Showing papers by "Technical University of Berlin published in 1995"

Inas/gaas pyramidal quantum dots: strain distribution, optical phonons, and electronic structure

Abstract: The strain distribution in and around pyramidal InAs/GaAs quantum dots (QD's) on a thin wetting layer fabricated recently with molecular-beam epitaxy, is simulated numerically. For comparison analytical solutions for the strain distribution in and around a pseudomorphic slab, cylinder, and sphere are given for isotropic materials, representing a guideline for the understanding of strain distribution in two-, one-, and zero-dimensional pseudomorphic nanostructures. For the pyramidal dots we find that the hydrostatic strain is mostly confined in the QD; in contrast part of the anisotropic strain is transferred from the QD into the barrier. The optical-phonon energies in the QD are estimated and agree perfectly with recent experimental findings. From the variation of the strain tensor the local band-gap modification is calculated. Piezoelectric effects are additionally taken into account. The three-dimensional effective-mass single-particle Schr\"odinger equation is solved for electrons and holes using the realistic confinement potentials. Since the QD's are in the strong confinement regime, the Coulomb interaction can be treated as a perturbation. The thus obtained electronic structure agrees with luminescence data. Additionally AlAs barriers are considered.

Ultranarrow Luminescence Lines from Single Quantum Dots.

Abstract: We report ultranarrow $(l0.15\mathrm{meV})$ cathodoluminescence lines originating from single InAs quantum dots in a GaAs matrix for temperatures up to 50 K, directly proving their $\ensuremath{\delta}$-function-like density of electronic states. The quantum dots have been prepared by molecular beam epitaxy utilizing a strain-induced self-organizing mechanism. A narrow dot size distribution of width $12\ifmmode\pm\else\textpm\fi{}1\mathrm{nm}$ is imaged by plan-view transmission electron microscopy. Cathodoluminescence images directly visualize individual dot positions and recombination from a single dot. A dense dot array $(\ensuremath{\sim}{10}^{11}\mathrm{dots}/{\mathrm{cm}}^{2})$ gives rise to a distinct absorption peak which almost coincides with the luminescence maximum.

Spontaneous ordering of arrays of coherent strained islands.

Abstract: The energetics of an array of three-dimensional coherent strained islands on a lattice-mismatched substrate is studied. The contribution of the edges of islands to the elastic relaxation energy always has a minimum as a function of the size of an island $L$, and the total energy $E(L)$ may have a minimum at an optimum size ${L}_{\mathrm{opt}}$. Among different arrays of islands on the (001) surface of a cubic crystal, the total energy is minimum for the 2D periodic square lattice with primitive lattice vectors along ``soft'' directions [100] and [010]. This is a stable array of islands which do not undergo ripening.

CH and CC Bond Activation by Bare Transition‐Metal Oxide Cations in the Gas Phase

Abstract: Over the last decade the gas-phase chemistry of bare transition-metal oxide cations MO+ has received considerable attention. This interest is primarily due to the particular role of metal oxides in the oxidation of organic compounds in a variety of chemical and biochemical transformations. At a molecular level the simplest model system for these processes deals with reactions of bare metal-oxide ions in the gas phase. Due to the high oxophilicities of the early transition metals, their monoxide cations MO+ do not mediate O-atom transfer to any organic compounds at all. In contrast, monoxide cations of late transition metal can oxygenate a variety of hydro-carbons, and the most reactive ions, MnO+, FeO+, NiO+, OsO+, and PtO+, even activate methane. Insight into the reaction mechanisms of these oxidation processes can be obtained by analysis of reaction kinetics, isotope effects, product distributions etc., and for the reactions of MO+ with alkanes the initial CH bond activation by MO+ is often rate-determining. Interestingly, the high reactivity of some MO+ ions is not always associated with a decrease in regioselectivity; for example, FeO+ ions induce regiospecific γ-CH bond activation of dialkylketones in the gas phase. The situation for the epoxidation of olefins in the gas phase turns out to be even more complex than for condensedphase analogues. This is primarily because the metal ion that mediates O-atom transfer to the olefin also catalyzes the isomerization of the epoxides formed, to afford the energetically more stable aldehydes or ketones. Aromatic compounds can also be hydroxylated by MO+ ions, and particularly the oxidation of benzene by bare FeO+ ions in the gas phase reveals striking parallels to the metabolism of arenes. Furthermore, the storing capabilities of ion cyclotron resonance mass spectrometers even permit the design of catalytic processes in which a single metal ion converts more than one substrate molecule into an oxygenated product in a sequence of strictly bimolecular reactions. The most outstanding examples are the Pt+-mediated oxidation of methane by molecular oxygen and the Co+-mediated hydroxylation of benzene by N2O as oxidant. Finally, the key features of the gas-phase reactions are compared with observations in condensed-phase systems in which metal oxides are anticipated as central intermediates. The result of this comparison is promising in the sense that, in general, the understanding of transition-metal-mediated oxidations in the gas phase may lead to a more uniform description of these processes at a molecular level. Ultimately, it is hoped that gas-phase studies will serve as one of the building blocks in the evolution of tailor-made catalysts.

