Probing AC electron compressibility of 3D HgTe and Bi2Se3 topological insulators at high electric fields : Evidence for excited massive surface states
Andreas Inhofer (LPA)

This thesis discusses the electronic compressibility of two representative three dimensional topological insulators : Strained mercury telluride (HgTe) and bismuth selenide (Bi2Se3).
I present low temperature phase-sensitive electron admittance data over a broad frequency range.
This allows to extract the quantum capacitance related to the density of states and the resistivity of the investigated materials.
I show that the response of an intrinsic topological insulator is dominated by topological surface states over a large energy range
exceeding the bulk material’s transport gap. This regime, named “Dirac screening” is characterized by an electron compressibility
proportional to the surface Fermi level and a high mobility. Subsequently, I investigate the limits of this regime.
At high energy and large perpendicular electric fields I observe the population of excited massive surface states.
Experimentally, these manifest themselves in multiple signatures : A drop in the electronic diffusion constant,
a peak in the conductivity, appearance of a second carrier type in magneto-transport and meta-stability in the charge-voltage relation.
A theoretical model based on a quasi-relativistic treatment of the surface Hamiltonian is presented.
It allows to identify the electric field and energy dependence of the massive surface states.
This thesis is complemented by experimental results on Bi2Se3 grown on boron nitride, where I demonstrate
the importance of clean surfaces for the study of electronic properties in topological insulators.

mettant en évidence l’importance de la pureté des interfaces d’isolants topologiques