By measuring the temperature dependence of the superlattice-spot intensity in reflection high-energy electron diffraction from a clean Si(111) surface during the phase transition between 7×7 and 1×1 structures, a hysteresis was found with a temperature difference of 5°C between heating and cooling processes. We conclude that a first-order phase transition is present. Hysteresis was barely observed for the (5×2 or 3 × 3) -1×1 phase transitions of Au-induced superstructures.
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