Anomalous infrared spectra of hybridized phonons in type-I clathrate Ba$_8$Ga$_{16}$Ge$_{30}$

TL;DR

This study investigates the optical conductivity of rattling phonons in Ba8Ga16Ge30, revealing anomalous temperature-dependent linewidth broadening and phonon softening due to impurity scattering in a hybridized phonon system.

Contribution

The paper provides detailed terahertz spectroscopy analysis of hybridized phonons in a type-I clathrate, highlighting unexpected phonon linewidth broadening at lower temperatures.

Findings

Phonon peak at 1.2 THz shows linewidth broadening with decreasing temperature.

Monotonic softening of the phonon peak observed as temperature decreases.

Enhanced tail structure in the spectra attributed to impurity scattering effects.

Abstract

The optical conductivity spectra of the rattling phonons in the clathrate Ba$_8$Ga$_{16}$Ge$_{30}$ are investigated in detail by use of the terahertz time-domain spectroscopy. The experiment has revealed that the lowest-lying vibrational mode of a Ba(2)$^{2+}$ ion consists of a sharp Lorentzian peak at 1.2 THz superimposed on a broad tail weighted in the lower frequency regime around 1.0 THz. With decreasing temperature, an unexpected linewidth broadening of the phonon peak is observed, together with monotonic softening of the phonon peak and the enhancement of the tail structure. These observed anomalies are discussed in terms of impurity scattering effects on the hybridized phonon system of rattling and acoustic phonons.