Phase transitions in two-dimensionally modulated systems

We discuss, within the Landau theory, the phase diagram of physical systems which display two-dimensionally modulated structures. Specifically, we consider a model appropriate for describing the phase transitions in rare-gas layers adsorbed on graphite. The phase diagram exhibits three phases: a disordered, fluidlike phase, an ordered phase registered with the underlying lattice, and an incommensurate phase. These phases are separated by three phase-transition lines which intersect at a multicritical point. The topology of the phase diagram and the nature of the various transitions are discussed and compared with the appropriate phase diagram for the charge-density-wave layered compounds $2H$-Ta${\mathrm{Se}}_2$ and $2H$-Nb${\mathrm{Se}}_2$.