Topological insulator to Dirac semimetal transition driven by sign change of spin-orbit coupling in thallium nitride

Based on the first-principles calculations, we reveal that TlN, a simple binary compound with wurtzite structure, is a three-dimensional (3D) topological insulator (TI) with effectively negative spin-orbit coupling lambda(eff) < 0, which makes it distinguished from other TIs by showing opposite spin-momentum locking effect in its surface states. The sign of lambda(eff) depends on the hybridization between N-2p and Tl-5d states, and can be tuned from negative to positive by lattice strain or chemical substitution, which drives the system into a Dirac semimetal with 3D Dirac cones in its bulk states. Such topological phase transition can be realized by electronic mechanism without breaking any crystal symmetry.