Dirac cone shift and potential fluctuations in a passivated In$_2$Se$_3$/Bi$_2$Se$_3$ topological interface state

The topological interface state of Bi$_2$Se$_3$ capped with In$_2$Se$_3$ is measured by gated THz cyclotron resonance. An observed shift of 70 meV in the position of the Dirac point towards mid-gap due to the physical properties of the trivial insulator In$_2$Se$_3$ on Bi$_2$Se$_3$ opens new possibilities in tailoring Dirac cone properties in topological insulators. Modulating and sweeping a semi-transparant gate while probing at terahertz frequencies in magnetic field enables characterization of the burried In$_2$Se$_3$/Bi$_2$Se$_3$ topological interface state, even in the presence of significant bulk conductivity. Near the Dirac point, the mobility is 3500 cm$^2$/V$\cdot$s with potential fluctuations of 60 meV. The scattering rate shows a precipitous drop with Fermi energy indicating decoupling of the surface states from bulk states. At Fermi energies above the conduction band edge, a plateau is observed in the real part of the Faraday angle that is 80 times flatter than the step size expected from a single Landau Level, quantized in units of the fine structure constant.