New insights on charge density wave and other fluctuations in blue bronze from NMR measurements

On the basis of extensive NMR measurements on a single crystal of Rbo.3MoO3 (blue bronze), we report several aspects of its charge density wave (CDW) and metallic phases that require a reinterpretation of some earlier results and indicate several aspects of the CDW fluctuations that will require further development of the theoretical models to describe them. From measurements of the spin-lattice relaxation rate (1/T1) of both 85Rb and 87Rb, it is clear that the dominant coupling responsible for 1/T1 is quadrupolar both above and below the CDW transition. This result contradicts the earlier interpretation that it was magnetic coupling to the conduction electrons above the transition. Also, there is no clear evidence of a coherence peak in 1/T1 below the transition. For most orientations of the external field relative to the crystalline axes, 1/T1 in the CDW phase is nearly constant across the spectrum of the central NMR transition, in contradiction to the prediction of the simple phason model for the fluctuations. Thus, there are additional fluctuation modes of the CDW that need to be included into the model for 1/T1.