Strain-Driven Metal-to-Insulator Transition and Ferroelectricity in WTe₂*

WTe₂, a two-dimensional (2D) van der Waals (vdW) material, has attracted interest for its potential to exhibit out-of-plane ferroelectricity. Despite being semimetallic, WTe₂ allows electric polarization distortion, making it a candidate for exploring 2D ferroelectric (FE) phenomena. In this study, we investigate the effects of mechanical strain on WTe₂ to induce out-of-plane polarization and FE behavior.

Using density functional theory (DFT), we apply biaxial strain to WTe₂ layers. Our results reveal a transition from a semimetallic to an insulating state via band gap opening. This strain-induced band gap is critical for ferroelectricity, as an insulating state is necessary for sustaining spontaneous polarization. Out-of-plane polarization is expected from interlayer sliding in this vdW material, enabling polarization reversal with minimal energy barriers.

Our findings focus on the successful induction of a band gap through biaxial strain, setting the stage for ferroelectricity in WTe₂. We plan to perform Berry phase calculations to quantify spontaneous polarization in the insulating phase. Additionally, we aim to investigate energy barriers associated with polarization switching via interlayer sliding, facilitating efficient control of the out-of-plane polarization state.

Furthermore, we intend to examine the robustness of FE instability under open-circuit boundary conditions (OCBC). In conventional FEs, depolarization fields under OCBC can suppress spontaneous polarization. By analyzing longitudinal optical (LO) phonon modes for soft-mode behavior, we hope to determine whether WTe₂ maintains its out-of-plane FE properties under such conditions.

Our preliminary results underscore the significant impact of biaxial strain on inducing out-of-plane polarization in WTe₂. This work not only demonstrates the feasibility of strain engineering to realize 2D ferroelectricity but also opens avenues for developing nanoscale electronic and memory devices that leverage the unique properties of vdW-layered materials.