At the heart of high-temperature super- conductivity

Electrons in cuprate superconductors behave in remarkable ways. They conduct electricity without resistance at record high temperatures, and when they cease to superconduct, they do not comply with our usual expectations for how a metal should behave. Now, a team led by Sherbrooke Professor and IQ Member Louis Taillefer reports in Nature a major finding that could explain these remarkable properties: a quantum critical point right at the heart of cuprate superconductors.

A time will come when superconductors will revolutionize our daily life, in areas as diverse as energy transmission, medicine, and communications. For this to happen, the critical temperature below which superconductivity occurs must first be increased up to room temperature. As of today, copper-oxide materials called “cuprates” are the most promising candidates for achieving that goal. However, these are materials where the weirdness of quantum mechanics seems to have created a perfect storm, creating a wealth of spectacular and anomalous properties that defy our understanding.

The most puzzling feature of cuprates is the “pseudogap phase”, a mysterious electronic phase of matter that coexists with superconductivity, and is considered one of the great enigmas in physics today. Elucidating the nature of that phase is thought to be the key to understand how electrons behave in these materials.

The team of researchers led by Taillefer and Thierry Klein at the Institut Néel in Grenoble, France, discovered a crucial ingredient for resolving the enigma: the pseudogap phase ends at a quantum critical point. This means that it harbours some kind of order, and that order is the key. In other materials, superconductivity has been found to spring from a quantum critical point where magnetic order ends, and magnetism is what causes the superconductivity and the other anomalous properties. In cuprates, there is no magnetic order, so it must be something else. Figuring out what type of unusual order is involved is now the big question.

The team’s discovery came from measurements of the specific heat at very low temperature, having first suppressed superconductivity with very large magnetic fields, using the powerful magnets at the Laboratoire National des Champs Magnétiques Intenses in Grenoble. The long-term project was a collaboration between Sherbrooke and Grenoble, involving three researchers who are members of the Laboratoire Circuits et Matériaux Quantiques, à Laboratoire International Associé created by the French CNRS in 2017, linking the Institut Quantique in Sherbrooke with several French laboratories. This Sherbrooke-France partnership is a good example of co-tutoring, whereby Taillefer and Klein jointly supervised PhD students Bastien Michon (Bastien Michon’s portrait) and Clément Girod, first and second authors on the Nature article.

“At Sherbrooke, we have explored the pseudogap phase with various probes, but to see the quantum critical point required the expertise of Thierry Klein and Christophe Marcenat in measuring specific heat in high magnetic fields”, says Taillefer. “A great complementarity!”.

The project is also an excellent illustration of the synergetic approach of the Canadian Institute for Advanced Research (CIFAR), whereby experimentalists, theorists and experts at materials synthesis are brought together to achieve a breakthrough. In this case, CIFAR’s Quantum materials program enabled Taillefer to obtain precious cuprate samples from Bruce Gaulin and Hidenori Takagi, respectively member and advisor in that program. Theoretical calculations were carried out by Simon Verret, a postdoc in the group of IQ member André-Marie Tremblay, a long-term member of the CIfAR program.

“Being able to convince Bruce Gaulin and his group at McMaster to synthesise cuprate samples at very high doping was crucial”, says Taillefer. “Thanks to CIFAR, a new collaborative axis between Sherbrooke and McMaster was opened”.

The Gordon & Betty Moore Foundation contributed to funding this project, via a grant to Taillefer as part of its program on Emergent Phenomena in Quantum Systems.