Suppression of charge and spin order in confined NdNiO$_3$ layers

Atomic-layer synthesis allows one to study and control the complex phase behavior correlated systems by controllably modifying dimensionality and interfacial constraints. To this end, the rare-earth nickelates ($R$NiO$_3$) embody a remarkable model system, as exhibited by the bulk metal-insulator and paramagnetic-antiferromagnetic ordering transitions, which are sensitive to structural and electronic conditions. We present evidence from transport, x-ray absorption, and resonant x-ray scattering measurements of NdNiO$_3$/NdAlO$_3$ superlattices of a suppression of charge and spin order induced by interfacial confinement. We find that the spectroscopic signatures of charge localization and antiferromagnetic ordering remain for NdNiO$_3$ layers thicker than a single unit cell. The disappearance of ground state order in single NdNiO$_3$ layers is attributed to enhanced $e_g$ orbital polarization from the interaction with the NdAlO$_3$ confining layers. We also observe a crossover region of thickness with no metal-insulator transition but distinct charge and spin ordering temperatures. These findings shed light on the interplay between competing ground states in nickelates and help guide efforts to controlling long-range order in such systems.