Spontaneous valley polarization and valley-nonequilibrium quantum anomalous Hall effect in Janus monolayer ScBrI

Spin Hall effect modulated by an electric field in asymmetric two-dimensional MoSiAs2Se

Spin current generation from charge current in nonmagnetic materials promises an energy-efficient scheme for manipulating magnetization in spintronic devices. In some asymmetric two-dimensional (2D) materials, the Rashba and valley effects coexist owing to strong spin-orbit coupling (SOC), which induces the spin Hall effect due to spin-momentum locking of both effects. Herein, we propose a new Janus structure MoSiAs2Se with both valley physics and the Rashba effect and reveal an effective way to modulate the properties of this structure. The results demonstrated that applying an external electric field is an effective means to modulating the electronic properties of MoSiAs2Se, leading to both type I-II phase transitions and semiconductor-metal phase transitions. Furthermore, the coexistence of the Rashba and valley effects in monolayer MoSiAs2Se contributes to the spin Hall effect (SHE). The magnitude and direction of spin Hall conductivity can also be manipulated with an out-of-plane electric field. Our results enrich the physics and materials of the Rashba and valley systems, opening new opportunities for the applications of 2D Janus materials in spintronic devices.

Tunable abundant valley Hall effect and chiral spin-valley locking in Janus monolayer VCGeN4

It is both conceptually and practically fascinating to explore fundamental research studies and practical applications of two-dimensional systems with the tunable abundant valley Hall effect. In this work, based on first-principles calculations, the tunable abundant valley Hall effect is proved to appear in Janus monolayer VCGeN4. When the magnetization is along the out-of-plane direction, VCGeN4 is an intrinsic ferromagnetic semiconductor with a valley feature. The intriguing spontaneous valley polarization exists in VCGeN4 due to the common influence of broken inversion and time-reversal symmetries, which makes it easier to realize the anomalous valley Hall effect. Furthermore, we observe that the valley-non-equilibrium quantum anomalous Hall effect is driven by external strain, which is located between two half-valley-metal states. When reversing the magnetization, the spin flipping makes the position of the edge state to change from one valley to another valley, demonstrating an intriguing behavior known as chiral spin-valley locking. Although the easy magnetic axis orientation is along the in-plane direction, we can utilize an external magnetic field to transform the magnetic axis orientation. Moreover, it is found that the valley state, electronic and magnetic properties can be well regulated by the electric field. Our works explore the mechanism of the tunable abundant valley Hall effect by applying an external strain and electric field, which provides a perfect platform to investigate the spin, valley, and topology.

Intrinsic edge states and strain-tunable spin textures in the Janus 1T-VTeCl monolayer

Using first-principles calculations and micro-magnetic simulations, we investigate the electronic structures, the effect of biaxial strain on the topological characteristics, magnetic anisotropy energy (MAE), Dzyaloshinskii-Moriya interaction (DMI) and spin textures in the Janus 1T phase VTeCl (1T-VTeCl) monolayer. Our results show that 1T-VTeCl has an intrinsic edge state, and a topological phase transition with a sizeable band gap is achieved by applying biaxial strain. Interestingly, the MAE can be switched from the in-plane to the off-plane with a compressive strain of -5%. Microscopically, the origin of MAE is mainly associated with the large spin-orbit coupling (SOC) from the heavy nonmagnetic Te atoms rather than that from the V atoms. Furthermore, the induced DMI (0.09 meV) can occur stabilizing magnetic merons without applying temperatures and magnetic fields. Then, the skyrmions, frustrated antiferromagnetism and vortex are induced after applying a suitable compressive strain. Our study provides compelling evidence that the 1T-VTeCl monolayer with topological properties holds great potential for application in spintronic devices, as well as information storage devices based on different magnetic phases achievable through strain engineering.

Novel valley character and tunable quasi-half-valley metal state in Janus monolayer VSiGeP4

The manipulation and regulation of valley characteristics have aroused widespread interest in emerging information fields and fundamental research. Realizing valley polarization is one crucial issue for spintronic and valleytronic applications, the concepts of a half-valley metal (HVM) and ferrovalley (FV) materials have been put forward. Then, to separate electron and hole carriers, a fresh concept of a quasi-HVM (QHVM) has been proposed, in which only one type of carrier is valley polarized for electron and hole carriers. Based on first-principles calculations, we demonstrate that the Janus monolayer VSiGeP4 has QHVM character. To well regulate the QHVM state, strain engineering is utilized to adjust the electronic and valley traits of monolayer VSiGeP4. In the discussed strain range, monolayer VSiGeP4 always favors the ferromagnetic ground state and out-of-plane magnetization, which ensures the appearance of spontaneous valley polarization. It is found that the QHVM state can be induced in different electronic correlations (U), and the strain can effectively tune the valley, magnetic, and electronic features to maintain the QHVM state under various U values. Our work opens up a new research idea in the design of multifunctional spintronic and valleytronic devices.

Quantum anomalous valley Hall effect in ferromagnetic MXenes with asymmetric functionalization

The potential to detect and manipulate the valley degree of freedom within two-dimensional hexagonal lattices possessing both inversion asymmetry and time-reversal symmetry is theoretically feasible. Intrinsic ferrovalley polarization in MXenes could be induced by asymmetric surface functionalization to break their inversion symmetry and the presence of spin-orbital coupling ensures their time-reversal symmetry. Our results indicate that the ferromagnetic Cr2COF MXene with Janus functionalization becomes an intrinsic Chern insulator with large spin-valley polarization and belongs to the family of quantum anomalous valley Hall effect (QAVHE) materials, based on Berry curvature and edge state calculations. Applying chemical engineering of functionalization to magnetic MXenes allows us to tune the structure-property relationship in 2D layers to obtain desirable spin-valley coupling. Our theoretical insight into the QAVHE on magnetic MXenes with asymmetry functionalization provides a new opportunity for valleytronics and spintronics.