Coexisting Giant Tunable Valley Polarization and Piezoelectric Response in FeO2SiGeN2 Monolayers

The valley-related multiple Hall effect and piezoelectric response are interesting transport characteristics in low-dimensional systems; however, few studies have reported their coexistence in a single system as well as their coupling relationships. By first-principles calculations, we propose a multifunctional Janus semiconductor, i.e., an FeO₂SiGeN₂ monolayer with a large valley polarization of about 120 meV and in-plane piezoelectric polarization with a d₁₁ of −0.71–4.03 pm/V. The magnetic anisotropy energy can be significantly regulated by the electronic correlation strength and strain, which can be attributed to the change in the competitive relationship with respect to the Fe-3d-resolved magnetic anisotropy energy brought about by external regulatory means. Electronic correlation strength can induce phase transitions in the Janus FeO₂SiGeN₂ monolayer from a ferrovalley to the quantum anomalous Hall phase, while the half-valley metallic state as the boundary of the phase transition can generate 100% spin and valley polarization. The related phase transition mechanism is analyzed based on the two-band strained k · p model. The presence of the piezoelectric strain coefficient d₁₁ in valleytronic materials makes the coupling between charge degrees of freedom and valley degrees of freedom possible, and the intrinsic electric field caused by the in-plane piezoelectric response provides a way to realize the piezoelectric anomalous valley Hall effect. This work may pave a way to find interesting materials with the valley-related multiple Hall effect and stimulate further experimental work related to valleytronics and piezotronics.