Tunable Weyl half-semimetals in two-dimensional iron-based materials MFeSe (M=Tl,In,Ga)

The layered iron chalcogenide materials have attracted considerable attention recently for their exotic superconductivity at relatively high temperatures. Topological phases are, however, seldom proposed in these materials. Based on density-functional theory calculations together with symmetry analysis, 100% spin-polarized Weyl semimetals, namely Weyl half semimetals (WHSMs), are predicted in two-dimensional (2D) TlFeSe and GaFeSe monolayers, built based on FeSe monolayers. The acquired Weyl fermions are protected by a nonsymmorphic symmetry. Dissimilarly, the InFeSe monolayer is found to be a quantum anomalous Hall (QAH) insulator with a large band gap (403 meV). By tuning the magnetization direction, the monolayers can vary from a WHSM to a QAH insulator or vice versa. The phase-transition mechanism is analyzed by using an effective $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ model. Our work provides a pathway to carry out the fascinating 2D WHSMs and the QAH effect in one material which will have promising applications in not only spintronics but also topological microelectronics.