Viscosity of quantum Hall fluids.

Abstract: The viscosity of quantum fluids with an energy gap at zero temperature is related to the adiabatic curvature on the space parametrizing flat background metrics. For quantum Hall fluids on two-dimensional tori, the quantum viscosity is computed. It turns out to be isotropic, constant, and proportional to the magnetic field strength.

Two‐State Reactivity in Organometallic Gas‐Phase Ion Chemistry

Abstract: In contrast to organic reactions, which can almost always be described in terms of a single multiplicity, in organometallic systems, quite often more than one state may be involved. The phenomenon of two states of different multiplicities that determine the minimum-energy pathway of a reaction is classified as two-state reactivity (TSR). As an example, the ion/molecule reactions of ‘bare’ transition-metal-monoxide cations with dihydrogen and hydrocarbons have been analyzed in terms of the corresponding potential-energy hypersurfaces. It turns out that, besides classical factors, such as the barrier heights, the spin-orbit coupling factor is essential, since curve crossing between the high- and low-spin states constitutes a distinct mechanistic step along the reaction coordinates. Thus, TSR may evolve as a new paradigm for describing the chemistry of coordinatively unsaturated transition-metal complexes. This concept may contribute to the understanding of organometallic chemistry in general and for the development of oxidation catalysts in particular.

Potential food applications of high-pressure effects on ice-water transitions

Abstract: Pressure depresses the freezing point of water and the melting point of ice, as well as enabling various high-density forms of ice to be obtained. These effects of pressure on the solid-liquid phase diagram of water have several potential applications in food technology, including pressure-assisted freezing, pressure-assisted thawing and non-frozen storage at low temperature (under pressure). Studies that have been published in these and in related areas are reviewed, and the potential applications and limitations are highlighted.

External optical feedback phenomena in semiconductor lasers

Abstract: In optical fiber networks, the semiconductor laser source may be subjected to unavoidable optical feedback from fiber pigtails, fiber connectors and other components, unless expensive optical isolators are used. Therefore, a good understanding of external optical feedback phenomena in semiconductor lasers is required. This paper will review these phenomena, including linewidth narrowing and broadening, mode hopping phenomena and the transition to the so-called coherence-collapse regime. In particular, laser designs with high endurance against optical feedback will be discussed.

Properties of the yellow luminescence in undoped GaN epitaxial layers

Abstract: Photoluminescence, time-integrated, time-resolved, and photoluminescence excitation spectroscopy have been employed to study the 2.2 eV (``yellow'') emission in undoped GaN epitaxial layers. It is best described by a recombination model involving shallow donors and deep donors of probably intrinsic origin. Optically detected magnetic resonance reveals the participation of the shallow donor based on the analysis of the g value and Lorentzian line shape.

Radiative states in type-II GaSb/GaAs quantum wells.

Abstract: We have studied optical properties of staggered band line-up (type-II) heterostructures based on strained GaSb sheets in a GaAs matrix. The giant valence-band offset characteristic to this heterojunction leads to an effective localization of holes in ultrathin GaSb layers. An intense photoluminescence (PL) line caused by radiative recombination of localized holes with electrons located in the nearby GaAs regions is observed. The separation of nonequilibrium electrons and holes in real space results in a dipole layer and, thus, in the formation of quantum wells for electrons in the vicinity of the GaSb layer. The luminescence maximum shifts towards higher photon energies with rising excitation density reflecting the increase in the electron quantization energy. A bimolecular recombination mechanism is revealed in PL and in time-resolved PL studies. In the case of pseudomorphic monolayer-thick GaSb layers, the radiative exciton ground state does not exist. Accordingly, small absorption coefficients and a featureless behavior of the band-to-band calorimetric absoprtion spectrum are found in the vicinity of ${\mathit{k}}_{\mathit{x},}$y=0. Remarkable enhancement of the absorption coefficient with a characteristic onset is observed for heavy holes with ${\mathit{k}}_{\mathit{x},}$yg0. Radiative states in the continuum of heavy-hole subbands are revealed also in temperature-dependent PL studies. The experimentally measured onset energies point out the importance of GaSb heavy- and light-hole mixing effects. We demonstrate intense luminescence from staggered band line-up GaSb-GaAs heterostructures up to room temperature.

