Evidence against strong correlation in 4d transition-metal oxides CaRuO₃ and SrRuO₃

We investigate the electronic structure of 4d transition-metal oxides CaRuO₃ and SrRuO₃. The analysis of the photoemission spectra reveals significantly weak electron correlation strength (U∕W∼0.2) as expected in 4d systems and resolves the long-standing issue that arose due to the prediction of large U∕W similar to 3d systems. It is shown that the bulk spectra, thermodynamic parameters, and optical properties in these systems can consistently be described using first-principles calculations. The observation of different surface and bulk electronic structures in these weakly correlated 4d systems is unusual.