Scanning tunnelling microscopy at liquid helium temperature was used to image potassium adsorbed on graphite at low coverage (≈0.02 monolayer). Single atoms appeared as protrusions on scanning tunnelling microscopic topographs. A statistical analysis of the position of the atoms demonstrated repulsion between adsorbates, which was quantified by comparison with molecular dynamics simulations. This gives access to the dipole moment of a single adsorbate, found to be 10.5D. Time-lapse imaging showed that long-range order was broken by thermally activated diffusion, with a 30meV barrier to hopping between graphite lattice sites.

Real-Time Imaging of K Atoms on Graphite: Interactions and Diffusion. Renard, J., Lundeberg, M.B., Folk, J.A., Pennec, Y.: Physical Review Letters, 2011, 106[15], 156101