Metal-insulator transition in $\mathrm{Pr}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$ and $\mathrm{Sm}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$ 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 $T_{\mathrm{MI}}=60\mathrm{K}$ and $16\mathrm{K}$, respectively, have been studied by means of optical spectroscopy. Their optical conductivity spectra have clearly revealed an energy gap of $\sim 10\mathrm{meV}$ 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.

Figure 1

(Color online) Optical reflectivity spectra $\left[R\left(\omega \right)\right]$ of $\mathrm{Pr}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$ (a) and $\mathrm{Sm}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$ (b) at different temperatures. Each inset shows $R\left(\omega \right)$ in a wider range of photon energies.

Figure 2

(Color online) Optical conductivity spectra $\sigma \left(\omega \right)$ of (a) $\mathrm{Pr}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$ and (b) $\mathrm{Sm}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$ at different temperatures. Each inset shows the integrated spectral weight $N_{\mathrm{eff}}\left(\omega \right)$ (see text). The low-energy portion of the spectra in (a) and (b) are shown in (c) and (d), respectively. For clarity, each spectra are offset by $2\times {10}^3{\Omega }^{-1}{\mathrm{cm}}^{-1}$ for (c) and $3\times {10}^3{\Omega }^{-1}{\mathrm{cm}}^{-1}$ for (d).

Figure 3

(Color online) (a) An example of the fitting for $\mathrm{Pr}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$, as discussed in the text. (b) The position of the gap excitation peak as a function of temperature. The superlattice diffraction intensity $\left({\mid \mathrm{F}\mid }^2\right)$ of $\mathrm{Pr}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$ (open circles) is also shown (Ref. 5) for comparison.

Figure 4

(Color online) The peak frequencies and the linewidths of phonon peaks in $\sigma \left(\omega \right)$ as a function of temperature for $\mathrm{Pr}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$ (a)–(c) and $\mathrm{Sm}{\mathrm{Ru}}_4{\mathrm{P}}_{12}$ (d)–(f).