"That's been the debate for 20 years – what is going on in the pseudogap phase?"Louis Taillefer, Université de Sherbrooke

Physicists from Canada and France have zoomed in on the quantum phase transition that could explain why copper oxides have such impressive superconducting powers.

Settling a 20-year debate in the field, they found that a mysterious quantum phase transition associated with the termination of a regime called the ‘pseudogap’ causes a sharp drop in the number of conducting electrons available to pair up for superconductivity. The team hypothesizes that whatever is happening at this critical point is probably the reason why cuprates support superconductivity at much higher temperatures than other materials – about half way to room temperature.

"It's very likely that the reason superconductivity grows in the first place, and the reason it grows so strongly, is because of that critical point," says Louis Taillefer from the Université de Sherbrooke in Canada and a senior fellow at the Canadian Institute for Advanced Research (CIFAR). The new findings are published in Nature.

The study combined the University of British Columbia's expertise in making copper oxide materials, known as cuprates, the Université de Sherbrooke's expertise at probing them, and the powerful magnetic fields produced at the Laboratoire National des Champs Magnétiques Intenses in Toulose, France.

This work forms part of a global effort to harness superconductivity – the transmission of electricity with zero resistance in certain materials – to greatly improve power efficiency in many technologies. Cuprates are the most promising materials for that purpose right now, but the community is faced with a formidable physics problem: understanding the mysterious ‘pseudogap’ phase. "That's been the debate for 20 years – what is going on in the pseudogap phase?" says Taillefer.

The mystery has remained unsolved for so long mainly because it becomes difficult to study what behaviors are taking place beneath superconductivity once it kicks in. By applying a magnetic field two million times stronger than the Earth's, the team of scientists managed to wipe out superconductivity in cuprate samples and then look closely into the pseudogap phase at temperatures near absolute zero (-273°C).

They found that, at the point of instability where the pseudogap sets in, the electronic structure of cuprates undergoes a radical change: the number of available electrons plummets six-fold. This marks a quantum phase transition – a fundamental change of behavior within the material.

The scientists believe this new work will shift the focus of future research, and will lead to a new understanding of the properties of superconductors. Taillefer says this finding points the way to discovering the nature of the critical point and its fluctuations, and then exploring how to make superconductivity work at room temperature.

The discovery follows intense research on the pseudogap mystery, after the same group of CIFAR researchers discovered the first signs of strange behavior by observing quantum oscillations in 2007. "The development at Toulouse of very low noise measurements, crucial for the discovery of quantum oscillations in 2007, and now recently the design and construction of our 90T magnet, together opened up a new window of capability, allowing us to look directly at the pseudogap critical point," says Cyril Proust from the Laboratoire National des Champs Magnétiques Intenses.

CIFAR Associate Fellow Subir Sachdev from Harvard University, who did not take part in the study, says the findings validate some of his recent theoretical research and set a clearer direction for future investigation of this critical point. "This gets me very excited about working on the theory of such a critical state," Sachdev says. "The new experiments really sharpen the picture."

Taillefer says the research would not have been possible without the collaborations fostered by CIFAR on quantum materials, both within Canada and internationally. "It's really a pure CIFAR story," he says. Doug Bonn at the University of British Columbia (UBC) adds that CIFAR's long-term support for collaborations on materials development and the many experimental techniques for studying these materials has advanced the field. "The UBC-Sherbrooke collaboration is a particularly successful and long-running example, with each new experimental discovery pushing harder on further development of the materials samples used in the experiments," he says.

"This breakthrough is an example of how sustained, global collaboration that brings together diverse expertise from across the world is the most powerful way to advance science and address important sustainability challenges," says CIFAR president and CEO Alan Bernstein.

This story is adapted from material from CIFAR, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.