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 (√3×√3)→(3×3) charge-density wave phase transition in the α phase of Sn on Ge(111) has been studied using a variable-temperature scanning tunneling microscope. Above 105 K, Ge substitutional defects stabilize regions with (3×3) symmetry that grow with decreasing temperature and can be described by a superposition of exponentially damped waves. At low temperatures, T<~105K defect-defect density-wave-mediated interactions force an alignment of the defects onto a honeycomb sublattice that supports the low-temperature (3×3) phase. This defect-mediated phase transition is completely reversible. The length scales involved in this defect-defect interaction dictate the domain size (≈10⁴ Å ²).