Directional-Locked Switching in Sliding Ferroelectrics Driven by Improper Mechanism.

Sliding ferroelectrics possess vertical polarization via stackings of monolayer van der Waals (vdW) materials, exhibiting energy efficiency and ultrafast switching dynamics. Here, we uncover an intriguing polarization-switching behavior in bilayer BN, wherein the interlayer sliding is locked toward a specific direction under uniaxial strain, unable to reverse motion. The in-plane driving force for guiding directional-locked sliding is determined by the emergent off-diagonal Born effective charge (BEC), along with its sign reversal when strain changes from compressive to tensile. The sign reversal of BEC is governed by an improper bilinear coupling between strain and in-plane polarization. Strikingly, the polarization switching barrier is drastically reduced and nearly vanishes under compressive strain. Microscopically, reduced charge density gradient is utilized to capture intricate vdW interactions, providing an intuitive understanding for the sign reversal of BEC and reduced switching barrier. Finally, we demonstrated promising applications in long-range structural precision control, dynamical multiferroicity and terahertz-light emission based on switching performances in sliding ferroelectrics.