Prediction for an intrinsic ferrovalley semiconductor in a Janus 2H-CeBrCl monolayer with a high Curie temperature and perpendicular magnetic anisotropy

Two-dimensional (2D) ferrovalley materials with intrinsic ferromagnetic ordering and spontaneous spin-valley splitting provide a great platform in nanoscale valleytronic devices. In this paper, the stability, mechanical, magnetic, transport, and valley properties of the Janus $2H$-CeBrCl monolayer are predicted based on density functional theory calculations. The Janus $2H$-CeBrCl monolayer exhibits an intrinsic ferromagnetic (FM) semiconductor character with a high Curie temperature (${T}_{c}$) of 540 K, a large piezoelectric coefficient ${d}_{11}$ of 2.95 pm/V, a robust perpendicular magnetic anisotropy (PMA), and a spontaneous valley polarization ($\mathrm{\ensuremath{\Delta}}\mathrm{V}$) of 29.1 meV. Applying the in-plane biaxial strain from $\ensuremath{-}6$ to 6%, the $2H$-CeBrCl monolayer still remains FM ground states, and the $\mathrm{\ensuremath{\Delta}}V$ and ${d}_{11}$ monotonically increase from $\ensuremath{-}18.1$ to 59.4 meV and from 1.56 to 4.03 pm/V, respectively. It is noted that a transition from PMA to in-plane magnetic anisotropy occurs at the tensile strain of 2%, which is due to the sign reversal contributed by the Ce-${d}_{xy}/{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals. The investigations of electrical transport show that the effective mass of the hole is isotropic, while the effective mass of the electron is anisotropic for the $2H$-CeBrCl monolayer, corresponding to mobilities of the hole and electron along the $x$ direction of 111.6 and 83.2 ${\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively. Moreover, the valley-contrast Berry curvature ${\mathrm{\ensuremath{\Omega}}}_{z}(k)$ and valley anomalous Hall effect can be clearly observed due to the breaking of time and space reversal symmetry in the $2H$-CeBrCl monolayer. Overall, the Janus $2H$-CeBrCl monolayer can be considered as an attractive 2D intrinsic ferrovalley material for nanoscale electronics and valleytronic devices.