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SIGLECNRS 14802 E / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
Observation of Volkov-Pankratov states in topological HgTe heterojunctions using high-frequency compressibility
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