Domain walls and adsorbate step interactions: An STM study of sulphur layers on Ru(0001)

The local configurations of chemisorbed sulphur on Ru(0001) were studied by STM at various coverages above Theta = 0.33 in phases with and without long range order. Sulphur was found to adsorb only in (or close to) threefold sites (fcc and hcp sites) at all coverages. Only four structural elements (p(2 x 2), (root 3 x root 3)R30 degrees, c(2 x 4) and (root 7 x root 7)R19.1 degrees) seem to exist in this system over the full coverage range. They form all long range ordered and one glass-like phase. These structural elements, only two of which coexist at a given coverage, are also combined to form domain wall phases and a high-order commensurate phase. The insertion of additional atoms into a pure phase of one structural element always results in domain wall formation. These walls repel each other in the domain wall phase at coverages 0.33 < theta <0.5 and form a long range ordered phase of striped superheavy domain walls. The walls in the c(2 x 4) phase for theta <0.5, however, are attractive, but instead of simple phase separation a short range ordered glass phase with units of c(2 x 4) and (root 7 x root 7)R19.1 degrees as structural elements results. Step morphologies as well as the local structure of the adsorbate were found to depend on average sulfur concentration. For the glass phase a coupling mechanism between different terraces could be identified.