Defects in the (√3x√3)↔(3x3) phase transition in the Pb/Si(111) system

The role played by point defects in the (root 3 x root 3 double left right arrow ( 3 x 3 ) phase transition that takes place in the 1/3 monolayer-Pb/Si (111) system has been investigated by means of variable temperature scanning tunneling microscopy. In this system, it has been possible to grow exceptionally large defect-free regions coexisting with more defective ones. These defective regions have between 2% and 5% of point defects, which are mainly substitutional Si adatoms. Our experiments show that the point defects produce a local perturbation of the surface, but discard them as the fundamental driving force of the phase transition. By tracking exactly the same surface regions with atomic resolution while varying the sample temperature from 40 to 200 K, we have observed that substitutional point defects are not mobile throughout the phase transition and our statistical analysis of the STM images reveals that they are randomly placed from 40 K to room temperature.