Two-dimensional phase transition mediated by extrinsic defects

We have investigated the (root 3 x root 3) to (3 x 3) phase transition in the a phase of Sn/Ge(111) with variable temperature STM at temperatures between 30 and 300 K. Point defects in the Sn film stabilize localized regions of the (3 x 3) phase, where the size is characterized by a temperature dependent length (exponential attenuation). The inverse of the attenuation length is a linear function of temperature showing that the phase transition occurs at 70 K. At low temperature a density wave mediated defect-defect interaction realigns the defects to be in registry with the (3 x 3) domains.