Observation of bond-orientational order in the incommensurate fluid phase of potassium chemisorbed on nickel (111)

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Abstract
Low-energy electron diffraction measurements for incommensurate potassium chemisorbed on Ni(111) indicate that at coverages between 0.17 and 0.22, the overlayer disorders into a fluid phase which has long-range bond-orientational order. The transition temperature depends on the mean density of the overlayer and varies from approximately 130 K at 0.17 monolayers to 230 K at 0.21 monolayers. The diffraction data for this orientationally ordered phase are consistent with the predictions made by the Kosterlitz-Thouless-Halperin-Nelson-Young theory for the intermediate hexatic phase in two-dimensional melting, but substrate effects or other intrinsic effects cannot be ruled out as the source of the orientational order. The orientationally ordered fluid exists over a very wide temperature range, and the transition is reversible and apparently continuous. Bond-orientational order is still observed as high as 50 K above the onset of the transition; an isotropic fluid is never observed at these coverages.