Tunable Coupling between Surface States of a Three-Dimensional Topological Insulator in the Quantum Hall Regime

Abstract

The paired top and bottom Dirac surface states, each associated with a half-integer quantum Hall (QH) effect, and a resultant integer QH conductance (νe 2 /h), are hallmarks of a three-dimensional (3D) topological insulator (TI). In a dual-gated system, chemical potentials of the paired surface states are controlled through separate gates. In this work, we establish tunable capacitive coupling between the surface states of a bulk-insulating TI BiSbTeSe2 and study the effect of this coupling on QH plateaus and Landau level (LL) fan diagram via dual-gate control. We observe non-linear QH transitions at low charge density in strongly-coupled surface states, which are related to the charge-density-dependent coupling strength. A splitting of the N= 0 LL at the charge neutrality point for thin devices (but thicker than the 2D limit) indicates inter-surface hybridization possibly beyond singleparticle effects. By applying capacitor charging models to the surface states, we explore their chemical potential as a function of charge density and extract the fundamental electronic quantity of LL energy gaps from dual-gated transport and capacitance measurements. These studies are essential for the realization of exotic quantum effects such as topological exciton condensation.

Intrinsic coupling between spatially-separated surface Fermi-arcs in Weyl orbit quantum Hall states

“…1e). This is different from the TI case, where the two surface orbits are perfectly isolated (C 12 /C 11 = 0) by the insulating bulk state in the three-dimensionally thick limit 21 . We note that reducing the film thickness in the TI case leads to finite coupling between the two surfaces (C 12 /C 11 ̸ = 0).…”

“…Even in such a case, the checkerboard pattern of the QH states ( Fig. 1c) only deforms from a tetragonal to a diamond pattern 21 , and is still distinct from the stripe pattern in the Weyl orbit case (Fig. 1d).…”

“…In the TI-like conventional case ( Fig. 1a), each of the two surfaces independently hosts a 2D electronic state [19][20][21] . The total QH filling factor ν can be expressed by simple summation of those of the two surfaces (ν 1 and ν 2 ),…”

“…The field effect applied on each surface modulates ν 1 or ν 2 only independently, thus giving a checkerboard QH plateau pattern of ν as depicted in Fig. 1c 21 . In the Weyl orbit case (Fig.…”

Emergent Helical Edge States in a Hybridized Three-Dimensional Topological Insulator

“…S12) presents a key feature resulting from the strong coupling between surface states. To better elucidate the electric field response, we converted the dual-gate voltages into the displacement field (D) versus total charge density (n) relation [34] , as presented in descending order of thickness in Fig. 4a-d.…”