Antiferromagnetically Enhanced ¹³ C Spin-Lattice Relaxation in K₃C₆₀ - No Evidence of Electronic Multi-Sites

We report a study of ¹³C NMR spin-lattice relaxation in the superconducting fulleride K₃C₆₀ at low temperatures (T). The experimentally obtained relaxation data, which are scaled to a T-independent relaxation shape, are found to be well reproduced by the anisotropy parameter α^spin ≡ A_iso^spin/A_ax^spin, where A_iso^spin and A_ax^spin are the isotropic and anisotropic part of the ¹³ C hyperfine coupling of conduction electrons, respectively. Simulation for a powder sample with various α^spin values indicates that the deviation from a single-exponential relaxation is largest when the hyperfine coupling is isotropic to some extent (i.e., α^spin ≈ 0.5) instead of purely anisotropic (i.e., α^spin = 0). These results justify a model based on the electronic single site. Moreover, from the validity of the extended Korringa relation with K(α) = 5.7, it is found that the electronic state is a Fermi liquid in which the relaxation rate is enhanced substantially by the antiferromagnetic electron interaction.