DISORDERING OF THE LOW-TEMPERATURE C(4X2) STRUCTURE OF GE(001) TO THE(2X1) RECONSTRUCTION - EVIDENCE FOR A MEAN-FIELD TRANSITION

The low-temperature c(4X2) structure of Ge(001) undergoes an order-dis order phase transition at temperatures in the range 174-194 K dependin g on the quality of the starting structure. The critical exponents bet a and gamma have been experimentally determined with x-ray diffraction measurements and found to be consistent with the mean-field approxima tion. Surface contamination from the residual gas tends to pin the ord ered structure causing increased critical temperatures and rounding of the transition.