NMR investigation on C₆₀ superconductors, Cs_xRb_3-xC₆₀(x=1,2)

¹³³Cs-NMR was observed in C₆₀ superconductors, Cs_xRb_3-xC₆₀ with x=1 and 2. It was found that Cs atoms are intercalated in octahedral interstitial sites of C₆₀ fcc lattice for x=1, while for x=2 in both of the octahedral and tetrahedral interstitial sites. The shift of the ¹³³Cs-NMR strongly suggested that the 6s electron of Cs atom is fully ionized and conduction electrons are mainly on the C₆₀ sites. ¹³C-NMR was observed to study electronic states at C₆₀ sites. It showed a monotonic increase of the superconducting transition temperature, Tc, with increasing Fermi level density of states, N(E_f), at carbon sites, where N(E_f)^-2 was assumed to be proportional to ¹³C spin-lattice relaxation time T₁ times temperature T : N(E_f)^-2∝ T₁T.