Electrostatic Coupling Between the Surface States of a Topological Insulator

We report electronic transport measurements on nanofabricated topological insulator Bi$_{\mathrm{1.5}}$Sb$_{\mathrm{0.5}}$Te$_{\mathrm{1.7}}$Se$_{\mathrm{1.3}}$ exfoliated devices with electrostatic top- and bottom-gate electrodes. We observe independent, ambipolar modulation of the device resistance on both the top and bottom surfaces. On thin devices, the bottom-gate capacitively couples to the top surface, indicating poor bulk screening which allows for surface-to-surface electrostatic coupling. We explain the data through a capacitance model and extract information about the surface and bulk density of states. Additionally, we show that the ambipolarity of the surface state resistance persists up to room temperature.