Topological insulators are exotic materials, discovered just a few years ago, that hold great promise for new kinds of electronic devices. The unusual behavior of electrons within them has been very difficult to study, but new techniques developed by a team of researchers at MIT could help unlock the mysteries of exactly how electrons move and react in these materials, opening up new possibilities for harnessing them.
For the first time, the MIT team has managed to create three-dimensional “movies” of electron behavior in a topological insulator, or TI. The movies can capture vanishingly small increments of time — down to the level of a few femtoseconds, or millionths of a billionth of a second — so that they can catch the motions of electrons as they scatter in response to a very short pulse of light.
Electrons normally have mass, just like many other fundamental particles, but when moving along the surface of TIs they move as if they were massless, like light — one of the extraordinary characteristics that give these new materials such promise for new technologies.
The dramatic new results are published this week in the journal Physical Review Letters, in a paper by MIT graduate student Yihua Wang, assistant professor of physics Nuh Gedik and six other researchers.
TIs are a class of materials with seemingly contradictory characteristics: The bulk of the material acts as an insulator, almost completely blocking any flow of electrons. But the surface of the material behaves as a very good conductor, like a metal, allowing electrons to travel freely. In fact, the surface is even more conductive than normal metals — allowing electrons to travel at almost the speed of light and to be unaffected by impurities in the material, which normally hinder their motion.
Because of these characteristics, TIs are seen as a promising new material for electronic circuits and data-storage devices. But developing such new devices requires a better understanding of exactly how electrons move around on and inside the TI, and how the surface electrons interact with those inside the material.
Last year, Gedik, Wang and others published a paper describing a new technique that allowed them to create instantaneous three-dimensional still images of the energy, momentum and spin of electrons within topological insulators. Until then, the only way to create such 3-D images was through a slow and painstaking technique that could reveal one point at a time on the surface. The material would then be rotated slightly so that an image of the next point could be taken, and so on, gradually building up the full image.