In a result that reads like a plot twist in the story of superconductivity, physicists at the University of Illinois Urbana-Champaign have found direct evidence that Cooper pairs — the bound electron duos that make superconductors lossless — can survive in a metal even after the superconducting phase itself has vanished. The discovery, reported in the Proceedings of the National Academy of Sciences, was made in uranium ditelluride, an unconventional superconductor that has rapidly become one of the most closely watched materials in condensed matter physics. The finding concerns a long-predicted but elusive state called a pair density wave, and it delivers the first convincing demonstration that this state can persist above the critical temperature at which ordinary superconductivity disappears.
The metaphor that the researchers themselves chose is memorable. Eduardo Fradkin, a physics professor at Illinois Grainger Engineering and a co-lead of the project, compared the phenomenon to the grin of the Cheshire Cat from Alice in Wonderland: the cat disappears, but its grin lingers in the air. In the same way, the pair density wave is the vestige of superconductivity that remains once the phase itself has faded. In a conventional superconductor, Cooper pairs appear only when the full superconducting transition takes place. Here, the experiments show that the pairs are already organized into spatially modulated patterns before the material ever enters its superconducting phase — a scenario that theorists proposed two decades ago but that had never been directly confirmed in a real material.
To appreciate why this matters, it helps to revisit the basic physics of superconductivity. When certain metals are cooled below a critical temperature, their free electrons condense into a collective low-energy quantum state that conducts electricity with zero resistance. But electrons are fermions, and quantum mechanics forbids identical fermions from occupying the same state. The way around this prohibition was explained in 1957 by Illinois physicists John Bardeen, Leon Cooper and Robert Schrieffer in the celebrated BCS theory. Each electron binds weakly to a partner through interactions mediated by the crystal lattice, forming a Cooper pair. Unlike single electrons, these pairs behave as bosons, particles that can happily pile into one shared quantum state. That bosonic condensation is the essence of superconductivity.
BCS theory accounted beautifully for every superconductor known until 1986, when the first unconventional superconductors appeared. These materials violate the assumptions of BCS theory, yet their electrons still pair up and condense, and understanding how remains one of the central open problems in the field. A recurring clue is that unconventional superconductors tend to host other ordered phases below their critical temperature. One common companion is the charge density wave, in which a fraction of the electrons arranges itself into a periodic spatial pattern, producing alternating regions of higher and lower electric charge. In 2007, Fradkin and his colleagues went a step further and theorized a more exotic companion: a pair density wave, in which the superconducting Cooper pairs themselves — normally spread uniformly through the metal — organize into a nonuniform, wave-like pattern.
One of the most striking predictions of that 2007 theory was that pair density waves should be able to exist above the superconducting critical temperature, implying that Cooper pairs can form and order even while the metal as a whole is not superconducting. Fradkin admits the idea was bold. The state is peculiar, he noted, and although there had been experimental hints over the years, no direct confirmation of the phase’s existence had been achieved. Part of the difficulty is diagnostic: as Julian May-Mann, a former Illinois graduate student who performed the study’s theoretical analysis, explained, pair density waves behave like conventional superconductors in some experiments and like charge density waves in others, so confirming one requires both high-quality data and careful theoretical interpretation.
The material that finally revealed the state is uranium ditelluride, which until 2019 was regarded as an ordinary metal. That year, researchers discovered a superconducting phase below 2 kelvins, and subsequent studies led physicists to suspect that uranium ditelluride is an elusive triplet-pair superconductor — one in which the paired electrons carry magnetic moments, unlike the spin-canceling pairs of BCS superconductors. The only confirmed triplet-pair super-phase in nature is superfluid helium-3, a system studied extensively by the late Illinois physicist Anthony Leggett, who won the Nobel Prize for that work. Fradkin cautions that the question is not completely settled, but he notes that the consensus is that uranium ditelluride belongs to this rare class.
The experimental thread began in the laboratory of Vidya Madhavan, a physics professor and department head at Illinois Grainger Engineering and the project’s other co-lead. Her group had identified charge density waves in uranium ditelluride using scanning tunneling microscopy, but they noticed something deeply strange: the waves could be destroyed by magnetic fields. A charge density wave is simply a collective electronic state modulated in space, and there is no reason for it to respond to magnetic fields, let alone be wiped out by them. When the experimentalists brought the data to Fradkin and his students, the theorists predicted that such behavior could occur if a pair density wave were also present in the system — and, as Madhavan put it, pair density waves were the best explanation available for such an unusual response.
