Universal Quantum Phenomenon Found in Strange Metals

A thin-film sample of a cuprate crystal called bismuth strontium calcium copper oxide mounted for resistivity measurements at the LNCMI in Toulouse, France.

A ubiquitous quantum phenomenon has been detected in a large class of superconducting materials, fueling a growing belief among physicists that an unknown organizing principle governs the collective behavior of particles and determines how they spread energy and information. Understanding this organizing principle could be a key into “quantum strangeness at its deepest level,” said Subir Sachdev, a theorist at Harvard University who was not involved with the new experiments.

The findings, reported today in Nature Physics by a team working at the University of Sherbrooke in Canada and the National Laboratory for Intense Magnetic Fields (LNCMI) in France, indicate that electrons inside a variety of ceramic crystals called “cuprates” seem to dissipate energy as quickly as possible, apparently bumping up against a fundamental quantum speed limit. And past studies, especially a 2013 paper in Science, found that other exotic superconducting compounds — strontium ruthenates, pnictides, tetramethyltetrathiafulvalenes and more — also burn energy at what appears to be a maximum allowed rate.

Strikingly, this speed limit is linked to the numerical value of Planck’s constant, the fundamental quantity of quantum mechanics representing the smallest possible action that can be taken in nature.

“When you see that, you know you’re touching on something very, very deep and fundamental,” said Louis Taillefer, a condensed matter physicist at Sherbrooke, who conducted the new cuprate experiment with his graduate student Anaëlle Legros, Cyril Proust of LNCMI, and 13 collaborators.

This energy-burning behavior occurs when the cuprates and other exotic compounds are in a “strange metal” phase, in which they resist the flow of electricity more than conventional metals. But when they’re cooled to a critical temperature, these strange metals transform into perfect, lossless conductors of electricity. Physicists have been struggling for 32 years to understand and control this powerful form of superconductivity, and the behavior of electrons in the preceding strange-metal phase is increasingly seen as a key part of the story.

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Images courtesy of: quantamagazine.org