Covalency-driven unusual metal-insulator transition in nickelates

Electronic structures of metallic $LaNiO_3$ and insulating $NdNiO_3$ are investigated using electron spectroscopies as well as band-structure and model-Hamiltonian calculations. It is shown that there is an increase in the transfer integral, $t_{pd}$ in $LaNiO_3$ compared to $NdNiO_3$ though $U_{dd}$ and A are very similar. A phase diagram obtained with a multiband Hubbard model explains the metal-insulator transition as well as the anomalous resistivity of $NdNiO_3$ under pressure while indicating various interesting possibilities near the metal-insulator phase line