David L. Cowan

A dangling‐silicon‐bond defect in topaz

Fast neutron bombardment of natural topaz crystals creates an optically active point defect with an absorption peak near 2 eV, coloring the crystal blue. This blue color anneals near 500 °C. The optical absorption is strongly dichroic, with a maximum when E is along the a direction in the orthorhombic lattice. We identify the defect as an oxygen vacancy in an isolated SiO4 tetrahedron, in the form of a doubly occupied dangling silicon bond. The defect is nonmagnetic, but electron spin resonance of its Frenkel partner implies an average oscillator strength f=0.012. A natural blue topaz is colored by this same defect, and electron and gamma irradiations produce a similar blue, but with strikingly different polarization dependences.

Effects of plasmenylethanolamine on the dynamic properties of the hydrocarbon region of mixed phosphatidylcholine-phosphatidylethanolamine aqueous dispersions. A spin label study

Spin-labeled aqueous dispersions of total phospholipid extracts from whole brains of hibernating hamsters and rats chronically consuming ethanol were compared with dispersions from control animals. Order parameter values and approximate rotational correlation times for the nitroxide spin labels indicated that ethanol consumption results in an adaptive decrease in bilayer membrane fluidity, while hibernation produces increases in fluidity. Since it has been proposed that changes in plasmenylethanolamine such as those seen with hibernation play a role in the homeoviscous adaptation of brain membranes, electron spin resonance studies using aqueous phospholipid dispersions containing equimolar mixtures of rat brain phosphatidylethanolamine and phosphatidylcholine, or synthetic dioleylphosphatidylcholine and dioleylphosphatidylethanolamine, and brain plasmenylethanolamine were performed. The molar amount of plasmenylethanolamine was varied within the ethanolamineglycerophospholipid fraction of each dispersion. Order parameter values of spin labels in liposomes containing brain phosphatidylcholine and phosphatidylethanolamine increased in parallel with increases in plasmenylethanolamine concentrations, indicating that fluidity was decreasing. Liposomes composed of synthetic dioleyl phospholipids exhibited biphasic changes in order parameter (S) values as plasmenylethanolamine replaced the diacyl form. Below 30% (mol%) plasmenylethanolamine, S values decreased, while above 30%, S values were seen to increase; indicating an initial fluidization, followed by a decrease in fluidity.

Cluster center formation in neutron‐damaged silicon

The production of vacancy clusters in neutron‐damaged silicon has been investigated by electron spin resonance. It is found that the production of P3 four vacancies and P6 di‐interstitials is independent of oxygen concentration, suggesting that these defects are formed in the primary cascade. Approximately two P3 centers were formed per primary cascade independent of the primary mean recoil energy for irradiations in varying fast neutron spectra. This suggests that these defects are associated with the Brinkman spike which terminates the cascade. Low energy primaries from thermal neutron capture and subsequent gamma recoil are very inefficient in producing these centers.

ESR dating of quartz from Exile Hill, Nevada

Abstract Quartz crystals from a geological site dated at 13 Myr contain paramagnetic lattice defects produced by exposure to in situ radiation. Two of these defects, both ESR active, contain information about the age of the mineral. From a γ-ray sensitive ESR signal of unknown identity we infer a lower age bound: T ⩾ 7 Myr. The superoxide radical (“peroxy”) defect, produced naturally by α recoil and reproduced artificially by fast-neutron irradiation, gives a compatible age estimate independent of γ dose: T = 8.7 ± 2.6 Myr.

Stochastic esr analysis of rat liver and hepatoma mitochondrial lipids

A commonly used model for the interaction of the motional narrowing of ESR lines is shown to be qualitatively misleading. An analysis of lipid extracts of mitochondrial preparations labeled with 12-nitroxide stearic acid produced linear plots of the logarithm of the correlation time versus the reciprocal of the absolute temperature when analyzed with stochastic computer simulations. However, when the data were analyzed with isotropic Lorentzian line shape approximations, nonlinear plots were obtained.

Critical currents at the second peak in irradiated YBCO

Abstract YBCO samples frequently show a second peak (SP) in critical current, j c , at a large field, B p . The origin of this technologically important effect is unclear, although a connection between the systematics of the SP and the disorder introduced by oxygen vacancies has been shown. The decrease in j c at fields above the peak is probably related to plastic flow in the vortex glass. We have examined the SP in an optimally doped single crystal of YBCO and in a nominally identical sample irradiated by fast neutrons. At 76 K, j c =2.5×10 5 A/cm 2 in the irradiated sample, showing an irradiation induced enhancement by about a factor of three. Both samples exhibit a strong SP effect. We use SQUID-based creep data at B B p and B > B p to investigate changes in vortex relaxation mechanisms as the peak is crossed. The creep data show a change in relaxation mechanism as one crosses the peak. On the high field side of the peak we obtain results in both samples that are in good agreement with a plastic flow mechanism involving one-loop depinning.

Flux Pinning and Defects in As-Grown and Neutron Irradiated Y-123 Crystals

A model of cooperative pinning where the pinning potential U(j) is inversely proportional to j accurately describes flux creep in high quality single crystals of Y-123 in the single fluxoid creep regime. In contrast, for strong pinning the pinning potential varies as (1-j/jmax)3/2. Plots of the function S(j)=dln(U)/dln(j) provide a sensitive test for distinguishing between these two pinning mechanisms. Several crystals grown and annealed under different conditions have been analyzed. We find that we can prepare single crystals in either pinning class, depending on the growth and annealing process. It is suggested that strong pinning occurs when the growth and annealing processing lead to the formation of precipitates with nearly optimum size distribution. Strong pinning effects are also observed in fast neutron irradiated samples.

Strong pinning in heat treated YBa2Cu3O7−δ

Abstract A number of authors have discussed the pinning of magnetic fluxoids in high temperature superconductors in terms of a cooperative process involving large numbers of defects. In particular, in a recent study of magnetic flux creep in high quality single crystals of YBa 2 Cu 3 O 7−δ , we showed that an appropriate collective pinning model accurately described the creep in that part of the H-T phase diagram where the creep is controlled by single fluxoid motion. Here we present data on similar crystals given an additional heat treatment at temperatures above 800°C. These crystals exhibit large critical currents that exceed the limit associated with collective pinning, suggesting the influence of strong pinning defects. An analysis of the pinning potential U(j) at large j provides further evidence for strong pinning. We suggest that the heat treatment introduces precipitates of nearly optimum size for pinning, leading to a combination of weak and strong pinning defects.

Flux creep in twinned, untwinned, and neutron irradiated Yba2Cu3O7☆

Abstract The creep in persistent magnetization M has been measured for twinned, unwinned and “rediation hardended” single crystals of YBa 2 Cu 3 O 7 . In all crystals, large measured flux velocitis show that avalanches paly an important role in relaxation from the critical state. In twinned samples the pinning is found to be very similar to that in untwinned samples even though M is significantly smaller-an apparent paradox that can be resolved by noting that M will be reduced by flux penetration along the twin boundaries. Irradiation produces a large increase in the activation energy as expected from the increase in critical current. The data imply that the irradiation-induced pinning sites are similar in strength to those present in as- grown samples.