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c0lo writes:

Quanta Magazine reports:

"This is a big shock," said Suchitra Sebastian, a condensed matter physicist at the University of Cambridge whose findings appeared today in an advance online edition of the journal Science. Insulators and metals are essentially opposites, she said. "But somehow, it's a material that's both. It's 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 atom's 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 field — a mobility that is only expected in materials that conduct electricity.

...

Amazingly, the observed deviation from the Lifshitz-Kosevich formula was presaged in 2010 by Sean Hartnoll and Diego Hofman, both then at Harvard University, in a paper that recast strongly correlated materials as higher-dimensional black holes, those infinitely steep curves in space-time predicted by Albert Einstein. In their paper, Hartnoll and Hofman investigated the effect of strong correlations in metals by calculating corresponding properties of their simpler black hole model — specifically, how long an electron could orbit the black hole before falling in. "I had calculated what would replace this Lifshitz-Kosevich formula in more exotic metals," said Hartnoll, who is now at Stanford University. "And indeed it seems that the form [Sebastian] has found can be matched with this formula that I derived."

Other links:

• topological insulators (insulator in its interior but whose surface contains conducting states),
• SmB₆ as topological insulator,
• arxiv entry of the cited article