NMR study in the superconducting silicon clathrate compound NaxBaySi46

Si-29, Na-23, Ba-137, and Ba-135 NMR experiments were carried out in the superconducting silicon clathrate compound NaxBaySi46 to study the electronic states above the superconducting transition temperature. We observed three distinct Si-29 signals with different Knight shift (2036, 862, and 720 ppm at 90 K with a small temperature dependence of similar to 10%), which were ascribed to inequivalent Si sites in the Si-46 structure. The results indicate that the conduction-electron spin and/or charge density is strongly site-dependent. At all the atomic sites, the Korringa relation in NMR relaxation was observed, indicative of metallic electronic structure with moderate electronic correlation The Na and Ba atoms are partially ionized in NaxBaySi46, which shows the noticeable difference from the system of alkali-metal-doped fullerides. The comparison with band calculations is discussed.