IN A DECEPTIVELY drab black crystal, physicists have stumbled upon a baffling behavior, one that appears to blur the line between the properties of metals, in which electrons flow freely, and those of insulators, in which electrons are effectively stuck in place. The crystal exhibits hallmarks of both simultaneously.
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This is a big shock, said Suchitra Sebastian, a condensed matter physicist at the University of Cambridge whose findings appeared this month in an advance online edition of the journal Science. Insulators and metals are essentially opposites, she said. But somehow, its a material thats both. Its contrary to everything that we know.
The material, a much-studied compound called samarium hexaboride or SmB6, is an insulator at very low temperatures, meaning it resists the flow of electricity. Its resistance implies that electrons (the building blocks of electric currents) cannot move through the crystal more than an atoms width in any direction. And yet, Sebastian and her collaborators observed electrons traversing orbits millions of atoms in diameter inside the crystal in response to a magnetic fielda mobility that is only expected in materials that conduct electricity. Calling to mind the famous wave-particle duality of quantum mechanics, the new evidence suggests SmB6 might be neither a textbook metal nor an insulator, Sebastian said, but something more complicated that we dont know how to imagine.
It is just a magnificent paradox, said Jan Zaanen, a condensed matter theorist at Leiden University in the Netherlands. On the basis of established wisdoms this cannot possibly happen, and henceforth completely new physics should be at work.
It is too soon to tell what, if anything, this new physics will be good for, but physicists like Victor Galitski, of the University of Maryland, College Park, say it is well worth the effort to find out. Oftentimes, he said, big discoveries are really puzzling things, like superconductivity. That phenomenon, discovered in 1911, took nearly half a century to understand, and it now generates the worlds most powerful magnets, such as those that accelerate particles through the 17-mile tunnel of the Large Hadron Collider in Switzerland.
Theorists have already begun to venture guesses as to what might be going on inside SmB6. One promising approach models the material as a higher-dimensional black hole.
