Showing papers by "University of Stuttgart published in 1982"

Optical properties of copper in silicon: Excitons bound to isoelectronic copper pairs

Abstract: Copper doping of silicon crystals results in an intense emission at 1.014 eV. The photoluminescence spectrum exhibits a characteristic structure consisting of a narrow nophonon line and equispaced lower-energy resonant-mode phonon replicas. The typical phonon energy is 7.0 meV. We observe isotope shifts of the lines which conclusively show that copper is incorporated in the luminescent defect. Combination with the observed quadratic dependence of the emission intensity on copper concentration leads us to suggest copper pairs as recombination centers. The symmetry of the pair as determined from uniaxial stress and Zeeman data is that of a $〈111〉$ configuration. The no-phonon line structure and the splitting in external fields indicate an exciton localized at an isoelectronic trap. The exciton is discussed in terms of an isoelectronic donor combining the present data with recent deep-level transient-spectroscopy results.

The3He(α,γ)7Be reaction and the solar neutrino problem

Abstract: The capture reaction3He(α,γ)7Be has been investigated in the energy range ofEc.m. =107 to 1,266 keV. The4He or3He beams of up to 300 μA particle current were incident on3He or4He gas targets, respectively. The gas target systems were all of the windowless and recirculating type. Excitation functions have been obtained with the use of an extended-static gas target, while the measurements ofγ-ray angular distributions involved a quasi-point supersonic jet system. The determination of absolute cross sections has been carried out with both types of gas target systems. Theγ-ray yields in the3He(α,γ)7Be reaction were detected using 80 cm3 Ge(Li) detectors. The data lead to a zero-energy intercept of the astrophysicalS(E) factor ofS(0)=0.30±0.03 keV-b. This result reduces the calculated solar neutrino rate by a factor of 1.76.

Spectroscopic study of vanadium in GaP and GaAs

Abstract: The near-infrared optical-absorption, luminescence, and luminescence-excitation spectra of liquid-encapsulation Czochralski-grown, vanadium- (V) doped GaP and GaAs have been investigated. The behavior of the characteristic V emission has been studied in magnetic fields up to 5.3 T. An analysis of the data within the framework of crystal-field theory indicates that the V-induced optical bands arise from crystal-field transitions within nonassociated ${V}^{2+}{3d}^{3}$ ions in the strong-field (low-spin) case. The electron-spinresonance spectrum of semi-insulating GaAs:V contains a characteristic signal which is identified as isolated ${V}^{3+}{3d}^{2}$. The characteristics of this signal show that V can act as a deep acceptor. The distribution coefficient of V in GaAs is close to that of Cr in GaAs.

Enolether, XIII. Die Acylierung von Enolethern mit reaktiven Carbonsäure‐chloriden

Abstract: Enolether 2 lassen sich mit aktivierten Saurechloriden wie den Chloressigsaurechloriden 1a und 1b, Cyanacetylchlorid (6c) oder Malonylchloriden 6a, b unter milden Bedingungen in guten Ausbeuten acylieren. Die Abhangigkeit der Produktbildung und der Ausbeuten vom Acylierungspotential der eingesetzten Saurechloride wird aufgezeigt. Die erhaltenen potentiellen β-Ketoaldehyde 4, 5 bzw. 1,3,5-Tricarbonylverbindungen 8 sind wertvolle Zwischenprodukte fur die Synthese von Heterocyclen. Enol Ethers, XIII. Acylation of Enol Ethers with Reactive Carbonyl Chlorides Enol ethers 2 are acylated under mild conditions with activated acyl chlorides, e.g. chloroacetyl chlorides 1a, b, cyanoacetyl chloride (6c), and malonic acid chlorides 6a,b in good yields. The products formed and the yield depend on the acylation potential of the acyl chlorides. The potential β-keto aldehydes 4, 5, and 1,3,5-tricarbonyl compounds 8 obtained are valuable intermediates for heterocyclic synthesis.

Pattern formation in morphogenesis. Analytical treatment of the Gierer-Meinhardt model on a sphere.

