  It is widely believed that carrier-density inhomogeneities ("electron-hole
puddles") in single-layer graphene on a substrate like quartz are due to
charged impurities located close to the graphene sheet. Here we demonstrate by
using a Kohn-Sham-Dirac density-functional scheme that corrugations in a real
sample are sufficient to determine electron-hole puddles on length scales that
are larger than the spatial resolution of state-of-the-art scanning tunneling
microscopy.
