Stability of graphene band structures against an external periodic perturbation: Na on graphene

The electronic structure of Na-adsorbed graphenes formed on the 6H-SiC(0001) substrate was studied using angle-resolved photoemission spectroscopy with synchrotron photons and ab initio pseudopotential calculations. It was found that the band of the graphenes sensitively changes upon Na adsorption especially at low temperature. With increasing Na dose, the pi band appears to be quickly diffused into the background at 85 K whereas it becomes significantly enhanced with its spectral intensity at room temperature (RT). A new parabolic band centered at k similar to 1.15 A(-1) also forms near Fermi energy with Na at 85 K while no such band was observed at RT. Such changes in the band structure are found to be reversible with temperature. The changes in the pi band of graphene are mainly driven by the Na-induced potential especially at low temperature where the potential becomes periodic due to the crystallized Na overlayer. The new parabolic band turns out to be the pi band of the underlying buffer layer partially filled by the charge transfer from Na adatoms. The increase in the hopping rate of Na adatoms at RT by 5 orders of magnitude prevents such a charge transfer, explaining the absence of the new band at RT.