Two-dimensional ferromagnetic semiconductors of rare-earth Janus 2H-GdIBr monolayers with large valley polarization

Based on a rare-earth Gd atom with 4f electrons, through first-principles calculations, we demonstrate that a Janus 2H-GdIBr monolayer exhibits an intrinsic ferromagnetic (FM) semiconductor character with an indirect band gap of 0.75 eV, a high Curie temperature T_c of 260 K, a significant magnetic moment of 8μ_B per f.u. (f.u. = formula unit), in-plane magnetic anisotropy (IMA) and a large spontaneous valley polarization of 118 meV. The MAE, inter-atomic distance or angle, and T_c can be efficiently modulated by in-plane strains and charge carrier doping. Under a strain range from -5% to 5% and charge carrier doping from -0.3 e to 0.3 e per f.u., the system still retains its FM ordering and the corresponding T_c can be modulated by strains from 233 K to 281 K and by charge carrier doping from 140 K to 245 K. Interestingly, under various strains, the matrix element differences (d, d), (d, d) and (p, p) of Gd atoms dominate the MAE behaviors, which originates from the competition between the contributions of the Gd-d orbitals, Gd-p orbitals, and p orbitals of halogen atoms based on the second-order perturbation theory. Inequivalent Dirac valleys are not energetically degenerate due to the time-reversal symmetry breaking in the Janus 2H-GdIBr monolayer. A considerable valley gap between the Berry curvature at the K and K' points provides an opportunity to selectively control the valley freedom and states. External tensile (compressive) strain further increases (decreases) the valley gap up to a maximum (minimum) value of 158 (37) meV, indicating that the valley polarization in the Janus 2H-GdIBr monolayer is robust to external strains. This study provides a novel paradigm and platform to design spintronic devices for next-generation quantum information technology.