DISORDERED SURFACE SHEETS IN SOLID MATTER NEAR BULK INSTABILITIES

In this article, I review surface-sensitive X-ray scattering experimen ts using evanescent X-ray scattering and asymptotic Bragg scattering t echniques which have been carried out in order to investigate the ther modynamic behaviour of the surface and the subsurface layers when the bulk of the system approaches a phase transition. I discuss the surfac e structures of the binary alloys Fe3Al and Cu3Au close to the order-d isorder transition, the surface of the molecular crystal NH4Br near th e critical temperature and ice single-crystal surfaces close the bulk melting paint. The observed surface phenomena range from surface disor der and surface-induced new universal critical behaviour to surface me lting and surface segregation. They all have in common that they are n ot bound to the outermost layers, but rather occur within a surface sh eet whose thickness can grow to a mesoscopic or even macroscopic size.