Mechanism for the metal-insulator transition in Sr2Ir1-xRuxO4

The mechanism for the metal-insulator (MI) transition in Sr2Ir1-xRuxO4 is studied through heat-capacity and magnetic-susceptibility measurements. For metallic Sr2RuO4, we find a large Sommerfeld constant γ0 of 29 (0.5) mJ/mol K2 and a correspondingly high Pauli susceptibility indicating a narrow bandwidth. As the 4d4+ Ru4+ is substituted by the 5d5 Ir4+, the temperature-independent magnetic susceptibility χ0 and γ0 initially increase, marking an increase in the density of states (DOS) at the Fermi level and a further narrowing of the band. For x<0.90, a rapid decrease in χ0 and a commensurate increase in the effective Curie moment peff suggest that the electron spins start to localize. The γ0 continues to increase up to x∼0.70, indicating a further enhancement of the DOS over this localized region, below which the full local moment of Ru4+ is observed. Finally, a gap in the DOS is developed for Sr2IrO4. 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.