Patterned Cooper pairs outlast superconductivity in uranium ditelluride

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 ^[13]. 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 ^[8]. 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 ^[9]. 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 ^[11]. So a periodic signal seen above the transition has to be shown to consist of pairs ^[11]. 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 ^[2]. 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 ^[2]. 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.