Spectral weight transfer of the optical conductivity in doped Mott insulators

Optical spectra have been systematically investigated for R1-xCaxTiO3 with various rare earth ions R (=La, Pr, Nd, Sm, and Y) as a typical three-dimensional correlated electron system, in which both the electon-correlation strength (U/W) and the band filling (n) can be varied. With a decrease of n from 1, the spectrum of the Mott-gap excitation collapses and the Drude part evolves. We found that the rate of the Drude-part evolution with the nominal hole-doping level (=1-n) is critically enhanced as U/W approaches the critical value (U/W)c for the bandwidth-controlled Mott transition.