Charles Pennington

Design and testing of high sensitivity microreceiver coil apparatus for nuclear magnetic resonance and imaging

We report the design and testing of a nuclear magnetic resonance (NMR) microcoil receiver apparatus, employing solenoidal microreceiver coils of dimensions of tens to hundreds of microns, using applied field of 9 T (proton resonance frequency 383 MHz). For the smallest receiver coils we attain sensitivity sufficient to observe proton NMR with signal to noise (S/N) one in a single scan applied to a ∼10 μm3 (10 fl) water sample, containing 7×1011 total proton spins. We also test the dependence of the S/N on important coil parameters, including coil composition and resistivity, turn spacing, and lead lengths.

Triaxial magnetic field gradient system for microcoil magnetic resonance imaging

There is a great advantage in signal to noise ratio (S/N) that can be obtained in nuclear magnetic resonance (NMR) experiments on very small samples (having spatial dimensions ∼100 μm or less) if one employs NMR “micro” receiver coils, “microcoils,” which are of similarly small dimensions. The gains in S/N could enable magnetic resonance imaging (MRI) microscopy with spatial resolution of ∼1–2 μm, much better than currently available. Such MRI microscopy however requires very strong (>10 T/m), rapidly switchable triaxial magnetic field gradients. Here, we report the design and construction of such a triaxial gradient system, producing gradients substantially greater than 15 T/m in all three directions, x, y, and z (and as high as 50 T/m for the x direction). The gradients are switchable within time ∼10 μs and adequately uniform (within 5% over a volume of [600μm3] for microcoil MRI of small samples.

θ-REDOR: an MAS NMR method to simplify multiple coupled heteronuclear spin systems

Abstract The measurement of heteronuclear dipolar couplings with rotational-echo, double-resonance NMR (REDOR) is straightforward for isolated S–I spin pairs. However, measuring the n dipolar couplings for an S spin coupled simultaneously to n I spins is not easy because of additional complications arising from the orientational dependencies between the n dipolar tensors. We present an important modification of REDOR, called θ -REDOR, that is useful for systems with multiple dipolar couplings. This new experiment recovers the n dipolar couplings while eliminating the complicating orientational dependence between the dipolar tensors of the REDOR experiment. A REDOR transform of data obtained by θ -REDOR provides the n dipolar frequencies.

Magnetic Field Independence of the Spin Gap in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

We report, for magnetic fields of 0, 8.8, and 14.8thinspthinspT, measurements of the temperature dependent {sup 63}Cu NMR spin lattice relaxation rate for near optimally doped YBa{sub 2}Cu {sub 3}O{sub 7{minus}{delta}} , near and above T{sub c} . In sharp contrast with previous work we find no magnetic field dependence. We discuss experimental issues arising in measurements of this required precision and implications of the experiment regarding issues including the spin gap or pseudogap. {copyright} {ital 1998} {ital The American Physical Society}

Positive-intrinsic-negative diode-based duplexer for microcoil nuclear magnetic resonance

Microcoil nuclear magnetic resonance (NMR), using receiver coils of diameters of order 100 μm, is increasingly employed to observe very small (∼0.3 nl) samples with high sensitivity. However, many experimental aspects of microcoil NMR differ greatly from conventional NMR. In particular, the duplexer is a device used to switch between the transmit and receive phases of the experiment. The conventional duplexer is a passive device employing crossed diodes, that switch automatically to transmit mode when high rf power is present. In microcoil NMR, however, the transmitter power is necessarily quite low, with voltages that do not greatly exceed characteristic diode voltage drops. Here we present the complete design and construction methods for a duplexer well suited to the special demands of microcoil NMR.

Advances toward MR microscopy of single biological cells

Here we report further progress toward the goal of achieving proton magnetic resonance imaging microscopy with resolution approaching a few micrometers in all three dimensions. We obtain proton images of a phantom sample — a microcapillary containing water and 39 μm diameter polymer microspheres — with a resolution of a few micrometers (perhaps about 5 μm) in all three spatial dimensions.

NMR studies of alkali C60 superconductors

The authors report13C,87Rb,39K, and133Cs nuclear magnetic resonance (NMR) measurements of lineshapes, Knight shifts, and spin-lattice relaxation rates in the normal and superconducting states of M3C60, where M3 = Rb3, K3, Rb2K, RbK2, Rb2Cs, and RbCs2. Measurements are used as a guide to a new ammonia solvent synthesis technique. Temperature dependence of the superconducting state electron spin susceptibility is found to follow BCS weak coupling predictions. The issue of the Hebel-Slichter coherence peak is addressed.

NMR spin echo experiments as probes of vortex dynamics in YBa2Cu3O7

Abstract We report 89 Y and 17 O NMR (9 Tesla) spin echo decay (T 2 ) measurements for YBa 2 Cu 3 O 7 . We correct the results to remove effects of 63,65 Cu dipolar coupling in order to isolate effects of vortex dynamics. We confirm vortex localization in the vortex solid state with rms displacements consistent with Langevin dynamical theory, but with motional dynamics at time scales some 10 6 times slower than predicted.