Mott-Hubbard gaps and their doping-induced collapse in strongly correlated pyrochlore ruthenates

Abstract

We investigate the variation of charge dynamics in the course of the filling-control metal-insulator transitions (MITs) for pyrochlore ${R}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ ($R$ being rare-earth ions). With widely changing of $R$ ions from Lu to Pr, the antiferromagnetic transition temperature systematically increases while the magnitude of the charge gap decreases toward zero at the hypothetical bandwidth-control Mott transition. It is attributable to the reduction of effective electron correlation in the Mott-Hubbard insulator regime. Doping holes give rise to the filling-control MITs from antiferromagnetic insulators to paramagnetic metals accompanied by the collapse of the Mott-Hubbard gaps. The doping-induced spectral weight transfer of the optical conductivity from the above-gap region to the in-gap region is nearly proportional to the doping level, whose rate is enhanced with the reduction of the electron correlation, in accord with the standard Mott criticality.