利用高频压缩性观测拓扑HgTe异质结中的Pankratov态
Observation of Volkov-Pankratov states in topological HgTe heterojunctions using high-frequency compressibility
摘要：
拓扑绝缘体维持狄拉克费米子表面态是体中能带反转的结果。这些态具有螺旋自旋极化和大费米速度的线性色散。在这篇文章中，我们报告了一组实验观察表明存在大量的表面状态。这些状态被限制在界面处，并且在高能量和/或高电场下支配平衡和输运性质。通过监测碲化汞拓扑绝缘体场效应电容器的交流导纳，我们获得了表面态的可压缩性和电导率在很宽的能量范围和电场。狄拉克表面态的特征在于可压缩性最小值，线性能量依赖性和高迁移率持续到远大于体的传输带隙的能量。新的功能，揭示了在高能量的签名，如电导峰，压缩性颠簸，一个强大的电荷亚稳性和霍尔电阻异常。这些功能点的存在下，激发的大规模的表面状态，负责一个强大的子带间散射的狄拉克状态和亚稳态散装载流子的成核。激发态的光谱与拓扑平凡半导体界面的唯象模型的预测一致。该模型考虑了有限的界面深度和电场的影响。激发态的存在对于理解拓扑相是必不可少的，并且为量子技术中的工程和开发拓扑资源开辟了一条路线。
英文摘要：
It is well established that topological insulators sustain Dirac fermion surface states as a consequence of band inversion in the bulk. These states have a helical spin polarization and a linear dispersion with large Fermi velocity. In this article we report on a set of experimental observations indicating the existence of massive surface states. These states are confined at the interface and dominate equilibrium and transport properties at high energy and/or high electric field. By monitoring the AC admittance of HgTe topological insulator field-effect capacitors, we access the compressibility and conductivity of surface states in a broad range of energy and electric fields. The Dirac surface states are characterized by a compressibility minimum, a linear energy dependence and a high mobility persisting up to energies much larger than the transport bandgap of the bulk. New features are revealed at high energies with signatures such as conductance peaks, compressibility bumps, a strong charge metastability and a Hall resistance anomaly. These features point to the existence of excited massive surface states, responsible for a strong intersubband scattering with the Dirac states and the nucleation of metastable bulk carriers. The spectrum of excited states agrees with predictions of a phenomenological model of the topological-trivial semiconductor interface. The model accounts for the finite interface depth and the effect of electric fields. The existence of excited topological states is essential for the understanding of topological phases and opens a route for engineering and exploiting topological resources in quantum technology.
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