Role of Enlarged Symmetry in Sliding Ferroelectrics

Van der Waals “sliding'' ferroelectric bilayers, whose electric polarization is locked to the interlayer alignment, have attracted considerable attention as novel non-volatile memory. The stability and switching properties of their ferroelectric order has so far been described in terms of an Ising-like binary polarization. However, it is a much larger translation symmetry that is broken in the polar state. Here we introduce a discrete statistical--mechanical model that emphasizes the effect of this larger symmetry. We show this model exhibits a richer phase diagram, including an intermediate critical phase of algebraically-correlated polarization and a Berezinskii--Kosterlitz--Thouless transition to the depolarized state. Our results indicate the need for future models of this system to account for the larger symmetry.