  There has been a long-standing debate about the mechanism of the unusual
superconductivity in alkali-intercalated fulleride superconductors. In this
work, using high-resolution angle-resolved photoemission spectroscopy, we
systematically investigate the electronic structures of superconducting K3C60
thin films. We observe a dispersive energy band crossing the Fermi level with
an occupied bandwidth of about 130 meV. The measured band structure shows
prominent quasiparticle kinks and a replica band involving high-energy
Jahn-Teller active Hg(8) phonon mode, reflecting strong electron-phonon
coupling in the system. The electron-phonon coupling constant is estimated to
be about 1.2, which dominates the quasiparticle mass renormalization. Moreover,
we observe an isotropic nodeless superconducting gap beyond the mean-field
estimation. Both the large electron-phonon coupling constant and large reduced
superconducting gap suggest a strong-coupling superconductivity in K3C60, while
the electronic correlation effect is indicated by the observation of a
waterfall-like band dispersion and the small bandwidth compared with the
effective Coulomb interaction. Our results not only directly visualize the
crucial band structure of superconducting fulleride but also provide important
insights into the mechanism of the unusual superconductivity.
