Science1 publisherNot yet confirmed elsewhere2 min readPublished
Pair density waves outlast superconductivity in uranium ditelluride
University of Illinois physicists report the first direct evidence that pair density waves in uranium ditelluride persist above its superconducting transition. If that holds, Cooper pairs in this metal form before it superconducts, unlike in conventional superconductors.
The Scientist · Science desk
What happened
- Eduardo Fradkin, the other co-lead, proposed the pair density wave with colleagues in 2007, a phase in which Cooper pairs are spread unevenly through the material.
- Earlier studies had seen pair density waves in other metals, where they coexisted with superconductivity.
- The research appeared in the Proceedings of the National Academy of Sciences, according to a University of Illinois Grainger College of Engineering release.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint Any working model of uranium ditelluride now has to let Cooper pairs exist above the superconducting transition, a step the conventional sequence of pairing at the transition does not allow.
- constraint Independent checks depend on crystal quality: the team credits new growth methods, so other groups need comparable samples before they can confirm or refute the above-transition signal.
- precedent Groups studying other unconventional superconductors now have a specific signature to look for: pair modulations above the transition that respond to field and temperature.
In the 1957 BCS theory of John Bardeen, Leon Cooper and Robert Schrieffer, electrons correlate through the metal's lattice and bind into Cooper pairs [9]. The pairs behave as bosons and condense together into the superconducting state [9]. The unconventional superconductors, found in 1986, break the theory's assumptions while still pairing and condensing [10]. How they work is still an open research question [10].
Fradkin proposed the pair density wave with colleagues in 2007 [12], 19 years before this release [14]. He called the waves "the Cheshire Cat's grin of superconductivity" [5]. "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," he said [6].
That interpretation depends on the ripple being made of pairs. Unconventional superconductors often host charge density waves, in which charge itself rises and falls periodically from region to region [11]. A modulation measured above the transition has to be told apart from one of those. Madhavan pointed to how the signal behaves as conditions change. "Thanks to new methods for growing higher-quality samples, we were able to observe spectral signatures that respond to temperature and magnetic fields exactly as pair density waves should," she said [7]. "We even showed that the modes persist above the temperature at which superconductivity disappears, a telltale theoretical prediction that has not been convincingly observed before now," she said [8].
The university's release does not give the transition temperature, how far above it the modes survive, how many samples were measured, or which instrument recorded the spectra [3]. Anyone wanting the effect size will need the PNAS paper itself [3].
The evidence so far concerns one compound [1]. If the result holds, I think its immediate use is as a constraint: whatever theory explains uranium ditelluride has to let pairs exist in the ordinary phase, above the point where superconductivity disappears [4]. Whether a pre-formed pair state shows up in other unconventional superconductors, and whether it helps anyone design new ones, is a question for experiments on other materials.
What to watch
- Whether the PNAS paper reports how far above the superconducting transition the modes persist, and across how many samples.
- Independent groups reproducing the above-transition signal in uranium ditelluride crystals grown in other labs.
- Searches for the same pre-formed pair signature in other unconventional superconductors.
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- [1]
Physicists at the University of Illinois Urbana-Champaign's Grainger College of Engineering uncovered evidence of an unusual form of superconducting behavior in uranium ditelluride; Cooper pairs can arrange into uneven patterns even when the material is no longer in its main superconducting state.
ReportedSupportedSource: University of Illinois Grainger College of Engineering release via ScienceDailyView cited source - [2]
Pair density waves were first predicted about 20 years ago, and previous studies have observed them alongside superconductivity in other metals.
- [3]
The research was published in the Proceedings of the National Academy of Sciences, according to a release from the University of Illinois Grainger College of Engineering.
- [4]
The research provides the first direct evidence that pair density waves in uranium ditelluride can remain in the material's ordinary phase after superconductivity has disappeared.
- [5]
"Pair density waves are the Cheshire Cat's grin of superconductivity," said Eduardo Fradkin, an Illinois Grainger Engineering physics professor and project co-lead.
- [6]
"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."
- [7]
"Thanks to new methods for growing higher-quality samples, we were able to observe spectral signatures that respond to temperature and magnetic fields exactly as pair density waves should," said Vidya Madhavan, an Illinois Grainger Engineering physics professor and the other project co-lead.
- [8]
"We even showed that the modes persist above the temperature at which superconductivity disappears, a telltale theoretical prediction that has not been convincingly observed before now."
- [9]
In 1957 Illinois physicists John Bardeen, Leon Cooper and Robert Schrieffer explained superconductivity with BCS theory: electrons become correlated through interactions with the lattice, pair into Cooper pairs, which behave as bosons and condense into a superconducting state.
- [10]
Unconventional superconductors, discovered in 1986, do not fit the assumptions behind BCS theory even though their electrons still form Cooper pairs and condense; understanding how they work remains an active area of research.
- [11]
Unconventional superconductors often host other electronic phases, such as charge density waves, in which some electrons form a repeating nonuniform pattern producing a periodic variation in charge.
- [12]
In 2007 Fradkin and colleagues proposed the pair density wave phase; Cooper pairs are normally distributed uniformly in a superconductor, but the researchers suggested they might be distributed unevenly.
- [13]
The University of Illinois release was dated October 7, 2026.
- [14]
The release came 19 years after Fradkin and colleagues proposed the pair density wave.
Sources
1 independent publisher whose own reporting we read for this story.
- sciencedaily.comPhysicists just found the ‘ghost’ of superconductivity
1 article · October 7, 2026
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Entities
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