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Title:Structure and motion of charge density waves, and NMR study of NbSe3
Author(s):Ross, Joseph Hansbro
Doctoral Committee Chair(s):Slichter, C.P.
Department / Program:Physics
Subject(s):charge density waves
Nuclear magnetic resonance (NMR)
electric field gradients
Abstract:The author reports Nb93 NMR studies of the charge density wave (COW) conductor, NbSe3 , using aligned, multicrystalline samples, observing the NMR with and without current flow in the crystals. Above 144 K, where no COW'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 COW phase transition, broadening is observed on the "yellow" site, plus a change in the Knight shift for that site. Below 59 K, the other COW 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 COW structure with a period N [less than / equal to] 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 COW displacement, at smaller applied voltages. The motional narrowing and saturation results demonstrate that the COW conduction phenomenon involves bulk movement of the COW. Analysis indicates that the COW 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. COW displacements were measured using spin-echo techniques combined with pulses of sample voltage. Results indicate an approximate 2° COW phase shift, at voltages near the conduction threshold. This is inconsistent with rigid classical COW motion, but could be consistent with the tunneling model of COW motion, or classical motion involving a highly deformable COW.
Issue Date:1986
Genre:Dissertation / Thesis
Rights Information:1986 Joseph Hansbro Ross, Jr.
Date Available in IDEALS:2011-06-02
Identifier in Online Catalog:994627

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