Science2 publishers3 min readPublished Updated
Patterned Cooper pairs outlast superconductivity in uranium ditelluride
Illinois physicists report the first direct evidence that pair density waves in uranium ditelluride outlive superconductivity, as a 2007 theory predicted. If the result holds, electrons in this metal pair up before superconductivity sets in, though so far the evidence covers only one material.
The Scientist · Science desk
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What happened
- The PNAS study, co-led by Illinois physics professors Eduardo Fradkin and Vidya Madhavan, also establishes that pair density waves exist in uranium ditelluride.
- Fradkin said earlier experiments had offered hints of the patterned pair state but no direct confirmation that it exists.
- In other metals, pair density waves had been found only where they coincided with superconductivity.
- BCS theory, from 1957, accounted for all observed superconducting behavior until unconventional superconductors were identified in 1986.
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Why it matters
- capability If the pairs form before the phase does, uranium ditelluride is a material where pairing can be studied apart from condensation. In BCS superconductors the two happen at a single transition.
- constraint Pair density waves look like charge density waves in some measurements, so the claim depends on the temperature and field response, and until another lab reproduces it the evidence is one team's samples.
- decision Groups studying other unconventional superconductors, where companion phases appear below the critical temperature, now have a tested reason to look for paired electrons above it as well.
The BCS account describes a sequence. Electrons bind in pairs through the metal's lattice, and the pairs, being bosons, can share one quantum state and condense into the superconducting phase [8]. In conventional superconductors the pairs form at the same transition as the phase itself [5]. Fradkin said uranium ditelluride does not follow that order: "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." [5]
Unconventional superconductors seem to share one feature. Below the critical temperature, other ordered phases appear alongside superconductivity, among them charge density waves, in which some of the electrons arrange into a periodic pattern of higher and lower charge [10]. The 2007 Illinois proposal said the Cooper pairs could form a pattern of their own, and that this pattern could exist above the critical temperature. That would mean paired electrons in a metal that is not superconducting [11]. Fradkin said pair density waves are "the Cheshire Cat's grin of superconductivity" and "the vestige that remains once the phase itself has disappeared." [4]
The experimental problem is telling a pattern of pairs from a pattern of charge. "PDWs are tricky to analyze in real materials because they behave like conventional superconductors in some experiments and like CDWs in others," reads a quoted passage in the phys.org report [13]. So a periodic signal seen above the transition has to be shown to consist of pairs [13]. Madhavan described the evidence: "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." [6]
The persistence result rests on that identification. "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," Madhavan said [7]. The phys.org account does not report how far above the transition the modes survive, how many samples were measured, or which spectroscopic technique produced the signatures [3]. I would want those numbers before treating the question as closed.
In uranium ditelluride, the result puts the origin of pairing in the ordinary phase above the transition, if Fradkin's interpretation holds [5]. That makes the normal state of this metal a place to study pairing on its own, separate from condensation. The paper itself concerns one material [3]. It does not test whether other unconventional superconductors also form their pairs ahead of the phase. I think the result justifies looking above the transition in those materials too, on the condition that another group reproduces the uranium ditelluride measurement first.
What to watch
- Independent reproduction of the above-transition modes in uranium ditelluride using samples grown by another group.
- The PNAS paper's figures on how far above the critical temperature the modes survive and how many samples showed them.
- Searches for pair density waves above the critical temperature in other unconventional superconductors.