Strain-modulated Mott transition in EuNiO³ ultrathin films

A series of ultrathin epitaxial films of EuNiO³ (ENO) were grown on a set of substrates traversing from compressive (-2.4%) to tensile (+2.5%) lattice mismatch. On moving from tensile to compressive strain, transport measurements demonstrate a successively suppressed Mott insulating behavior eventually resulting in a complete suppression of the insulating state at high compressive strain. Corroborating these findings, resonant soft x-ray absorption spectroscopy at the Ni L_3,2 edge reveals the presence of a strong multiplet splitting in the tensile strained samples that progressively weakens with increasing compressive strain. Combined with cluster calculations, the results show how cumulatively enhanced covalency (i.e., bandwidth) between Ni d and O p orbital derived states leads to the emergent metallic ground state not attainable in the bulk ENO.