Alignment tensor versus director: Description of defects in nematic liquid crystals.

Abstract: The core structure of nematic defects cannot be represented by the usual director field, but requires the description by the full orial calculus.

Measuring correlations in symbol sequences

Abstract: The paper is devoted to relations between correlation functions and mutual information. It is shown that, in sequences over an alphabet of λ symbols, statistical dependences are measured by (λ − 1)2 independent parameters. However, not all of them can be determined by autocorrelation functions. Appropriate sets of correlation functions (including crosscorrelations) are introduced, which allow the detection of all dependences. The results are exemplified for binary, ternary, and quaternary symbol sequences. As an application, it is discussed that a nonuniform codon usage in protein-coding DNA sequences introduces periodic correlations even at distances in the order of 1000 base pairs.

Credibility, Information Preferences, and Information Interests

Abstract: The Seveso Directive of the European Union demands that information be provided to the public by companies and authorities about facts, risks, and behaviors related to hazardous facilities, in particular chemical facilities. On behalf of the Commission of the European Communities, a survey was run in five European countries on the credibility of various information sources. This article describes the results of the German study. 430 persons were interviewed with a questionnaire of 50 items, in particular about their perceptions and evaluations of technical risks, the credibility of sources of information about chemical risks, their preferences for receiving risk information from these sources, and their interests in receiving information. Major findings are great differences in credibility, differentiated information preferences, and strong information interests. Surprisingly, credibility played only a minor role with regard to the respondents’information preferences and interests.

Continuous shock structure in extended thermodynamics

Abstract: It is shown by numerical calculations that in extended thermodynamics of 13, 14, 20, and 21 moments a continuous shock structure exists up to a critical Mach number. The critical Mach number increases by increasing the number of moments; the value runs from 1.65 for 13 moments up to 1.887 for 21 moments.

First record of the family Dromaeosauridae (Dinosauria: Theropoda) in the Cretaceous of Gondwana (Wadi Milk Formation, northern Sudan)

Abstract: The Cenomanian Wadi Milk Formation in Sudan yielded a rich continental vertebrate fauna. Apart from big theropod fragments, some disarticulated elements of a pes and a tooth indicate the presence of a dromaeosaurid theropod in the Late Cretaceous of Sudan. This is the first record of this theropod family from Gondwanian continents.

Algebraic high-level net transformation systems

Abstract: The concept of algebraic high-level net transformation systems combines two important lines of research recently introduced in the literature: algebraic high-level nets (AHL-nets for short) and high-level replacement systems (HLR-systems for short). In both cases a categorical formulation of the corresponding theory has turned out to be highly important and is also a good basis for the integration of these concepts in this paper.AHL-nets combine Petri nets with algebraic specifications and provide a powerful specification technique for distributed systems including data types and processes.HLR-systems are transformation systems for high-level structures such as graphs, hypergraphs, algebraic specifications and different kinds of Petri nets. The theory of HLRsystems - formulated already in a categorical framework - is applied in this paper to AHLnets. Thus we obtain AHL-net transformation systems as an instantiation of HLR-systems to AHL-nets. This allows us to build up AHL-nets from basic components and to transform the net structure using rules or productions in the sense of graph grammars. This concept is illustrated by extending the well-known example of ‘dining philosophers’. We are able to show that AHL-net-transformation systems satisfy several important compatibility properties. On the one hand we obtain a local Church-Rosser and Parallelism Theorem, which is well-known for graph grammars and has recently been generalized to HLR-systems. This allows us to analyse concurrency in AHL-nets not only on the token level but also on the level of transformations of the net structure. On the other hand, we consider the ‘fusion’ and ‘union’ constructions for high-level structures, motivated by corresponding concepts for high-level Petri nets in the literature, and we show compatibility of these constructions with derivations of HLR-systems in general and AHL-nettransformations in particular. This means compatibility of vertical and horizontal structuring in terms of software development.

