Wetting phenomena in bcc binary alloys

The wetting behavior at the free (100) surface of a bcc binary alloy exhibiting a continuous order-disorder transition in the bulk is investigated theoretically, using a model equivalent to a bcc Ising antiferromagnet in an external magnetic field. Its salient feature is the generation of an effective ordering surface field , g1 6= 0, for certain symmetry-breaking surface orientations like (100). Such a field, coupling to the local order parameter at the surface, does not only crucially affect the surface critical behavior at bulk criticality, but also leads to the occurrence of wetting phenomena below the critical temperature Tc. Starting from the mean-field theory for the lattice model and making a continuum approximation, a suitable Landau-Ginzburg model is derived. Explicit results for the dependence of its parameters (e.g., of g1) on the microscopic interaction constants are obtained. Utilizing these in conjunction with Landau theory, the wetting phase diagram is calculated.