NMR studies of silicon clathrate compounds

Abstract

29Si and 137Ba NMR measurements were applied to silicon clathrate compound BagAgxSi46−x (x=0∼6) to reveal the electronic structure and the superconducting mechanism. The large Knight shifts in the superconducting samples indicated the large density of states at Si and Ba sites. In Ba8Si46, the temperature dependence of the Knight shift at Si site in the normal state was reproduced by an activation type plus constant between 50 K and 300 K, which suggests that the electrons at the Fermi level in the sharp band are thermally exited to the Si46 states. The opening of an energy gap was confirmed from the decrease of the spin-lattice relaxation time of 29Si NMR with decreasing temperature. The temperature dependences of the Knight shift and the spin-spin relaxation rate are unconventional below the superconducting transition temperature.