Electronic Properties of thin films of strained HgTe.

Strained HgTe is a small band gap 3D topological insulator. Bulk HgTe therefore has 2D Dirac-like surface states that are protected by the time-reversal symmetry. Because of the small gap, the surface state localization length is however fairly large and the separation between bulk and surface effects in the thin film samples that are available experimentally is not always clear. With this motivation we have examined the multi-band envelope description of strained HgTe thin films with thicknesses between 10 and 100 nm, accounting carefully for both electrostatic and exchange-correlation interaction effects, to provide a basis for interpreting measurements of surface and bulk state electronic properties that are tuned by varying gate voltages. We find that the films respond like semiconductors to gates that induce electron gases, but like metals to gates that induce hole gases. We are able to provide a simple explanation for recent experiments that characterize HgTe thin films using capacitance and thermopower measurements. In particular, we find that it is not necessary to invoke phonon drag to explain the large difference in thermopower between electron and hole cases.