  Ferroelectric-dielectric superlattices have attracted renewed interest for
their ability to frustrate the polar order, leading to the emergence of exotic
polar textures. The electrostatic depolarization, thought to be responsible for
the complex polar textures in these superlattices can be alleviated by
replacing the dielectric layer with a metallic one. One would thus expect that
a close to uniform polarization state be recovered in the ferroelectric layer.
However, here we show, using Density Functional Theory calculations, that
antipolar motions may still appear in superlattices combining multiferroic
BiFeO3 and metallic SrRuO3 perovskite layers. We find that a complex oxygen
octahedra tilt order, a so-called nanotwin phase, exists in BiFeO3/SrRuO3
superlattices and competes with a more conventional phase. It leads to a
doubling of the chemical period along the out-of-plane direction, owing to the
presence of an oxygen octahedra tilt wave pattern and antipolar motions caused
by trilinear energy couplings. We also show that out-of-plane polar
displacements in the BiFeO3 layer may reverse the (anti)polar displacements
thanks to a strong quadrilinear coupling term. The oxygen tilt-driven couplings
identified here reveal new ways to engineer and control polar displacements in
superlattice based polar metals and hybrid improper (anti)ferroelectrics.
