Origin of arc shape of LEED streaks on Li adsorbed on Cu(001) surface at lower coverage

We considered the origin of the arc-shaped streaks connecting usual diffraction spots observed in an LEED pattern, for a Li system adsorbed on a Cu(001) surface at low coverage, Θ<1/2. We noted a condition that the natural distance between adsorbed atoms, b_nat, of less than - in particular, - is consistent with the formation of the arced streaks. Given this condition, adsorbed atoms fill the surface and form a structure, which is one of the 'ladder structures', at Θ ≤ 3/5. Even for Θ<1/2, the atoms form the ladder structure locally. We could observe atomic pairs with second-neighbour distance having a c(2 × 2) structure unit in the ladder structure (where the distance is basically d ≤ 2a), that were shrunk and tilted. These shrunk and tilted pairs produce straight streaks extending from the M(± 1/2, ± 1/2) points, with a tilted angle in the vicinity of M points. Considering the other arrangements of the second-neighbour pairs, which have a different inclination, a variety of straight streaks exists. The envelope function of these straight streaks is nothing but the arc shape observed in the experiment.