Today it is widely accepted that defects and impurities affect not only transport and optical properties of a system but also its critical behavior. This perturbation in charge distribution affects only nearest and second nearest neighbors of the defects. When the temperature is lowered the extent of these 3x3 waves grows continuously and the resultant STM image is a superposition of waves from each defect. This phase can no longer be described as a superposition of decaying waves and is therefore different from the high temperature phase. Tc is significantly lower than the 210 K deduced previously from LEED intensity measurements which cannot distinguish between defect induced perturbations with 3x3 symmetry described above and a true 3x3 reconstruction of the Sn film.
Defects are very important in the charge density wave phase too.
They predefine the position and orientation of the domains. One of the most interesting things, which we observed, is the correlation of the defects below the phase transition temperature. In the low temperature phase, Ge substitutional atoms do not occupy charge density maxima inside one domain. At room temperature and even at temperatures as low as 165 K, however, the defects are randomly distributed. We attribute this somewhat surprising observation to a collective interaction between the Ge defects mediated by density waves in the Sn layer.