DOMAIN-WALLS AND ADSORBATE STEP INTERACTIONS - AN STM STUDY OF SULFURLAYERS ON RU(0001)

The local configurations of chemisorbed sulphur on Ru(0001) were studi ed by STM at various coverages above Theta = 0.33 in phases with and w ithout long range order. Sulphur was found to adsorb only in (or close to) threefold sites (fcc and hcp sites) at all coverages. Only four s tructural elements (p(2 x 2), (root 3 x root 3)R30 degrees, c(2 x 4) a nd (root 7 x root 7)R19.1 degrees) seem to exist in this system over t he 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 hi gh-order commensurate phase. The insertion of additional atoms into a pure phase of one structural element always results in domain wall for mation. These walls repel each other in the domain wall phase at cover ages 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 s hort range ordered glass phase with units of c(2 x 4) and (root 7 x ro ot 7)R19.1 degrees as structural elements results. Step morphologies a s well as the local structure of the adsorbate were found to depend on average sulfur concentration. For the glass phase a coupling mechanis m between different terraces could be identified.