Nuclear Magnetic Resonance of $A_3{\mathrm{C}}_{60}$ Superconductors

We report field and temperature dependent multinuclear NMR Knight shift and $T_1$ investigations of $A_3{\mathrm{C}}_{60}$ superconductors ($A=\mathrm{alkali}\mathrm{metal}$). Suppression of the Hebel-Slichter peak in $\frac{1}{T_{1}T}$ is shown to result from large applied magnetic fields. The normal state temperature variation of $T_{1}T$ is shown to be associated with thermal expansion of the lattice, and Stoner enhancement of the density of states is inferred. Knight shift results indicate the weak coupling limit of Bardeen-Cooper-Schrieffer superconductivity.