Abundant topological phases in hydrogenated group-IV binary alloy compounds.

Based on the widely studied two-dimensional layered materials, new materials with unique properties can be acquired by stacking the layered materials with different styles. By using density-functional calculations and symmetry analysis, we here present a route to produce abundant topologically nontrivial electronic states in three-dimensional compounds made of stacked monolayer hydrogenated group-IV binary alloys. Triply degenerate point (TDP) semimetals and Dirac semimetals are found in the hydrogenated SnPb compounds with different stacking configurations. The TDP semimetal is characterized by two sets of near-Dirac TDPs, stemming from the inversion of the s (p ) and p bands from the Pb (and also Sn) atoms and protected by a double point group symmetry. Type-I and type-II states, and one essential Dirac semimetal state, coexist in a hydrogenated SnPb compound. Hourglass surface states are also observed in this case. For the hydrogenated GeSn and GePb compounds, normal insulators and weak topological insulators (and also Dirac semimetals) are obtained, respectively. The topological nature of the states is identified by calculations of topological indexes as well as surface states. With these extremely rich topological phases, the studied compounds offer an ideal material platform for realizing topological semimetals and insulators in experiments.