The valley-polarized quantum anomalous Hall effect (VP-QAHE) in topological materials, which usually is induced by applying external manipulations, has attracted intensive attention. Here, we predict the formation and regulation of the intrinsic VP-QAHE in ferromagnetic Janus monolayer Fe2SSe. Spontaneous valley polarization (VP) appears without external manipulations due to the Janus structure in monolayer Fe2SSe. The spontaneous VP in addition to the nonzero Chern number in Fe2SSe confirm the intrinsic VP-QAHE. Besides, the topologically protected chiral-spin-valley locking edge states can be regulated by reversing the magnetization. Topological phase transitions between metal, half-metal, topological insulator, and ferrovalley phases can be obtained by applying biaxial strains in Fe2SSe, and the nontrivial band gap reaches up to 441 meV. Also, the topological phase with the VP-QAHE is robust under certain conditions. Both the intrinsic VP-QAHE and controllable topological phase transitions can be achieved in Janus monolayer Fe2SSe, which provides an avenue for the applications of dissipationless valleytronic devices.

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The limitation of suitable anomalous valley Hall effect (AVHE) materials has seriously hindered the booming development and the widespread application of valleytronics.Here, through the first-principles calculations, we propose a MXene monolayer Y 3 N 2 O 2 with spontaneous valley polarization (VP) of 21.3 meV, which induces intrinsic AVHE. The VP can be modulated linearly, which provides a route of effective control of the valley signals.Importantly, VP can be enhanced by adjusting up the spin−orbit coupling (SOC) based on a SOC Hamiltonian model and the first-principles calculations. From this physics underlying, we substitute the Y atom with the La atom and further propose the monolayer La 3 N 2 O 2 , in which the heavy atom La will provide stronger SOC than Y atom. The spontaneous VP in La 3 N 2 O 2 is enhanced to 100.4 meV, so AVHE can be easily achieved. Our work not only provides compelling candidates for AVHE materials but also offers a novel mindset for finding suitable valleytronic devices.

Transition metal nitrogen halides (TMNHs) are environmentally friendly and widely studied van der Waals two-dimension (2D) materials. However, there are still few 2D TMNHs with magnetic properties that have been synthesized, which greatly limits the further applications of them in spintronics and valleytronics. Here, we substitute Hf atoms by magnetic elements V and Cr in β-type TMNHs Hf2N2Cl2, which evolved into ferromagnetic half-metals and ferrovalley semiconductors MHfN2Cl2 (M = V, Cr). Meanwhile, the valley splitting (VS) properties and magnetic properties can be effectively regulated by modulating the strong correlation effect of transition metals and applying biaxial strain. VS in VHfN2Cl2 increases linearly with the increasing strain and the U value; however, VS is robust to the strain and decreases linearly with the increasing U value in CrHfN2Cl2, and the maximum VS reaches to 175 and 62 meV for the two monolayers. CrHfN2Cl2 remains robustly ferromagnetic under different strain and U values, while there is magnetic phase transition in VHfN2Cl2 from ferromagnetic to antiferromagnetic, accompanied by a half-metal-to-semiconductor transition. The Curie temperatures of VHfN2Cl2 and CrHfN2Cl2 can be increased by a maximum of 2.5 and 2.36 times under modulation. This opens different ideas for the study of bimetallic TMNHs in spintronics and valleytronics.

Prediction and synthesis of two-dimensional high transition temperature (TC) superconductors is an area of extensive research. Based on calculations of the electronic structures and lattice dynamics, we predict that graphene-like layered monolayer LiC12 is a π-electrons mediated Bardeen-Cooper-Schrieffer-type superconductor. Monolayer LiC12 is theoretically stable and expected to be synthesized experimentally. From the band structures and the phonon dispersion spectrum, it is found that the saddle point of π-bonding bands induces large density of states at the Fermi energy level. There is strongly coupled between the vibration mode in the in-plane direction of the lithium atoms and the π-electrons of carbon atoms, which induces the high-TC superconductivity in LiC12. The TC can reach to 41 K under an applied 10% biaxial tensile strain based on the anisotropic Eliashberg equation. Our results show that monolayer LiC12 is a good candidate as π-electrons mediated electron-phonon coupling high-TC superconductor.