Faculty Bibliography

We describe methods for the measurement of translational diffusion in very large static magnetic field gradients by NMR. The techniques use a 'hole-burning' sequence that, with the use of fringe field gradients of 42 T/m, can image diffusion along one dimension on a submicron scale. Two varieties of this method are demonstrated, including a particularly efficient mode called the 'hole-comb,' in which multiple diffusion times comprising an entire diffusive evolution can be measured within the span of a single detected slice. The advantages and disadvantages of these methods are discussed, as well as their potential for addressing non-Fickian diffusion, diffusion in restricted media, and spatially inhomogeneous diffusion

Combined theoretical and experimental studies are reported on alkali ion-cryptand interactions which affect charge transport in liquid electrolytes. Encapsulation of the cation by a cryptand cage can significantly improve the ionic conductivity, by reducing the fraction of time in which the cation is bound to its counterion. Comparison between experimental conductivities and NMR measurements of diffusion rates of Li and F in the amorphous LiMPSA electrolyte suggest that in the presence of the C222 cryptand, Li is bound to MPSA about 75% of the time. A hybrid classical/quantum methodology is used to model dynamics and electronic structure of a number of cryptand-alkali ion-MPSA and cryptand-ion-cryptand systems in an effort to extract general features which may be useful in designing electrolytes with improved performance.

NMR measurements of the linewidth of planar O-17 in near-optimally doped YBa2Cu3O7-delta in the vortex lattice state are reported. We find that the lineshape, i.e. magnetic field probability distribution, is much broader than what is expected based on the calculated distribution arising from the vortex supercurrents. We can qualitatively account for this anomalously large NMR linewidth measured at low temperatures by postulating that the broadening is caused by antiferromagnetism in the vortex core. (C) 2003 Elsevier Science B.V. All rights reserved.

We report spatially resolved nuclear-magnetic-resonance measurements on a high-temperature superconductor that indicate the presence of correlated antiferromagnetic fluctuations in the vortex core. The nuclear-spin-lattice relaxation rate 1/T-17(1) of planar O-17, in near-optimally doped YB2Cu3O7-delta, was measured. Outside of the core, ((T1T)-T-17)(-1) is independent of temperature consistent with theoretical predictions for a d-wave superconductor. In the vortex core, ((T1T)-T-17)(-1) increases with decreasing temperature following an antiferromagnetic Curie-Weiss law.

We report on a solution to the problem of phase noise in nuclear magnetic resonance (NMR) experiments, Phase noise refers to the variation in the phases of NMR signals from successive acquisitions due to an unstable applied field. Such a situation exists in high-field resistive Bitter magnets and, for sufficiently long timescales, can cause serious signal degradation upon signal averaging. An inductive shield, formed by a highly conducting metal tube placed around the sample and along the applied field. provides screening of the AC components of the applied field and thereby retains phase coherence over long periods', Although Simple in principle there are technical difficulties for practical implementation of this method. We Present demonstrations, of the utility of this approach. In particular, we show a significant extension of the effective transverse coherence time of the C-13 resonance in doubly C-13-labeled glycerol in a resistive Bitter magnet. This was accomplished through the use of a highly conducting aluminum shield. cooled to 4 K with liquid helium. (C) 2002 Elsevier Science (USA). All rights reserved.

Signals suffering from phase noise, such as spin echoes from NMR experiments in a fluctuating magnetic field, can be underestimated by traditional signal averaging. Averaging in the polar rather than in the Cartesian domain can solve this problem. However, because of background noise, there is an inherent drawback in the polar averaging technique: polar averaging under conditions of a low initial signal to noise will cause these same signals to be overestimated. We studied this problem with both numerical simulations and experiments. The quantitative results allow not only an assessment of the applicability of polar averaging for a given experiment but also possible corrections of the overestimate. (C) 2002 Wiley Periodicals, Inc.