Physicists discover Cooper pairs persist above superconductivity threshold in uranium ditelluride

A team of physicists at the University of Illinois Urbana-Champaign has observed something that theory predicted two decades ago but experiments have never quite confirmed: Cooper pairs—the paired electrons that make superconductivity possible—can exist and organize themselves even after the superconducting state itself has vanished. The discovery, published in the Proceedings of the National Academy of Sciences, centers on an unusual metal called uranium ditelluride and reveals a hidden layer of order in how electrons behave at extreme cold.

Superconductivity has been understood since 1957 through what physicists call BCS theory, named after John Bardeen, Leon Cooper, and Robert Schrieffer—all of whom worked at Illinois. The theory explains how electrons, which normally repel each other, can pair up and flow through a metal with zero electrical resistance once the temperature drops below a critical threshold. The pairing works because two electrons together behave differently than one electron alone; they become bosons instead of fermions, which means they can occupy the same quantum state without the usual quantum mechanical prohibition. But in 1986, physicists discovered "unconventional" superconductors that didn't fit the BCS mold, yet still worked. Understanding these materials has remained an open problem.

In 2007, Eduardo Fradkin and colleagues at Illinois proposed something stranger still: Cooper pairs might not always be uniformly distributed through a metal. Instead, they could organize into periodic patterns—regions where pairs bunch together alternating with regions where they thin out. These patterns, called pair density waves, had a peculiar theoretical prediction attached: they could exist above the temperature at which superconductivity itself disappears. "Pair density waves are the Cheshire Cat's grin of superconductivity," Fradkin said. "They are the vestige that remains once the phase itself has disappeared." For nearly two decades, no one had directly observed this.

Uranium ditelluride entered the picture in 2019 when researchers discovered it becomes superconducting below 2 kelvins—an extraordinarily cold temperature, just a few degrees above absolute zero. The metal appeared to be a triplet-pair superconductor, a rare type where the paired electrons carry magnetic moments, similar to superfluid helium-3. Vidya Madhavan's experimental group at Illinois began studying it with scanning tunneling microscopy, a technique that images the surface of materials at atomic scales. They found charge density waves—patterns of electrons organizing themselves spatially—but something odd happened: magnetic fields could destroy these waves, which shouldn't be possible if they were ordinary charge density waves. The explanation that fit was pair density waves.

To test this hypothesis, the team needed better samples. Pair density waves are delicate structures that only form in highly regular crystals; impurities scatter the signal like fog obscuring a distant light. Collaborators provided uranium ditelluride samples grown using a new molten flux method, and the researchers deployed a vector magnetic field scanning tunneling microscope—equipment that can apply magnetic fields in any direction and measure how the material responds. This capability mattered because uranium ditelluride is anisotropic, meaning its properties differ depending on direction. By systematically varying both the strength and direction of the magnetic field alongside temperature, the team could track how electronic modes evolved.

The data told a clear story. Certain spectral signatures responded to temperature and magnetic fields exactly as pair density wave theory predicted. But the crucial finding was this: modes persisted above the critical temperature, above the point where superconductivity itself ceased to exist. Once the superconducting phase vanished, these modes should have vanished too—unless Cooper pairs were forming in a different kind of order, one that didn't require the full superconducting state. The researchers confirmed that no explanation based solely on charge density waves could account for the pattern they observed. The physics required pair density waves.

Zhen Zhu, a postdoctoral researcher who carried out the experiments, noted that multiple independent measurements—temperature dependence, magnetic field response, and the quality of the samples themselves—all converged on the same picture. "As an experimentalist, one of the most satisfying things is when several independent measurements begin to tell the same story," he said. The work opens questions about what other unconventional superconductors might harbor similar hidden orders, and whether understanding pair density waves could lead to new materials or applications. For now, the discovery confirms that the quantum world of electrons at extreme cold is far stranger and more layered than the standard theory suggested.