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The Cii3Au(100) surface has been studied close to the bulk order-disorder transition temperature To by x-ray diffraction under total external reflection. The order-parameter profile which has been probed in various depths is analyzed in terms of a surface-induced disorder transition. It is demonstrated that the temperature dependence of the superlattice intensity is consistent with the assumption of a disordered surface layer which exists between the ordered bulk and the vacuum at temperatures below To and grows logarithmically as To is approached. PACS numbers: 68.35.Rh, 61.55.Hg, 64.70.KbThe thermodynamic behavior of a semi-infinite system undergoing a first-order phase transition is particularly interesting close to its free surface, because the broken translational symmetry in the atomic arrangement and the atomic interactions may allow new phase transitions. l At the (100) surface of the binary alloy CU3AU, a continuous behavior of the surface order parameter in the neighborhood of its well-known order-disorder transition at To -663 K has been observed with LEED. 2 Lipowsky and co-workers 3 have analyzed semi-infinite systems within mean-field theory and found so-called surface-induced disorder (SID) transitions which are intimately related to wetting phenomena. 4 It turns out that at temperatures T < To a disordered layer intervenes between the vacuum and the bulk [ Fig. 1FIG. 1. Order-parameter profiles m(z)/mb associated with surface-induced disorder. The coordinate z measures the distance from the surface (r-"0). E,b is the bulk correlation length and mb the bulk order parameter, (mb is set equal to unity in the main text.) (a) Onset of surface disordering; (b) advanced surface disordering with a delocalized interface at mean position z =L from the surface. which grows to a macroscopic size as To is approached [ Fig. 1(b)]. By this process the interface between the disordered and the ordered phases becomes delocalized according to L,=L 0 ln(lA), (0 as long as short-range interactions govern the phenomena. (L t is the mean position of the interface from the surface and t = \ -T/To the reduced temperature.) In the framework of this theory the continuous surface disordering originates from the exponential tail of the order-parameter profile which vanishes continuously at z =0 as the interface moves into the bulk. This situation corresponds in the context of wetting phenomena to a complete wetting at the wetting transition temperature (see, e.g., Ref. 4).In this Letter we consider the Cu3Au(100) surface. Although the continuous surface order parameter as found in the early work by Sundaram et al. 2 has meanwhile been confirmed, 5,6 the predicted growth of a disordered surface layer in the presence of the SID transition has not yet been observed. Since surface-induced disorder does not necessarily imply the existence of an interface derealization transition, 7 it is highly desirable to subject this theoretical model to an experimental test. In this work we have studied the order parameter close to the Cu3Au(l...

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