Giant plateau in the terahertz Faraday angle in gated Bi₂Se ₃

We report gated terahertz Faraday angle measurements on epitaxial Bi ₂Se₃ thin films capped with In₂Se₃. A plateau is observed in the real part of the Faraday angle at an onset gate voltage corresponding to no band bending at the surface, which persists into accumulation. The plateau is two orders of magnitude flatter than the step size expected from a single Landau level in the low-frequency limit, quantized in units of the fine structure constant. At 8 T, the plateau extends over a range of gate voltage that spans an electron density greater than 14 times the quantum flux density. Both the imaginary part of the Faraday angle and transmission measurements indicate dissipative off-axis and longitudinal conductivity channels associated with the plateau.