Significance of Off-Center Rattling for Emerging Low-lying THz Modes in type-I Clathrates

Abstract

We show that the distinct differences of low-lying THz-frequency dynamics between type-I clathrates with on-center and off-center guest ions naturally follow from a theoretical model taking into account essential features of the dynamics of rattling guest ions. Our model analysis demonstrates the drastic change from the conventional dynamics shown by on-center systems to the peculiar dynamics of off-center systems in a unified manner. We claim that glass-like plateau thermal conductivities observed for off-center systems stem from the flattening of acoustic phonon dispersion in the regime |k|<|G|/4. The mechanism is applicable to other systems such as glasses or relaxers.

References in corpus (2)

• Glasslike vs. crystalline thermal conductivity in carrier-tuned Ba8Ga16X30 clathrates (X = Ge, Sn)
• Interacting Dipoles in Type-I Clathrates: Why Glass-like though Crystal?

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