Maybe not so old after all

Abstract

of electrons is linked to their spin (intrinsic angular momentum), and this coupling acts to delocalize electrons. Topological-insulator surfaces are effectively at the limit of infinite spin-orbit coupling, and no localization can occur 8 until the impurity density is so high as to destroy the bulk insulator. The origin of this anti-localization effect is that an electron moving at these surfaces is never perfectly turned back by an impurity, unlike Brave Sir Robin in the film Monty Python and the Holy Grail: "When danger reared its ugly head,/He bravely turned his tail and fled. " Although the impurity can change the electron's direction and the conductivity is finite-not infinite as in a superconductor-a 180° about-face is impossible (Fig. 1). This suppression of perfect backscattering turns out to change the effect of coherence between multiple scattering events from localizing to anti-localizing. The anti-localization effect is observed by Roushan et al. 1 in surface microscopy images of the topological insulator Bi 1−x Sb x using the interference of electron waves reflected from a defect or impurity in the material. An intuitive picture for this technique is that the wave pattern from a pebble falling into a pond could be used to study not only the pebble but also the nature of water waves. Recent observation 9 of a simpler anti-localization effect in the electrical conductance along the one-dimensional edge of a planar insulator was the first evidence for the 'quantum spin Hall' phase, which is a topological insulator in two dimensions rather than three. A weaker form of this anti-localization effect can be observed in graphene if the scattering potential is very smooth, but ultimately electrons in graphene are localized by impurities. Another property of graphene is that it is a semi-metal: unlike in an insulator, graphene has no gap in the energy of electronic states, but the density of metallic electrons vanishes in pure graphene. In most topological insulators, the bulk chemistry leads to a metallic surface rather than a semi-metallic one. But by precise chemical modification of both the bulk and surface of a bismuth-tellurium-based topological insula-tor, Bi 2 Te 3 , Hsieh et al. 2 show that it is possible to bring the surface to the semi-metallic state. They also show that the semi-metallic state retains its strong coupling of orbital motion and spin by examining the spin of electrons moving in each direction along the surface. Hsieh and colleagues' discovery suggests that topological-insulator surfaces may compete with graphene in several potential electronic applications. One advantage of tuning the material to the semi-metallic state is that an applied electrical field can create charge carriers of either positive or negative charge (holes and electrons, respectively). This electrical control is possible because, in graphene and the new materials synthesized by Hsieh et al. 2 , the semi-metallic electronic structure sits at the boundary between hole-like and electron-like metals. In current silicon-based electronics, control of the charge of the majority carriers is accomplished permanently by chemical manipulation. Being able to switch electrically between electron and hole carriers leads to new flexibility in device design. The result of Hsieh et al. 2 also has implications for fundamental science. One example is that using an electrical field to create electrons and holes at the top and bottom surfaces of a thin layer of a topological insulator is predicted to lead to a superconductor-like phase driven by electrostatic binding of electrons and holes 10. The ability to create semi-metals at topologi-cal-insulator surfaces and the demonstration 1 of their invulnerability to localization are key steps in unlocking the scientific and technological potential of these remarkable materials. ■ Joel Moore is in the