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 73Ge-NMR at cage sites and by 135/137Ba-NMR at guest sites at ambient pressure (P=0) and 2.7 GPa. The measurements of the nuclear spin-lattice relaxation rate 1/T1 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(EF), at P. It is this increase in N(EF) that increases the superconducting transition temperature Tc from 0.24 K at P=0 to 3.8 K at P - 2.7 GPa. (author)