Study of Excitations in Structurally Incommensurately Modulated Solids by Means of Nuclear Magnetic Resonance

Abstract

The dynamics of the incommensurate (IC) modulation is investigated for several one-dimensionally incommensurately modulated crystals near the transition to the normal high temperature phase at the temperature T-i by means of quadrupolar perturbed nuclear magnetic resonance (NMR) spectra and nuclear spin-lattice relaxation. All results can be described consistently in terms of a static modulation in the IC phase without any indication for `floating' or large scale fluctuations of the modulation wave. The critical exponents derived from the NMR line shape and the relaxation times T-1 very nicely fit to the universality class of the 3d-XY model. For Rb2ZnBr4 and Rb2ZnCl4 crystals the characteristic frequency of the critical dynamics of the order parameter (OP) slows down below the Larmor frequency used in the Rb-87 NMR relaxation time measurements. In this particular case one can derive the characteristic frequency of the critical dynamics of the OP above T-i and of the phason below T-i from the Larmor frequency dependence of T-1.