Beyond the High-Speed Hard Drive: Topological Insulators Open a Path to Room-Temperature Spintronics

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 Alexei 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.”The TI in question is bismuth selenide, Bi2Se3, on whose surface electrons can flow at room temperature, making it an attractive candidate for practical applications like spintronics devices, plus farther-out ones like quantum computers. Much of the research on electron-phonon coupling in Bi2Se3 was conducted at beamline 12.0.1 by a team including Fedorov, led by Tonica Valla of Brookhaven National Laboratory. Their results are reported in Physical Review Letters. …

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