| The quantum spin Hall (QSH) phase exhibits spontaneous quantized (charge/spin) conductance in the absence of external magnetic field, as a result of strong spin-orbit coupling. However, due to the large bulk contribution in 3D TIs, it is difficult to observe these new states. Here we first demonstrate a selective control of two surfaces conductions (top/bottom) in epitaxially grown Bi2Te3 thin films by gate-dependent Shubnikov-de Hass oscillations. By sweeping the gate voltage, the Fermi level of the top surface is selectively tuned while the bottom surface conduction remains unchanged due to strong electric-field screening effects arising from the high dielectric constant of Bi2Te3. In addition, the total bulk conduction can be modulated from n- to p-type with a varying gate bias. We further investigate BixSb2-xTe 3 as it provides lower bulk contribution and enhanced two surface conductions. These Sb-doped materials provide us a good opportunity to see the quantum spin Hall result. |
