Enhancement of electron-phonon coupling in Cs-overlayered intercalated bilayer graphene

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on cesium (Cs) intercalated bilayer graphene with a Cs overlayer (Cs-C₈CsC₈). Low-energy electron diffraction shows a (2 × 2) pattern consistent with intercalation of a Cs layer similar to bulk C₈Cs, in addition to the signature of a nearly commensurate superstructure created by the Cs overlayer. ARPES results reveal folding of the π bands due to the periodic (2 × 2) potential of the intercalated Cs atoms, together with a free-electron-like state at the point. Significant mass renormalization is observed in the band dispersion near the Fermi level, indicative of strong electron-phonon coupling. Based on analysis of the self-energy, we find anisotropic electron-phonon coupling with an estimated strength of ± 0.02 in the K- direction, and in the K-M direction. This coupling is much larger than that of other doped graphenes, and comparable to superconducting bulk GICs. We attribute this large electron-phonon coupling constant to the presence of the Cs overlayer, which highly dopes bands, and creates a structure similar to stage-I graphite intercalation compounds.