Roles of the magnetic field and electric current in thermally activated domain wall motion in a submicrometer magnetic strip with perpendicular magnetic anisotropy

We have experimentally studied micrometer-scale domain wall (DW) motion driven by a magnetic field and an electric current in a Co/Pt multilayer strip with perpendicular magnetic anisotropy. The thermal activation energy for DW motion, along with its scaling with the driving field and current, has been extracted directly from the temperature dependence of the DW velocity. The injection of DC current resulted in an enhancement of the DW velocity independent of the current polarity, but produced no measurable change in the activation energy barrier. Through this analysis, the observed current-induced DW velocity enhancement can be entirely and unambiguously attributed to Joule heating.     

Stereoselective synthesis of 3-azido-2,3-dideoxy-D-ribose derivatives and its utilization for the synthesis of anti-HIV nucleosides

Detail account of the synthesis of 3′-azido nucleosides utilizing 3-azido-2,3-dideoxy-D-ribose derivative 7 as the key intermediate was described. The key intermediate 7 was synthesized from D-mannitol in 8 steps in a preparative scale. The Michael reaction of the azide group with α,β-unsaturated-γ-butyrolactone 4 was affected by the steric bulkiness of the substituent at the 5-O position. A bulky t-butyldiphenylsilyl substitution at 5-O gave almost exclusively the α-azido adduct 5b , while unsubstitution at 5-O produced 1:1 mixture of α-and β-adducts. The ratio of α to β anomers in the condensation between azido acetate 7a and pyrimidine bases for the preparation of AZT and AZDU was greatly influenced by the solvent and the Lewis acid catalyst used. In the synthesis of 12 (AZDU, CS-87), the combination of dichloroethane and trimethylsilyl triflate gave an optimal result, while in the case of 14 (AZT), various conditions gave similar ratio of α,β anomers. The azido intermediate 7b was also utilized for the synthesis of several 3′-azido purine-like nucleosides 16–27 . The glycosylation was also affected by the Lewis acid catalyst. Boron trifluoride etherate gave the desired N₁-glycosylated compounds in which the α-anomer was major, but other catalysts such as trimethylsilyl triflate or stannic chloride produced N₂-glycosylated compounds as the major products. The newly synthesized purine-like compounds have been tested against HIV, however, none of them showed any significant activity.     

Determination of the elemental homogeneity of selected biological organs by 2MeV proton induced X-ray emission analysis

The use of finite size sub-samples to derive elemental concentrations which are representative of a tissue or organ as a whole are subject to errors arising from the heterogeneous nature of biological specimens. Proton induced X-ray emission (PIXE) analysis is employed to identify element variances in porcine liver, kidney and heart and sampling factors, which are the minimum mass of sample required to reduce concentration variability to a given level, are calculated. This analysis highlights the inhomogeneous nature of biological specimens and the need for clearly defined sampling protocols.     

A kinetic method of reactivity ratio measurement in olefin copolymerization with Ziegler–Natta catalysts

An efficient kinetic method of reactivity ratio measurement for binary copolymerization of gaseous monomers⁴ that does not require the measurement of copolymer composition and can be applied to the data obtained in a single copolymerization run was modified and applied to olefin copolymerization with heterogeneous Ziegler–Natta catalysts. Estimated r₁ and r₂ values for pairs like ethylene-propylene and propylene-1-butene agree favorably with published data.     

Rate constants for the reaction of Cl atoms with O3 at temperatures from 298 to 184 K

Using the standard, low pressure, discharge‐flow technique, with resonance fluorescence in the vacuum ultraviolet to observe Cl atoms, rate constants have been determined for the reaction of Cl atoms with O₃ at temperatures down to 184 K. The measured rate constants for 298–184 K fit the Arrhenius expression k(T) = (3.1 ± 1.3₅) × 10^?11 exp((?280 ±100 K)/T) cm³ molecule^?1 s^?1. The results extend the data on this key atmospheric reaction to slightly lower temperatures. The data are in fairly good agreement with those currently in the literature but suggest that the rate constant is approximately 15% lower than that given by currently recommended rate expressions at the lowest temperatures found in the stratosphere.© 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 104–109, 2002     

EVIDENCE FOR BRANCHING IN THE PHOTOCYCLE OF BACTERIORHODOPSIN AND CONCENTRATION CHANGES OF LATE INTERMEDIATE FORMS

Abstract— Flash photolysis transients of bacteriorhodopsin were recorded with a spectrograph -multielement photodiode array combination and the recordings were analyzed to determine the concentrations of bacteriorhodopsin intermediates "M" and "O" relative to the amount of "bR" cycling (pH 7.1,10–40°C). Estimated concentration time courses were simulated with solutions to two kinetic decay models which could account for photocycle temperature dependence. A unidirectional unbranched decay model overpredicts our estimated levels of [O(r)], whereas a model branched at the "M" intermediate describes each of the later intermediate levels well (with no evidence for an independent "N" form). Our results are consistent with "M" decay regulating the level and rates of change of [bR (t)] and (bR(f)]- and also suggest that two temperature-dependent pathways form "bR" from "M", one directly, and the other indirectly through "O".     

Determination of solid surface tension from particle-substrate pull-off forces measured with the atomic force microscope

Atomic force microscopy (AFM) is capable of solid surface characterization at the microscopic and submicroscopic scales. It can also be used for the determination of surface tension of solids (gamma) from pull-off force (F) measurements, followed by analysis of the measured F values using contact mechanics theoretical models. Although a majority of the literature gamma results was obtained using either Johnson-Kendall-Roberts (JKR) or Derjaguin-Muller-Toporov (DMT) models, re-analysis of the published experimental data presented in this paper indicates that these models are regularly misused. Additional complication in determination of gamma values using the AFM technique is that the measured pull-off forces have poor reproducibility. Reproducible and meaningful F values can be obtained with strict control over AFM experimental conditions during the pull-off force measurements (low humidity level, controlled and known loads) for high quality substrates and probes (surfaces should be free of heterogeneity, roughness, and contamination). Any probe or substrate imperfections complicate the interpretation of experimental results and often reduce the quality of the generated data. In this review, surface imperfection in terms of roughness and heterogeneity that influence the pull-off force are analyzed based upon the contact mechanics models. Simple correlations are proposed that could guide in selection and preparation of AFM probes and substrates for gamma determination and selection of loading conditions during the pull-off force measurements. Finally, the possibility of AFM measurements of solid surface tension using materials with rough surfaces is discussed.     

1.1-W cw Cr:LiSAF laser pumped by a 1-cm diode array

We demonstrate 1.1-W cw output power from a diode-laser array-pumped Cr:LiSAF laser based on a concept that allows for pumping low-gain solid-state lasers at reduced temperature rise. We discuss scaling to higher powers as a function of diode power and define a figure of merit for evaluating given diode lasers as pump sources for low-gain solid-state lasers.