Strange new materials experimentally identified just a few years ago are now driving research in condensed-matter physics around the world. First theorized and then discovered by researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and their colleagues in other institutions, these "strong 3-D topological insulators" -- TIs for short -- are seemingly mundane semiconductors with startling properties. For starters, picture a good insulator on the inside that's a good conductor on its surface -- something like a copper-coated bowling ball.
A topological insulator's surface is not an ordinary metal, however. The direction and spin of the surface electrons are locked together and change in concert. And perhaps the most surprising prediction is that the surface electrons cannot be scattered by defects or other perturbations and thus meet little or no resistance as they travel. In the jargon, the surface states remain "topologically protected" -- they can't scatter without breaking the rules of quantum mechanics.
"One way that electrons lose mobility is by scattering on phonons," says Alexis Fedorov, staff scientist for beamline 12.0.1 of Berkeley Lab's Advanced Light Source (ALS). Phonons are the quantized vibrational energy of crystalline materials, treated mathematically as particles. "Our recent work on a particularly promising topological insulator shows that its surface electrons hardly couple with phonons at all. So there's no impediment to developing this TI for spintronics and other applications."
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