A New Family of Two-Dimensional Topological Materials: CdX (X = F, Cl, Br, and I)

Two-dimensional (2D) transition-metal halides have attracted great interest owing to their versatile applications in electronics, optoelectronics, and renewable energy storage/conversions. Using first-principles calculations, it is proposed that a new series of 2D transition-metal halide CdX (X=F, Cl, Br, and I) monolayers with honeycomb lattice structure show topological properties. When omitting the spin-orbital coupling (SOC) effect, all of them behave as Dirac semimetal whose Fermi surface is composed of two Dirac points at high symmetry K and K' points in Brillouin zone. When considering the SOC effect, CdCl, CdBr, and CdI monolayers behave as topological insulators with global band gap, whereas CdF is converted from Dirac semimetal into topological metal with local band gap. The nontrivial topological properties are further proved by their nontrivial edge states. The fascinating properties of the CdX (X=F, Cl, Br, and I) monolayers show their potential for future quantum computing and next generation of high-speed electronic devices.