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For three-dimensional (3D) topological insulators that have a layered structure, strain was used to control critical physical properties. Here, we show that tensile strain decreases bulk carrier density while accentuating transport of topological surface state using temperature-dependent resistance and magneto-resistance measurements, terahertz-time domain spectroscopy and density functional theory calculations. The induced strain was confirmed by transmittance X-ray scattering measurements. The results show the possibility of reversible topological surface state device control using structural deformation.

Topological insulator (TI), a band insulator with topologically protected edge states, is one of the most interesting materials in the field of condensed matter. Bismuth selenide (Bi 2 Se 3 ) is the most spotlighted threedimensional TI material; it has a Dirac cone at each top and bottom surface and a relatively wide bandgap. For application, suppression of the bulk effect is crucial, but in ultrathin TI materials, with thicknesses less than 3 QL, the finite size effect works on the linear dispersion of the surface states, so that the surface band has a finite bandgap because of the hybridization between the top and bottom surface states and Rashba splitting, resulting from the structure inversion asymmetry. Here, we studied the gapless top surface Dirac state of strained 3 QL Bi 2 Se 3 /graphene heterostructures. A strain caused by the graphene layer reduces the bandgap of surface states, and the band bending resulting from the charge transfer at the Bi 2 Se 3 −graphene interface induces localization of surface states to each top and bottom layer to suppress the overlap of the two surface states. In addition, we verified the independent transport channel of the top surface Dirac state in Bi 2 Se 3 /graphene heterostructures by measuring the magneto-conductance. Our findings suggest that the strain and the proximity effect in TI/non-TI heterostructures may be feasible ways to engineer the topological surface states beyond the physical and topological thickness limit.

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