IMPURITY PINNING AND THERMALLY EXCITED COLLECTIVE MOTIONS IN INCOMMENSURATELY MODULATED STRUCTURES

Randomly quenched impurities pin the modulation wave of incommensurate (I) structures and prevent its free motion. However, rapid thermal fl uctuations reduce the pinning frequency (or, in the case of structural ly incommensurate insulators (SIIS) the phason gap DELTArho), and they have a significant effect on the spin-lattice relaxation time T1 and the NMR and NQR lineshapes. A T1 and lineshape model is developed for the i phase of SIIS. This model, which includes impurity pinning and t hermal fluctuations, is used to explain Cl-35(1) T1 and experimental l ineshape data for a pure Rb2ZnCl4 Crystal. The results indicate that, while the impurities are of a strong-pinning symmetry-breaking type ve ry close to the paraelectric-to-incommensurate transition temperature T(I), thermal fluctuations significantly reduced the impurity-induced pinning.