Pulse sequences that permit selective detection of moving spins in a magnetic resonance image have been developed. Experiments were performed by the authors to produce projected angiographic data without the use of contrast agents, with the intensity of each image pixel determined by the macroscopic velocity of the detected spins. With this method, suppression of nonmoving spins is essentially complete, yielding a high dynamic range in signal intensity for detected vessels. Selective detection of moving spins is not dependent on pulsatile flow. Consequently, not only arterial structures, but also venous structures can easily be visualized. High-resolution angiographic images can be obtained by combining the flow experiment with surface coil techniques.
The spin-Peierls transition is considered as a progressive spin-lattice dimerization occurring below a transition temperature in a system of one-dimensional antiferromagnetic Heisenberg chains. In the simplest theories, the transition is second order and the ground state is a singlet with a magnetic gap. The historical origins and theoretical development of the concept are examined. Magnetic susceptibility and EPR measurements on the 7T-donor-acceptor compounds TTF· MS4C.(CF3) 4 (M =Cu, Au; TTF is tetrathiafulvalene) are reported. These compounds exhibit clearly the characteristics of the spin-Peierls transition in reasonably good agreement with a mean-field theory. The susceptibility of each compound has a broad maximum near 50 K, while the transitions occur at 12 and 2.1 K for M =Cu and Au, respectively. EPR linewidth observations over a broad temperature range are examined. Areas for further experimental and theoretical work are indicated, and a critical comparison is made of related observations on other materials.
The density of states in four superconductors, lead, tin, indium, and aluminum, has been studied using the tunneling technique. The experimental results agree remarkably well with the Bardeen-Cooper-Schrieffer theory; however, two exceptions were found. The energy gap is not as sharp in the experiment as in the theory, but this may merely be due to imperfect samples. The density of states in lead has definite but small divergences from the theory.
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