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Turning up the heat on superconductors

by CIFAR Mar 3 / 14

For the first time researchers made direct measurement of the critical magnetic field in cuprates, the most promising materials for superconductivity. This breakthrough resolves an enigma that has baffled researchers for 20 years and clears the way for major advances.

The study is published today in the prestigious journal Nature Communications.

Louis Taillefer, Director of CIFAR’s Quantum Materials program, works with a dilution refrigerator that cools down superconductors and other quantum materials to a few millidegrees above absolute zero.
Photo courtesy of La Tribune / Imacom by Jocelyn Riendeau

The research was led by a team at the Université de Sherbrooke that included Louis Taillefer, Senior Fellow and Director of the program in Quantum Materials. Others authors included Senior Fellows Walter N. Hardy, Ruixing Liang and Doug Bonn of the University of British Columbia, and Associate Cyril Proust of the Laboratoire National des Champs Magnetiques Pulses (LCNMP).

When some materials are cooled to very low temperature, barely above absolute zero (‑273 °C), they become superconductors, and their electrical and magnetic properties change radically. They acquire a nearly magical property: they carry electricity perfectly, without any energy loss.

The most promising superconducting materials are copper oxides, also called cuprates. They are, at present, the materials that become superconductors at the highest temperature, specifically -150 °C, which is halfway between absolute zero and ambient temperature.

“If this state could persist at ambient temperature, it would profoundly transform our technological world,” maintains Louis Taillefer, holder of the Canada Research Chair in Quantum Materials and the study’s senior investigator. The transmission of electricity around the world would be radically changed, for example. “This great dream will become possible when scientists understand how to increase the maximum value of the critical temperature by a factor of two or more.”

The team has just identified one of the main mechanisms limiting the critical temperature of cuprates, which opens a new direction in determining how to increase it.

In addition to their critical temperature, another fundamental property of superconductors is their critical magnetic field. In order to measure the critical field of cuprates, the team investigated their capacity to conduct heat. A material’s heat conductivity turns out to be very sensitive to the onset of superconductivity.

“The critical field’s signature immediately became apparent in our data,” says Nicolas Doiron-Leyraud, a Sherbrooke researchers and co-author of the paper. “We observed a sudden drop in the critical field below a certain concentration of electrons.”

Says Taillefer, “For 20 years, scientists have wondered what mechanism is responsible for the drop in critical temperature when the concentration of electrons in a cuprate material drops below a certain level. Up until now, two major scenarios were in the running.”

The first scenario attributes the drop to the fact that the metal—that is, the cuprate—is gradually becoming an insulator. Electrons don’t move in insulators, so they can no longer form mobile pairs. The second scenario attributes the drop in critical temperature to the sudden appearance in the material of a distinct electronic phase that enters into competition with the superconductivity and weakens it.

“Since 1995, the scientific community has been strongly leaning in favor of the first scenario. Our work now unequivocally demonstrates that the second scenario is at work. That opens a whole new path for increasing the critical temperature at which superconductivity can occur: the competing phase has to be eliminated,” Taillefer says.