Two-dimensional MX family of Dirac materials and quantum spin Hall insulators with tunable electronic and topological properties*

A novel class of two dimensional MX (M=Be, Mg, Zn and Cd, X = Cl, Br and I) compounds with an unbalanced chemical formula was proposed by using data-driven method and first-principle calculations, exhibiting Dirac states with ultra-high Fermi velocities emerged from the interplay of incomplete charge transfer and hexagonal bipartite lattice. Intrinsically being time-reversal-invariant topological insulators when considering spin-orbital coupling, some members of this family offer exciting opportunities to host superconductivity in a highly tunable topological matrix. The electronic and topological properties are found to be highly tunable and amenable to modulation via anion-layer substitution and vertical electric field. The presence of sizable spin-orbital-coupling band gaps, ultra-high carrier mobilities, and small effective masses makes the MX family promising for electronics and spintronics applications.