Observation of Volkov-Pankratov states in topological HgTe heterojunctions using high-frequency compressibility

• Published in: Physical review. B 2017-11, Vol.96 (19)
• Main Authors: Inhofer, A, Tchoumakov, S, Assaf, B A, Fève, G, Berroir, J M, Jouffrey, V, Carpentier, D, Goerbig, M O, Plaçais, B, Bendias, K, Mahler, D M, Bocquillon, E, Schlereth, R, Brüne, C, Buhmann, H, Molenkamp, L W
• Format: Article
• Language: English
• Subjects: Charge transfer; Charge transport; Compressibility; Dependence; Electric fields; Electrical impedance; Heterojunctions; Mathematical models; Polarization (spin alignment); Resistance; Topological insulators
• ISSN: 2469-9950; 2469-9969
• DOI: 10.1103/PhysRevB.96.195104

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.