Domain walls in pseudo spin systems

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Abstract
Reviewed is the description of modulated systems and especially of their planar defects within the frame of the ANNNI model. After a sketch of the reduction of the original lattice dynamical formulation to this discrete pseudo spin model, the derivation of discommensuration properties and their use for deducing the characteristics of structural transitions are described. Explicitly dealt with are: general (meta-)stability criteria for modulated phases and their explicit forms for special structures; fixed point expansions for the equilibrium equations; profiles, free energies and free interaction energies of discommensurations and their significance for the location and the order of phase transitions and for the discussion of structure branchings and their accumulation points. The applicability of the ANNNI model to actual systems is demonstrated by deriving the amply structured temperature vs. interaction ratio phase diagram of BCCD. In this context it is stressed that both theoretical considerations and experimental findings show that an ANNNI model treatment of actual materials generally requires effective interactions depending on parameters like temperature or pressure.