Synopsis: Not your typical insulator

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Illustration: Alan Stonebraker

The realization that even mundane band insulators can have unusual conducting edge states similar to what is seen in the quantum Hall effect, but without an external magnetic field, came as a huge surprise to the condensed matter community [1]. A number of studies have explored the unique properties of these so-called topological insulators. For example, while it is well known that in two dimensions a metal becomes an insulator in the presence of disorder, it is still a matter of debate whether this transition in a topological insulator occurs in the same way.

Writing in Physical Review B, Hideaki Obuse of RIKEN in Wako, Japan, and colleagues in the U.S. and Switzerland show how the topological state betrays itself after all. In conventional two-dimensional metals, the wave functions of the electrons at the metal-insulator transition are neither localized or delocalized, but are instead described by universal, largely model-independent critical exponents. While certain classes of models have not seen any difference in the scaling exponent of the diverging localization length in a topological insulator compared to a conventional metal, Obuse et al. show that even in this class of models, near the edge of a two-dimensional topological insulator the scaling properties of the electron wave functions are unique.

The authors argue that the distinction they predict can be directly tested in recently discovered two-dimensional topological insulator HgTe/(Hg, Cd)Te quantum wells. – Ashot Melikyan

[1] S. C. Zhang, Physics 1, 6 (2008).