Sub-Poissonian Distribution Of Cs And K Ions In The Valleys Of Hbn/Ru(0001)

The moire pattern of hBN/Ru(0001) provides a template for the adsorption of the alkali metals Cs and K: Scanning tunneling microscopy reveals that the adatoms prefer to adsorb in the valley areas of the moire unit cell. Distinctive from transition metal adatoms that tend to agglomerate, the alkali metal adatoms avoid each other within the valleys, leading to a sharper distribution than the Poisson statistics expected for noninteracting particles. This sub-Poissonian distribution is further explored via Monte Carlo simulations. The average charge q of the alkali metal adatoms is extracted for different alkali metal concentrations lambda, which reveals a monotonically decreasing q(lambda) dependence. We explain this dependence with the energy trade-off between the charge transfer from the alkali metals to the more electron-affine Ru(0001) substrate and the static electric field created by this charge transfer.