Title: Converting topological insulators into topological metals within the tetradymite family,
Abstract: We report the electronic band structures and concomitant Fermi surfaces for a
family of exfoliable tetradymite compounds with the formula $T_2$$Ch_2$$Pn$,
obtained as a modification to the well-known topological insulator binaries
Bi$_2$(Se,Te)$_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 Bi$_2$Se$_3$, where the surface Dirac bands are at the
$\Gamma-$point, for (Zr,Hf)$_2$Te$_2$(P,As) there are Dirac cones of strong
topological character around both the $\bar {\Gamma}$- and $\bar {M}$-points
which are above and below the Fermi energy, respectively. For Ti$_2$Te$_2$P the
surface state is predicted to exist only around the $\bar {M}$-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).