Faculty Bibliography

Spin-lattice relaxation time (T(1)) measurements are often time-consuming due to the need to measure the full equilibrium magnetization with a long wait time. However, any magnetization recovery can be decomposed into pure recovery and pure decay components, the latter of which lends itself to a much simpler and faster extraction of T(1). We demonstrate several pulse sequences that accomplish this decomposition experimentally and illustrate its applications in a steady magnetic field gradient, and in materials possessing a broad distribution of T(1)

The multiple-modulation-multiple-echo sequence, previously used for rapid measurement of diffusion, is extended to a method for single shot imaging. Removing the gradient switching requirement during the application of RF pulses by a constant frequency encoding gradient can shorten experiment time for ultrafast imaging. However, having the gradient on during the pulses gives rise to echo shape variations from off-resonance effects, which make the image reconstruction difficult. In this paper, we propose a simple method to deconvolve the echo shape variation from the true one-dimensional image. This method is extended to two-dimensional imaging by adding phase encoding gradients between echoes during the acquisition period to phase encode each echo separately. Slice selection is achieved by a frequency selective pulse at the beginning of the sequence. Imaging speed is mainly limited by the phase encoding gradients' switching times and echo overlap when echo spacing is very short. This technique can produce a single-shot image of sub-millimeter resolution in 5 ms

The B-11 NMR spectra in polycrystalline MgB2 were measured for several magnetic fields (1.97 and 3.15 T) as a function of temperature from 5 to 40 K. The composite spectra in the superconducting state can be understood in terms of anisotropy of the upper critical field, gamma(H), which is determined to be 5.4 at low temperature. Using Brandt's algorithm (Brandt 1997 Phys. Rev. Lett. 78 2208) the full spectrum, including satellites, was simulated for the temperature 8 K and a magnetic field of 1.97 T. The penetration depth. was determined to be 1152 +/- 50 angstrom, and the anisotropy of the penetration depth, gamma(lambda), was estimated to be close to one at low temperature. Therefore, our findings establish that there are two different anisotropies for upper critical field and penetration depth at low temperatures

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.

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.

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.

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