Low-temperature heat capacity of Sr8Ga16Ge30 and Ba8Ga16Ge30: Tunneling states and electron-phonon interaction in clathrates

Tunneling states of Sr8Ga16Ge30 (SGG) and Ba8Ga16Ge30 (BGG) are studied in detail using single crystals with a variety of carrier concentrations from the view point of heat capacity (C-p). An important excessive contribution alpha T due to the tunneling states is found besides gamma T-e of the conduction electrons. The value of alpha is accurately deduced to be 10.1 +/- 1.5 mJ/mol K-2 for SGG while such a value is as small as 0.8 +/- 0.4 mJ/mol K-2 for BGG. From the temperature evolution of C-p as a function of carrier concentration, the effective masses, one of the most important physical parameters for evaluating electron-phonon interactions lambda(e-ph), are accurately estimated to be 1.68m(0) and 1.01m(0) for SGG and BGG, respectively. The lambda(e-ph) seems to be enhanced when the anharmonic rattling modes are involved.