Strain-modulated Mott transition in EuNiO${}_3$ ultrathin films

A series of ultrathin epitaxial films of EuNiO${}_3$ (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.

Figure 1

Partial phase diagram for the family of rare-earth nickelates of different $A$ sites (data from Refs. 28 and 29). The dotted line represents the change from orthorhombic ($Pbnm$) to rhomohedral ($R\overline{3}C$) symmetry.

Figure 2

(a) XRD data for 15 uc ENO samples on various substrates. The arrows indicate the film peaks. The shifting from the bulk lattice value (indicated by the dashed line) is apparent for the compressively strained samples. Note for SLAO the (0 0 6) rod is scanned due to the tetragonal structure. The data have been artificially shifted vertically to ease inspection. (b) RSM for a 35 uc ENO film grown on NGO showing the film is coherently strained.

Figure 3

(a) Transport data for 15 uc ENO samples on various substrates. The arrow indicates the direction of increasing compressive strain. (b) $d\ln \left(\rho \right)/d(1/T)$ data for the films. Arrows indicate the location of $T^{*}$. (c) $T^{*}$ for various strains.

Figure 4

(a) XAS data for 15 uc ENO samples on various substrates showing the change in the multiplet peak with strain. The data have been artificially shifted vertically to ease inspection. (b) The experimentally obtained peak splitting along with the theoretically obtained corresponding CT energy. Note a small peak, indicated by the asterisks, around $\sim$ 851 eV corresponding to the La M${}_4$ edge appears for the substrates containing La.