Insulator-semimetal transitions from annihilating and restoring topological surface Dirac points

Pairwise merging and annihilation of Dirac points is known to give a topological transition from a semimetal to an insulator. Here, we report transport studies on thin films of BiSbTeSe₂ (BSTS), which in its bulk form is a three-dimensional topological insulator (3D TI) that hosts gapless (semi-metallic) Dirac fermion surface states (SS). When the sample is reduced to ~10 nm thick, the hybridization between opposite surfaces opens a gap thus annihilates the Dirac points, giving rise to an insulating behavior. We observe that an in-plane magnetic field can drive the system again towards a metallic behavior, with a prominent negative magnetoresistance and a temperature-insensitive resistivity close to h/2e² at the charge neutral point. This is in general agreement with a predicted in-plane magnetic field induced restoration of Dirac points, split in the momentum space and giving rise to a tunable graphene-like band structure. Such topological transitions of Dirac cones as controlled by the TI thickness and magnetic fields can be exploited for topological electronics and spintronics applications.