Dynamics in incommensurate phases studied by NMR: Theory and relaxation measurements on Rb87 satellites in Rb2ZnCl4

A general formalism is developed for describing nuclear-spin-lattice relaxation for quadrupolar perturbed nuclear magnetic resonance (NMR) in structurally incommensurately (IC) modulated crystals in terms of the elementary excitations of these systems. Our discussion deviates from previous ones as regards the identification of the amplitudon and phason induced relaxation rates 1/T1A and 1/T1φ in the plane wave limit as well as the temperature dependence of T1φ in the soliton limit. Measurements are reported for the spin-lattice relaxation of the Rb87 satellite transitions in the normal, incommensurate, and commensurate phases of the prototype incommensurate system Rb2ZnCl4. At ambient temperature the variation of the nuclear magnetic relaxation rate is measured over the incommensurately broadened distribution of resonance frequencies for two crystal orientations. The T1 model developed is applied to these data, making use of the Fourier series of the static electric field gradient determined from the NMR satellite spectra in the IC phase. A strict agreement is found. The temperature dependence of T1 is determined in that crystal orientation where a well-defined assignment of T1A and T1φ can be given. In the low-temperature part of the incommensurate phase, our data reflect the local softening of phase fluctuations in the discommensurations or the flattening out of the corresponding acoustic branch what is in accordance with our theoretical prediction and in contrast to previous results. The relevance of phason gaps previously derived from NMR data is discussed. © 1994 The American Physical Society.