Technology

Topological insulators

Print edition : January 10, 2014

The crystal structure and successfully grown single crystals of BiTeCl. Photo: Tokyo Institute of Technology

A NEW compound developed at the Tokyo Institute of Technology (Tokyo Tech) shows highly unusual conducting properties that could be used in future electronic components.

Ordinary insulating solids, such as diamond, have energy bands that are fully occupied by electrons. The conducting band is so far away from the valence band in diamond that electrons do not have sufficient energy to move across the “band gap” and, therefore, no electric current can be carried.

In recent years, researchers have become interested in materials called topological insulators (TIs), which act as insulators on the inside but are highly conductive on their surfaces. In TIs, there is an exceptionally strong interaction between an electron’s spin and its motion. This inverts the energy gap between occupied and empty states so that electrons at the surface can flow across the gap. These properties are intrinsic to the material, meaning a TI remains conductive even if its surface is not perfect.

An international team of scientists from Japan, the United Kingdom and the United States, led by Takao Sasagawa at Tokyo Tech, successfully developed a new TI from bismuth, tellurium and chlorine (BiTeCl), which exhibits many topological effects that have not been seen experimentally before.

The work was published in a recent issue of the journal Nature Physics.

“The metallic surface state of a TI is similar to graphene in that the electron mobility is remarkably high,” explained Sasagawa, whose team optimised the laboratory growing conditions to produce single crystals of BiTeCl. The team then split each single crystal to obtain two different surfaces, one Te and one Cl, and observed their electronic structures using spectroscopy.

diode

The composition of the TI’s top and bottom crystal surfaces are such that their charge carriers are opposite, leading to polarisation. The TI can therefore be used as a diode, allowing current flow in only one direction. It also exhibits pyroelectric capabilities, meaning that it can generate a temporary voltage when heated or cooled.

BiTeCl shows promise as a platform for other topological phenomena and may have applications at high temperatures. The discovery could also have significant implications for the future development of quantum-based technologies, Sasagawa said.

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