EVANESCENT X-RAYS PROBING SURFACE-DOMINATED PHASE TRANSITIONS

This article reviews the scattering of evanescent x-ray waves as they occur inside a solid in the regime of total external reflection. The theoretical and experimental details of glancing angle scattering and the use of synchrotron radiation are discussed. It is shown how strong surface disorder and surface roughness affect the Bragg scattering of these exponentially damped waves and how these scattering signals can be interpreted in a straightforward way within the distorted wave Born approximation (DWBA). The application of this novel surface technique to bulk phase transitions allows the observation of surface-dominated behaviour. This is illustrated via two examples:. 1. The discontinuous order-disorder transition in Cu3Au has been studied near the (001) and (111) surface by evanescent x-rays. It turns out that the order parameter close to the free surface of the alloy decays in a very pronounced way upon approaching the transition temperature. The quantitative analysis of the evanescent superlattice intensity is compatible with a wetting phenomenon. 2. The critical phenomena associated with the continuous order-disorder transition in Fe3Al become distinctly modified due to the presence of a free surface. By applying the evanescent wave method three new universal surface exponents β1, γ11 and η‖ could be determined. This allows for the first time a critical exprimental test of surface scaling relations which turn out to be in good agreement with the experimental findings. At the surface of binary alloys surface segregation phenomena occur which complicate the experimental observation of near-surface ordering and disordering phenomena as well as the theoretical models which have to account for these effects. Some implications of these surface effects is briefly presented.