Strain-engineered topological phase transitions in ferrovalley 2H−RuCl2 monolayer

Ferrovalley and topology are two basic concepts in both fundamental research fields and emerging device applications. So far, reports are extremely scarce regarding the coupling of these two scenarios in a single material system. By first-principles calculations and Monte Carlo simulations, we predict a highly stable intrinsic ferrovalley $2H\text{\ensuremath{-}}{\mathrm{RuCl}}_{\text{2}}$ semiconductor with an easy out-of-plane magnetization and a high Curie temperature of 260 K. Due to the combination of exchange interaction and spin-orbit coupling, the system exhibits a spontaneous valley polarization of 240 meV in the top valence band, which is further confirmed by a first-order perturbation theory. Moreover, its out-of-plane ferromagnetic ordering can remain rather robust and survive up to 547 K under $\ensuremath{-}5$% compressive strain well above room temperature. On the other hand, the application of tensile strain can drive the system from the ferrovalley phase into a half-valley-metal state with 100% spin and valley polarizations, followed then by a quantum anomalous Hall (QAH) phase in support of scarce high-temperature QAH effect. Beyond around 2% tensile strain, the system is switched to the in-plane magnetization. If the magnetization is steered to the out-of-plane direction through overcoming the magnetic anisotropy energy barriers, the system is further transitioned from the QAH phase to another half-valley-metal state with the rise of tensile strain, and finally restores to the initial ferrovalley phase. Once synthesized, monolayer $2H\text{\ensuremath{-}}{\mathrm{RuCl}}_{\text{2}}$ would find many important applications in spintronic, valleytronic, and topological electronic nanodevices.