Connecting bulk symmetry and orbital polarization in strained RNiO ₃ ultrathin films

We examine the structural and electronic properties of LaNiO₃ and NdNiO₃ epitaxial thin films grown on cubic (001) SrTiO ₃ from the viewpoint of bulk crystal symmetry and misfit strain. X-ray scattering and polarization-dependent x-ray absorption spectroscopy measurements are performed to determine the crystal symmetry and extract the local Ni 3d orbital response, respectively, to understand the strain-induced distortions of the bulk structure. A strain-induced orbital polarization is found in NdNiO₃ films, but is absent in LaNiO₃ films. The difference in electronic structure is attributed to the bulk thermodynamic phases through group theoretical methods, which reveals that thin film perovskites retain a "memory" of their preferred electronic and structural configurations.