Physicists at the University of Illinois Urbana-Champaign's Grainger College of Engineering have uncovered evidence of an unusual form of superconducting behavior in uranium ditelluride. Their experiments show that Cooper pairs, the paired electrons that make superconductivity possible, can arrange themselves into uneven patterns even when the material is no longer in its main superconducting state. These patterns are called pair density waves (PDWs).
Scientists first predicted them about 20 years ago, and previous studies have observed them alongside superconductivity in other metals. The new research, published in the Proceedings of the National Academy of Sciences, goes further. It provides the first direct evidence that PDWs in uranium ditelluride can remain in the material's "ordinary" phase after superconductivity has disappeared.
"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." How Unconventional Superconductors Behave Superconductivity occurs when a metal can carry electricity with zero resistance after being cooled below a critical temperature. In this state, the material's free electrons settle into a low-energy quantum state. That creates a fundamental challenge.
Electrons belong to a group of particles called fermions, and quantum mechanics prevents fermions from gathering in the same state. Superconductivity gets around this restriction through a more complicated process involving pairs of electrons. In 1957, Illinois physicists John Bardeen, Leon Cooper and Robert Schrieffer explained this behavior with what became known as BCS theory.
According to their model, electrons first become correlated through interactions with the metal's underlying lattice. One electron then pairs with another to create a Cooper pair. Unlike individual electrons, Cooper pairs behave as bosons.
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