Spectral Evolution in(Ca,Sr)RuO3near the Mott-Hubbard Transition

We investigated optical properties of $(\mathrm{Ca},\mathrm{Sr}){\mathrm{RuO}}_3$ films on the borderline of a metal-insulator transition. Our results show all of the predicted characteristics for a metallic Mott-Hubbard system, including (i) a mass enhancement in dc limit, (ii) a $U/2$ excitation, and (iii) a $U$ excitation. Also, self-consistency is exploited within the Gutzwiller-Brinkman-Rice picture for the Mott transition. Our finding displays that electron correlation should be important even in $4d$ materials. However, low frequency behaviors of electrodynamic quantities suggest extra scattering mechanisms in addition to the Mott-Hubbard correlation.