¹³C spin-lattice relaxation study on the electronic properties of K₃C₆₀

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 T_c, are well reproduced by only the anisotropy parameter (α^spin) for the hyperfine coupling that was derived from a line-shape analysis. From the simulation for various values of α^spin, 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 T_c, a broadened Hebel-Slichter coherence peak is observed, which means s-wave Cooper pairing. The T dependence of [(T₁)_isoT]^-1 below T_c is well reproduced by a theoretical prediction with the superconducting gap 2Δ(0)/k_BT_c = 4.3. The origin of the isotropic hyperfine coupling is also discussed.