Perfect cocycles through stochastic differential equations

Abstract: We prove that if ϕ is a random dynamical system (cocycle) for whicht→ϕ(t, ω)x is a semimartingale, then it is generated by a stochastic differential equation driven by a vector field valued semimartingale with stationary increment (helix), and conversely. This relation is succinctly expressed as “semimartingale cocycle=exp(semimartingale helix)”. To implement it we lift stochastic calculus from the traditional one-sided time ℝ to two-sided timeT=ℝ and make this consistent with ergodic theory. We also prove a general theorem on the perfection of a crude cocycle, thus solving a problem which was open for more than ten years.

Chemical composition measurements of a nickel-base superalloy by atom probe field ion microscopy

Abstract: Single crystals of the rhenium-containing nickel-base superalloy CMSX-4 were investigated using a time of flight atom probe. The distribution of the alloying elements in the two phases, matrix (γ phase) and precipitate (γ′ phase), was studied. The high spatial resolution of the atom probe allows to analyse the γ−γ′ interface. The transition from matrix to γ′ phase takes place within one atomic layer. The boundary is enriched with titanium. Layer-by-layer analysis of (001) planes of the L 1 2 ordered γ′ phase reveals alternating Ni- and Al-rich planes. The probability of site occupation by different elements could be evaluated. An approximate formula of the γ′ phase is given by (Ni 81 Ti 3 Ta 2 Cr 5 Co 7 W 2 ) 3 (Ni 20 Al 70 Ta 6 Co 2 W 2 ). Rhenium is found mainly in the matrix and has a tendency to build clusters.

Microbial transformations and biological effects of fungicide-derived benzothiazoles determined in industrial wastewater.

Abstract: Benzothiazole (BT), 2-mercaptobenzothiazole (MBT), and 2-(methylthio)benzothiazole (MTBT) were determined as degradation products of the fungicide 2-(thiocyanomethylthio)benzothiazole (TCMTB) in tannery wastewater and are shown to be incompletely removed (75%) in an anaerobic and aerobic wastewater treatment pilot plant. Average total concentration of these benzothiazoles is 5.7 μmol L -1 in the untreated wastewater and 1.4 μmol L-' after aerobic treatment. Aerobic batch degradation tests revealed that TCMTB is transformed to MBT. MBT is primarily methylated to MTBT, which was not further degradable. BT was aerobically degraded along unknown pathways. Potential effects of benzothiazoles entering the aquatic environment are illustrated by luminescence inhibition of Vibrio fischeri (= Photobacterium phosphoreum ; EC 50 between 0.03 μmol L -1 for TCMTB and 32 μmol L -1 for BT) and growth inhibition of the same organism with TCMTB and MBT. MBT, BT, and MTBT at concentrations of 0.1- 0.2 μmol L -1 inhibit nitrification on sediment columns and mixed culture respiration determined as BOD (0.6-11 μmol L -1 ). It is concluded that 2-substituted benzothiazoles employed in industrial processes are not completely removable by biological wastewater treatment and are of concern for aquatic environment due to their limited biodegradability and potential toxicity.

A Comparative Study On Color Edge Detection

Abstract: Several approaches of different complexity already exist to edge detection in color images. Nevertheless, the question remains of how different are the results when employing computational costly techniques instead of simple ones. This paper presents a comparative study on different approaches to color edge detection. The approaches are based on the Sobel operator, the Laplace operator, the Mexican Hat operator, different realizations of the Cumani operator, and the Alshatti-Lambert operator. Furthermore, we present an efficient algorithm for implementing the Cumani operator. All operators have been applied to several synthetic and real images. The results are presented in this paper. We show that the quality of the results increases if Gaussian masks of larger width are used in the derivation process instead of simple 3 x 3 masks as suggested in the underlying papers. Moreover, multiresolution approaches can be applied to color images when using Gaussian masks with different standard deviations in the edge detection scheme.

High degree of polyandry in Apis dorsata queens detected by DNA microsatellite variability

Abstract: Workers of six colonies of the giant honeybee Apis dorsata from Sabah, Malaysia (five colonies) and Java (one colony) were genotyped using single locus DNA fingerprinting. The colonies from Sabah nested in colony aggregations of 5 and 28 nests respectively on two trees. Three DNA microsatellite loci (A14, A76, A88) with a total of 27 alleles provided sufficient genetic variability to classify the workers into distinct sub-families revealing the degree of polyandry of the queens. Queens mated on average with 30.17 ± 5.98 drones with a range from 19 to 53. The average effective number of matings per queen was 25.56 ± 11.63. In the total sample of 192 workers, 22 individuals were found that were not offspring of the colony's queen. Three of these were potentially drifted offspring workers from genotyped queens of colonies nesting on the same tree.