Dominance of the spin-dipolar NMR relaxation mechanism in fullerene superconductors

We have performed ab initio calculations for the NMR relaxation rate in doped fullerenes and find that the spin-dipolar relaxation mechanism dominates the orbital and Fermi-contact mechanisms. The reason is that the states at the Fermi level are ppπ-like so that the carbon orbitals have almost exclusively radial p character. With the values 7.2 and 8.1 states /(eV spin molecule) for the bare densities of states at the Fermi level for, respectively, ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$, plus the ab initio value 1.7 eV spin (C atom) for the effective Stoner exchange parameter, good agreement is obtained with the experimental relaxation rates.