As anyone with a healthy fear of sticking their fingers into a plug socket will know, the behaviour of electrons in different materials varies dramatically. The first “electronic phases” of matter to be defined were the electrical conductor and insulator, and then came the semiconductor, the magnet and more exotic phases such as the superconductor. Recent work has, however, now uncovered a new electronic phase called a topological insulator. Putting the name to one side for now, the meaning of which will become clear later, what is really getting everyone excited is the behaviour of materials in this phase. Strangely, they can insulate on the inside but conduct on the outside – acting like a thick plastic cable covered with a layer of metal, except that the material is actually the same throughout. What is more, the conducting electrons arrange themselves into spin-up electrons travelling in one direction, and spin-down electrons travelling in the other; this “spin current” is a milestone in the realization of practical “spintronics”.
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