Metal-Insulator Transitions and Non-Fermi Liquid Behaviors in 5d Perovskite Iridates

Transition metal oxides, in particular, 3d or 4d perovskites, have provided diverse emergent physics that originates from the coupling of various degrees of freedom such as spin, lattice, charge, orbital, and also disorder. 5d perovskites form a distinct class because they have strong spin-orbit coupling that introduces to the system an additional energy scale that is comparable to bandwidth and Coulomb correlation. Consequent new physics includes novel Jeff = 1/2 Mott insulators, metal-insulator transitions, spin liquids, and topological insulators. After highlighting some of the phenomena appearing in the Ruddlesden-Popper iridate series Srn+1IrnO3n+1 (n = 1, 2, and ?), we focus on the transport properties of perovskite SrIrO3. Using epitaxial thin films on various substrates, we demonstrate that metal-insulator transitions can be induced in perovskite SrIrO3 by reducing its thickness or by imposing compressive strain. The metal-insulator transition driven by thickness reduction is due to disorder, but the metal-insulator transition driven by compressive strain is accompanied by peculiar non-Fermi liquid behaviors, possibly due to the delicate interplay between correlation, disorder, and spin-orbit coupling. We examine various theoretical frameworks to understand the non-Fermi liquid physics and metal-insulator transition that occurs in SrIrO3 and offer the Mott-Anderson-Griffiths scenario as a possible solution.