Order-disorder transition of the(3×3)Sn/Ge(111) phase

We have measured the long-range order of the α phase of Sn on the Ge(111) surface throughout the $(3\mathrm{}\times 3)\to (\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ phase transition. The transition has been found of the order-disorder type with a critical temperature $T_c\sim 220\hspace{0.5em}\mathrm{K}.$ The expected three-state Potts critical exponents are shown to be consistent with the observed power-law dependence of the $(3\mathrm{}\times 3)$ order parameter and its correlation length close to $T_c,$ thus excluding a charge-density wave driven phase transition.