ALBERT

Notes: The effect of applying an electric field across a quasi-phase-matched frequency doubling lithium tantalate waveguide has been investigated. The waveguide was fabricated by a two-stage ion exchange process in pyrophosphoric acid. An electric field of 2 kV/mm was found to shift the phase matching wavelength by 0.05 nm. It is estimated that more optimized waveguides could produce wavelength shifts of ±4 nm for an applied electric field of ±20 kV/mm and could compensate for temperature variations of ±67 °C. © 1996 American Institute of Physics.

Notes: The formation of amorphous layers in GaAs during ion bombardment at elevated temperatures, where dynamic annealing of radiation-induced defects is substantial, is shown to be extremely sensitive to the implantation temperature. For example, we have found that a temperature change of only 6 °C can change the residual damage from small clusters barely visible by conventional transmission electron microscopy and Rutherford backscattering to a thick amorphous layer. The temperature at which this occurs is strongly dependent upon the ion flux. © 1997 American Institute of Physics.

Notes: The simplest model for the contribution of pore surfaces to nuclear magnetic resonance (NMR) relaxation of a pore fluid gives R, the average relaxation rate minus the bulk rate, equal to a constant ρ, the velocity at which nuclear magnetization flows out of the pore fluid at the surfaces, times the pore-space surface-to-volume ratio S/V. Although ρ can vary widely, a great variety of porous media exhibit ρ values of the order of a few μm/s for longitudinal relaxation when S/V is measured by gas adsorption by the Brunauer, Emmett, and Teller (BET) method or high pressure mercury injection. For samples with wide distributions of relaxation rates it is of interest to find what functions of the relaxation data correlate best with S/V measurements and how different relaxation parameters relate to each other. Longitudinal relaxation data were taken for 77 sandstone samples of different origin, which had been cleaned and saturated with brine. After the NMR measurements the samples were dried and surface areas measured by BET. The samples have S/V from 1.5 to 150 (μm)−1, porosity from 3% to 28%, and permeability from less than 0.1 mD to more than 1 D. Longitudinal relaxation data were taken from 400 μs to 6 s and analyzed in many different ways, including stretched-exponential fits and multiexponential fits up to five components. S/V and ln(S/V) were correlated with various relaxation rates derived from these computed parameters.In principle, the relaxation parameter to use with a ρ value is the average rate, which is initial slope divided by initial amplitude, namely, R(0), where R(t)=(d/dt)ln S(t) at t=0 and S(t) is the relaxing signal. One can extrapolate an n component fit to t=0 to get Rn(0), but very good signal quality is required even to get small short components reliably for t well within the times covered by the data. Over half of the points have ρ's within a factor of 2 of the minimum value 0.9 μm/s when the average rate of a five-component fit to the data is used. There are numerous points with ρ up to 7 μm/s, but none of the high-ρ points are for samples with high S/V. All samples with high S/V have wide distributions of relaxation rates, but not vice versa. The best simple correlation with ln(S/V) was ln(S/V)≈1.81 ln(R33)−5.73, where R33 is the highest rate of a three-component fit without regard to the corresponding amplitude, and where S/V is in (μm)−1 and rate in s−1. This result was unexpected. This fit does not represent proportionality to a velocity ρ and does not correspond to any obvious physical model, but it can be of practical interest to estimate in a very simple and noninvasive manner S/V at the BET scale in sandstones. © 1996 American Institute of Physics.

Notes: An investigation was made of the defect characteristics of single-crystal Bi12SiO20 (BSO) grown by both Czochralski (Cz) and hydrothermal transport methods. Only Cz-grown BSO is photorefracting using a coherent pumping source centered around 500 nm, as undoped hydrothermal BSO is transparent throughout the visible spectrum. Thermally stimulated current (TSC) studies in conjunction with temperature-dependent optical-absorption measurements and room-temperature photoconductivity data all indicate that the hydrothermal material is near intrinsic in terms of its low defect content. TSC measurements made below room temperature indicate that concentrations of traps of activation energy 〈0.7 eV are a factor of 103 smaller in hydrothermal than in Cz BSO. At least six different defects were identified in the TSC measurements. Temperature-dependent optical-absorption measurements indicate two Urbach band tails for Cz-grown materials that are not observed in the hydrothermal materials. Cz material of lower purity also possesses an impurity band tail which can be observed through temperature dependent optical absorption measurements. Comparison of Cz material with hydrothermal BSO of similar impurity content suggests that the BiSi defect responsible for the 500 nm absorption may be complexed with an impurity such as Fe or V. In the absence of this defect, however, these impurities have no effect on the absorption. An additional defect in the TSC data is also related to a transition metal impurity. The results indicate that the photoconductivity associated with the photorefractive effect in Cz material must proceed via a trap-hopping conduction mechanism that is missing in the intrinsic material.

