Realizing Dirac and Weyl fermions in magnetically tunable Zintl materials

Recent classification efforts encompassing crystalline symmetries have revealed rich possibilities for solid-state systems to support a tapestry of exotic topological states. However, finding materials that realize such states remains a daunting challenge. Here, we show how the interplay of topology, symmetry, and magnetism combined with doping and external electric and magnetic field controls can be used to drive the SrIn₂As₂ materials family into various topological phases. Our first-principles calculations and symmetry analysis reveal that SrIn₂As₂ is a dual topological insulator with Z₂=(1;000) and mirror Chern number C_M=-1. Its isostructural and isovalent antiferromagnetic cousin EuIn₂As₂ is found to be an axion insulator (Z₄ = 2). The broken time-reversal symmetry via Eu doping results in a higher-order or topological crystalline insulator state in Sr_1-xEu_xIn₂As_2, depending on the orientation of the magnetic easy axis. We also find that antiferromagnetic EuIn₂P₂ is a trivial insulator (Z₄ = 0). It undergoes a magnetic-field-driven transition to an ideal Weyl fermion or nodal fermion state (Z₄ = 1) with an applied magnetic field. Our study identifies Sr_1-xEu_xIn₂(As, P)₂ as a tunable materials platform for investigating the physics and applications of Weyl and nodal fermions in the scaffolding of crystalline and axion insulator states.