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We study the critical dynamics of crystals which undergo a second-order phasetransition from a high-temperature normal phase to a structurallyincommensurate (IC) modulated phase. We give a comprehensive description of thecritical dynamics of such systems, e.g. valid for crystals of the A_2BX_4family. Using an extended renormalization scheme, we present a framework inwhich we analyze the phases above and below the critical temperature T_I. AboveT_I, the crossover from the critical behavior to the mean-field regime isstudied. Specifically, the resulting width of the critical region isinvestigated. In the IC modulated phase, we consider explicitly the coupling ofthe order parameter modes to one-loop order. Here the Goldstone anomalies andtheir effect on measurable quantities are investigated. We show their relationwith the postulated phason gap. While the theory can be applied to a variety ofexperiments, we concentrate on quadrupole-perturbed nuclear magnetic resonance(NMR) experiments. We find excellent agreement with these dynamicalmeasurements and provide answers for some questions that arose from recentresults.Comment: 21 pages, revtex, 17 figures included (16 as .ps files

Critical dynamics at incommensurate phase transitions and NMR relaxation experiments