Order-disorder transition of the (3x3)Sn/Ge(111) phase

We have measured the long-range order of the ? phase of Sn on the Ge(111) surface throughout the (3×3)->(?3×?3)R30° phase transition. The transition has been found of the order-disorder type with a critical temperature Tc~220 K. The expected three-state Potts critical exponents are shown to be consistent with the observed power-law dependence of the (3×3) order parameter and its correlation length close to Tc, thus excluding a charge-density wave driven phase transition.