The Si(111) 7 × 7 TO "1 × 1" transition

We have made a detailed study of the Si(111) 7 × 7 to "1 × 1" transition at 1140 K using low-energy electron diffraction. Excess diffuse scattering appears during the transition whose temperature dependence and magnitude are consistent with the loss of the 7th order superlattice intensities showing the transition is to a disordered surface phase rather than a true unreconstructed ×1. This strongly favors the class of models for the 7 × 7 selvedge involving an ordered arrangement of subunits like vacancies or adatoms rather than models with periodic distortion. The superlattice intensity shows the transition is continuous and reversible at least for temperatures below 0.995 T_c. The critical exponent for the long-range order is β = 0.11 ± 0.015 similar to those for 2D Ising or Potts models. However, the system does not show the critical scattering properties expected for simple models. The relaxation time seems to diverge near T_c but only for negative temperature increments.