Moiré beatings in graphene on Ru(0001)

The moir\'e superstructure of a single layer of carbon on ruthenium, where 25×25 graphene honeycombs sit on 23×23 substrate unit cells, is determined from first principles. The density functional theory (DFT) calculations predict two kinds of structural units, Ω and Y, in the supercell, which are identified as moir\'e beatings or moirons. The related topographic bucklings, or ``hills,'' have distinct carbon conformations and a height of 1.16 {}. The different moirons are observed with scanning tunneling microscopy (STM), and surface x-ray diffraction (SXRD) also discriminates the two. This connects ab initio DFT calculations with STM and SXRD experiments in unit cells containing more than 4000 atoms.