Two-dimensional rare-earth Janus 2H-GdXY (X, Y=Cl, Br, I; X ≠ Y) monolayers: Bipolar ferromagnetic semiconductors with high Curie temperature and large valley polarization

Two-dimensional (2D) ferromagnetic semiconductors hold great interest due to their potential applications for nanoscale electronic devices. In this paper, the Janus 2H-GdXY (X, Y =Cl, Br, I, X not equal Y ) monolayers with rare -earth element Gd (4 f7 +/- 5d1) are predicted by first-principles calculations. Small exfoliation energy of less than 0.25 J/m2 and excellent dynamical/thermal stabilities can be confirmed for the Janus 2H-GdXY monolayers, which exhibit the bipolar magnetic semiconductor character with high Curie temperatures above 260 K and large spin-orbit coupling effect, and can be further transformed into the half-semiconductor phase under proper tensile strains (5-6%). In addition, in-plane magnetic anisotropy can be observed in the 2H-GdICl and 2H-GdIBr monolayers. On the contrary, the 2H-GdBrCl monolayer exhibits perpendicular magnetic anisotropy character, which originates from the competition between Gd-p/d and halogen atom -p orbitals. Calculated valley optical actions of the Janus 2H-GdXY monolayers exhibit distinguished valley-selective circular dichroisms, which is expected to realize the special valley excitation by polarized light. Spontaneously, valley-Zeeman effect in the valance band for the Janus 2H-GdXY monolayers induces a giant valley splitting of 60-120 meV, which is also robust against various external biaxial strains. A tunable valley degree of freedom in Janus 2H-GdXY systems is very necessary for encoding and processing information.