Stabilization of sliding ferroelectricity through exciton condensation.

Sliding ferroelectricity is a phenomenon that arises from the insurgence of spontaneous electronic polarization perpendicular to the layers of two-dimensional systems upon the relative sliding of the atomic layer constituents. Because of the weak van der Waals interactions between layers, sliding and the associated symmetry breaking can occur at low energy cost in materials such as transition-metal dichalcogenides. Here we discuss theoretically the origin and quantitative understanding of the phenomenon by focusing on a prototype structure, the WTe bilayer, where sliding ferroelectricity was first experimentally observed. We show that excitonic effects induce relevant energy band renormalizations in the ground state, and exciton condensation contributes significantly to stabilizing ferroelectricity upon sliding, beyond previous predictions that disregard electron-hole interaction effects. Enhanced excitonic effects in 2D and van der Waals sliding are general phenomena that point to sliding ferroelectricity as relevant for a broad class of important materials, where the intrinsic electric dipole can couple with other quantum phenomena and, in turn, an external electric field can control the quantum phases through ferroelectricity in unexplored ways.