Quantum Cooper pairs linger in uranium ditelluride post-superconductivity

Physicists at the University of Illinois Urbana-Champaign’s Grainger College of Engineering have observed Cooper pairs persisting in the metal uranium ditelluride even after superconductivity ceases. These pairs organize into patterns called pair density waves, predicted two decades ago but never before directly observed to remain once superconductivity vanishes.

“Pair density waves are the Cheshire Cat’s grin of superconductivity,” explains Eduardo Fradkin, an Illinois Grainger Engineering physics professor and project co-lead; they are a vestige of the superconducting phase itself. This research, published in the Proceedings of the National Academy of Sciences, offers new insight into unconventional superconductivity and how electrons can condense into a low-energy quantum state.

Uranium Ditelluride Reveals Cooper Pair Patterns Beyond Superconductivity

Uranium ditelluride exhibits an unusual property. Cooper pairs within the material persist even after the loss of superconductivity, a phenomenon previously unobserved directly in any substance. Researchers used a vector magnetic field scanning tunneling microscope to observe these lingering pairs, a capability made possible by newly developed equipment sensitive to the material’s anisotropic nature.

Zhen Zhu, a postdoctoral research associate at Illinois Grainger Engineering, explained the importance of this instrumentation, stating, “A key advantage of our experiment was the newly developed vector-magnet equipment, which provides an unusually large magnetic-field range along multiple directions.” This allowed for detailed examination of the crystals from multiple orientations, which was important for detecting the subtle patterns indicative of pair density waves. The discovery builds on theoretical predictions made two decades ago regarding pair density waves, or PDWs, and confirms their existence in uranium ditelluride beyond the superconducting phase.

Prior to 2019, uranium ditelluride was considered a standard metal, but the subsequent discovery of a superconducting phase below 2 kelvins prompted further investigation. Initial studies suggested it might be a rare triplet-pair superconductor, a state similar to superfluid helium-3 extensively studied by the late Nobel laureate Anthony Leggett. However, the observation of PDWs offers a new perspective on the material’s behavior, suggesting that Cooper pairs organize into these patterns even before the onset of superconductivity. Obtaining conclusive evidence required overcoming significant material challenges.

Early attempts to detect PDWs were hampered by impurities in the uranium ditelluride samples, obscuring the delicate patterns. Madhavan explained the difficulty, saying, Collaborations with materials scientists were essential to produce high-quality crystals using a novel molten flux method, enabling clear observation of the PDWs.

The researchers believe that the PDW-based explanation provides a consistent understanding of the experimental data, resolving discrepancies that arose with previous models. Any alternative explanation relying solely on charge density waves, in which electrons organize into nonuniform patterns, is incompatible with the observed principles. Madhavan summarized the team’s conclusion, stating, “There aren’t many possible explanations for something like this, and pair density waves are the best one I can think of.” This finding opens new avenues for exploring unconventional superconductivity and understanding the fundamental mechanisms governing Cooper pair formation and behavior in complex materials.

Pair Density Waves: A Theoretical Prediction Confirmed in Experiment

The team’s experimental setup allowed for systematic variation of both magnetic field magnitude and direction, alongside temperature adjustments, enabling them to track the evolution of the PDWs and confirm their intrinsic nature. “This capability is particularly well suited to uranium ditelluride, whose superconducting upper critical field is strongly anisotropic. 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, an Illinois Grainger Engineering physics postdoctoral research associate.

Vector STM Identifies PDW Signatures in Uranium Ditelluride Samples

High-resolution scanning tunneling microscopy, coupled with a newly developed vector magnet, enabled researchers to observe spectral signatures confirming the presence of pair density waves in uranium ditelluride even after the material ceased to superconduct. This observation directly links to the persistence of Cooper pairs beyond the superconducting critical temperature, a phenomenon previously unseen and long theorized.

Thanks to new methods for growing higher-quality samples, the team was able to observe these signatures responding to temperature and magnetic fields exactly as pair density waves should, according to the published findings. The ability to probe the material’s response across various orientations was essential for accurately characterizing the pair density waves and distinguishing them from other potential electronic states.

The team’s success hinged on overcoming limitations imposed by material impurities that previously obscured data. “It would have been like trying to spot a light in a cloud of fog. But we succeeded with the better samples because of our new vector magnetic field equipment.” This improvement in sample quality, achieved through collaborations with materials scientists employing a new molten flux method, allowed for clearer observation of the subtle signatures of PDWs.

The resulting data revealed modes that responded to both temperature and magnetic field in a manner consistent with theoretical predictions for pair density waves, including their suppression under strong magnetic fields. In conventional superconductors, Cooper pairs form when the full phase transition occurs, but, in this system, their observation in PDWs above the transition point shows that they are formed beforehand in a different state.”

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