The alkali-doped solid materials A 3 C 60 (where A is an alkali metal), which are superconductors with transition temperatures among the highest known apart from the high-T c cuprates, are among the most exciting outgrowths of the discovery of the family of fullerene molecules. The structural, electronic, and superconducting properties of the alkali fullerides have been subjects of great controversy. In this article the authors review nuclear magnetic resonance (NMR) investigations of the alkali fullerides and of undoped C 60 . They show that, although the NMR data certainly provide evidence for unusual static and dynamic structural properties, there is little evidence for unusual normal-and superconducting-state electronic properties, such as strong correlations in the normal state or nonphononic mechanisms of superconductivity. [S0034-6861(96)

The authors have used ' Y NMR to measure the internal magnetic field of a sample of YBa:Cu30& z (T, =90 K) in the superconducting state to correct for the effects of the Meissner shielding currents. They used this information to separate the magnetic-shift tensor into orbital (chemical-shift) and spin (Knight-shift) contributions. They find that the temperature dependence of the chain Cu Knight shift fits the classic Yosida function of weak-coupling, orbital-s-state, spinsinglet BCS theory. For the planes, the Knight shift also requires a spin singlet, but with a strongcoupling Yosida function. The best fit is for an orbital s state, but an orbital d state is also possible.They find a zero-temperature gap b(0) of 1.76k& T, for the chains. For the planes, 6(0) can range from 1.9k8T, to 3.lk~T, .

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The indirect nuclear spin-spin coupling between Cu nuclei in the CuC>2 planes of YBa2Cu307-5, deduced from 63 Cu transverse relaxation, is shown to yield information about the wave-vector dependence of the real part of the planar-Cu static electron-spin susceptibility. The coupling is evaluated with no adjustable parameters using the antiferromagnetic Fermi-liquid theory of Millis, Monien, and Pines, providing a new test of that model. At 100 K, the theoretical relaxation time is 190 ± 75 jusec versus the experimental 130± 10 /isec. PACS numbers: 74.70.Vy, 74.30.Gn, 76.60.Es, 76.60.Gv NMR has proven to be a valuable tool for the study of both the normal and the superconducting states of YBa2Cu307-^, especially through studies of the Knight shift and spin-lattice relaxation. 1 We analyze another aspect of its use in dealing with a crucial issue concerning the appropriate description of the Q1O2 planes. Measurements 2 of transverse relaxation of the 63 Cu nuclei in the planes have shown that there is a nuclear spin-spin coupling an order of magnitude larger than would be expected from conventional nuclear dipolar coupling, requiring that there be an additional nuclear spin-spin coupling mechanism. Such an additional nuclear spin-spin coupling is well known in molecules (the so-called J coupling seen in high-resolution NMR) and solids (for example, the RKKY coupling of metals) where it arises from the hyperfine coupling of the nuclear spins to the electron spins of the valence electrons. 3 The strength of the coupling is calculated using perturbation theory in which the valence electrons are described by molecular or band wave functions, respectively. A major issue for high-temperature superconductors is finding the proper description of the valence electrons. One model which has been very successful in understanding the Knight shift and spin-lattice relaxation is to represent the electrons of the CuC>2 planes as an antiferromagnetic Fermi liquid. Using the Millis, Monien, and Pines formulation of this model, 4 we calculate the extra nuclearnuclear coupling as a test of that description of the CuC>2 planes. While introducing no adjustable parameters, we find a theoretical transverse relaxation time of 190 ±75 //sec compared to the experimental 130 ± 10 jusec. 2 There are two broad classes of theories of the CuC>2 planes: the "one-component" and "two-component" pictures. In the two-component picture one thinks of two separate systems; a set of Cu 2+ ions, and a conduction band made up of holes in oxygen p orbitals. Recent experimental and theoretical advances, however, favor the one-component picture. The key insight for this description, given by Hammel et al. 5 and developed by Shastry, 6 is that one may obtain differing spin-lattice relaxations for 17 0 and 63 Cu by invoking a spin-wave-vector-dependent hyperfine coupling of each nuclear species to temperature-dependent antiferromagnetic fluctuations. Bulut et al., 7 Mila and Rice, 8 Millis, Monien, and Pines (MMP), 4 and Lu et al. 9 have each presented theoret...