Surface Hubbard U of alkalifullerides

We report a combined photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES) study of distilled, phase pure films of C(60) and the monomeric fullerides Cs(6)C(60), Cs(4)C(60) and fcc RbC(60). The separation between the highest energy PES and lowest energy IPES features, which is a measure of the barrier to hopping, is 1.45 eV in Cs(4)C(60) and 0.7 eV in RbC(60). This difference is large enough to explain, in a correlated electron picture, the reported differences in electronic mobility between the two stoichoimetries. From the PES-IPES energy separation, the value of the Hubbard U is estimated to be 1.5 eV in closed-shell C(60) and Cs(6)C(60), while in Cs(4)C(60) and RbC(60) such value is reduced to similar to 1 and 0.7 eV, respectively. This trend can be only partially understood taking into account the different molecular polarizability and crystal structure of the various stoichiometries. The relatively low values found for open-shell compounds indicate that the bulk Hubbard U is smaller in open-shell fullerides than usually believed, which might help explain superconductivity and the observation of spin-singlets in odd-stoichiometry fullerides.