2-DIMENSIONAL NUCLEAR-MAGNETIC-RESONANCE STUDY OF PHASON AND AMPLITUDON RELAXATION MECHANISMS IN STRUCTURALLY INCOMMENSURATE RB2ZNCL4

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 we 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 Rb-87 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 3.