Observation of Volkov-Pankratov states in topological HgTe heterojunctions using high-frequency compressibility
Observation of Volkov-Pankratov states in topological HgTe heterojunctions using high-frequency compressibility
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DOI：
10.1103/physrevb.96.195104
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发表时间：
2017-11-01
影响因子：
3.7
通讯作者：
Molenkamp, L. W.
中科院分区：
文献类型：
--
作者：
Inhofer, A.;Tchoumakov, S.;Molenkamp, L. W.
作为整体能带反转的结果，拓扑绝缘体支持狄拉克-费米子表面态，这是公认的。这些态具有螺旋自旋极化和大费米速度的线性色散。我们报告了一组支持附加质量表面态存在的实验观测结果。这些态还受到拓扑平凡的半导体异质结处的能带反转的限制。虽然在了解这种连接的拓扑性质之前，Volkov和Pankratov(VP)首次提出了它们，但它们并没有被实验识别。在这里，我们确定了它们在高电场输运性质上的特征。通过监测HgTe拓扑绝缘子场效应电容的交流导纳，我们得到了表面态在广泛的能量和电场范围内的可压缩性和电导率。Dirac态的特征是可压缩性最小，与能量线性相关，以及高迁移率，持续到能量远大于块体的输运带隙。在较高的能量下，我们在电导、电荷亚稳性和霍尔电阻方面观察到了多种反常行为，表明大量表面态在输运散射和电荷转移中对体相的贡献。这些反常的光谱与光滑拓扑异质结中VP态的唯象模型的预言一致。该模型考虑了有限界面深度、包括狄拉克屏蔽在内的电场的影响，并预测了第一VP态的能量。大质量表面态是拓扑异质结的一个标志，它的理解对于输运研究和应用是至关重要的。
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. We report on a set of experimental observations supporting the existence of additional massive surface states. These states are also confined by the band inversion at a topological-trivial semiconductor heterojunction. While first introduced by Volkov and Pankratov (VP) before the understanding of the topological nature of such a junction, they were not experimentally identified. Here we identify their signatures on transport properties at 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 energies and electric fields. The Dirac states are characterized by a compressibility minimum, a linear energy dependence, and a high mobility persisting up to energies much larger than the transport band gap of the bulk. At higher energies, we observe multiple anomalous behaviors in conductance, charge metastability, and Hall resistance that point towards the contribution of massive surface states in transport scattering and charge transfer to the bulk. The spectrum of these anomalies agrees with predictions of a phenomenological model of VP states in a smooth topological heterojunction. The model accounts for the finite interface depth, the effect of electric fields including Dirac screening, and predicts the energy of the first VP state. The massive surface states are a hallmark of topological heterojunctions, whose understanding is crucial for transport studies and applications.