Exciting New Research on Topological Insulators

Topological insulators have become one of the hottest topics in physics. These new materials act as both insulators and conductors, with their interior preventing the flow of electrical currents while their edges or surfaces allow the movement of a charge.

Notably for electronic devices, the surfaces of topological insulators can also enable the transport of spin-polarized electrons, and prevent the "scattering" typically associated with power consumption. This is important because with scattering, electrons deviate from their trajectory, resulting in dissipation.

Expanding the horizons for electronic devices, quite literally, researchers from the U.S. and Australia have shown that using a ‘nanoribbon’ approach to developing topological insulators can expand the surface area of these materials and can thus allow them to be more easily turned on and off to control surface state and conduction.

Topological insulator nanoribbons provide larger surface-to-volume ratios, which in turn allow researchers to manipulate the surfaces by external means. Till date, researchers have faced many challenges in working with the material mainly due to small surfaces.

The work, done by UCLA's Henry Samueli School of Engineering and Applied Science and Australia's University of Queensland’s materials division, demonstrated control over the surface-conduction channels in topological insulator nanoribbons made of bismuth telluride.

"Our finding enables a variety of opportunities in building potential new-generation, low-dissipation nanoelectronic and spintronic devices, from magnetic sensing to storage," said Dr. Kang L. Wang, who led the team’s work. Dr. Wang is the Raytheon Professor of Electrical Engineering at UCLA Engineering.

Bismuth telluride is a known thermoelectric material, and has been predicted to be an excellent topological insulator. Topological insulator nanoribbons offer a new approach to working with bismuth telluride and have also been predicted to be an excellent topological insulator.

Dr. Wang and his team used thin bismuth telluride nanoribbons as conducting channels in field-effect transistor structures. These rely on an electric field to control the Fermi level and hence the conductivity of a channel. With this approach, researchers showed for the first time a unanimous approach for controlling surface states in topological insulators.

"We have demonstrated a clear surface conduction by partially removing the bulk conduction using an external electric field," said Faxian Xiu, a UCLA staff research associate and lead author of the study. By properly tuning the gate voltage, very high surface conduction was achieved, up to 51 percent, which represents the highest values in topological insulators, he added.

"This research is very exciting because of the possibility to build nanodevices with a novel operating principle," said Wang, who is also associate director of the California NanoSystems Institute (CNSI) at UCLA. "Very similar to the development of graphene, the topological insulators could be made into high-speed transistors and ultra–high-sensitivity sensors."

The new findings shed light on the controllability of the surface spin states in topological insulator nanoribbons and demonstrate significant progress toward high surface electric conditions for practical device applications. The next step for Wang's team is to produce high-speed devices based on their discovery.

"The ideal scenario is to achieve 100 percent surface conduction with a complete insulating state in the bulk," Xiu said. "Based on the current work, we are targeting high-performance transistors with power consumption that is much less than the conventional complementary metal-oxide semiconductors (CMOS) technology used typically in today's electronics."

The work appears in Nature Nanotechnology in the Feb 13, 2011 issue.

The research on topological insulators was pioneered by FENA's Shoucheng Zhang, a professor of physics at Stanford University. UCLA/Australia team members included: Jin Zou, a professor of materials engineering at the University of Queensland; and Yong Wang, a Queensland International Fellow; and Zou's team at the division of materials at the University of Queensland.

The study was funded by the Focus Center Research Program—Center on Functional Engineered Nano Architectonics (FENA) at UCLA Engineering; the U.S. Defense Advanced Research Projects Agency (DARPA); and the Australian Research Council.


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