This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Physicists with the University of Illinois Urbana-Champaign's Grainger College of Engineering have identified a new form of superconducting behavior. Experiments on the metal uranium ditelluride reveal that Cooper pairs, the composite electron units responsible for superconductivity, can organize into nonuniform patterns that exist even when the main superconducting phase is absent.
These patterns, known as pair density waves (PDWs), were first predicted 20 years ago and have been found to coincide with superconductivity in other metals. But new research published in the Proceedings of the National Academy of Sciences not only shows that PDWs exist in uranium ditelluride but also provides the first direct evidence that they can persist in the "ordinary" phase once superconductivity vanishes. "Pair density waves are the Cheshire Cat's grin of superconductivity," said Eduardo Fradkin, an Illinois Grainger Engineering physics professor and a project co-lead.
"They are the vestige that remains once the phase itself has disappeared. 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." "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. "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." The phenomenon of superconductivity, in which metals conduct electricity with zero resistance, arises because the metal's free electrons condense into a low-energy quantum state when cooled below a critical temperature.
However, electrons belong to a class of particles called fermions, meaning the laws of quantum physics prohibit them from coalescing into the same state. The mechanism by which they are allowed to condense in superconductivity is complex and subtle. This complexity was first explained in 1957 by Illinois physicists John Bardeen, Leon Cooper and Robert Schrieffer in a model now known as the BCS theory.
They proposed that electrons condense by first correlating through the metal's underlying lattice. Each electron binds to another electron, forming units called Cooper pairs. Unlike single electrons, Cooper pairs belong to a class of particles called bosons, so they can coexist in the same state with no quantum repulsion.
The pairs are then free to condense into the superconducting state. BCS theory successfully accounted for all observed superconducting behavior until "unconventional" superconductors were identified in 1986. Their structures are incompatible with the assumptions of BCS theory, but electrons still form Cooper pairs and condense.
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