Origin of the different electronic structure of Rh- and Ru-doped Sr2IrO4

One way to induce insulator-to-metal transitions in the spin-orbit Mott insulator ${\mathrm{Sr}}₂\mathrm{Ir}{\mathrm{O}}₄$ is to substitute iridium with transition metals (Ru, Rh). However, this creates intriguing inhomogeneous metallic states, which cannot be described by a simple doping effect. We detail the electronic structure of the Ru-doped case with angle-resolved photoemission and show that, in contrast to Rh, it cannot be connected to the undoped case by a rigid shift. We further identify bands below ${E}_F$ coexisting with the metallic ones that we assign to nonbonding Ir sites. We rationalize the differences between Rh and Ru by a different hybridization with oxygen, which mediates the coupling to Ir and sensitively affects the effective doping. We argue that the spin-orbit coupling does not control either the charge transfer or the transition threshold.