We report field and temperature dependent multinuclear NMR Knight shift and Tl investigations of A3C60 superconductors (A = alkali metal). Suppression of the Hebel-Slichter peak in I/T, T is shown to result from large applied magnetic fields. The normal state temperature variation of T&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. PACS numbers: 74.25.Nf, 74.70.Wz, 76.60.k Nuclear magnetic resonance (NMR) has historically provided crucial tests of proposed theories of both conventional [1] and exotic [2,3] forms of superconductivity. Alkali-fulleride compounds [4,5] (A3C6o, A = alkali metal), with transition temperatures approaching the limit that can be obtained from Bardeen-Cooper-Schrieffer (BCS) theory, provide the most recent challenge. Tycko et al. [6] reported ' C NMR investigations of K 3C 60 and Rb 3C 6O which uncovered unusual properties, including the absence or near absence of the Hebel-Slichter (HS) coherence peak [7] in the spin-lattice relaxation. In this Letter we report field and temperature dependent NMR measurements at multiple atomic sites (' C, Rb, and ' Cs) of A3C6o superconductors which resolve this and several other outstanding issues: We find that the suppression of the HS coherence peak results from the application of large magnetic fields, and that the peak is clearly present at low fields. We show that the unusual temperature dependence of the normal state spin-lattice relaxation (T~T) is associated with thermal expansion of the lattice, and that Stoner enhancement is implied. Finally, we use the NMR Knight shift to measure the superconducting state spin susceptibility, which supports the BCS weak coupling limit. A3C6Q samples [8] were characterized by x-ray diffraction and NMR. Zero-field cooled, 10 G SQUID susceptibility measurements on the Rb2CsC60 sample used in Tl measurements yielded T, = 31 K (onset), and shielding fractions of -100% at 5 K and -50% at 28 K. ' C and Cs T&'s were obtained using the saturation-recovery se-87 cc quence. For spin-2 Rb only the "central" transition (2 to -2) is observed; we use the inversion recovery sequence and a well-known double exponential fit. ' C relaxation follows a stretched exponential [6], exp[ -(t/T~)t ], and P = 0.85 provides an excellent fit for all T ) 10 K. Temperature dependent Rb, ' Cs, and ' C spinlattice relaxation rates I/T~T for Rb2CsC6p at 8.8 T, shown in Fig. 1, demonstrate that the T~' s for all three nucIear species have the same temperature dependence. Their identical behavior indicates that all three nuclei 0.1 0.01 pooa o o Cl~~0 oooo o o

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The 87 Rb NMR line shape of the Rb 3 C 60 superconductor contains three distinct peaks: one associated with octahedrally coordinated Rb in the fcc lattice of C 60 molecules and two others, labeled T and TЈ, both associated with tetrahedrally coordinated Rb. This contrasts with the accepted crystal structure, in which all tetrahedral Rb sites are equivalent. We report multinuclear single and double resonance NMR experiments which probe for effects which could lead to the unexpected splitting, and discuss implications for electronic structure.