Researchers at the University of Illinois Urbana-Champaign have identified a new form of superconducting behavior. Their experiments reveal that Cooper pairs, the composite electron units responsible for superconductivity, can organize into patterns known as pair density waves that persist even when the main superconducting phase is absent.
This research is published in a PNAS paper titled "Evidence of intertwined pair density and charge density wave orders in UTe 2."
What are pair density waves?
Pair density waves, or PDWs, are nonuniform spatial patterns of Cooper pairs. In ordinary superconductors, these electron pairs are distributed evenly throughout the metal. But in certain materials, they can arrange themselves into periodic patterns with regions of higher and lower pair density. PDWs were first predicted in 2007 by Illinois physicist Eduardo Fradkin and his colleagues. A striking feature of their theory was that PDWs could exist above the critical temperature where superconductivity normally disappears.
Fradkin describes PDWs as "the Cheshire Cat's grin of superconductivity." Just as the cat's smile lingers after the cat itself has vanished, PDWs represent a vestige of superconductivity that remains once the full phase has disappeared. The new research provides the first direct evidence that this peculiar state can indeed persist in the "ordinary" phase above the transition point.
The study focused on uranium ditelluride, a metal that becomes superconducting below 2 kelvins. The researchers identified density waves in uranium ditelluride using scanning tunneling microscopy. The data revealed spectral signatures that responded to temperature and magnetic fields exactly as PDWs should, including modes that continued to exist above the critical temperature where superconductivity vanished. This observation confirmed the theoretical prediction that Cooper pairs can form beforehand in a different state.
This discovery matters because it resolves a long-standing theoretical prediction and could guide the search for new superconducting materials with practical applications, such as lossless power transmission or advanced computing devices.