Observation of ultraslow translational diffusion in metallic lithium by magnetic resonance

Journal ArticleThe theory of a new magnetic-resonance technique for studying the ultra slow motion of atoms was presented in a previous paper. In this paper, we present its experimental confirmation for the case of translational diffusion in lithium metal. By this technique the mean time between atomic jumps r can be measured provided that r is less than the spin-lattice relaxation time Ti, permitting study of much slower rates of motion than previously has been possible using magnetic resonance. For lithium metal we have measured over nine orders of magnitude from r = 10- 9 sec to T = 1 sec, thereby extending by nearly five decades the results previously obtained by Holcomb and Norberg using conventional techniques. We have applied a new spin-temperature theory to the analysis of our low-temperature results in the range of its validity, TI>T>T2. By studying the variation of our relaxation time with the rf field strength Hi, we have unambiguously demonstrated the validity of the spin-temperature theory and the invalidity of perturbation theories in describing relaxation due to infrequent atomic motions in weak applied fields

Study of utraslow atomic motions by magnetic resonance

Journal ArticleMagnetic resonance has been widely used to study phenomena such as atomic diffusion and molecular reorientation. It is applicable when the mean time, r, between atomic jumps is either (a) sufficiently short to narrow the resonance line width or (b) of the correct magnitude to produce spin-lattice relaxation. Case (a) occurs when r is less than 1/Aco where Au> is the rigid lattice linewidth. Case (b) occurs when T is of the order of l/co0 , where oo0 is the Larmor frequency. Typically, case (a) is found when r < 100 /isec, case (b) when r ~ 10 ~ 8 sec. In this Letter we report a new, experimentally simple technique which enables us to study motions of a much slower rate, the criterion being roughly T < T 1 where T1 is the spin-lattice relaxation time

Low-field relaxation and the study of ultraslow atomic motions by magnetic resonance

Journal ArticleConventional resonance enables one to study motion of atoms by measurement of line width when the mean time r between jumps is less than 1/Aco, where Aa> is the rigid lattice line width, or by measurement of the spin-lattice relaxation time, Ti, when r ^ l / coo , where coo is the Larmor frequency. We describe a new technique applicable when T<TI. It is therefore applicable to the study of very slow motion. The method is analogous to measuring T\ with coo = 0. However, we are able to keep cu0 in the megacycle region by performing the experiments in the reference frame rotating at the Larmor frequency. Analysis of the technique requires solution of the problem of the effect of infrequent motion on the nuclear relaxation time when the applied static field is comparable to the local field. The relaxation time is then comparable to r, indicating that jumps are strong "collisions" for the spins. The case of strong "collisions" is not treated in the conventional treatment of Bloembergen, Purcell, and Pound. We solve the problem by use of the concept of spin temperature and the sudden approximation. Explicit formulas are given for the nuclear relaxation in the laboratory for weak static fields, and in the rotating frame for alternating fields of the order of or less than the local field. We treat both diffusional motion and molecular reorientation

Two-component uniform spin susceptibility in superconducting HgBa2_{2}CuO4+δ_{4+\delta} single crystals determined with 63^{63}Cu and 199^{199}Hg NMR

$^{63}$Cu and $^{199}$Hg NMR shifts for an optimally and underdoped HgBa$_{2}$CuO$_{4+\delta}$ single crystal are reported, and the temperature dependence dictates a two-component description of the uniform spin susceptibility. The first component, associated with the pseudogap phenomenon in the NMR shifts, decreases already at room temperature and continues to drop as the temperature is lowered, without a drastic change at the transition temperature into the superconducting state. The second component is temperature independent above the superconducting transition temperature and vanishes rapidly below it. It increases with doping and is a substantial part of the total spin susceptibility measured at both nuclei

Magnetic properties of pure and Gd doped EuO probed by NMR

An Eu NMR study in the ferromagnetic phase of pure and Gd doped EuO was performed. A complete description of the NMR lineshape of pure EuO allowed for the influence of doping EuO with Gd impurities to be highlighted. The presence of a temperature dependent static magnetic inhomogeneity in Gd doped EuO was demonstrated by studying the temperature dependence of the lineshapes. The results suggest that the inhomogeneity in 0.6% Gd doped EuO is linked to colossal magnetoresistance. The measurement of the spin-lattice relaxation times as a function of temperature led to the determination of the value of the exchange integral J as a function of Gd doping. It was found that J is temperature independent and spatially homogeneous for all the samples and that its value increases abruptly with increasing Gd doping.Comment: 14 pages, 10 figures, to be published in Physical Review

NMR study of the structure and motion of charge density waves in NbSe3

The authors report NMR measurements of the Nb93 resonance in the charge-density-wave (CDW) conductor NbSe3 on aligned, multicrystalline samples. They have observed the resonance with and without current flow in the sample at temperatures above 59 K, where a single incommensurate CDW is present. Results include (1) a demonstration that the CDW is not discommensurate, (2) quantitative measurements of CDW displacements below threshold, and (3) evidence, including current-induced motional narrowing, of CDW motion throughout the entire sample, above the threshold

NMR studies of NbSe3: Electronic structures, static charge-density-wave measurements, and observations of the moving charge-density wave

We describe a series of Nb93 NMR studies of the charge-density-wave (CDW) conductor NbSe3. Using an aligned, multicrystalline sample, we have measured the complete shift tensors for the three Nb sites in the normal state. The results confirm a picture in which one chain is essentially insulating, and the other two are conducting. Studies below the 144-K CDW transition give well-resolved CDW-broadened line shapes, which indicate that the CDW is incommensurate with little or no lock-in to the lattice. Further NMR measurements performed with the CDW driven by electrical currents give information about the moving CDW. Motional narrowing of the NMR line indicates that the CDW conduction phenomena are associated with bulk motion of the NbSe3 CDW. On the other hand, the motion is irregular at all voltages, as indicated by the large voltage required for motional narrowing, the short T2, and the behavior of the magnetization under current-induced saturation. Furthermore, NMR spin-echo measurements of the low-voltage dielectric response indicate a spatially nonuniform response of the CDW under its pinning barriers