Strong coupling of rattling phonon to conduction electrons in semimetallic type-I clathrate Ba⁸Ga¹⁶Sn³⁰

We report studies of ^117,119Sn and ^69,71Ga nuclear magnetic resonance (NMR) to investigate electron-phonon interactions due to the off-center rattling of Ba atoms in the semi-metallic type-I Ba ₈Ga₁₆Sn₃₀clathrate with n-type charge carriers in the temperature range of 0.4-200 K. The Ga NMR spin-lattice relaxation rate has an anomalous enhancement at approximately 10 K, indicating that the Ba atoms exhibit thermal rattling motion in an anharmonic potential. From calculations using a two-phonon Raman process model for a one-dimensional anharmonic Hamiltonian within a quasi-harmonic approximation, significant broadening of the phonon lifetime and softening of the phonon frequency are found to be essential to reproduce the temperature dependence of the NMR spin-lattice relaxation rate. The characteristic energy of the renormalized phonon frequency w _r≈ 21K was obtained for the rattling state, in agreement with the values estimated by heat capacity analysis and other techniques. These results point to strong coupling of the rattling phonon to conduction electrons in semimetallic type-I Ba₈Ga₁₆Sn₃₀.