Emergent Metal–Insulator Transitions Associated with Electronic Inhomogeneities in Low-Dimensional Complex Oxides

Abstract

A prominent feature of complex oxides is the coexistence of competing electronic phases. The separation of metallic and insulating phases is believed to be responsible for a variety of emergent transport phenomena, including quantum criticality in ruthenates and colossal magnetoresistance (CMR) in manganites. Interestingly, the phase boundaries between neighboring phases can often be displaced by small perturbations such as chemical doping, heating, stress, and electric or magnetic field, leading to intriguing metal–insulator transitions (MITs). The association of the emergent MIT with electronic inhomogeneities is particularly pronounced in low-dimensional materials which are uniquely suited to studying the MIT and phase evolutions in response to modification of the order parameters. Here, we present a few examples to illustrate the intimate interplay between emergent MIT and the competing electronic phases in functional metal oxide materials, including a percolative MIT near the critical temperature of the Mott transition in a Mn-doped bilayer ruthenate Sr3Ru2O7 crystal surface, and the abrupt conductance changes and reemergent MIT in manganite nanowires of La5/8 − x Pr x Ca3/8MnO3. This experimental research has benefited from new developments in the fabrication and characterization of low-dimensional oxide materials and nanostructures. A rare glimpse of the microscopic phase separation, the dynamic phase percolation, and the strain-tuned MIT has been provided. The results indicate the critical role of electron–lattice interactions in phase separation and suggest that the origin of phase coexistence is much more strongly influenced by strain than local chemical inhomogeneity, both for ruthenates and manganites.