Optical conductivity of rattling phonons in type-l clathrates Ba_8Ga_(16)Ge_(30) and Ba_8Ga_(16)Sn_(30)

摘要

We review our dynamical property measurements of rattling phonons in type-I clathrate compounds, the quasi-on-center Ba_8Ga_(16)Ge_(30) (BGG) and off-center Ba_8Gai_(16)Sn_(30) (BGS), using a terahertz time-domain spectrometer. The lowest-lying vibrational modes of rattling Ba ions in the oversized cage show anomalous temperature dependence in their spectra. For BGG, the temperature dependence is mostly consistent with a local anharmonic potential approximation that predicts softening towards low temperature. On the other hand, for BGS, a single broad peak of off-center rattling phonons splits into two subpeaks below 120 K, and, with further lowering temperature, the spectra show anomalous broadening. While the splitting can be understood by the double well potential, the linewidth broadening must be attributed to some relevant interactions with such excitations as acoustic phonons and doped carriers.