P. W. Stephens, P. A. Heiney, R. J. Birgeneau, P. M. Horn, D. E. Moncton, and G. S. Brown, Phys. Rev. B29, 3512-3532 (1984).
We report a detailed high-resolution, x-ray scattering study of the commensurate-incommensurate transition of Kr on ZYX exfoliated graphite powder. As the Kr coverage is increased, the transition proceeds as follows: A sharp diffraction line from the commensurate phase drops in intensity and is replaced by a diffuse incommensurate line. The scattering from this disordered phase sharpens continuously as it moves to higher wave vector. We have followed this evolution up to 4% compression, where the line shape is adequately described by a power-law correlation function characteristic of two-dimensional solids. In one sample, there was significant two-phase coexistence region which we ascribe to a distribution of critical points. No evidence for a uniaxially compressed phase is seen. The weakly incommensurate diffraction line is accompanied by a satellite which may arise from either a domain-wall superlattice or a weak strain modulation of the overlayer. A detailed description of domain lattice models is presented. The loss of long-range order at the transition is discussed in the light of current theories.
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