Length scales of interfacial coupling between metal and insulator phases in oxides

Understanding the mechanisms that control the metal to insulator transition (MIT) in correlated electron systems is one of the major challenges in condensed matter physics. Moreover, remarkably little is known about the characteristic lengths scale over which a metallic or insulating region can be established and the physics that sets this length scale. In this work, we use experimental and theoretical methods to design and study superlattices of two distinct rare earth nickelate oxides SmNiO₃ and NdNiO₃ that in bulk form show a MIT at two very different temperatures (400 K and 200 K, respectively). We find that, depending on the superlattice periodicity, these new complex oxide superlattices display different MIT behavior than that of the "bulk" materials. We show that the length scale of the metal-insulator transition in the superlattices is set not by the length scale of the propagation of structural motifs across the two materials, which ab-initio calculations and STEM analysis suggest is minimal, but rather by the balance between the energy cost of the boundary between a metal and an insulator and the energy gain of the bulk phases (Domínguez, C., Georgescu, A.B., Mundet, B. et al. Nat. Mater. 19, 1182–1187 (2020)).