Thermal fluctuations of charge-density waves studied by NMR

Thermal fluctuations of a pinned incommensurate charge-density wave in zero external electric field were studied by NMR. Low-frequency random incohere nt fluctuations of the amplitude and phase of the modulation wave affect th e shape of the homogeneous decay of the transverse nuclear-spin magnetizati on. Phase fluctuations induce a characteristic exponential decay with the e xponent proportional to the time variable as t(3/2) whereas the amplitude f luctuations result in a decay linear in t. The two types of fluctuations sh ow very different temperature behavior. The phase fluctuations exhibit a we ak T dependence and are present in the whole incommensurate phase, whereas the amplitude fluctuations are significant only in the close vicinity of th e Peierls transition temperature, where they behave critically. The above e ffects are predicted on the basis of Landau theory using the assumption tha t the spectral density of phase fluctuations exhibits a central peak to acc ount for the slow relaxational processes and memory effects in the presence of impurities. Theoretical predictions are compared to the Rb-87 NMR exper iment in blue bronze Rb0.3MoO3, and a good agreement between the theory and experiment was found. The results show that the thermally induced phase fl uctuations are slow and take place on a spatial scale of a small fraction o f the charge-density-wave wavelength, thus on the subnanometric scale. [S01 63-1829(99)13929-8].