Role of defects in two-dimensional phase transitions: An STM study of the Sn/Ge(111) system

The influence of Ge substitutional defects and vacancies on the (root 3 x r oot 3)-->(3 x 3) charge-density wave phase transition in the alpha phase of Sn on Ge(111) has been studied using a variable-temperature scanning tunne ling microscope. Above 105 K, Ge substitutional defects stabilize regions w ith (3 x 3) symmetry that grow with decreasing temperature and can be descr ibed by a superposition of exponentially damped waves. At low temperatures, T less than or equal to 105 K defect-defect density-wave-mediated interact ions force an alignment of the defects onto a honeycomb sublattice that sup ports the low-temperature (3 x 3) phase. This defect-mediated phase transit ion is completely reversible. The length scales involved in this defect-def ect interaction dictate the domain size (approximate to 10(4) Angstrom(2)).