Theory of the memory effect in thiourea. Defect density waves in modulated systems

A simple Landau-Ginzburg theory is shown to account qualitatively and semi-quantitatively for the memory effect anomalies of the susceptibility and the birefringence in thiourea. The importance of the gradient amplitude coupling energy in thiourea is emphasized. The possibility of creating a defect-induced locked incommensurate phase around any arbitrary temperature within the modulated phase is demonstrated. Such a locked phase will appear as a consequence of the condensation of a periodic defect concentration if extrinsic mobile impurities are allowed to interact for a sufficient time with a static modulation. We briefly discuss some aspects of such defect density waves in the physics of modulated structures.