Order-disorder transition of the (3X3) Sn/Ge(111) phase

We have measured the long-range order of the alpha phase of Sn on the Ge(111) surface throughout the (3 x 3) --> (root3 x root3)R30 degrees phase transition. The transition has been found of the order-disorder type with a critical temperature T-c similar to 220 K. The expected three-state Potts critical exponents are shown to be consistent with the observed power-law dependence of the (3 x 3) order parameter and its correlation length close to T-c, thus excluding a charge-density wave driven phase transition.