16 June 2009

Novel material could revolutionize electronics

by Kate Melville

Physicists at the Department of Energy's (DOE) SLAC National Accelerator Laboratory and Stanford University have confirmed the existence of an exotic type of material that could provide dramatically faster, more efficient computer chips. Recently-predicted and now confirmed, bismuth telluride allows electrons on its surface to travel with no loss of energy at room temperatures and can be fabricated using existing semiconductor technologies.

It was previously predicted that several bismuth and antimony compounds would act as topological insulators at room-temperature. Now, the new paper confirms that prediction in bismuth telluride. Physicists Yulin Chen, Zhi-Xun Shen and colleagues recently tested the behavior of electrons in bismuth telluride and their results, appearing in Science Express, show a clear signature of what is called a topological insulator, a material that enables the free flow of electrons across its surface with no loss of energy.

The experimenters examined bismuth telluride using X-rays and saw the clear signature of a topological insulator. Not only that, the group discovered that the reality of bismuth telluride was even better than theory. "The theorists were very close," Chen said, "but there was a quantitative difference." The experiments showed that bismuth telluride could tolerate even higher temperatures than theorists had predicted. "This means that the material is closer to application than we thought," Chen said.

The material's surprising qualities are possible thanks to surprisingly well-behaved electrons. The quantum spin of each electron is aligned with the electron's motion - a phenomenon called the quantum spin Hall effect. This alignment is a key component in creating spintronics devices, new kinds of devices that go beyond standard electronics. "When you hit something, there's usually scattering, some possibility of bouncing back," explained co-researcher Xiaoliang Qi. "But the quantum spin Hall effect means that you can't reflect to exactly the reverse path." As a consequence, electrons flow without resistance. Put a voltage on a topological insulator, and this special spin current will flow without heating the material or dissipating.

But topological insulators aren't conventional superconductors, as they can only carry small currents; but they could pave the way for a paradigm shift in microchip development. "This could lead to new applications of spintronics, or using the electron spin to carry information," Qi said. "Whether or not it can build better wires, I'm optimistic it can lead to new devices, transistors, and spintronics devices."

Fortunately, bismuth telluride is fairly simple to grow and work with. "It's a three-dimensional material, so it's easy to fabricate with the current mature semiconductor technology. It's also easy to dope - you can tune the properties relatively easily," noted Chen.

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Source: Department of Energy's SLAC National Accelerator Laboratory