Observing electrons in topological insulators could lead to new electronic devices

The team created three dimensional 'movies' that capture increments of time down to the level of femtoseconds and catch the motions of electrons as they scatter in response to a short pulse of light. With TIs, the bulk of the material acts as an insulator, but the surface is even more conductive than normal metals, allowing electrons to travel at almost the speed of light and be unaffected by impurities in the material. TIs could therefore enable new electronic circuits and data storage devices. The technique developed by the researchers uses a short pulse of laser light to energise the material, causing electrons to scatter, and a second, slightly delayed pulse to illuminate it and produce an image. The process is then repeated, with the second laser pulse delayed by ever increasing increments of just a few femtoseconds. Each resulting image shows the response of the electrons to the beam after a corresponding interval. These images can then be assembled into a movie that shows how the response changes with time. By using this technique, the researchers say they have already discovered interactions between a TI's surface and bulk electrons that had never been seen before, revealing new details of how TIs work. "With this 3d movie, in real time we can visualise how one population of electrons [those on the surface] scatters into the other population [inside the material]," said assistant professor of physics Nuh Gedik. "This is very important to understand." The researchers found that the interaction between the two is mediated by sound waves, and that this interaction happens much more intensely at high temperatures. According to the team, understanding this interaction will guide future work and applications of these materials.