Strain-Gradient Engineering of Topological, Nonlinear Optical, and Spin Chiral Responses in MnBi2Te4

Strain gradients arising from nonuniform interlayer expansion in layered van der Waals materials simultaneously modulate interlayer coupling and break inversion symmetry, yet their potential as a control knob for topological quantum materials remains unexplored. Using first-principles calculations, we demonstrate that graded interlayer expansion characterized by an effective gap-strain gradient in five-septuple-layer MnBi2Te4 drives three distinct physical responses: a topological phase transition from a quantum anomalous Hall state (C = 1) to a trivial insulator (C = 0), a second-harmonic-generation response in the mid-to-far-infrared spectral range, and a Rashba-type chiral spin texture with opposite inner-outer helicities. All three responses share a common microscopic origin in the layer-selective redistribution of interlayer hybridization, which weakens the Bi-Te orbital inversion near the Γ point and generates an out-of-plane polar field. Our findings establish strain-gradient engineering as a design principle for coordinating topological, nonlinear optical, and spin-textural functionalities in layered quantum materials.