Feb 17 2014
Three physicists at the Université de Sherbrooke led an international team to first 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 in the prestigious journal Nature Communications.
A Dream Destination: Superconductivity at Ambient Temperature
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.
So, for now, these materials must still be cooled down to extremely low temperatures before they become superconducting. "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.
A Million Times Stronger than the Earth's Magnetic Field
In addition to their critical temperature, another fundamental property of superconductors is their critical magnetic field. What is its value in cuprates?
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 very first direct measurement of this critical field in cuprates was made possible as the result of a novel approach developed by the group of researchers working on the physics of quantum materials at the Université de Sherbrooke.
"The key to our discovery," says Nicolas Doiron-Leyraud, "was developing equipment at Sherbrooke that can make such measurements under very strong magnetic fields." The team then traveled to specialized laboratories in Tallahassee, Florida, and Grenoble, France, where magnetic fields up to 1 million times the earth's field are produced.
"Once there, we realized that it was the first time that anyone had made such an attempt, explains Gaël Grissonnanche, PhD student in physics and first author on the paper. The first measurements on the first day… and it worked!"
A Clear Signature
"The critical field's signature immediately became apparent in our data," says Nicolas Doiron-Leyraud. It was this new measurement that led the Sherbrooke group to make its major discovery. "We observed a sudden drop in the critical field below a certain concentration of electrons" explains Doiron-Leyraud. Louis Taillefer, who also directs the Quantum Materials program at the Canadian Institute for Advanced Research, puts this discovery into perspective. "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," concludes Louis Taillefer.