Absence of critical thickness in an ultrathin improper ferroelectric film.

We study the ferroelectric stability and surface structural properties of an oxygen-terminated hexagonal YMnO3 ultrathin film using density functional theory. Under an open circuit boundary condition, the ferroelectric state with the spontaneous polarization normal to the (0001) surface is found to be metastable in a single domain state despite the presence of a depolarizing field. Our results highlight that improper ferroelectric ultrathin films can have rather unique properties that are distinct from those of very thin films of ordinary ferroelectrics.