  The electronic structure and selected zone-center phonons in Yb graphite
intercalation compound (YbC6) are investigated {theoretically} using density
functional calculations and LDA+U approximation for Coulomb correlations in the
f shell, and experimentally using angle-resolved photoemission. We find that
both in LDA and LDA+U approach the Yb f states are fully occupied providing no
evidence for mixed-valent behavior. The obtained theoretical results are in
good agreement with photoemission experiments. The 4f states are considerably
hybridized both with the Yb 5d and C 2p states resulting in a substantial
admixture of Yb f at the Fermi level. Soft Yb phonons, given a noticeable
presence of the Yb states at the Fermi level, are probably responsible for the
superconductivity recently reported in YbC6.
