Probing glasslike excitations in single-crystalline Sr₈Ga ₁₆Ge₃₀ by specific heat and thermal conductivity

We report the thermal conductivity κ and specific heat C measurements down to 0.3K on single-crystalline samples of n-type Sr₈Ga ₁₆Ge₃₀ and p-type Ba₈Ga₁₆Ge ₃₀ clathrates. The overall temperature dependence of κ(T) of both samples with a plateau at about 20K and a quadratic T dependence below 1K is well described by the soft-potential model. Using the soft-potential parameter of 4.4K derived from the analysis of κ(T) for Sr ₈Ga₁₆Ge₃₀, the broad maximum of C/T³ centered at 7K 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.