ic mechanism it is not known whether the 9. D. L. Novikov, V A. Gubanov. A J. Freeman, Physica C 191, 399 (1 992). in the matrix 10. S. Chakravarty, M. P. Gelfand, S. Kivelson, Scisponding to hopping on (13C, ,12C, , ) , , ence 254, 970 (1 991) v.2 --Versus I t 6 , a n d -' i~6 i clusters wa give rise 11. G. Baskaran and E ' Tossatti. Curr. Sci. 61, 33 (1991). to significant variations in Tc' Or whether 12. J. Bardeen, L N. Cooper, J. R. Schrieffer, Phys. these differences are averaged in the mean-Rev. 108. 1175 (1957). field model used to estimate T,. Calcula-13. P. Morel and P. W. Anderson, rbid. 125. 1263tions for the electronic model are needed to address this essential issue. Our experimental isotope effect data ohtained on a series of specifically substituted Rb3C6, superconductors provide an unambiguous value for the isotope shift in completely substituted fullerene superconductors and elucidate a unique difference between intermolecular and intramolecular isotopic substitution. A theoretical understanding of these experimental results may lead to a single viable mechanism for superconductivity in the fullerenes.

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Magnetic circular x-ray dichroism measurements at the Co Ls and L2 edges in Co/Pd multilayers give first experimental evidence for a greatly enhanced orbital moment compared to that in bulk Co, confirming recent theoretical predictions. Our results also show that, in general, there is no simple relationship between the dichroism signal and either the spin moment or the total moment. This is verified by a simple model calculation.PACS numbers: 78.70.Dm, 78.20.Ls The breaking of symmetry and/or the change in nearest-neighbor coordination near surfaces and interfaces introduces many interesting phenomena in magnetic materials. Among them, the existence of perpendicular magnetic anisotropy [1] and enhanced spin [2] and/or orbital magnetic moments [3 -6] has attracted much attention.The understanding of magnetic anisotropy in thin films is an interesting scientific problem with great technological importance in areas like magnetic and magneto-optical recording. Since the microscopic origin of magnetocrystalline anisotropy is the spin-orbit interaction, which depends on both the spin and orbital angular momenta, direct, independent measurements of both quantities are highly desirable. Such measurements also allow direct comparison with theoretical predictions. The separation of orbital and spin moments is a difficult problem for most established techniques, e.g. , neutron scattering. In principle, magnetic x-ray scattering is capable of separating the spin and orbital momentum densities [7], but a conclusive demonstration is still lacking. Here we demonstrate by application of a recently derived powerful sum rule [8] that magnetic circular x-ray dichroism (MCXD) measurements allow a quantitative determination of orbital magnetic moments. In particular, we determine the orbital moment of Co in Co/Pd multilayers, systems with great promise for use as future magnetooptical media.The MCXD technique measures the difference in absorption between right-and left-handed circularly polarized x rays near a core absorption edge in magnetic materials [9, 10]. For transition metals dichroism measurements typically involve p~d core-to-valence excitations near Ls and/or L2 edges. Based on a simple one-electron picture it has generally been assumed that the so-measured MCXD signal is proportional to the difference in the spin-up and spin-down occupation in the empty d shell of the absorbing atom, i.e. , to the local magnetic moment on the probed atom [9]. This led to the notion that MCXD can determine element specific magnetic moments [11,12], assuming that the proportionality constant is transferable across different chemical and structural environments of a given atom. Recent results for the pure 3d transition metals Fe, Co, and Ni [13,14] suggested a more complicated situation however, in particular a breakdown of the one-electron picture [13, 15]. A particularly important development is the recent derivation of a sum rule by Thole et aL [8] which links the sum of the Ls and Lz dichroism intensities to the local orbital moment. ...

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