73Ge- and135/137Ba-NMR Studies of Clathrate Superconductor Ba24Ge100

We report on the normal-state properties of a superconducting type-III clathrate compound Ba 24 Ge 100 investigated by 73 Ge-NMR at cage sites and by 135/137 Ba-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/ T 1 T 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/ T 1 T at high temperatures is markedly suppressed at P =2.7 GPa. As a result of the suppression of the successive structural transformations, the 1/ T 1 T =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 ∼2.7 GPa.