Probing the uniaxial strain-dependent valley drift and Berry curvature in monolayer MoSi2N4

We use ab initio calculations and theoretical analysis to investigate the influence of uniaxial tensile strain on valley drifts and Berry curvatures in the monolayer ${\mathrm{MoSi}}₂{\mathrm{N}}₄$, a prototypical septuple atomic layered two-dimensional material. The low energy electron and hole valleys drift far off the $\mathrm{K}/{\mathrm{K}}^{\ensuremath{'}}$ point under uniaxial strains. The direction and strength of valley drift strongly depend on the nature of the charge carrier and uniaxial strain with a more substantial response along the zigzag path. Our findings exhibit the pivotal role of microscopic orbital contribution and symmetry lowering. The changing geometric properties of Bloch states affect the Berry curvatures and circular dichroism. Specifically, Berry curvature dipole is significantly enhanced under the tensile strain along armchair and zigzag directions. Meanwhile, the particle-hole asymmetry arising from nonequivalent electron and hole valley drifts relaxes the selection rules, thus reducing the degree of circular polarization up to $\ensuremath{\sim}0.98$. Therefore, strain engineering of valley physics in the monolayer ${\mathrm{MoSi}}₂{\mathrm{N}}₄$ is of prime importance for valleytronics.