Jost Andreas, Bendias Michel, Böttcher Jan, Hankiewicz Ewelina, Brüne Christoph, Buhmann Hartmut, Molenkamp Laurens W, Maan Jan C, Zeitler Uli, Hussey Nigel, Wiedmann Steffen
High Field Magnet Laboratory-European Magnetic Field Laboratory, Radboud University, 6525 ED Nijmegen, The Netherlands.
Institute for Molecules and Materials, Radboud University, 6525 ED Nijmegen, The Netherlands.
Proc Natl Acad Sci U S A. 2017 Mar 28;114(13):3381-3386. doi: 10.1073/pnas.1611663114. Epub 2017 Mar 9.
Topological insulators are a new class of materials with an insulating bulk and topologically protected metallic surface states. Although it is widely assumed that these surface states display a Dirac-type dispersion that is symmetric above and below the Dirac point, this exact equivalence across the Fermi level has yet to be established experimentally. Here, we present a detailed transport study of the 3D topological insulator-strained HgTe that strongly challenges this prevailing viewpoint. First, we establish the existence of exclusively surface-dominated transport via the observation of an ambipolar surface quantum Hall effect and quantum oscillations in the Seebeck and Nernst effect. Second, we show that, whereas the thermopower is diffusion driven for surface electrons, both diffusion and phonon drag contributions are essential for the hole surface carriers. This distinct behavior 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 with electrons. These findings show that the metallic surface states in topological insulators can exhibit both strong electron-hole asymmetry and a strong deviation from a linear dispersion but remain topologically protected.
拓扑绝缘体是一类新型材料，其体相绝缘而表面态具有拓扑保护的金属特性。尽管人们普遍认为这些表面态呈现出狄拉克型色散，在狄拉克点上下是对称的，但费米能级两侧的这种精确等效性尚未得到实验证实。在此，我们对三维拓扑绝缘体——应变碲化汞进行了详细的输运研究，这对这一主流观点提出了强烈挑战。首先，通过观测双极性表面量子霍尔效应以及塞贝克效应和能斯特效应中的量子振荡，我们证实了仅存在表面主导的输运。其次，我们表明，虽然表面电子的热电势由扩散驱动，但对于空穴表面载流子而言，扩散和声子拖拽贡献都至关重要。表面态色散关系强烈偏离线性，空穴的色散比电子平坦得多，这解释了热电响应中的这种独特行为。这些发现表明，拓扑绝缘体中的金属表面态既能表现出强烈的电子 - 空穴不对称性，又能强烈偏离线性色散，但仍受到拓扑保护。
