We investigated the transport and optical properties of $\mathrm{Sr}{\mathrm{Ti}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_3$ $(0⩽x⩽1)$ which shows a metal-insulator transition at $x$ about 0.5 at room temperature. While $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ is a ferromagnetic metal with 4 $t_{2g}$ electrons, $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ is a band gap insulator with an optical gap of about $3.7\mathrm{eV}$. Because the $t_{2g}$ states of Ru and Ti are well decoupled, the electronic structure of $\mathrm{Sr}{\mathrm{Ti}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_3$ near the Fermi level is dominated by Ru $t_{2g}$ states. The temperature dependent transport data of samples near the transition showed characteristics of various localization phenomena. The optical conductivity spectra showed that as $\mathrm{Sr}{\mathrm{Ti}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_3$ changes from a metal to an insulator, the Drude-like peak decreases and evolves into an incoherent peak, which shifts to higher energy gradually and disappears. This evolution of the optical conductivity could not be explained by either disorder or correlation mechanisms. From our transport and optical data, we could categorize six kinds of electronic states in this system depending on $x$: a correlated metal $(x\sim 1.0)$, a disordered metal $(x\sim 0.7)$, an Anderson insulator $(x\sim 0.5)$, a soft Coulomb gap insulator $(x\sim 0.4)$, a disordered correlation insulator $(x\sim 0.2)$, and a band insulator $(x\sim 0.0)$. To understand these electronic structure evolutions, the disorder and the electron correlation effects should be considered together. We believe that $\mathrm{Sr}{\mathrm{Ti}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_3$ is a prototype system experiencing a Mott-Hubbard like transition in the Ru $t_{2g}$ alloy band, which is derived from the combined effects of the disorder and the electron correlation.

• Received 22 September 2004

DOI:https://doi.org/10.1103/PhysRevB.71.125104