Anisotropic electron-phonon coupling in doped graphene

The effects of doping single layer graphene are investigated by mapping the valence band in the vicinity of EF using angle-resolved photoemission spectroscopy (ARPES). The carrier concentration was varied from 0.04 -- 1.05 electrons per unit cell with the deposition of Ca and K at low temperatures. As the doping increases there is an enhancement of the electron-phonon coupling along certain high symmetry directions. Changes in electron-phonon coupling parameter, lambda, shows that the systems goes through a transition from the weak-coupling regime to the strong-coupling regime.