Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa, USA.
Sci Rep. 2012;2:669. doi: 10.1038/srep00669. Epub 2012 Sep 18.
Topological insulators show unique properties resulting from massless, Dirac-like surface states that are protected by time-reversal symmetry. Theory predicts that the surface states exhibit a quantum spin Hall effect with counter-propagating electrons carrying opposite spins in the absence of an external magnetic field. However, to date, the revelation of these states through conventional transport measurements remains a significant challenge owing to the predominance of bulk carriers. Here, we report on an experimental observation of Shubnikov-de Haas oscillations in quantum capacitance measurements, which originate from topological helical states. Unlike the traditional transport approach, the quantum capacitance measurements are remarkably alleviated from bulk interference at high excitation frequencies, thus enabling a distinction between the surface and bulk. We also demonstrate easy access to the surface states at relatively high temperatures up to 60 K. Our approach may eventually facilitate an exciting exploration of exotic topological properties at room temperature.
拓扑绝缘体具有独特的性质，源于无质量的狄拉克型表面态，这些表面态受时间反演对称性保护。理论预测，在没有外磁场的情况下，表面态表现出量子自旋霍尔效应，具有相反自旋的反向传播电子。然而，迄今为止，由于体载流子的主导地位，通过传统的输运测量来揭示这些状态仍然是一个重大挑战。在这里，我们报告了在量子电容测量中观察到的 Shubnikov-de Haas 振荡的实验结果，这些振荡源于拓扑螺旋态。与传统的输运方法不同，量子电容测量在高激发频率下显著减轻了体干扰，从而能够区分表面和体。我们还证明了在相对较高的温度（高达 60 K）下很容易进入表面态。我们的方法最终可能有助于在室温下探索奇异的拓扑性质。
