Two-dimensional nuclear magnetic resonance study of phason and amplitudon relaxation mechanisms in structurally incommensurate Rb2ZnCl4

Abstract

In structurally incommensurate systems phason and amplitudon thermal fluctuations of the frozen-in modulation wave provide for two different nuclear magnetic resonance (NMR) spin-lattice relaxation mechanisms. These two relaxation mechanisms influence different parts of the inhomogeneously broadened NMR absorption spectrum differently. They can be resolved in principle by a conventional one-dimensional measurement of the spin-lattice relaxation time T1 over the NMR lineshape. Here the authors show that a two-dimensional NMR spin-lattice relaxation technique provides a very convenient way of determining the variation of T1 over the lineshape by the use of normalized contour plots. The results for the central transition of 87Rb in Rb2ZnCl4 show that the measured phason-induced spin-lattice relaxation time is temperature-independent. The much less efficient amplitudon relaxation mechanism is bypassed in part by the cross-relaxation process of the amplitudon-relaxed parts to the phason-relaxed parts of the NMR lineshape. The theoretical prediction for the total variation of T1 over the lineshape for the cross-relaxation process agrees with the experimental value of three.