SHERBROOKE, QC, Feb. 14, 2014 /CNW Telbec/ - 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
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 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.
SOURCE: Université de Sherbrooke
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Isabelle Huard, Media-Relations Officer
Communications Department | Université de Sherbrooke
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