73Ge- and 135/137Ba-NMR Studies of Clathrate Superconductor Ba24Ge100

We report on the normal-state properties of a superconducting type-III clathrate compound Ba24Ge100 investigated by Ge-73-NMR at cage sites and by Ba-135/137-NMR at guest sites at ambient pressure (P = 0) and 2.7 GPa. The measurements of the nuclear spin-lattice relaxation rate 1/T-1 have revealed a significant increase in 1/T1T of approximately 200 K at P = 0, which may be relevant to the rattling motion of Ba atoms in Ge cages. This increase in 1/T1T at high temperatures is markedly suppressed at P = 2.7 GPa. As a result of the suppression of the successive structural transformations, the 1/T1T = const. in a low-temperature regime is increased, proving the increase in the density of states at the Fermi level, N(E-F), at P. It is this increase in N(E-F) that increases the superconducting transition temperature T-c from 0.24 K at P = 0 to 3.8 K at P similar to 2.7 GPa.