Properties of a double ising model: Phase diagrams, discommensurations, and accumulation points of structure branchings

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Abstract
A crystal exhibiting structures modulated in c direction is described by assigning every atom to a plane perpendicular to c, introducing two Ising spins per layer unit cell which specify the state of that cell, and providing for frustrated interactions. Starting from the Hamiltonian, the MFA free energy is given, from which phase diagrams in temperature-interactions-space and especially the properties of discommensurations are derived. To this purpose, the fixed point expansion of the four dimensional mapping representing the equilibrium equations is used to determine profiles, free energies and free interaction energies of discommensurations in different phases. As an example for the derivation of special phase diagram features from these results, the location of an accumulation point of structure branchings at the boundary of the q = 1/6 phase is determined.