Topological electronic structure and Weyl semimetal in the TlBiSe2 class of semiconductors

We present an analysis of bulk and surface electronic structures of thallium-based ternary III-V-VI₂ series of compounds TlMQ₂, where M=Bi or Sb and Q=S, Se, or Te, using the ab initio density functional theory framework. Based on parity analysis and (111) surface electronic structure, we predict TlSbSe₂, TlSbTe₂, TlBiSe₂, and TlBiTe₂ to be nontrivial topological insulators with a single Dirac cone at the Γ point and TlSbS₂ and TlBiS₂ to be trivial band insulators. Our predicted topological phases agree well with available angle-resolved photoemission spectroscopy measurements, in particular, the topological phase changes between TlBiSe₂ and TlBiS₂. Moreover, we propose that Weyl semimetal can be realized at the topological critical point in TlBi(S_1-xSe_x)₂ and TlBi(S_1-xTe_x)₂ alloys by breaking the inversion symmetry in the layer-by-layer growth in the order Tl-Se(Te)-Bi-S, yielding six Dirac cones centered along the Γ-L directions in the bulk band structure.