Abstract: We first treat the Gierer-Meinhardt equations by linear stability analysis to determine the critical parameter, at which the homogeneous distributions of activator and inhibitor concentrations become unstable. We find two types of instabilities: one leading to spatial pattern formation and another one leading to temporal oscillations. We consider the case where two instabilities are present. Using the method of generalized Ginzburg-Landau equations introduced earlier we then analyze the nonlinear equations. As we are mainly interested in spatial pattern formation on a sphere we consider the problem under an appropriate constraint. Combining the two occurring solutions we find patterns well-known in biology, such as a gradient system and temporal oscillations.

Electron correlations in the ground state of diamond

Abstract: The correlated ground-state wave function and energy of diamond are calculated. The calculations are done in order to demonstrate that electron correlations in solids can be treated with the same accuracy as in small systems. This is achieved by applying a recently developed Local Approach to the correlation problem. The approach requires a SCF calculation as a starting point. We present such a calculation for diamond by applying a method due to Stoll and Preuss which uses localization potentials. The correlation energy is analysed in terms of inter- and intraatomic correlation energy contributions. The results are used to calculate the binding energy of diamond.

Intuitonistic Basis for Non-Monotonic Logic

Abstract: McDermott and Doyle [4] suggested a system, denoted by ⊢, of non-monotonic logic. This notion was intended to formalise non-monotonic reasoning as involved in real situations and in artificial intelligence. McDermott and Doyle also list in their paper several difficulties and problems in their approach. Their semantics seems to be inadequate and there are several counterintuitive results obtained in their system.

A robust and efficient protocol for checking the availability of remote sites

Abstract: A robust and efficient protocol for checking the availability of remote sites is described. A remote site is available if it has not crashed and if the communication facilities are able to transmit messages to and from this site. The presented protocol is robust against any number of site crashes and communication break-downs including network partitioning. We prove that the protocol is minimal in the given context. Some applications such as the recovery of multi-site-transactionsare given.

Development Support Systems

Abstract: It may once have been true that engineers were very successful in automating every job except their own, but the situation has changed rapidly in recent years. Development support systems now automate engineering activities which can be performed better and more cheaply by computers than by people. Today, engineering is becoming more attractive and satisfying because \"slave\" tasks, such as establishing documentation, can be done by computer , and the advent of workbench systems to support the development process will further impact engineering work, changing not only the habits of the individual engineer but also the structure and function of companies and educational institutions. But there are still many difficulties associated with the development of computer software and hardware systems. For example: * The \"engineeringware\" to specify, design, implement , test, and maintain such systems is costly, time consuming, and difficult to manage. * Computer software and hardware are often non-responsive to user requirements. * More often than not, the documentation proves to be inconsistent and incomplete. * System reliability and integrity may be jeopardized by modifications and changes made during the development process. These difficulties may be overcome, at least in part, through automation of engineering activities. Terms defined. By development we mean all engineering activities, whether related to hardware and software or their operation and use, that are needed to create a man-machine system and keep it operating. As shown in Figure 1, there are two phases of system development. The predelivery phase is usually divided into five stages: requirements analysis, system specification, design, implementation and coding, and test and integration. The postdelivery phase comprises not only maintenance in the usual sense (error and failure diagnosis and repair) but also, more importantly, the enhancement of the system and its adaptation to new environments. This phase, in which the stages of requirements analysis, specification, design, implementation and coding, and test and integration are iterated several times,1 could be renamed the \"evolution phase.\" The man-machine system to be developed may be, for example, a distributed microcomputer system in which software, processors, peripheral hardware, sensors, ac-tuators, and human operators interact to control an industrial process. There are two ways in which a computer can aid the developer. One is to build software tools that support specific activities. Such tools might include an assembler, a text editor, a program to generate the layout for a printed circuit board, or a check-out program. …

The activation energy of crystallization of amorphous Fe 40 Ni 40 P 14 B 6

Abstract: The isothermal crystallization of Fe40Ni40P14B6 (Metglas 2826) has been studied by transmission electron microscopy, using static observations of partially crystallized ribbons at room temperature and in situ dynamic registration of the crystallization process at elevated temperatures. At all temperatures crystallization takes place by the nucleation and growth of individual crystals. Analysis of the transformation kinetics allowed to determine the nucleation rates and the activation energy for crystal growth. The growth velocity of the crystal phase was found to be controlled by the diffusion coefficient of phosphorus in this alloy withD 0=2.5×1010±1cm{swu2}/s andQ=(3.4±0.15)eV.

