Abstract
The 3d transition metal (M) perovskite oxides exhibit a remarkable array of properties, including novel forms of superconductivity, magnetism and multi-ferroicity. Strain can have a profound effect on many of these properties. This is due to the localized nature of the M 3d orbitals, where even small changes in the M-O bond lengths and M-O-M bond angles produced by strain can be used to tune the 3d-O 2p hybridization, creating large changes in electronic structure. A new route is presented to strain the M-O bonds in epitaxial two-dimensional perovskite films by tailoring local electrostatic dipolar interactions within every formula unit via atomic layer-by-layer synthesis. The response of the O anions to the resulting dipole electric fields distorts the M-O bonds by more than 10%, without changing substrate strain or chemical composition. This distortion is largest for the apical oxygen atoms (O-ap), and alters the transition metal valence state via self-doping without chemical substitution.