Spectral Weight Transfer of the Optical Conductivity in Doped Mott Insulators

Optical spectra have been systematically investigated for $R_{1-x}{\mathrm{Ca}}_x{\mathrm{TiO}}_3$ with various rare earth ions $R$ ( $=\mathrm{La}$, 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 $\delta$ ( $=1-n$) is critically enhanced as $U/W$ approaches the critical value $(U/W{)}_c$ for the bandwidth-controlled Mott transition.