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 transitio n at the (001) surface of Cu3Au. We have measured the specular and the supe rlattice (-1, 0) Bragg intensities in the 140 K less than or equal to T les s than or equal to 800 K temperature range. We have performed an accurate d etermination of the Debye-Waller factor, including anharmonic contributions . Thanks to the extreme surface sensitivity of helium diffraction, the supe rlattice 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 predict ions within the surface-induced disorder model.