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.

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.