Synopsis: Bulk carriers cannot take the strain

Mercury telluride—a semimetal in bulk—shows signs of becoming a topological insulator under strain.

Mercury Telluride ($\text{HgTe}$) has an “inverted” band structure, where the valence band rises above the conduction band. As such, it was the first material to be identified as a candidate topological insulator—an exotic insulator that has conducting states along its surface or edge that are unusually robust against defects and scattering.

Since bulk $\text{HgTe}$ is a semimetal, the properties of a topological insulator can only be observed by opening up a gap in its band structure. Accordingly, the conducting surface states were first found along the edges of two-dimensional $\text{HgTe}$ quantum wells, where the reduced symmetry of the well lifted the semimetallic character of the band structure. Analogously, three-dimensional (3D) topological insulating behavior should be observable in the surface states of bulk $\text{HgTe}$, provided a gap in the bulk electronic structure can be induced.

Now, Christoph Brüne and colleagues at Universität Würzburg, Germany, and Stanford University, US, report in Physical Review Letters that they have observed quantized Hall conductance on the surface of a $70$-$\text{nm}$-thick sample of $\text{HgTe}$—a signature that this sample is a 3D topological insulator. When deposited on $\text{CdTe}$, the lattice mismatch between the $\text{HgTe}$ film and substrate creates a strain that opens the necessary band gap. In contrast to more-studied 3D topological insulators, such as ${\text{Bi}}_{2}{\text{Se}}_{3}$, ${\text{Sb}}_{2}{\text{Te}}_{3}$, and ${\text{Bi}}_{2}{\text{Te}}_{3}$, bulk $\text{HgTe}$ has a very low background doping, so that the strained bulk layer corresponds to an intrinsic topological insulator.

The next challenge for the group is to observe the exotic effects predicted to show up in the topologiocal surface states, such as quantized topological magnetoelectricity, the existence of magnetic monopoles, and the creation of Majorana bound states. – Daniel Ucko

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