In this work, Janus monolayers are predicted for a new 2D MA2Z4 family by means of first-principles calculations. The predicted MSiGeN4 (M = Mo and W) monolayers exhibit dynamic, thermodynamic and mechanical stability, and they are indirect band-gap semiconductors.

Coexistence of intrinsic ferrovalley (FV) and nontrivial band topology attracts intensive interest both for its fundamental physics and for its potential applications, namely valley-polarized quantum anomalous Hall insulator (VQAHI). Here, based on first-principles calculations by using generalized gradient approximation plus U (GGA+U) approach, the VQAHI induced by electronic correlation or strain can occur in monolayer RuBr2. For perpendicular magnetic anisotropy (PMA), the ferrovalley (FV) to half-valley-metal (HVM) to quantum anomalous Hall (QAH) to HVM to FV transitions can be driven by increasing electron correlation U. However, there are no special QAH states and valley polarization for in-plane magnetic anisotropy. By calculating actual magnetic anisotropy energy (MAE), the VQAHI indeed can exist between two HVM states due to PMA, a unit Chern number/a chiral edge state and spontaneous valley polarization. The increasing U can induce VQAHI, which can be explained by sign-reversible Berry curvature or band inversion between dxy/dx2-y2and dz2orbitals. Even though the real U falls outside the range, the VQAHI can be achieved by strain. Taking U=2.25 eV as a concrete case, the monolayer RuBr2 can change from a common ferromagentic (FM) semiconductor to VQAHI under about 0.985 compressive strain. It is noted that the edge states of VQAHI are chiral-spin-valley locking, which can achieve complete spin and valley polarizations for low-dissipation electronics devices. Both energy band gap and valley splitting of VQAHI in monolayer RuBr2 are higher than the thermal energy of room temperature (25 meV), which is key at room temperature for device applications. It is found that electronic correlation or strain have important effects on Curie temperature of monolayer RuBr2. These results can be readily extended to other monolayer MXY (M = Ru, Os; X/Y=Cl, Br I).

Motived by experimentally synthesized (Hong Y. L. et al., Science, 369 (2020) 670), the intrinsic piezoelectricity in monolayer ( , W, Cr, Ti, Zr and Hf) are studied by density functional theory (DFT). Among the six monolayers, has the best piezoelectric strain coefficient d 11 of 1.24 pm/V, and the second is 1.15 pm/V for . Taking as a example, strain engineering is applied to improve d 11. It is found that tensile biaxial strain can enhance d 11 of , and the d 11 at 4% strain can improve by 107% with respect to the unstrained one. By replacing the N by P or As in , the d 11 can be raised substantially. For and , the d 11 is as high as 4.93 pm/V and 6.23 pm/V, which is mainly due to smaller and very small minus or positive ionic contribution to piezoelectric stress coefficient e 11 with respect to . The discovery of this piezoelectricity in monolayer enables active sensing, actuating and new electronic components for nanoscale devices, and is recommended for experimental exploration.

The septuple-atomic-layer $\mathrm{VSi_2P_4}$ with the same structure of experimentally synthesized $\mathrm{MoSi_2N_4}$ is predicted to be a spin-gapless semiconductor (SGS) with generalized gradient approximation (GGA). In this work, the biaxial strain...