Two-dimensional phase transition mediated by extrinsic defects

We have investigated the (root 3 x root 3) to (3 x 3) phase transition in t he a phase of Sn/Ge(111) with variable temperature STM at temperatures betw een 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 depe ndent length (exponential attenuation). The inverse of the attenuation leng th is a linear function of temperature showing that the phase transition oc curs at 70 K. At low temperature a density wave mediated defect-defect inte raction realigns the defects to be in registry with the (3 x 3) domains.