Converting topological insulators into topological metals within the tetradymite family

We report the electronic band structures and concomitant Fermi surfaces for a family of exfoliable tetradymite compounds with the formula T2Ch2Pn, obtained as a modification to the well-known topological insulator binaries Bi2(Se,Te)3 by replacing one chalcogen (Ch) with a pnictogen (Pn) and Bi with the tetravalent transition metals T= Ti, Zr, or Hf. This imbalances the electron count and results in layered metals characterized by relatively high carrier mobilities and bulk two-dimensional Fermi surfaces whose topography is well-described by first-principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states. In contrast to Bi2Se3, where the surface Dirac bands are at the Γ point, for (Zr,Hf)2Te2(P,As) there are Dirac cones of strong topological character around both the Γ and M points, which are above and below the Fermi energy, respectively. For Ti2Te2P, the surface state is predicted to exist only around the M point. In agreement with these predictions, the surface states that are located below the Fermi energy are observed by angle-resolved photoemission spectroscopy measurements, revealing that they coexist with the bulk metallic state. Thus this family of materials provides a foundation upon which to develop novel phenomena that exploit both the bulk and surface states (e.g., topological superconductivity).