Faculty Bibliography

Puzzling aspects of high-transition-temperature (high-Tc) superconductors include the prevalence of magnetism in the normal state and the persistence of superconductivity in high magnetic fields. Superconductivity and magnetism generally are thought to be incompatible, based on what is known about conventional superconductors. Recent results, however, indicate that antiferromagnetism can appear in the superconducting state of a high-Tc superconductor in the presence of an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines, each of which represents a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there. Here we report the results of a high-field nuclear-magnetic-resonance (NMR) imaging experiment in which we spatially resolve the electronic structure of near-optimally doped YBa2Cu3O7-delta inside and outside vortex cores. Outside the cores, we find strong antiferromagnetic fluctuations, whereas inside we detect electronic states that are rather different from those found in conventional superconductors

We have studied the temporal instability of a high field resistive Bitter magnet through nuclear magnetic resonance (NMR). This instability leads to transverse spin decoherence in repeated and accumulated NMR experiments as is normally performed during signal averaging. We demonstrate this effect via Hahn echo and Carr--Purcell--Meiboom--Gill (CPMG) transverse relaxation experiments in a 23-T resistive magnet. Quantitative analysis was found to be consistent with separate measurements of the magnetic field frequency fluctuation spectrum, as well as with independent NMR experiments performed in a magnetic field with a controlled instability. Finally, the CPMG sequence with short pulse delays is shown to be successful in recovering the intrinsic spin--spin relaxation even in the presence of magnetic field temporal instability

We have carried out nuclear magnetic resonance (NMR) studies on a new class of electrolyte consisting of a lithium salt (LiMPSA) and a macrocycle ([2.2.2] cryptand). NMR diffusion coefficient measurements have determined the motion of lithium cations (Li-7-NMR), polyatomic anions (MPSA) (F-19-NMR), and encapsulating macrocycles ( H-1-NMR), with the striking result that they are equal over a large temperature range. Diffusion coefficients as low as (1.0+/-0.1) 10(-9) cm(2)/s have been measured through the use of a 42 T/m fringe-field gradient. The magnitudes of the anion and cryptand diffusion were found to be consistent with a free-volume picture for translational diffusion. The transport of the lithium cations was interpreted as taking place mainly through association with large anions (MPSA) and encapsulating macrocycles ([2.2.2] cryptand), with averaged residence time fractions of 77 and 23 %. respectively. NMR spin-lattice relaxation time measurements of Li-7 and F-19 spins have also been performed.

The NMR spin-lattice relaxation rate, T-1(-1), can be measured precisely by progressive saturation. This efficient technique is useful when T-1 is long and the NMR signal is weak. We derive the quasiequilibrium spin response to excitation in the case of a Zeeman spectrum in the presence of quadrupolar interactions. Exact solutions for the recovery of magnetization under the influence of purely magnetic fluctuations for I = 1/2, 3/2, and 5/2 are presented. This is the general solution to a problem that has been previously solved only for the I=1/2 case. An important example for the application of this technique is O-17 NMR in cuprate superconductors (l = 5/2) we show comparisons of the theory with the relaxation measured for high-temperature superconducting materials and the NMR-rates measured by this technique across the vortex-broadened spectrum at low temperature.