Analysis of 13 C-NMR spectra in C 60 superconductors: Hyperfine coupling constants, electronic correlation effect, and magnetic penetration depth

A 13 C-NMR anisotropic hyperfine coupling tensor was determined as 2(Ϫ1.68, Ϫ1.68, 3.37) ϫ10 6 rad/sec for C 60 3Ϫ in A 3 C 60 superconductors, where A is an alkali metal, by analyzing 13 C-NMR spectra below 85 K. Combined with an isotropic coupling constant of (2ϫ0.69)ϫ10 6 rad/sec, the 2s and 2p characters of the electronic wave functions at the Fermi level were deduced. The results were compatible with local-density-approximation band calculations. From a simulation of 13 C-NMR spectra at superconducting state, the traceless chemical ͑orbital͒ shift tensor and isotropic chemical shift were determined as ͑67, 34, Ϫ101͒ ppm and ϳ150 ppm, respectively. An estimated magnetic penetration depth is larger than 570 nm in K 3 C 60 . Furthermore, the modified Korringa relation, T 1 TK 2 ϳ␤S ͑with Knight shift K, spin-lattice relaxation time T 1 , and Korringa constant S͒, clearly showed the existence of weak but substantial antiferromagnetic spin fluctuation in A 3 C 60 ; ␤ϭ0.40-0.58 with an error of Ϯ20%. The Stoner enhancement factor was also determined as 1-1.5 from a comparison between spin susceptibility obtained from NMR and band-calculation results. ͓S0163-1829͑98͒00542-6