LOW-FREQUENCY PHASON AND AMPLITUDON DYNAMICS IN THE INCOMMENSURATE PHASE OF RB2ZNCL4

It is shown that the phason-induced Rb-87 NMR spin-lattice relaxation rate in the incommensurate phase of Rb2ZnCl4 depends on the Larmor fre quency, whereas the amplitudon-induced rate does not. The results are described consistently by relaxatory phason and amplitudon fluctuation s. The relaxatory phason gap is found to be on the order of magnitude of about 10 MHz, in contrast to previous estimates from NMR studies. O ur data also allow a determination of the temperature dependence of th e amplitudon relaxation frequency.