Directional preference in particle motion: Self-trapping of vacancies in an ordered adsorbate layer

Single vacancies within the dense 3O(2×2)/Ru(001) layer are found to display highly directional diffusion characteristics that leads to a self-trapping of this quasiparticle to within three neighboring sites. While a high mobility within these few sites is found, vacancy diffusion across the surface is very slow (ratio >1000:1). The nonzero motion of the entire triangular unit proceeds in the usual stochastical way and demonstrates that intrinsic structural defects of the substrate lattice are not involved in the self-trapping process. It is demonstrated that the observed motional constraint is due to the local symmetry of the oxygen adsorption sites, which implies a directional dependence of diffusion barriers for vacancy motion within the (2×2) unit cell.