Prediction of the two-dimensional ferromagnetic semiconductor Janus 2H-ZrTeI monolayer with large valley and piezoelectric polarizations.

Two-dimensional room-temperature Janus ferrovalley semiconductors with valley polarization and piezoelectric polarization offer new perspectives for designing multifunctional nanodevices. Herein, using first-principles calculations, we predict that the Janus 2H-ZrTeI monolayer is an intrinsic ferromagnetic semiconductor with in-plane magnetic anisotropy and a Curie temperature of 111 K. The Janus ZrTeI monolayer possesses a significant valley polarization of 141 meV due to time-reversal and inversion symmetry breaking. Based on the valley-contrasting Berry curvature, the anomalous valley Hall effect can be observed under an in-plane electric field. Meanwhile, the breaking of the inversion symmetry and mirror symmetry results in large longitudinal and transverse piezoelectric coefficients. By applying biaxial strain, the Janus 2H-ZrTeI monolayer can also be transformed into a Weyl nodal line semimetal. Furthermore, bilayers of ZrTeI with AB and BA stacking configurations allow the coexistence of valley polarization and ferroelectricity, enabling the manipulation of magnetism, ferroelectric polarization, and valley polarization through interlayer sliding. Our work provides a platform for studying valley polarization, piezoelectricity, and multiferroic coupling, which is significant for the application of multifunctional devices.