Diffusionlike Motion of the Modulation Wave in Structurally Incommensurate Systems: A NQR Study

³⁵ClNQR measurements of the spin-echo decay in pure Rb₂ZnCl₄ exhibit an exponential t³ diffusional time dependence, which is the first clear evidence for the existence of slow random-walk-type motion of the incommensurate modulation wave close to the transition temperature TI. This phenomenon can be described by an effective diffusion constant Df in a reduced real space, which is settled as the projection of the direct space onto 1/4 the period length of the modulation wave. The onset of the incommensurate phase is accompanied by nonzero Df values, which rapidly decrease on going away from TI.