Non-Korringa ¹³ C Nuclear Relaxation in the Normal State of the K ₃ C ₆₀ Superconductor

We present 13 C NMR results on K 3 C 60 showing that nuclear relaxation is exponential only above 250 K where molecular motion narrows the NMR line. The non-exponential nuclear relaxation at low temperatures is an intrinsic property of the frozen state and is associated with intramolecular variations of the electron spin density distribution. Dipolar coupling to the electron spins at the Fermi level of the carbon pπ-t 1 u band is identified as the dominant source of 13 C relaxation. The measured mean value of ( T 1 T ) -1 is determined not only by the total density of states at the Fermi level, but also by the intramolecular electron spin density distribution.