Metal-insulator transition in Pr Ru4 P12 and Sm Ru4 P12 investigated by optical spectroscopy

Electronic structures of the filled-skutterudite compounds $\mathrm{Pr}{\mathrm{Ru}}_{4}{\mathrm{P}}_{12}$ and $\mathrm{Sm}{\mathrm{Ru}}_{4}{\mathrm{P}}_{12}$, which undergo a metal-insulator transition (MIT) at ${T}_{\mathrm{MI}}=60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $16\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, have been studied by means of optical spectroscopy. Their optical conductivity spectra have clearly revealed an energy gap of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ below ${T}_{\mathrm{MI}}$. The detailed temperature and energy dependences of the energy gap are shown, which give much more information about the gap compared with that from the electrical resistivity experiment. For $\mathrm{Pr}{\mathrm{Ru}}_{4}{\mathrm{P}}_{12}$, in addition, optical phonon peaks in the spectra show anomalies upon the MIT, including broadening and shifts at ${T}_{\mathrm{MI}}$ and an appearance of new peaks below ${T}_{\mathrm{MI}}$. These results are discussed in terms of density waves or orbital ordering previously predicted for these compounds.