Strain relaxation in small adsorbate islands: O on W(110)

The stress-induced lattice changes in a p(1x2) ordered oxygen layer on W(110) are measured by low-energy electron diffraction. We have observed that small oxygen islands show mismatch with the underlying lattice. Our results indicate that along [1 (1) over bar0], the average mismatch inversely scales with the island size as 1/L for all oxygen coverages up to 0.5 ML, while along [001], it is significant only for the smallest oxygen islands and scales as a higher power of the inverse island size. The behavior along [1 (1) over bar0] is described by a one-dimensional finite-size Frenkel-Kontorova model. By using this model, together with the calculated force constants, we make a quantitative estimate of the change in surface stress upon oxygen adsorption. The result is consistent with our ab initio calculations, which give a relative compressive stress of -4.72 N/m along [1 (1) over bar0] and a minute relative tensile stress of 0.15 N/m along [001]. The scaling along [001] is qualitatively explained as an effect induced by the lattice relaxation in the [1 (1) over bar0] direction.