Proving it required overcoming a formidable experimental obstacle: pair density waves are delicate and form only in highly regular crystals, and impurities in earlier samples had obscured the signals entirely, like trying to spot a light in a cloud of fog. Collaborators supplied higher-quality uranium ditelluride crystals grown with a new molten flux method, and the team examined them with a vector magnetic field scanning tunneling microscope, an instrument that probes how material surfaces respond to magnetic fields applied in arbitrary directions. That capability proved essential because uranium ditelluride is strongly anisotropic — its superconducting upper critical field depends heavily on direction — so the researchers needed to sweep the field’s magnitude and orientation systematically while varying temperature. Zhen Zhu, a postdoctoral research associate who carried out the experiments, emphasized that tracking how the observed modes evolved under these combined controls built confidence that the behavior was intrinsic to the material rather than an artifact.
The payoff came in the form of modes that responded to temperature and magnetic field exactly as pair density waves should, including their destruction under magnetic effects. Most tellingly, some of these modes continued to exist above the critical temperature, in the temperature range where the main superconducting phase no longer existed. May-Mann pointed out that foundational principles of condensed matter physics constrain how different phases can appear and disappear as temperature changes or fields are applied, and any explanation based on a charge density wave alone clashes with those principles. The pair density wave interpretation, by contrast, provides a consistent account of everything the team observed. In Fradkin’s framing, the pairs are formed beforehand, in a different state, and the pair density wave is simply the grin that outlasts the cat.
There are caveats, and the researchers are candid about them. Scanning tunneling microscopy characterizes only a material’s surface, and it is possible for the interior of a crystal to behave differently from its boundary. Even so, Fradkin argues that the surface results give a very strong hint at what is happening inside the bulk, and the team is optimistic that the finding opens new research directions, both for confirming pair density waves in other unconventional superconductors and for exploring how triplet pairing reshapes the landscape of quantum phases. For Zhu, the satisfaction lies in the convergence: the temperature dependence, the magnetic-field dependence and the improved sample quality all told the same story at once. The work was supported by the U.S. Department of Energy’s Office of Science, and its contributors include Yudi Huang, Kaiming Liu, Zheyu Wu, Shanta Saha, Johnpierre Paglione, Alexander Eaton, Andrej Cabala and Michal Vališka. What began as a theoretical proposal sketched on a limb twenty years ago now stands as measured fact — and the grin, it turns out, was there all along, waiting for instruments sharp enough to see it.
Subject of Research: Pair density wave order and Cooper pairing above the superconducting critical temperature in uranium ditelluride
Article Title: The Cheshire Cat’s grin: Cooper pairs found above superconducting critical temperature in pair density waves
Article References: The Cheshire Cat’s grin: Cooper pairs found above superconducting critical temperature in pair density waves. (n.d.). Original publication
Image Credits: AI Generated
DOI: Not provided
Keywords: superconductivity, Cooper pairs, pair density wave, uranium ditelluride, unconventional superconductors, charge density wave, scanning tunneling microscopy, triplet pairing, BCS theory, condensed matter physics, critical temperature, quantum materials
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Katie Riggs. (October 2, 2026). Ghostly Cooper Pairs Persist Above Superconductivity’s Critical Temperature in Uranium Ditelluride. Scienmag. https://scienmag.com/ghostly-cooper-pairs-persist-above-superconductivitys-critical-temperature-in-uranium-ditelluride/
Katie Riggs. “Ghostly Cooper Pairs Persist Above Superconductivity’s Critical Temperature in Uranium Ditelluride.” Scienmag, 2 October 2026, https://scienmag.com/ghostly-cooper-pairs-persist-above-superconductivitys-critical-temperature-in-uranium-ditelluride/. Accessed 2 October 2026.
Katie Riggs. “Ghostly Cooper Pairs Persist Above Superconductivity’s Critical Temperature in Uranium Ditelluride.” Scienmag. October 2, 2026. https://scienmag.com/ghostly-cooper-pairs-persist-above-superconductivitys-critical-temperature-in-uranium-ditelluride/
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Tags: BCS theorycharge density waveCondensed matter physicsCooper pairsCooper pairs above critical temperaturecritical temperatureexperimental evidence of Cooper pair persistencehigh-temperature superconductivity phenomenalong-predicted quantum stateslossless electron pairingpair density wavepersistent electron pairsQuantum materialsquantum states beyond superconducting phaseresidual superconductivityscanning tunneling microscopySuperconductivitysuperconductivity phase transitiontriplet pairingUnconventional superconductorunconventional superconductorsuranium ditelluride
