Physicists found a ‘Cheshire Cat’s grin’ of superconductivity that lingers after the superconductivity is gone

Eduardo Fradkin, a theoretical physicist at the University of Illinois Urbana-Champaign, calls pair density waves “the Cheshire Cat’s grin of superconductivity,” the part that stays visible after the rest of the phase has vanished. His group and the experimental team of Vidya Madhavan now report evidence for that grin in a real crystal of uranium ditelluride, a material that superconducts only at temperatures just above absolute zero.

The patterns they measured persist above the temperature at which the material stops being a superconductor. The Illinois authors describe it as the first direct evidence of such waves in a material’s ordinary, non-superconducting phase, a behavior Fradkin and colleagues predicted in 2007.

Pair density waves in UTe2

Superconductivity begins when electrons form Cooper pairs and condense into a single low-energy state. In a pair density wave, the pairs do not spread evenly. Their density rises and falls across the crystal in a regular pattern. The Illinois Materials Research Laboratory release, dated Sept. 9, 2026, says such waves had been seen alongside superconductivity in other metals but never convincingly beyond it. Fradkin puts the idea in one line: the waves “are the vestige that remains once the phase itself has disappeared.” Madhavan, the department head who leads the experimental side, says the team showed the modes persist above the temperature where superconductivity disappears, which theory had predicted but experiments had not convincingly observed.

The setting is the gap between conventional and unconventional superconductors. The 1957 theory of Bardeen, Cooper and Schrieffer explained how electrons pair through vibrations of the crystal lattice, but materials identified in 1986 break its assumptions, and many of them host other ordered phases, among them charge density waves and pair density waves, below the critical temperature. Fradkin’s 2007 prediction placed pair density waves in that family, with the unusual feature that they should outlast the uniform superconducting state.

Uranium ditelluride, UTe2, was treated as an ordinary metal until 2019, when a superconducting phase turned up below 2 kelvins. It is believed to be a spin-triplet superconductor, a rare type in which the paired electrons carry magnetic moments. A University of Maryland account notes that the compound’s pairs move in swirling patterns instead of the straight-line motion of conventional superconductors, and that it withstood a 35-tesla magnetic field before superconductivity broke down. Superfluid helium-3 is the only confirmed triplet-pair system, which the Illinois release ties to Anthony Leggett’s Nobel-winning work, and Fradkin says he would not call the triplet question for UTe2 completely settled, though consensus favors it.

Vector-field microscopy on cleaner crystals

Madhavan’s group first used scanning tunneling microscopy to map charge density waves, ripples in the electron density, in earlier UTe2 data. Magnetic fields destroyed those waves in a way charge density waves alone would not be expected to allow, and the team proposed pair density waves as the explanation. Impurities in older samples hid the signal, so the experiment needed crystals grown by a new molten flux method, since pair density waves form only in highly regular crystals.

Zhen Zhu, a postdoctoral researcher, carried out the measurements with a vector-field microscope that can apply a magnetic field in any direction. Direction matters because UTe2 responds differently along different crystal axes. Julian May-Mann, a former graduate student, handled the theoretical analysis. The experiments were supported by the U.S. Department of Energy’s Office of Basic Energy Sciences.

Signals that outlast the transition

In the preprint of the paper, “Evidence of intertwined pair density and charge density wave orders in UTe2,” the authors report one set of modulations that disappears near the superconducting transition and a second that persists above it. The persistent set is suppressed by magnetic field anisotropically, and its critical fields follow the same directional pattern as the upper critical field of the superconductor. The authors read that match as evidence for a pair density wave above the bulk transition temperature.

An abstract the group presented at the American Physical Society’s March 2026 meeting puts numbers on it: the transition temperature is about 2.1 kelvins, and the persistent charge-density component survives to about 4.8 kelvins. The Illinois release does not state how far above the transition the signal extends, so those figures belong to the meeting abstract.

The work appeared in Proceedings of the National Academy of Sciences under DOI 10.1073/pnas.2602117123; a ScienceDaily version of the release circulated on Oct. 7.

The researchers are careful about what a scanning tunneling microscope can show. It measures surface effects, and a material’s interior can differ from its surface, so the team calls the results a strong hint about the bulk, not proof. Pair density waves are also hard to isolate because they look like conventional superconductivity in some experiments and like charge density waves in others. Whether the same lingering signal exists inside the crystal is the measurement the surface data cannot make.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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