Defect‐density waves and memory effects in modulated systems

We review some recent theoretical and experimental results concerning the memory effects observed in some modulated systems. Thiourea has been extensively investigated and we present new results in this system. The Defect-Density Wave (DDW) where the memory is printed induces lock-in effects in a way similar to Umklapp terms. New experimental results on the metastability of this extrinsic DDW show a striking similarity to the behaviour of memory effects displayed by the intrinsic disorder of spin-glasses for example. These memory effects have been observed in very different modulated systems but an understanding of their precise origin is still lacking; this is essential for a better understanding of the dynamics of the DDW which plays the role of a staggered field with adjustable wavelength and amplitude: interesting physics should be done with this new physical object.