Dec 05, 2011
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| This diagram illustrates how lasers can be used to control an electric
current on these new materials. Electrons (blue spheres) travel, as if
on a highway, in different directions, with their axis of spin (arrows)
aligned differently according to the direction of travel. A circularly
polarized laser beam (left) affects only electrons going in one
direction, removing them from the flow, leaving a net flow — an electric
current — going the other way. Photo: Gedik Group |
Exotic materials called topological insulators, discovered just a few years ago, have yielded some of their secrets to a team of MIT researchers. For the first time, the team showed that light can be used to obtain information about the spin of electrons flowing over the material’s surface, and has even found a way to control these electron movements by varying the polarization of a light source.
The materials could open up possibilities for a new kind of devices based on spintronics, which makes use of a characteristic of electrons called spin, instead of using their electrical charge the way electronic devices do. It could also allow for much faster control of existing technologies such as magnetic data storage.
Topological insulators are materials that possess paradoxical properties. The three-dimensional bulk of the material behaves just like a conventional insulator (such as quartz or glass), which blocks the movement of electric currents. Yet the material’s outer surface behaves as an extremely good conductor, allowing electricity to flow freely.
The key to understanding the properties of any solid material is to analyze the behavior of electrons within the material — in particular determining what combinations of energy, momentum and spin are possible for these electrons, explains MIT assistant professor of physics Nuh Gedik, senior author of two recent papers describing the new findings. This set of combinations is what determines a material’s key properties — such as whether it is a metal or not, or whether it is transparent or opaque. “It’s very important, but it’s very challenging to measure,” Gedik says.
The traditional way of measuring this is to shine a light on a chunk of the solid material: The light knocks electrons out of the solid, and their energy, momentum and spin can be measured once they are ejected. The challenge, Gedik says, is that such measurements just give you data for one particular point. In order to fill in additional points on this landscape, the traditional approach is to rotate the material slightly, take another reading, then rotate it again, and so on — a very slow process.
Gedik and his team, including graduate students Yihua Wang and James McIver, and MIT Pappalardo postdoctoral fellow David Hsieh, instead devised a method that can provide a detailed three-dimensional mapping of the electron energy, momentum and spin states all at once. They did this by using short, intense pulses of circularly polarized laser light whose time of travel can be precisely measured.
By using this new technique, the MIT researchers were able to image how the spin and motion are related, for electrons travelling in all different directions and with
different momenta, all in a fraction of the time it would take using alternative methods, Wang says. This method was described in a paper by Gedik and his team that appeared Nov. 11 in the journal Physical Review Letters.
In addition to demonstrating this novel method and showing its effectiveness, Gedik says, “we learned something that was not expected.” They found that instead of the spin being precisely aligned perpendicular to the direction of the electrons’ motion, when the electrons moved with higher energies there was an unexpected tilt, a sort of warping of the expected alignment. Understanding that distortion “will be important when these materials are used in new technologies,” Gedik says.
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