Epitaxial-strain-induced polar-to-nonpolar transitions in layered oxides.

Epitaxial strain can induce collective phenomena and new functionalities in complex oxide thin films. Strong coupling between strain and polar lattice modes can stabilize new ferroelectric phases from nonpolar dielectrics or enhance electric polarizations and Curie temperatures. Recently, strain has also been exploited to induce novel metal-insulator transitions and magnetic reconstructions through its coupling to nonpolar modes, including rotations of BO6 transition-metal octahedra. Although large strains are thought to induce ferroelectricity, here we demonstrate a polar-to-nonpolar transition in (001) films of layered A3B2O7 hybrid-improper ferroelectrics with experimentally accessible biaxial strains. We show the origin of the transition originates from the interplay of trilinear-related lattice mode interactions active in the layered oxides, and those interactions are directly strain tunable. Our results call for a careful re-examination of the role of strain-polarization coupling in ferroelectric films with nontrivial anharmonicities and offer a route to search for new functionalities in layered oxides.