Switchable valley polarization and quantum anomalous Hall state in the VN2X2Y2 nanosheets (X = group-III and Y = group-VI elements)

Motivated by the recent synthesis of MA2Z4 family materials, we perform a first-principles investigation on the structural stability and electronic properties of VN2X2Y2 nanosheets (X = B-Ga, Y = O-Te), which are the isostructural and isoelectronic analogues of the VSi2N4 system. We find that among the 16 possible III–VI combinations, there are four XY compositions, i.e., XY = BO, BS, AlO, and GaO, that can form stable VN2X2Y2 nanosheets with robust dynamic and thermal stabilities. Intrinsic ferromagnetism appears in these VN2X2Y2 nanosheets, among which the VN2B2S2 and VN2Ga2O2 ones possess an easy out-of-plane magnetization. Thus, unlike the VSi2N4 system, the VN2B2S2 and VN2Ga2O2 nanosheets exhibit spontaneous valley polarization with a sizeable polarization value in the bottom conduction band. Through the strain modulation, the valley polarization can be switched from the bottom conduction band to the top valence band, which is closely related to the strain-induced band inversion. Due to the existence of valley polarization, the band inversion occurs at different strains for the two valleys. Therefore, in the critical strain region, an intriguing quantum anomalous Hall state is present in the strained VN2X2Y2 nanosheets, which is characterized by a nonzero Chern number of C = 1. Such nontrivial topology is further confirmed by a quantized Hall conductance and a single gapless edge state in the bulk gap. Our study demonstrates as the cousin of MA2Z4 materials, the III–VI VN2X2Y2 nanosheets possess robust structural stability, peculiar electronic and topological properties, which have potential applications in electronics, spintronics, and valleytronics.