Obtaining decoupled surface transport by gating. (a)–(b) Gate-voltage dependences of sheet resistivity ρ_xx and Hall coefficient R_H for a (Bi_1−xSb_x)₂Te₃ (BST) thin film [panel (a)] and a Bi₂Te₃ (BT) thin film [panel (b)]. Both films are 10 nm thick and can be tuned into ambipolar regime. (c) Magnetoconductivities of the BST film (symbols) and their best fits to the Eq. (1) (lines) for a set of gate voltages. The curves for gate voltages other than −20 V are shifted vertically for a clearer view. (d) Gate-voltage dependence of the extracted α for both samples. Here, V_G0 is the gate voltage at which R_H = 0, and may be approximately regarded as the charge neutrality point. The insets show the schematic band diagrams for the case of strong bulk-surface coupling. All of the data were taken at T = 1.6 K.