DISORDERED SURFACE SHEETS IN SOLID MATTER NEAR BULK INSTABILITIES

In this article, I review surface-sensitive X-ray scattering experiments using evanescent X-ray scattering and asymptotic Bragg scattering techniques which have been carried out in order to investigate the thermodynamic behaviour of the surface and the subsurface layers when the bulk of the system approaches a phase transition. I discuss the surface structures of the binary alloys Fe3Al and Cu3Au close to the order-disorder transition, the surface of the molecular crystal NH4Br near the critical temperature and ice single-crystal surfaces close the bulk melting paint. The observed surface phenomena range from surface disorder and surface-induced new universal critical behaviour to surface melting and surface segregation. They all have in common that they are not bound to the outermost layers, but rather occur within a surface sheet whose thickness can grow to a mesoscopic or even macroscopic size.