Moire beatings in graphene on Ru(0001)

The moire superstructure of a single layer of carbon on ruthenium, where 25 x 25 graphene honeycombs sit on 23 x 23 substrate unit cells, is determined from first principles. The density functional theory (DFT) calculations predict two kinds of structural units, Omega and Y, in the supercell, which are identified as moire beatings or moirons. The related topographic bucklings, or hills, have distinct carbon conformations and a height of 1.16 angstrom. 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.