Local magnetic properties of rare-earth ͑RE͒ atoms encapsulated in fullerenes have been characterized using x-ray magnetic circular dichroism and x-ray absorption spectroscopy ͑XAS͒. The orbital and spin contributions of the magnetic moment have been determined through sum rules and theoretical model calculations, and have been found to be highly reduced compared to those of the corresponding free RE 3ϩ ions. Crystal-field and hybridization effects have been investigated by the way of calculations to simulate the effect of the carbon cage on the RE; both hypotheses have reproduced the experimental spectra resulting also in a significant reduction of the orbital and spin moments. While isotropic XAS spectra have confirmed a roughly trivalent state for the RE metals, a back electron transfer from the cage to the metal has been quantified. A paramagnetic coupling has been found between the metal centers from 6 K to 300 K.

We detect a significant angle-dependence in the core level and valence line shapes of photoelectron spectra of single crystal K 3 C 60 . This allows the identification of bulk and surface components in the data, and allows us to explain the anomalous line shapes observed for this system. The states near the Fermi level are associated with the bulk of the sample. There is strong evidence of an insulating surface layer, which we ascribe to intermolecular electron correlations. These results simplify the interpretation of previous, apparently conflicting observations.

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The simple cubic phase of a RbC 60 thin film has been studied using photoelectron spectroscopy. The simple cubic-to-dimer transition is found to be reversible at the film surface.A sharp Fermi edge is observed and a lower limit of 0.5 eV is found for the surface Hubbard U, pointing to a strongly-correlated metallic character of thin-film simple cubic RbC 60 . A molecular charge state is identified in the valence band and core level photoemission spectra which arises from C 60 2-anions and contributes to the spectral intensity at the Fermi level. PACS: 71.20.Tx, 71.27.+a, 73.25.+i, 79.60.Dp * Corresponding author. Electronic address: roberto.macovez@icfo.es 2 Alkali fullerides, stoichiometric compounds of Buckminsterfullerene (C 60 ) with alkali atoms, exhibit a large variety of insulating, metallic, magnetic and superconducting behaviors as a function of alkali concentration [1][2][3][4][5][6][7][8][9][10]. Such a wide range of physical properties partially stems from the proximity of cubic alkali fullerides to a metal-insulator transition [1] governed by the interplay of intramolecular interactions, in particular the inter-electron repulsion [2] and the Jahn-Teller vibronic coupling [3,4]. While even-stoichiometry fullerides such as Na 2 C 60 and Rb 4 C 60 are diamagnetic Mott-Jahn-Teller insulators [5,6], odd-stoichiometry compounds are metals and in some cases even superconductors [7][8][9][10]. Understanding the unconventional conduction properties of these materials is crucial for the fundamental description of molecular solids and narrow-band correlated systems in general.The lowest stoichiometry AC 60 fullerides (where A=K, Rb, Cs) display within a single composition a range of electronic properties and phases as a function of thermal treatment [11].Two distinct cubic phases with differing metallic properties are observed: a face-centered cubic (fcc) structure of rapidly spinning molecules [12], which is thermodynamically stable above 400 K, and a metastable simple cubic (sc) phase, obtained by fast-cooling the fcc phase to below 100 K, which only differs from the high-temperature structure in that the orientation of the C 60 monomers is fixed with respect to the cubic axes [13]. Upon annealing to 200 K, the sc structure transforms irreversibly into metastable (C 60 ) 2 2-dimers [14,15], which can also be obtained by fast-cooling the fcc phase to below room temperature. A weakly-conducting phase of polymer chains is thermodynamically stable at and below room temperature [16,17].Optical conductivity measurements on the fcc phase indicate metallic character [18], while NMR and ESR investigations showed a high degree of localization of the LUMO electrons on single molecules [12,17], suggesting that the bulk fcc phase is a strongly correlated metal. The bulk sc 3 phase is a metal [13] with unusual features: in CsC 60 a partial spin gap was observed by NMR, and ascribed to the presence of a minority of stable doubly-charged molecules in spin-singlet state [19,20]. The same authors suggest that similar ...

Using angle-dependent photoemission spectra of core and valence levels we show that metallic, single crystal K3C60 is terminated by an insulating or weakly-conducting surface layer. We attribute this to the effects of strong intermolecular correlations combined with the average surface charge state. Several controversies on the electronic structure are thereby resolved.