Structure and Motion of Charge Density Waves: AN NMR Study of NIOBIUM-SELENIDE(3)

The author reports Nb('93) NMR studies of the charge density wave (CDW) conductor, NbSe(,3), using aligned, multicrystalline samples, observing the NMR with and without current flow in the crystals. Above 144 K, where no CDW's are present, an orientation and field-dependence NMR study is presented. Three Nb NMR sites are identified, and associated with the three known structural sites, through the symmetry of the NMR shifts. Numerical calculations of the electric field gradients are also presented. Combined with the NMR results, these indicate that the "red" site is essentially insulating, while the other two sites have important conduction electron contributions. Below 144 K, a CDW phase transition, broadening is observed on the "yellow" site, plus a change in the Knight shift for that site. Below 59 K, the other CDW phase transition, comparable changes are observed on the "orange" site. A "rotated-field-gradient" model is presented to account for the line shape above 77 K, and no evidence of discommensurations, or of a commensurate CDW structure with a period N <= 22, are found, from detailed studies of the NMR line shapes. Results of current-flow NMR studies at 77 K include the observation of motional narrowing and the saturation of the magnetization, with applied voltages above the conduction threshold, and measurements of the CDW displacement, at smaller applied voltages. The motional narrowing and saturation results demonstrate that the CDW conduction phenomenon involves bulk movement of the CDW. Analysis indicates that the CDW moves with a nearly uniform velocity at large sample voltages, but the motion can be described as a sequence of jumps at lower sample voltages. Measurements of the T(,2) relaxation time, though, indicate that low-velocity modes are present even at the highest sample currents studied. CDW displacements were measured using spin-echo techniques combined with pulses of sample voltage. Results indicate an approximate 2(DEGREES) CDW phase shift, at voltages near the conduction threshold. This is inconsistent with rigid classical CDW motion, but could be consistent with the tunneling model of CDW motion, or classical motion involving a highly deformable CDW.