K. Iwamoto, S. Kushibiki, H. Honda, S. Kajitani, T. Mori, H. Matsumoto, N. Toyota, K. Suekuni, M. A. Avila, T. Takabatake
By improving experimental techniques in our terahertz time-domain spectroscopy, we have readdressed the optical conductivity spectra for rattling phonons strongly hybridized with the cage acoustic phonons in the title clathrate. In particular, the lowest-lying spectra consist of a sharp Lorentzian peak at 1.2 THz superimposed on a broad tail weighted in the lower frequency regime around 1.0 THz. The phonon softening monotonically induced by decreasing temperature is well explained by anharmonicity in rattling vibrations. On the other hand, at low temperatures, the optical weight globally shifts to lower frequency and accompanies a broadening of the phonon peak. These newly found anomalies are discussed by impurity scattering effects on the hybridized phonon system.
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http://arxiv.org/abs/1206.0073
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