Slaving principle for stochastic differential equations with additive and multiplicative noise and for discrete noisy maps

Abstract: We first treat multidimensional nonlinear noisy maps. We assume that the variables can be split into two classes of variablesu ands so that the linearized equations would give rise to growth or decay foru ands, respectively. We show how the slaved variabless can be explicitly expressed by the order parametersu by making use of the fully nonlinear equations. By taking the limit of vanishing time steps and using a Wiener process and the Ito calculus we derive the corresponding formulas for stochastic differential equations (including multiplicative noise). In this way a high-dimensional problem can be reduced to a problem of much lower dimensions described again by stochastic equations of theIto type. A similar procedure holds for theStratonovich calculus.

Efficient Synthesis of 8-Oxabicyclo[3.2.1]oct-6-en-3-ones, Preparatively Useful [4 + 3]-Cycloadducts

Abstract: The complete manuscript of this communication appears in: Angew. Chem. Suppl. 1982, 241. DOI:10.1002/anie.198202410

Phonon scattering at siliconcrystal surfaces

Abstract: Our observations of the reflection or backscattering of high-frequency phonons (v =280 GHz to 1 THz) at silicon-solid interfaces disagree significantly with predictions from the acoustic mismatch model. Interfaces composed of materials theoretically wellmatched, show high scattering experimentally. In contrast, interfaces theoretically poorly matched, show less phonon scattering than expected. Generally, this is best expressed by the fact that the interface scattering ranges from roughly 30–60% for different phonon modes with little dependence on the material covering the silicon crystal and different techniques of interface preparations. Thus, our experiments indicate that the well-known Kapitza anomaly of the phonon scattering at solid-liquid helium interfaces is not a special case; the same anomaly appears to be present at all tested interfaces. Our experiments are compared with detailed calculations which either assume pure specular or pure diffusive scattering. In these calculations the influence of the crystal anisotropy for the phonon propagation (phonon focussing) is included. This comparison shows, especially for the free silicon surface, that phonons are completely diffuse scattered. Hence, the acoustic mismatched model relying on specular reflection cannot be applied to the real silicon interface. The frequency dependence of phonon scattering at a free silicon interface indicates the existence of at least two different diffusive scattering mechanisms. Within our experimental limits in these two scattering processes the phonons are elastically scattered.

First high-repetition-rate Thomson scattering for fusion plasmas

Abstract: Electron temperature and density measurements by Thomson scattering were performed for the first time for the whole duration of a tokamak discharge. A Nd:YAG laser of 60 pulses per second at 1.06 μm was used in ASDEX in combination with silicon avalanche photodiode detectors. Density calibration was done by rotational anti-Stokes Raman scattering from hydrogen. The system is used for measurements at electron densities as low as 3 × 1012 cm−3.

Nonlinear relaxation and fluctuations of damped quantum systems

Abstract: The paper reexamines the treatment of irreversible quantum systems by master equations. Shortcomings of the conventional theory of quantum Markov processes pointed out by Talkner are analyzed. It is shown that a frequently used quantum regression hypothesis is not correct, in general. A new generalized master equation determining the relaxation to equilibrium is derived by means of time-dependent projection operator techniques. It is shown that this master equation also determines the time evolution of equilibrium correlations and response functions. The Markovian approximation is discussed, and a new type of Markovian limit, the Brownian motion limit, is introduced besides the weak coupling limit. The shortcomings of the conventional approach are resolved by deriving new formulae for the time evolution of the correlation and response functions of a quantum Markov process. The symmetries of the process are emphasized, and it is shown how the fluctuation-dissipation theorem and the detailed balance symmetry emerge from the master equation approach.