Thermal fluctuations and NMR spectra of incommensurate systems

In the presence of thermal fluctuations the incommensurate NMR line shape is a convolution of a static inhomogeneous with a dynamic homogeneous line shape which can be determined separately by 2D NMR. The form of the dynamic line shape and its variation over the inhomogeneous NMR spectrum permit a separate determination of the relative sizes of the phason and amplitudon fluctuations, compared to the static part of the order parameter. ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ 2D spectra of ultrapure ${\mathrm{Rb}}_2$ ${\mathrm{ZnCl}}_4$ agree with the above theory and show the existence of a temperature range where the incommensurate splitting induced by the static part of the order parameter is averaged out by thermal fluctuations.