Surface vs. bulk order/disorder transition in a binary alloy.: A He diffraction study of the Cu3Au(001) surface

We present a helium diffraction study of the order/disorder phase transition at the (001) surface of Cu3Au. We have measured the specular and the superlattice (-1, 0) Bragg intensities in the 140 K less than or equal to T less than or equal to 800 K temperature range. We have performed an accurate determination of the Debye-Waller factor, including anharmonic contributions. Thanks to the extreme surface sensitivity of helium diffraction, the superlattice peak intensity provides an experimental determination of the long-range order parameter M at the topmost surface layer. The surface critical temperature is found to be T-c(s) = 664 +/- 2 K. The value of the critical exponent beta = 0.45 +/- 0.05 is in good agreement with theoretical predictions within the surface-induced disorder model.