Electrostatic Coupling between Two Surfaces of a Topological Insulator Nanodevice

TL;DR

This study investigates the electrostatic coupling between the top and bottom surfaces of a topological insulator nanodevice, revealing tunable surface states, inter-surface capacitance, and magnetic field effects on surface state gaps.

Contribution

It demonstrates independent control of surface states in dual-gated topological insulator devices and introduces a charging model to measure inter-surface capacitance and energy-density relationships.

Findings

Surface states are independently tunable to the Dirac point.

Finite capacitive coupling exists between the surface states.

High magnetic fields induce a surface state band gap.

Abstract

We report on electronic transport measurements of dual-gated nano-devices of the low-carrier density topological insulator Bi1.5Sb0.5Te1.7Se1.3. In all devices the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the inter-surface capacitance $C_{TI}$ and the surface state energy-density relationship $\mu$(n), which is found to be consistent with independent ARPES measurements. At high magnetic fields, increased field penetration through the surface states is observed, strongly suggestive of the opening of a surface state band gap due to broken time-reversal symmetry.