Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films

X-ray photoelectron diffraction is used to directly probe the intracell polar atomic distortion and tetragonality associated with ferroelectricity in ultrathin epitaxial PbTiO₃ films. Our measurements, combined with ab initio calculations, unambiguously demonstrate noncentrosymmetry in films a few unit cells thick, imply that films as thin as three unit cells still preserve a ferroelectric polar distortion, and also show that there is no thick paraelectric dead layer at the surface.