Probing Glasslike Excitations in Single-Crystalline Sr₈Ga₁₆Ge₃₀ by Specific Heat and Thermal Conductivity

Abstract

We report the thermal conductivity κ and specific heat C measurements down to 0.3 K on single-crystalline samples of n-type Sr₈Ga₁₆Ge₃₀ and p-type Ba₈Ga₁₆Ge₃₀ clathrates. The overall temperature dependence of κ(T) of both samples with a plateau at about 20 K and a quadratic T dependence below 1 K is well described by the soft-potential model. Using the soft-potential parameter of 4.4 K derived from the analysis of κ(T) for Sr₈Ga₁₆Ge₃₀, the broad maximum of C⁄T³ centered at 7 K is also well reproduced. The density of states of quasilocalized vibrational modes for Sr atoms in Sr₈Ga₁₆Ge₃₀ has a maximum at 0.7 THz, in agreement with the low-energy peak in a previous Raman scattering study.