Electron–hole asymmetry of the topological surface states in strained HgTe
Researchers from the University of Würzburg (EP3 and TP4) and the High Field Magnet Laboratory (HFML) Nijmegen have investigated the electrical and thermal transport properties of the three-dimensional topological insulator strained HgTe. They demonstrate the existence of exclusively surface-dominated transport and find in thermopower experiments that the metallic surface states, which are widely assumed to display a Dirac-type dispersion, exhibit a strong deviation from a linear dispersion across the Fermi level but remain topologically protected. The results have been published in the Proceedings of the National Academy of Science (PNAS).
Topological insulators are a new class of materials with an insulating bulk and topologically protected metallic surface states, thus distinct from conventional states of matter such as metals, semiconductors and insulators.
In this project, the charge carrier properties of the surface states in a topological insulator have been investigated by tuning the Fermi energy in the bulk band gap to have access to both surface electron and hole charge carriers (Figure 1). It has been found that the thermoelectric power of the election-like surface states is diffusion driven in contrast to hole-like surface carriers where both diffusion thermopower and phonon drag are essential. This is reflected in the magnitude of the signal and its temperature dependence (see Figure 2a and b). This distinct behaviour in the thermoelectric response is explained by a strong deviation from the linear dispersion relation for the surface states, with a much flatter dispersion for holes compared to electrons making three-dimensional topological insulators distinct from other Dirac systems such as graphene with a symmetric dispersion for electrons and holes.
The exclusively surface-dominated transport manifests itself via the observation of an ambipolar surface quantum Hall effect (Figure 2c), a phenomenon that can only be observed if charge carriers are confined to two dimensions.
Figure 2: (a) Thermopower Sxx and Nernst effect Sxy at 0.2 T at 1.5 K. (b) Sxx as a function of temperature for electrons and holes (strong phonon peak for holes at 12 K). (c) Ambipolar surface quantum Hall effect (integer numbers are filling factors).
Electron–hole asymmetry of the topological surface states in strained HgTe, A. Jost, M. Bendias, J. Bottcher, E.M. Hankiewicz, C. Brune, H. Buhmann, L. W. Molenkamp, J. C. Maan, U. Zeitler, N.E. Hussey and S. Wiedmann, PNAS Early Edition (2017), 1 - 6