Optical probe of the metal-insulator transition in pyrochlore-type molybdate

We have investigated a metal-insulator transition in a $4d$-electron system $R_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ $(R=\mathrm{Sm}$ and Y) by means of optical spectroscopy. The electronic structure is reconstructed over an energy range of several electron volt upon the transition. The insulating ${\mathrm{Y}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ compound shows an optical gap of ca. 0.1 eV at 10 K and the temperature dependence of optical conductivity is rather weak. On the other hand, metallic ${\mathrm{Sm}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ exhibits a tiny Drude-peak and strong mid-infrared absorption at 10 K and the conductivity strongly depends on temperature. These observations demonstrate the importance of correlation effect also in $4d$-electron system with a wider bandwidth than $3d$-electron system.