Anisotropic electron-phonon coupling in Rb-intercalated bilayer graphene

We have performed high-resolution angle-resolved photoemission spectroscopy on Rb-intercalated bilayer graphene, which is regarded as the thinnest limit of Rb-intercalated graphite (Rb-GIC). We have clearly observed a velocity renormalization of the π^∗bands near the Fermi level, and electron doping from Rb atoms to the graphene layers. We attribute the renormalization to strong coupling of electrons with graphene's in-plane optical phonon mode. The electron-phonon coupling constant has been measured, and found to exhibit a strong momentum anisotropy similar to that of bulk GIC superconductors. This result provides evidence for universal electron-phonon coupling in intercalated multi-layer graphene and bulk GICs.