Strain-induced topological insulator phase transition in HgSe

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Using ab initio electronic structure calculations we investigate the change of the band structure and the ν₀ topological invariant in HgSe (noncentrosymmetric system) under two different types of uniaxial strain along the [001] and [110] directions, respectively. Both compressive [001] and [110] strain lead to the opening of a (crystal field) band gap (with a maximum value of about 37 meV) in the vicinity of Γ, and the concomitant formation of a camel-back- (inverse camel-back-) shaped valence (conduction) band along the direction perpendicular to the strain with a minimum (maximum) at Γ. We find that the Z₂ invariant ν₀=1 which demonstrates conclusively that HgSe is a strong topological insulator (TI). With further increase of the strain the band gap decreases, vanishing at a critical strain value (which depends on the strain type) where HgSe undergoes a transition from a strong TI to a trivial (normal) insulator. HgSe exhibits a similar behavior under a tensile [110] uniaxial strain. On the other hand, HgSe remains a normal insulator by applying a [001] tensile uniaxial strain. Complementary electronic structure calculations of the nonpolar (110) surface under compressive [110] tensile strain show two Dirac cones at the Γ point whose spin chiral states are associated with the top and bottom slab surfaces.