Variation of the charge dynamics in bandwidth-and filling-controlled metal-insulator transitions of pyrochlore-type molybdates

The systematics of the bandwidth- and filling-controlled metal-insulator transitions (MITs) have been investigated for ${\mathrm{R}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ family ($R=\mathrm{Nd}$, Sm, Eu, Gd, Dy, and Ho) by infrared spectroscopy. The substantial role of electron correlation in driving the MIT is verified. With changing the $R$ ionic radius $(r)$ or equivalently the one-electron bandwidth $(W)$, the MIT occurs in a continuous manner at ${r}_{c}\ensuremath{\approx}r(R=\mathrm{Gd})$. The $T=0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ gap continuously vanishes as $\ensuremath{\Delta}\ensuremath{\propto}({r}_{c}\ensuremath{-}r)$, while at the metallic side the linear decrease of Drude weight is followed toward ${r}_{c}$. In the metallic compounds, some of the infrared-active phonon modes show remarkably large Fano asymmetry correlating with the Drude weight. These Mo-O-Mo bending modes strongly couple to the conduction electrons via effective modulation of the bandwidth. Even for $r\ensuremath{\lesssim}{r}_{c}$ a minimal level of hole doping closes the correlation gap, for example, the barely insulating ${\mathrm{Gd}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ is turned to an incoherent metal by 5% partial substitution of ${\mathrm{Gd}}^{3+}$ with ${\mathrm{Ca}}^{2+}$. However, even on further doping no coherent electronic states are formed, indicating the role of the disorder-induced localization effect besides the dominant correlation effects.