Real-space electron transfer by thermionic emission in GaAsAlxGa1−xAs heterostructures: Analytical model for large layer widths

Calculations are presented for negative differential resistance (NDR) and switching in layered GaAsAl_xGa_1?xAs heterostructures with a high electric field parallel to the interface. The mechanism is based on thermionic emission of hot electrons from the GaAs layers into the Al_xGa_1?xAs layers. An analytical model is obtained in the limit of relatively large layer widths (400 Å or wider). The method of moments is employed to solve the Boltzmann equation, assuming a position-dependent electron temperature and Quasi-Fermi level in the Al_xGa_1?xAs layers, and a position-independent electron temperature and Quasi-Fermi level in the narrower GaAs layer. Thermal conduction of hot electrons from the GaAs layer into the Al_xGa_1?xAs layers is taken into account. The results of the calculations show that the threshold electric field for the onset of NDR and the peak-to-valley ratio can be controlled to a large extent by adjusting the mobility of the Al_xGa_1?xAs layer, the layer dimensions, and the potential barrier (Al mole fraction in the Al_xGa_1?xAs).     

Superfluidity in thin4He films: Comparison of onset on particular crystal facets and on disordered substrates

Superfluid flow in unsaturated⁴He films on highly ordered pyrolytic graphite, (001) cleaved MgO, epitaxial gold on mica, polycrystalline gold, and glass has been studied by the heat transport technique. Using a criterion for onset suggested by the Kosterlitz-Thouless theory, we confirm that the locus of onset in the variablesT andp/p ₀ is substrate independent within experimental error, whereas the film thickness at onset varies widely with substrate. The reason for this is not understood. Several aspects of the Kosterlitz-Thouless theory are verified: On polycrystalline gold the film flow impedance decreases approaching the transition from the normal side, consistent with the dependence calculated by Ambegaokar et al. On the superfluid side of onset, power law relations are found between temperature gradient and current, in agreement with the prediction based on thermal dissociation of vortex pairs in the velocity field. The exponents are consistent with the expected variation of the renormalized superfluid density with coverage, including a Nelson-Kosterlitz jump smeared by finite-velocity effects. On MgO we obtain only an upper limit for the film coverage at onset; its consistency with the Nelson-Kosterlitz jump depends on the interpretation of neutron scattering data.Supported in part by NSF grant DMR 78-22697.     

Two software tools for plane-wave scattering on nonspherical particles in the German Aerospace Center's Virtual Laboratory

We present the methodological background, the range of applicability, and the on-line usage of two software packages, MIESCHKA and CYL, which we have developed for light-scattering analysis on nonspherical particles. MIESCHKA solves Maxwell's equations in a rigorous way but is restricted to axisymmetric geometries, whereas CYL is an approximation for finite columns with nonspherical cross sections. We have established an easy on-line access to both of these programs through the Virtual Laboratory. Its generic software infrastructure was designed to simplify the web-based usage and to support the intercomparability of scientific software.     

The gas-cooled fast breeder reactor critical experiments program

This paper presents results of measurements and calculations of physics parameters in the first gas-cooled fast breeder reactor (GCFR) critical assemblies in the US, a program of experiments conducted on the ZPR-9 facility at Argonne National Laboratory. Through a progressive three-phase series of assemblies, the major features unique to GCFR physics due to the gaseous coolant, and the resulting hard neutron spectrum and greater leakage, were investigated. Phases I and II were simple-geometry, uniform-core assemblies providing tests of nuclear data and GCFR design methods for fast reactors with large void fractions. The Phase III core simulates a GCFR design with three enrichment zones. This report primarily concerns the results obtained in Phase II.In addition to the usual central indices, reaction rate mappings, etc. these initial studies have provided the first experimental data on reactivity coefficients relevant to GCFR safety, such as worths of fuel, control, and cladding materials, Doppler effect, and coolant (helium) depressurization worth. Effects of steam ingress into coolant channels (due to a hypothesized steam generator leak) were simulated using polyethylene. The physics information obtained is providing a valuable base for verification of GCFR design and safety analyses.     

POLYCHAR-10 World Forum on Polymer Applications &; Theory in 2002

A report from POLYCHAR-10 is provided. An introduction to the following papers presented at the POLYCHAR-10 Forum is given. A small selection of the authors and titles of the papers presented is listed. Prizes awarded are listed along with the composition of the Prize Committee. Some research groups of high activity which are represented at POLYCHAR every year are named. Electronic Publication     

Forschungsmethoden der Wirtschaftsinformatik

The german-speaking IS community sees itself as a methodological pluralist discipline. With respect to recent debates (e.g. on rigor versus relevance) an exact knowledge of its methodological profile becomes increasingly important. This contribution presents the findings of a content analysis of 300 articles published in the journal WIRTSCHAFTSINFORMATIK in the last ten years. As one result, the study identified six major methods of the german-speaking IS community: argumentative, conceptional and formal deductive research, prototyping, case studies and quantitative cross sectional studies. It could also be shown, that the methodological profile continually changed during the last ten years. The article also compares its findings with the current methodological profile of the anglo-american IS community based on a secondary data analysis.     

A contingency model for the allocation of media content in publishing companies

Today, articles and papers can be published in many different media and the allocation of content by publishing firms is vital for their operational efficiency, time-to-market, and customer retention. Past studies in MIS research have focused on the allocation of other objects than content, e.g., data or hardware, with different distribution and integration requirements. Such studies have ignored the allocation of media content. We developed and tested a structural model of media content allocation behavior of publishing companies. Drawing on contingency theory, we hypothesized that organizational structure (centralization, formalization, and specialization), size, distribution of IT decision-making, and information processing requirements influenced the degree of content distribution and integration. The model was tested using data from a survey of 115 companies in the book, newspaper, and magazine publishing industries. While content distribution was primarily contingent upon organizational formalization, organizational specialization, IT decision-making, and information processing requirements, only organizational standardization and information processing requirements were found to be significant in affecting the way that publishing companies carried out content integration.     

Study of Martensitic Phase Transformation in a NiTiCu Thin‐Film Shape‐Memory Alloy Using Photoelectron Emission Microscopy

Thermally induced martensitic phase transformation in a polycrystalline NiTiCu thin‐film shape‐memory alloy is probed using photoelectron emission microscopy (PEEM). In situ PEEM images reveal distinct changes in microstructure and photoemission intensity at the phase‐transition temperatures. In particular, images of the low‐temperature, martensite phase are brighter than that of the high‐temperature, austenite phase, because of the lower work function of the martensite. UV photoelectron spectroscopy shows that the effective work‐function changes by about 0.16 eV during thermal cycling. In situ PEEM images also show that the network of trenches observed on the room‐temperature film disappears suddenly during heating and reappears suddenly during subsequent cooling. These trenches are also characterized using atomic force microscopy at selected temperatures. The implications of these observations with respect to the spatial distribution of phases during thermal cycling in this thin‐film shape‐memory alloy are discussed.