The effect of Ge substitutional defects and vacancies, upon the (√3 x √3) > (3 x 3) charge-density wave phase transition in the α-phase of Sn on Ge(111), was studied by using a variable-temperature scanning tunnelling microscope. Above 105K, the Ge substitutional defects stabilized regions with (3 x 3) symmetry that grew with decreasing temperature and could be described by a superposition of exponentially damped waves. At temperatures below 105K, defect-defect density-wave mediated interactions imposed an alignment of the defects onto a honeycomb sub-lattice that supported the low-temperature (3 x 3) phase. This defect-mediated phase transition was completely reversible. The length scales which were involved in this defect-defect interaction fixed the domain size at about 10000Å2.

Role of Defects in Two-Dimensional Phase Transitions - an STM Study of the Sn/Ge(111) System A.V.Melechko, J.Braun, H.H.Weitering, E.W.Plummer: Physical Review B, 2000, 61[3], 2235-45