Metal-insulator transition in a disordered and correlated SrT1-xRuxO3 system:: Changes in transport properties, optical spectra, and electronic structure -: art. no. 125104

We investigated the transport and optical properties of SrTi1-xRuxO3 (0 <= x <= 1) which shows a metal-insulator transition at x about 0.5 at room temperature. While SrRuO3 is a ferromagnetic metal with 4 t(2g) electrons, SrTiO3 is a band gap insulator with an optical gap of about 3.7 eV. Because the t(2g) states of Ru and Ti are well decoupled, the electronic structure of SrTi1-xRuxO3 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 SrTi1-xRuxO3 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 similar to 1.0), a disordered metal (x similar to 0.7), an Anderson insulator (x similar to 0.5), a soft Coulomb gap insulator (x similar to 0.4), a disordered correlation insulator (x similar to 0.2), and a band insulator (x similar to 0.0). To understand these electronic structure evolutions, the disorder and the electron correlation effects should be considered together. We believe that SrTi1-xRuxO3 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.