Evidence for molecular Kondo effect in rare-earth metallocenes from f-occupancy and magnetic susceptibility

The Kondo effect describes a quasibound singlet state formed by the interaction between a local magnetic moment on a lattice site and states at the Fermi level. In theory, a similar interaction should occur in the single molecule cerium bis-cyclooctatetraene (cerocene), where the configuration interaction between the $f$-orbitals and the carbon p($\pi$)-orbitals create an intermediate valence state with a very high Kondo temperature, $T_K$ [Dolg et al., J. Chem. Phys. {\bf 94}, 3011 (1991)]. Very little experimental evidence exists for such a state, however. We report Ce $L_3$-edge x-ray absorption near-edge structure (XANES) measurements of the $f$-occupation that demonstrate intermediate valence, with an $f$-occupancy of $n_f\sim0.8$. Moreover, magnetic susceptibility $\chi(T)$ measurements demonstrate that the cerium in cerocene is paramagnetic, with a temperature-independent $\chi(T<300 K)=1.5\times10^{-4}$ emu/mol. These data are consistent with a $T_K$ of about 5000~K. A similar set of data on a series of ytterbium bis-cyclopentadienyl molecules supports this claim, except with a range of $T_K$'s from 800 K to greater than 1500 K. Taken together, these data are strong evidence of a molecular Kondo effect in these insulating systems and give the first indication of how to tune such Kondo interactions.