Mechanism for the metal-insulator transition in Sr2Ir1-xRuxO4.

Abstract

The mechanism for the metal-insulator (MI) transition in ${\mathrm{Sr}}_{2}$${\mathrm{Ir}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ru}}_{\mathit{x}}$${\mathrm{O}}_{4}$ is studied through heat-capacity and magnetic-susceptibility measurements. For metallic ${\mathrm{Sr}}_{2}$${\mathrm{RuO}}_{4}$, we find a large Sommerfeld constant ${\ensuremath{\gamma}}_{0}$ of 29 (0.5) mJ/mol ${\mathrm{K}}^{2}$ and a correspondingly high Pauli susceptibility indicating a narrow bandwidth. As the 4${\mathit{d}}^{4+}$ ${\mathrm{Ru}}^{4+}$ is substituted by the 5${\mathit{d}}^{5}$ ${\mathrm{Ir}}^{4+}$, the temperature-independent magnetic susceptibility ${\mathrm{\ensuremath{\chi}}}_{0}$ and ${\ensuremath{\gamma}}_{0}$ initially increase, marking an increase in the density of states (DOS) at the Fermi level and a further narrowing of the band. For x0.90, a rapid decrease in ${\mathrm{\ensuremath{\chi}}}_{0}$ and a commensurate increase in the effective Curie moment ${\mathit{p}}_{\mathrm{eff}}$ suggest that the electron spins start to localize. The ${\ensuremath{\gamma}}_{0}$ continues to increase up to x\ensuremath{\sim}0.70, indicating a further enhancement of the DOS over this localized region, below which the full local moment of ${\mathrm{Ru}}^{4+}$ is observed. Finally, a gap in the DOS is developed for ${\mathrm{Sr}}_{2}$${\mathrm{IrO}}_{4}$. Our results suggest that the MI transition is caused by perturbations of the weakly overlapping orbitals due to changes in the crystal structure induced by Ir doping.