Surface symmetry-breaking and strain effects on orbital occupancy in transition metal perovskite epitaxial films

The electron occupancy of 3d-orbitals determines the properties of transition metal oxides. This can be achieved, for example, through thin-film heterostructure engineering of ABO3 oxides, enabling emerging properties at interfaces. Interestingly, epitaxial strain may break the degeneracy of 3d-eg and t2g orbitals, thus favoring a particular orbital filling with consequences for functional properties. Here we disclose the effects of symmetry breaking at free surfaces of ABO3 perovskite epitaxial films and show that it can be combined with substrate-induced epitaxial strain to tailor at will the electron occupancy of in-plane and out-of-plane surface electronic orbitals. We use X-ray linear dichroism to monitor the relative contributions of surface, strain and atomic terminations to the occupancy of 3z2-r2 and x2-y2 orbitals in La2/3Sr1/3MnO3 films. These findings open the possibility of an active tuning of surface electronic and magnetic properties as well as chemical properties (catalytic reactivity, wettability and so on).