya Madhavan's experimental group at Illinois examined the material with a vector magnetic field scanning tunneling microscope (STM), a tool that probes a surface's response to magnetic fields applied from many directions. Earlier data had hinted at something odd: charge density waves — periodic patterns of electric charge — that magnetic fields could destroy. Charge waves alone have no reason to respond to magnets.
"There aren't many possible explanations for something like this, and pair density waves are the best one I can think of," Madhavan said.
The breakthrough came from cleaner crystals. Collaborators grew higher-quality samples using a new molten flux method, because pair density waves are extremely delicate and need highly regular crystals to form.
"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."
The new measurements revealed spectral modes that responded to temperature and magnetic fields exactly as pair density waves should — including the decisive result: some modes persisted above the temperature at which superconductivity disappeared.
The evidence stacks up from several independent angles:
- Modes tracked with temperature behaved as PDW theory predicts, not as ordinary charge waves.
- Magnetic fields destroyed the modes, a signature of paired electrons and incompatible with charge-only explanations.
- The modes survived above the superconducting critical temperature.
"As an experimentalist, one of the most satisfying things is when several independent measurements begin to tell the same story," said Zhen Zhu, the postdoctoral researcher who carried out the experiments.
Why does this matter for superconductivity research?
Fradkin and colleagues predicted pair density waves in 2007, about 20 years after the framework for conventional superconductivity was settled. Conventional superconductors follow BCS theory, formulated at Illinois in 1957 by John Bardeen, Leon Cooper and Robert Schrieffer: electrons pair up through interactions with the metal's lattice and then condense together into a frictionless state.
Unconventional superconductors, discovered in 1986, break those assumptions. Uranium ditelluride may be among the rarest of them: a suspected triplet-pair superconductor, where paired electrons carry magnetic moments. The only confirmed analogue is superfluid helium-3, studied by the late Illinois physicist and 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.
For researchers in the field, the takeaway is practical: pairing in this material begins in a distinct state above the transition, so models that assume pairing starts only at the superconducting phase boundary will miss what actually happens.
What are the limits of the result?
The team flags one constraint clearly. STM imaging measures only a material's surface, so the experiment cannot directly confirm what happens inside the bulk of the crystal.
"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.
Julian May-Mann, who worked on the theoretical analysis, added that any explanation relying only on charge density waves conflicts with established principles governing how phases appear and disappear under changing temperature or magnetic field.
The work, supported by the U.S. Department of Energy's Office of Science, points experimenters toward a new benchmark: watching electron pairs organize before superconductivity switches on.
via mrl.illinois.edu (Original)
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News editor covering regulation, policy and their impact on general science news at SciBeat.
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