TiSOH monolayer: A ferromagnetic semiconductor with multiple topological properties

Two-dimensional (2D) Janus ferromagnetic (FM) materials have recently attracted considerable interest due to their intriguing properties. Their structural asymmetry and the resulting electronic structures endow them with interesting physical quantities (such as Berry curvature and Dzyaloshinskii–Moriya interaction, DMI), which can induce a variety of topological phenomena. In this work, we theoretically predict a Janus TiSOH monolayer using first-principles calculations. Our results show that TiSOH is a FM semiconductor with a bandgap of ∼0.4 eV. The intrinsic polarity not only results in a large out-of-plane electric dipole of 0.247 eÅ and sizable piezoelectric coefficients (d11 ∼3.95 and d31 ∼2.37 pm/V), but also induces finite Berry curvatures at the K+ and K− valleys, as well as a sizable DMI (∼ 0.5 meV). When the spin polarization is aligned along the out-of-plane direction, a notable valley splitting of ∼57 meV occurs, which enables an anomalous valley Hall effect under suitable hole doping. Under ∼1.7% in-plane strain, band inversion occurs at the K+ valley, resulting in a Chern number of –1, which indicates a quantum anomalous Hall state. Additionally, applying 0.5% in-plane strain and a 1.3 T out-of-plane magnetic field leads to skyrmions with a size of ∼2.4 nm in the FM background. These findings not only suggest that the TiSOH monolayer is a promising candidate material for multifunctional spintronic devices, but also provide guidance for the design of 2D topological magnets.