Research Highlight

Subject Category: Inorganic, composite and hybrid materials

NPG Asia Materials research highlight; doi:10.1038/asiamat.2010.188
Published online 29 November 2010

Topological insulators: Ternary triumph

A topological insulator state has been observed in a ternary chalcogenide.

Topological insulators are one of the materials of the moment. In these unusual substances, the bulk behaves like an insulator, whereas the surface acts like a conductor. In addition to a host of practical applications, topological insulators are particularly important because they enable scientists to investigate a plethora of exotic states. Akio Kimura from Hiroshima University in Japan and an international team of scientists from Japan, Russia and Spain have now observed a topological insulator phase in a new family of materials — ternary compounds based on thallium.1 These compounds are anticipated to produce a more robust topological state than has been achieved in the previously investigated binary compounds.

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Fig. 1: Electrons in topological insulators are able to flow only at the edges of the material, not in the bulk.
© 2010 K. Kuroda

The electronic properties of a material are largely determined by its intrinsic electron band structure, which describes the energy levels an electron needs to travel through the medium. Insulators, for example, are classified by a wide band of ‘forbidden’ electron energies. At the surface of a topological insulator, however, electron states exist within this gap. “Importantly, these electrons are both ‘massless’ and magnetic,” explains Kimura. “The beauty of these states is that they are robust against the deleterious effects of scattering by impurities.”

Much work has been done on two-dimensional materials such as graphene and HgTe/CdTe quantum wells, and previous studies on three-dimensional topological insulators have focused on binary compounds of the element bismuth, particularly bismuth selenide. However, the band structure of bismuth selenide is such that the electrons can easily scatter from the surface state to a bulk state, destroying the topological transport. Instead, Kimura and his co-workers investigated the ternary compound thallium bismuth selenide (TlBiSe2) and confirmed that it is a more reliable topological insulator.

The researchers examined their samples by a technique known as angle-resolved photoemission spectroscopy using radiation from a tunable synchrotron. The photo-excited electrons produced in these experiments provided key information about the material’s electronic band structure, specifically that the surface state has a signature feature known as a ‘Dirac cone’. “We confirmed that the surface Dirac cone is well isolated from the bulk band structure,” says Kimura, “which means that surface electron current cannot be absorbed deeper in the material.”

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References

  1. Kuroda, K., Ye, M., Kimura, A., Eremeev, S.V., Krasovskii, E.E., Chulkov, E.V., Ueda, Y., Miyamoto, K., Okuda, T., Shimada, K., Namatame, H. & Taniguchi, M. Experimental realization of a three-dimensional topological insulator phase in ternary chalcogenide TlBiSe2. Phys. Rev. Lett. 105, 146801 (2010). | Article | PubMed | ChemPort |

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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