Notes: The emission intensity, wavelength, and lifetime of the fluorophore nitrobenzoxadiazole dipalmitoylphosphatidylethanolamine (NBD-PE) are sensitive to the local environmental structure when this species is present as a component of an amphiphilic membrane. Alterations of the physical and electrostatic structure of a membrane can result in changes in the fluorescence signal owing to changes in the extent of self-quenching of the probe. To investigate self-quenching, NBD-PE was incorporated into monolayers and vesicles composed of Egg phosphatidylcholine at concentrations of 0.1 to 50 mol %. Monolayer samples were dipcast onto glass slides at a pressure of 35 mN m−1. Both the integrated intensity per fluorophore (quantum yield) from vesicles and dipcast monolayers, and the mean fluorescence lifetime from vesicles decreased as the concentration of fluorophore in the membranes was increased. At all concentrations studied the decay of NBD-PE fluorescence was fitted to two discrete exponentials, and both lifetime components were observed to change with concentration. The complexity of the fluorescence decay did not permit the use of standard theoretical models such as the Klafter–Blumen or Stern–Volmer equations which are normally employed to describe changes in fluorescence lifetime with changes in quencher concentration. Instead, a phenomenological approach was used to develop an empirical model of fluorescence self-quenching which could describe the observed alterations in the fluorescence lifetime and intensity. The model was based on a combination of Perrin quenching and Förster energy transfer. The fluorescence data was fit by a model wherein NBD-PE formed nonemissive trap sites with a critical radius of Rc=1.0±0.1 nm (Perrin quenching), with Förster energy transfer occurring to the trap sites with an R0 value of 2.55±0.10 nm as determined from spectral overlap integrals.

Notes: Various time-resolved experiments on optically detected magnetic resonance (ODMR) of single pentacene molecules in p-terphenyl at 1.8 K are described and discussed with the help of a model based on the optical-microwave Bloch equations. Intersystem crossing rates for this system are deduced from the ODMR spectra, from the fluorescence correlation function, which varies nonmonotonically with the microwave power, and from the analysis of the fluorescence recovery transients. We also discuss the enhancement of the ODMR effect on using fluorescence photons from single molecules as a time base to trigger the microwaves in fluorescence recovery experiments.

Notes: Cardiac fibroblasts play a central role in the maintenance of extracellular matrix in the normal heart and as mediators of inflammatory and fibrotic myocardial remodeling in the injured and failing heart. In this review, we evaluate the cardiac fibroblast as a therapeutic target in heart disease. Unique features of cardiac fibroblast cell biology are discussed in relation to normal and pathophysiological cardiac function. The contribution of cardiac fibrosis as an independent risk factor in the outcome of heart failure is considered. Candidate drug therapies that derive benefit from actions on cardiac fibroblasts are summarized, including inhibitors of angiotensin-aldosterone systems, endothelin receptor antagonists, statins, anticytokine therapies, matrix metalloproteinase inhibitors, and novel antifibrotic/anti-inflammatory agents. These findings point the way to future challenges in cardiac fibroblast biology and pharmacotherapy.

Notes: Nonalloyed ohmic contacts of Pt/Ti to 5×1018 cm−3 doped p-InGaAsP (λg =1.3 μm) have been fabricated by rapid thermal processing of sputtered and e-gun-deposited metallizations. While the former as-deposited had a rectifying characteristic, the latter showed ohmic behavior prior to any heat treatment, with a specific contact resistance of 4×10−3 Ω cm2. Rapid thermal processing at temperatures higher than 400 °C caused the formation of ohmic contacts for the sputtered metals also, but with the evaporated metals producing slightly lower contact resistance. The lowest specific contact resistance values of 3.6–5.5×10−4 Ω cm2 for evaporated and sputtered metallizations, respectively, were achieved in both cases as a result of heating at 450 °C for 30 s. These heating conditions produced only a limited reaction at the Ti/InGaAsP interface, which was sharper for the e-gun-deposited contact, but had a significant effect on the stresses in the Ti/Pt bilayer. In both the sputtered and electron gun evaporated samples, the stresses were inverted from tensile as-deposited to compressive with values of 2.4×109 and 1.0×109 dyn cm−2, respectively, as a result of the heat treatment.

Notes: Well-sintered snow examined in a scanning electron microscope revealed a newly observed morphological structure that protrudes into the pore space along ice grain boundaries. We have termed this a "grain boundary ridge." Grain boundary diffusion is a sintering process that occurs at the interface of two crystals, whereby mass migrates from the center of the contact to the surface of the bond. Since mass tends to sublimate from sharp features toward smaller curvature surfaces through vapor diffusion, a ridge developed by grain boundary diffusion will readily sacrifice mass to the surrounding ice surfaces. A mass balance between vapor and grain boundary diffusion based on the observed geometry is considered. This analysis indicates grain boundary diffusion may play a far more significant role than generally acknowledged. While this study was restricted to ice, it may have implications for other crystalline materials. © 2001 American Institute of Physics.

Notes: Radiation-induced changes in the magnetization of sintered Nd-Fe-B permanent magnets are known to vary widely among specimens produced by different manufacturers. Samples of Nd-Fe-B MAGNEQUENCH magnets, which are made from melt-spun ribbons, have not been studied and show a much reduced sensitivity to neutron irradiation than do sintered Nd-Fe-B magnets. All melt-spun ribbon-based MAGNEQUENCH magnets, i.e., epoxy-bonded, hot-pressed, and die-upset magnets, show essentially the same slow decrease in magnetic remanence with neutron dose. Measurements of the open-circuit remanence Br/Br 0 at various times during the irradiation show a decay of only 1.5% of the preirradiated value for the MAGNEQUENCH magnets after 1 h of irradiation, or a dose of 1.4×1016 neutrons/cm2, compared to a 4.6% drop in remanence for the best sintered Nd-Fe-B magnet (Sumitomo 30H) with the same irradiation dose. Moreover, after 5.3 h of irradiation, the remanence drops by only 3% for the MAGNEQUENCH magnets. Magnets made from melt-spun ribbons are thus the least sensitive to neutron irradiation so far measured for Nd-Fe-B permanent magnets, but are somewhat more sensitive than samarium-cobalt magnets.