Library

Notes: Simulation methods like DSMC are efficient tools to compute rarefied gas flows. Using supercomputers it is possible to include various real gas effects like vibrational energies or chemical reactions in a gas mixture. Nevertheless it is still necessary to improve the accuracy of the current simulation methods in order to reduce the computational effort. To support this task the paper presents a comparison of the classical DSMC method with the so-called Finite Pointset Method. This new approach was developed during several years in the framework of the European space project HERMES. The comparison given in the paper is based on two different test cases: a spatially homogeneous relaxation problem and a two-dimensional axisymmetric flow problem at high Mach numbers. © 1995 American Institute of Physics.

Notes: Deep level analysis in the ohmic regime of ungated n-InGaAs/p-InP junction field-effect transistor structures was made by low-frequency noise measurements. The noise spectra exhibit two deep trap levels in the n-InGaAs channel with activation energies of 0.49 and 0.37 eV. The related capture cross sections are 7×10−15 and 4×10−16 cm2, respectively.

Notes: Strong carrier confinement in the active region of field-effect transistors is important for submicron devices. It is shown that in InGaAs junction field-effect transistors (JFETs) the introduction of a p-type InP buffer layer with high p-type dopant concentration (NA=2×1017 cm−3) supresses short-channel effects even in the submicron gate regime. By lowering the p-type concentration below 1017 cm−3 and reducing the thickness of the buffer layer, higher output conductances, threshold voltage shifts, and worse pinch-off behavior result especially at shorter gate lengths. InGaAs JFETs without a buffer layer exhibit tremendous short-channel effects. It is concluded that high potential barriers at the channel substrate heterointerface are necessary for control of InGaAs JFET performance over a wide range of gate length.

Notes: Band discontinuities in anisotype Pb1−xEuxSe/PbSe single heterojunctions are determined using frequency-dependent admittance analysis. A four component small signal equivalent circuit model is proposed to describe the diode admittance. Good agreement between the simulation results and experimental data have been obtained. The band discontinuities are evaluated at 130 K for x〈0.05.

Notes: Oak decline is attributable to predisposing conditions of both biotic and abiotic origin, arid to numerous insects, pathogens, and abiotic events that act as intermediate or ultimate agents of mortality. The precise causal factors vary with circumstances, but the fundamental role of the tree genotype in the process appears to be one of predisposition. Trees are predisposed to oak decline when they are genetically incapable of surviving the stresses natural to the habitats they occupy. Quercus rubra is discussed as a ease study for which there is a considerable amount of genetic and ecological information. This species is characterized by (1) unusual sensitivity in growth to small-scale environmental heterogeneity and (2) unusually high levels of within-population variation in growth rate. Using growth rate as the best-available surrogate measure of ecological fitness, it is clear that high levels of genetic variability are important, even necessary, for the ecological success of populations of this species. Differential adaptation of genotypes to microsites within even very small areas is likely. To minimize susceptibility to oak decline, forest practices should favour (1) the maintenance of high levels of genetic variation, (2) the use of local provenances in artificial regeneration, and (3) large seedling populations. A further implication is that high rates of seedling mortality in plantations may be normal and even desirable for long-term forest health.

Notes: Two clones of 5-year-old Norway spruce [Picea abies (L.) Karst.] were exposed to two atmospheric concentrations of CO2 (350 and 750 μmol mol−1) and O3 (20 and 75nmolmol−1) in a phytotron at the GSF-Forschung-szentrum (Munich) over the course of a single season (April to October). The phytotron was programmed to recreate an artificial climate similar to that at a high elevation site in the Inner Bavarian Forest, and trees were grown in large containers of forest soil fertilized to achieve contrasting levels of potassium nutrition, designated well-fertilized or K-deficient. Measurements of the rate of net CO2 assimilation were made on individual needle year age classes over the course of the season, chlorophyll fluorescence kinetics were recorded after approximately 23 weeks, and seasonal changes in non-structural carbohydrate composition of the current year's foliage were monitored. Ozone was found to have contrasting effects on the rate of net CO2 assimilation in different needle age classes. After c. 5 months of fumigation, elevated O3 increased (by 33%) the rate of photosynthesis in the current year's needles. However, O3 depressed (by 30%) the photo-synthetic rate of the previous year's needles throughout the period of exposure. Chlorophyll fluorescence measurements indicated that changes in photosystem II electron transport played no significant role in the effects of O3 on photosynthesis. The reasons for the contrasting effects of O3 on needles of different ages are discussed in the light of other recent findings. Although O3 enhanced the rate at which CO2 was fixed in the current year's foliage, this was not reflected in increases in the non-structural carbohydrate content of the needles. The transfer of ambient CO2-grown trees to a CO2-enriched atmosphere resulted in marked stimulation in the photosynthetic rate of current and previous year's foliage. However, following expansion of the current year's growth, the photosynthetic rate of the previous year's foliage declined. The extent of photosynthetic adjustment in response to prolonged exposure to elevated CO2 depended upon the clone, providing evidence of intraspecific variation in the long-term response of photosynthesis to elevated CO2. The increase in photosynthesis induced by CO2 enrichment was associated with increased foliar concentrations of glucose, fructose and starch (but no change in sucrose) in the new growth. CO2 enrichment significantly enhanced the photosynthetic rate of K-deficient needles, but there was a strong CO2soil interaction in the current year's needles, indicating that the long-term response of trees to a high CO2 environment may depend on soil fertility. Although the rate of photosynthesis and non-structural carbohydrate content of the new needles were increased in O3-treated plants grown at higher levels of CO2, there was no evidence that elevated CO2 provided additional protection against O3 damage. Simultaneous exposure to elevated O3 modified the effects of elevated CO2 on needle photosynthesis and non-structural carbohydrate content, emphasizing the need to take into account not only soil nutrient status but also the impact of concurrent increases in photochemical oxidant pollution in any serious consideration of the effects of climate change on plant production.

Notes: Abstract  The different Fe2+ lattice sites in iron-rich chlorites have been characterized by Mössbauer spectroscopy and molecular orbital calculations in local density approximation. The Mössbauer measurements were recorded at 77 K within a small velocity range (±3.5 mm s−1) to provide high energy resolution. Additionally, measurements were recorded in a wider velocity range (±10.5 mm s−1) at temperatures of 140, 200, and 250 K in an applied field (7 T) parallel to the γ-beam. The zero-field spectra were analyzed with discrete Lorentzian-shaped quadrupole doublets to account for the Fe2+ sites M1, M2, and M3 and with a quadrupole distribution for Fe3+ sites. Such a procedure is justified by the results obtained from MO calculations, which reveal that different anion (OH−) distributions in the first coordination sphere of M1, M2, and M3 positions have more influence on the Fe2+ quadrupole splitting than cationic disorder. The spectra recorded in applied field were analyzed in the spin-Hamiltonian approximation, yielding a negative sign for the electric field gradient (efg) of Fe2+ in the M1, M2, and M3 positions. The results of the MO calculations are in quantitative agreement with experiment and reveal that differences in the quadrupole splittings (ΔE Q ), their temperature dependence and in the isomer shifts (δ) of Fe2+ in M1, M2, and M3 positions can theoretically by justified. Therefore, the combined Mössbauer and MO investigation shows that the three Fe2+ lattice sites in the chlorites investigated here can be discriminated according to their ΔE Q -δ parameter pairs. With the calculated average iron-oxygen bond strength, the MO study provides an explanation for the observed trend that the population of the three lattice sites by Fe2+ increases according to the relation M1 〈 M2 〈 M3.