Low-temperature heat capacity of Sr₈Ga₁₆Ge ₃₀ and Ba₈Ga₁₆Ge₃₀: Tunneling states and electron-phonon interaction in clathrates

Tunneling states of Sr8 Ga16 Ge30 (SGG) and Ba8 Ga16 Ge30 (BGG) are studied in detail using single crystals with a variety of carrier concentrations from the view point of heat capacity (Cp). An important excessive contribution αT due to the tunneling states is found besides γe T of the conduction electrons. The value of α is accurately deduced to be 10.1±1.5mJ/mol K2 for SGG while such a value is as small as 0.8±0.4mJ/mol K2 for BGG. From the temperature evolution of Cp as a function of carrier concentration, the effective masses, one of the most important physical parameters for evaluating electron-phonon interactions λe-ph, are accurately estimated to be 1.68 m0 and 1.01 m0 for SGG and BGG, respectively. The λe-ph seems to be enhanced when the anharmonic rattling modes are involved.