A series of ultra-thin 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 Ni L3,2 edge reveal the presence of a strong multiplet splitting in the tensile strained samples that progressively weakens with increasing compressive strain. Combined with the ab initio cluster calculations, the results show how comulatively 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.Complex transition metal oxides with correlated carriers have been at the forefront of condensed matter research towards the understanding of fundamental physics underlying several remarkable physical phenomena including high temperature superconductivity, colossal magnetoresistance, multiferroicity, and the thermally induced metal-insulator transition (MIT) [1][2][3][4][5][6] . In particular, the temperature driven MIT in correlated oxides has garnered a strong investigation effort over last several decades. Understanding of the MIT and its control by external stimuli such as pressure, magnetic field, light, confinement, or chemical doping is not only interesting from the fundamental physics point of view 7-10 , but also demonstrate great opportunities for future electronic devices 11 . On the way to those goals, recent advances in material synthesis by using strain engineering have opened a new dimension in controlling the materials properties. Additionally, the epitaxial relation has been used to stabilize new structural and electronic phases in the form of ultra thin films of coherently strained materials 12 . In such epitaxially stabilized structures, the effect of the lattice modulation on the materials properties can be quite dramatic 13,14,[16][17][18][19][20][21][22][23][24][25][26][27] and is of particular current interest as applications continue to accelerate towards ultra-thin films and heterostructures.The rare-earth (RE) nickelates RNiO 3 (R = Pr, Nd, Eu, ...) in their bulk form, with Ni having formal +3 oxidation state (t 6 2g e 1 g ), all, except for R = La, display a metal-insulator transition, while the nature of the transition and its temperature (T M IT ) depends strongly on the choice of the rare-earth cation as shown in Fig. 1 28-30 . Specifically, the most distorted members with R= Lu, Y, Eu and Sm, etc. first exhibit a second order MIT at higher temperature accompanied by the development of a possible charge ordered state 31-35 while the magnetic moments remain disordered across the transition. Upon further cooling, these compounds undergo another second order transition characterized by a E -type antiferromagnetism. In sharp contrast,...
