Large Gap Topological Insulating Phase and Anisotropic Rashba and Chiral Spin Textures in Monolayer Zintl A2MX2

Abstract

A family of two-dimensional (2D) materials, particularly the complex-structured Zintl phase compounds, has attracted tremendous research attention because of tunable material properties and exceptional applications. Utilizing first-principles computations under the hybrid functional approach, we performed a systematic study on A2MX2 (A = Ca, Sr, or Yb; M = Zn or Cd; X = P, As, Sb, or Bi). Among the three surface terminations considered, the metal element XM-termination (T2) was found to be the most stable structural phase with the lowest total ground state energies. Thermodynamic stability was further confirmed through phonon dispersion and formation energy calculations. Surprisingly, the T2 monolayer Sr2CdBi2 was found to host the topological insulating phase with a sizable bandgap of 556 meV, as well as a Z2 topological invariance of 1. The corresponding topologically protected edge states link the conduction and valence bands. Moreover, the topological phase is driven by the spin–orbit coupling, which causes the inverted bands at Γ point close to the Fermi level concerning the Cd-s + Bi2-s and Bi2-px + py orbitals. Moreover, Rashba and chiral spin-splittings were also observed. The computed Rashba strengths along Γ-M (αRΓ-M) are 1.970, 0.676, and 0.669 eV·Å, whereas the computed values along Γ-K (αRΓ-K) are 1.701, 0.731, and 0.646 eV·Å, for Ca2ZnAs2, Sr2CdBi2, and Yb2ZnAs2, respectively. Our study provides fundamental knowledge to further experimental investigations and synthesis, which may lead to electronic applications of A2MX2 compounds in quantum computing or spintronics.