PHASE-DIAGRAM OF THE SQUARE-LATTICE MODEL WITH 1Q AND 2Q INCOMMENSURATE MODULATIONS

A two-dimensional square-lattice model of particles with displacive degrees of freedom and interacting via a potential with harmonic and anharmonic fourth-order terms, one local and the second having the character of the three-body forces, has been considered. The calculated phase diagrams, characterized by the wave vectors along the [k, 0], [1/2, k], and [k, k] directions, possess incommensurate and commensurate one-dimensional stripe 1q and two-dimensional 2q phases. The stable 2q modulated phases, which look like superpositions of two stripe phases, occur along the [k, 0] and [1/2, k] directions, and close to the (1/2, 0) point of the reciprocal lattice, provided the magnitude of the three-body anharmonic potential exceeds a critical value.