Electronic States and Superconductivity in Alkali-Intercalated Fullerides : ^<13>C-NMR Study in Na_2RbC_<60>, Na_2CsC_<60>, K_3C_<60>, K_2RbC_<60>, K_2CsC_<60>, KRbCsC_<60>, Rb_2CsC_<60> and RbCs_2C_<60>

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A systematic ^<13>C NMR experiment is reported in alkali-fullerides A_3C_<60> (where A_3 is alkali-metals) between superconducting transition temperature T_c and 〜100 K. Careful ^<13>C NMR spin-lattice relaxation time study clarified the properties of the conduction electron wave functions at Fermi surface. An upper limit of Stoner exchange parameter was estimated to be 〜3 eV/(spin-carbon), which is far from spin-wave instability criterion. Based on these studies, a variation of the density of states at Fermi level, N(E_f), with T_c was deduced. The results were analyzed in terms of Mc-Millan formula, leading to weak or medium electron-phonon coupling constants and relevant phonon energy larger than 600 K. It was strongly suggested that the A_3C_<60> superconductor is described within a framework of the conventional phonon-mediated BCS superconductivity.
社団法人日本物理学会の論文
1994-03-15