Saturday, February 27, 2016

"The Quantum Secret to Superconductivity"

Serious science.
From Quanta Magazine:

In a virtuoso experiment, physicists have revealed details of a “quantum critical point” that underlies high-temperature superconductivity.
The 90-tesla magnet system at the National Laboratory for Intense Magnetic Fields in Toulouse, France.
The energy equivalent of several kilograms of TNT surged into the coil, bathing the 0.003-carat crystal in its bore in one of the strongest magnetic fields ever generated. 
From the magnet came a small boom like the sound of a foot stomping, said engineer Jérôme Béard — but thankfully, no explosion. His calculations held up.

With that magnetic blast and a subsequent series of identical ones executed last winter, researchers at the National Laboratory for Intense Magnetic Fields (LNCMI) in Toulouse, France, uncovered a key property of the crystal, a matte-black ceramic in a class of materials called cuprates that are the most potent superconductors known. The findings, reported today in the journal Nature, provide a major clue about the inner workings of cuprates, and may help scientists understand how these materials allow electricity to flow freely at relatively high temperatures.

“Technically amazing,” said J.C. Séamus Davis, an experimental physicist with appointments at Cornell University, St. Andrews University in Scotland, and Brookhaven National Laboratory who was not involved in the experiment. “The paper is a masterpiece.”

The experimental team, led by LNCMI staff scientist Cyril Proust and Louis Taillefer of the University of Sherbrooke in Canada, used their 90-tesla magnet — which creates a magnetic field nearly two million times as strong as the one enshrouding Earth — to momentarily strip away superconductivity in their cuprate sample. This revealed details of the underlying phase from which the behavior seems to arise.

With the veil lifted, the scientists discovered a sharp change in behavior at what appears to be a “quantum critical point” in cuprates, reminiscent of the freezing point of water. Theorists have long speculated that such a quantum critical point might exist, and that it could play a key role in superconductivity, said Andrey Chubukov, a condensed-matter theorist at the University of Minnesota. “One thing is to say this; another thing is to measure it,” Chubukov said.

Superconductivity is a phenomenon in which electricity flows without resistance from the material it moves through, so that no energy is lost in the process. It occurs when electrons (the negatively charged carriers of electricity) unite to form pairs, balancing each other’s properties in a way that allows them all to move in unison. The phase in which this happens is delicate, typically occurring only when a material is cooled to rock-bottom temperatures. But if wires could be engineered to act as superconductors at room temperature, experts say the lossless electrical transmission would greatly reduce global energy usage and usher in a host of new technologies, such as magnetically levitating vehicles and cheap water-purification systems. 

The force driving superconductivity is strongest in cuprates. As IBM researchers Georg Bednorz and K. Alexander Müller discovered in 1986 (in work that earned them Nobel Prizes the following year), cuprates superconduct at much higher temperatures than other materials, suggesting that their electrons are paired by a different and stronger glue. But cuprates must still be cooled below minus 100 degrees Celsius before they become superconductive. The glue must be further strengthened if superconductors’ operating temperatures are to be dialed up. For 30 years scientists have asked: What is the glue — or, more precisely, the quantum mechanical interaction between electrons — that causes superconductivity to arise in cuprates?...MUCH MORE