The sliding phase transition in ferroelectric van der Waals bilayers

The discovery of ferroelectricity in van der Waals-stacked bilayers with out-of-plane polarization substantially expands the family of ferroelectric materials and has triggered tremendous interest. The unconventional dipolar order arises from the locking of the sliding motion of weakly coupled monolayers and ultralow potential barrier for switching (~meV per unit cell). Surprisingly, despite the extremely low barrier such “sliding ferroelectricity” can be stable above room temperature. In contrast, the order of two-dimensional van der Waals ferromagnets rarely survive at elevated temperatures. Here we present a theoretical picture that explains the conundrum of sliding ferroelectricity and the modelling of various associated observables. We attribute the robustness of the ferroelectricity recently observed in h-BN and WTe₂ bilayers to large monolayer in-plane stiffness. A soft “sliding phonon” of the bilayers triggers the phase transition at high Curie temperatures as an interplay between switching barrier and intralayer elasticity. This mechanism also holds for structural phase transitions in non-ferroelectric bilayers for which we predict anomalies of the specific heat at the critical temperature.