13C spin-lattice relaxation was measured for a superconducting fulleride K3C60. It is found that non-single exponential relaxation (NSER) data at low temperatures (T < 55 K), which have a T-independent shape above and below Tc, are well reproduced by only the anisotropy parameter (αspin) for the hyperfine coupling that was derived from a line-shape analysis. From the simulation for various values of αspin, the shape of the NSER, as well as the asymmetric line shapes, is found to be caused by the anisotropy of an electronic single-site rather than electronic multi-sites. By extracting the isotropic part of relaxation time, (T1)iso, from the NSER, it is found that an extended Korringa relation holds up to 300 K with K(α) = 7.4, and that the T dependence of [(T1)isoT]-1 results from a decrease in the density of states. Below Tc, a broadened Hebel-Slichter coherence peak is observed, which means s-wave Cooper pairing. The T dependence of [(T1)isoT]-1 below Tc is well reproduced by a theoretical prediction with the superconducting gap 2Δ(0)/kBTc = 4.3. The origin of the isotropic hyperfine coupling is also discussed.
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