  Surface-state mediated interactions between adsorbates on surfaces can be
exploited for the fabrication of self-organized nanostructures such as
two-dimensional superlattices of adatoms. Using angle-resolved photoemission we
provide experimental evidence that these interactions can be drastically
modified by adsorbate-induced alterations in the surface potential barrier.
This, in turn, will cause significant changes in the ordering of the
adsorbates. For the studied case example of Cs adatoms on Ag(111) our
momentum-resolved measurements reveal the surface state Fermi wave vector to be
increased by as much as $\sim$100% for coverages around 0.03 ML. Our results
unravel the origin for the hitherto puzzling and unexpectedly small lattice
constant in the adatom superlattice observed for this system.
