Sunday, July 26, 2009

New exotic material could revolutionize electronics

Announcements of "breakthroughs" like this often don't pan out. This one might be a little more promising. It involves confirmation of a new, fairly simple material – bismuth telluride (Bi2Te3), a compound of bismuth and tellurium – that exhibits what is known as the "quantum spin Hall effect" and could have a dramatic impact on electronics, computing, and other significant areas of technology.

One reason for the optimism is that the technology for working with bismuth telluride is similar to that for commonplace semiconductors like silicon.

New exotic material could revolutionize electronics (6/15/09)
SLAC National Accelerator Laboratory and Stanford University have confirmed the existence of a type of material that could one day provide dramatically faster, more efficient computer chips.

Recently-predicted and much-sought, the material allows electrons on its surface to travel with no loss of energy at room temperatures and can be fabricated using existing semiconductor technologies. Such material could provide a leap in microchip speeds, and even become the bedrock of an entirely new kind of computing industry based on spintronics, the next evolution of electronics.

Materials with the properties of bismuth telluride have been predicted theoretically, and in this case the predictions have been pretty accurate. In other words, physicists already have a good understanding of the material's properties.
This magic is 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 theorist Xiaoliang Qi. "But the quantum spin Hall effect means that you can't reflect to exactly the reverse path." As a dramatic 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.

Practical implementations of spintronics have been avidly sought, because the technolgy takes advantage for the first time of an electron's spin, as opposed to its electric charge. This may enable the manufacture of much faster and denser forms of digital information storage devices, and even more exotic things like quantum computers.

The quantum spin Hall effect mentioned above is a quantum version of a non-quantum effect, the spin Hall effect, known for about ten years. That, in turn, is an analog of the classical Hall effect, which has been known since 1879.

In the classical Hall effect, a voltage difference is produced in an electrical conductor transverse to an electrical current in the conductor. A magnetic field is also produced perpendicular to the current.

In the quantum spin Hall effect, there is also an electric current, and in fact electrons flow without dissipating heat. Consequently, for example, transistors that take advantage of the effect could be much more efficient than existing semiconductor transistors.

Importantly, in bismuth telluride, the effect occurs at much higher temperatures than those at which known superconducting materials work. This will make practical applications much easier.

The predictions on which the discovery is based are quite recent, having been published only in May of this year – see Super-efficient Transistor Material Predicted.

Research abstract:

Experimental Realization of a Three-Dimensional Topological Insulator, Bi2Te3
Three-dimensional topological insulators are a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. By investigating the surface state of Bi2Te3 with angle-resolved photoemission spectroscopy, we demonstrate that the surface state consists of a single nondegenerate Dirac cone. Furthermore, with appropriate hole doping, the Fermi level can be tuned to intersect only the surface states, indicating a full energy gap for the bulk states. Our results establish that Bi2Te3 is a simple model system for the three-dimensional topological insulator with a single Dirac cone on the surface. The large bulk gap of Bi2Te3 also points to promising potential for high-temperature spintronics applications.

Further reading:

Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface – abstract of May 2009 research paper in Nature Physics reporting predictions of topological insulators

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