Weyl nodal loop semimetals and tunable quantum anomalous Hall states in two-dimensional ferromagnetic cerium monohalides

Quantum anomalous Hall (QAH) effect with dissipationless edge channels offers innovative insight for designing the next-generation low-power electronic devices. Based on first-principles calculations and the tight-binding (TB) model, we predict rich QAH states with a tunable Chern number in single-layer ferromagnetic cerium monohalides $\mathrm{Ce}X$ ($X$ = Cl, Br, I). These stable ferromagnetic single-layer materials have isotropic magnetocrystalline anisotropy in the $x\text{\ensuremath{-}}y$ plane, which favors the adjustment of the topological state with an external magnetic field. A distinct Weyl nodal loop exists in the band structure of the $\mathrm{Ce}X$ single layers without spin-orbit coupling (SOC). When SOC is included and all mirror symmetries are broken, QAH state can be realized. Intriguingly, QAH states with varying Chern number ($C=\ifmmode\pm\else\textpm\fi{}1$), two-dimensional Weyl semimetals and band gap periodically manifest as the magnetization direction rotates in the $x\text{\ensuremath{-}}y$ plane. Furthermore, A TB model based on Slater-Koster framework is constructed to explain the origin of nontrivial band structure in $\mathrm{Ce}X$ single layers. The $\mathrm{Ce}X$ single layers exhibit remarkable topological states, providing an excellent platform for exploring low-power spintronic devices.