Strain-tuning of Dirac states at the SnTe (001) surface

The topological crystalline insulator SnTe belongs to the recently discovered class of materials in which a crystalline symmetry ensures the existence of topologically protected Dirac like surface states. In contrast to topological insulators, this symmetry can be broken via deformations of the crystal. This opens up new possibilities of manipulating the Dirac states and inducing a controllable gap. Here, we have employed density-functional theory to investigate the response of the Dirac states to lattice deformations [1]. The (001) surface exhibits four Dirac cones which lie at non-time-reversal-invariant points close to X, along the projection of the (110) and (110) mirror planes. Our calculations show that a gap of up to approx 30 meV can be introduced via lattice deformations that break at least one of these mirror symmetries. Remarkably, distortions at the surface only can already open up the gap, even though bulk properties are not changed. The gap is formed at either all four or just two cones, depending on the direction of the displacement vector, making it possible to create a state where gaped and non-gaped Dirac cones coexist. Notably, if the whole slab is distorted, bulk bands are being pushed into the gap making the whole system metallic. [1] M. Drüppel et al, Phys. Rev. B 90, 155312 (2014)