We present evidence that the metal-insulator transition (MIT) in a tensile-strained ${\mathrm{NdNiO}}_3$ (NNO) film is facilitated by a redistribution of electronic density and that it neither requires Ni charge disproportionation nor a symmetry change [U. Staub et al., Phys. Rev. Lett. 88, 126402 (2002); R. Jaramillo et al., Nat. Phys. 10, 304 (2014)]. Given that epitaxial tensile strain in thin NNO films induces preferential occupancy of the $e_g$ $d_{{\mathit{x}}^2-{\mathit{y}}^2}$ orbital we propose that the larger transfer integral of this orbital state with the O $2p$ orbital state mediates a redistribution of electronic density from the Ni atom. A decrease in the Ni $d_{{\mathit{x}}^2-{\mathit{y}}^2}$ orbital occupation is directly observed by resonant inelastic x-ray scattering below the MIT temperature. Furthermore, an increase in the Nd charge occupancy is measured by x-ray absorption at the Nd $L_3$ edge. Both spin-orbit coupling and crystal field effects combine to break the degeneracy of the Nd $5d$ states, shifting the energy of the Nd $e_g$ $d_{{\mathit{x}}^2-{\mathit{y}}^2}$ orbit towards the Fermi level, allowing the $A$ site to become an active acceptor during the MIT. This work identifies the relocation of electrons from the Ni $3d$ to the Nd $5d$ orbitals across the MIT. We propose that the insulating gap opens between the Ni $3d$ and O $2p$ states, resulting from Ni $3d$ electron localization. The transition seems to be neither a purely Mott-Hubbard transition nor a simple charge transfer.

• Received 10 December 2014

DOI:https://doi.org/10.1103/PhysRevLett.115.036401