Showing papers by "University of Stuttgart published in 1985"

A theoretical model of phase transitions in human hand movements

Abstract: Earlier experimental studies by one of us (Kelso, 1981a, 1984) have shown that abrupt phase transitions occur in human hand movements under the influence of scalar changes in cycling frequency. Beyond a critical frequency the originally prepared out-of-phase, antisymmetric mode is replaced by a symmetrical, in-phase mode involving simultaneous activation of homologous muscle groups. Qualitavely, these phase transitions are analogous to gait shifts in animal locomotion as well as phenomena common to other physical and biological systems in which new “modes” or spatiotemporal patterns arise when the system is parametrically scaled beyond its equilibrium state (Haken, 1983). In this paper a theoretical model, using concepts central to the interdisciplinary field of synergetics and nonlinear oscillator theory, is developed, which reproduces (among other features) the dramatic change in coordinative pattern observed between the hands.

Dislocation-Related Photoluminescence in Silicon

Abstract: Photoluminescence is studied in silicon, deformed in a well-defined and reproducible way. Usual deformation conditions (high temperature, low stress) result in sharp spectra of the D 1 through D4 lines as recently described in the literature. New lines D5 and D6 emerge for predeformation as above and subsequent low-temperature, high- stress deformation. Another new sharp line, D 12, is observed when both the familiar and the novel lines appear simultaneously. Annealing for 1 h at TA > 300 ~ causes all new lines to disappear and the D1-D4 spectra to reappear. Quantitative annealing and TEM micrographs suggest that D 5 is related to straight dislocations and D 6 to stacking faults, whereas D l-D4 are due to relaxed dislocations. Photoluminescence under uniaxial stress shows that D1/D2 originate in tetragonal defects with random orientation relative to (100) directions, whereas D6 stems from triclinic centers, preferentially oriented - as are the D3/D4 centers. We conclude that the D3/D4 and the D5 and D6 defects are closely related, whereas the independent D l/D2 centers might be deformation-produced point defects in the strain region of dislocations.

Hot holes in naphthalene: High, electric-field-dependent mobilities

Abstract: Intrinsic time-of-flight hole mobilities (${\ensuremath{\mu}}^{+}$) were obtained in naphthalene single crystals down to 4.2 K. Between 300 and 150 K the tensor component ${\ensuremath{\mu}}_{\mathrm{aa}}^{+}$ increases with decreasing temperature T, obeying a \ensuremath{\mu}\ensuremath{\propto}${T}^{n}$ dependence, with n=-2.9. At low temperature the hole transport becomes nonlinear (sub-Ohmic) with the hole velocity tending to saturate with increasing electric field, E, at about 2\ifmmode\times\else\texttimes\fi{}${10}^{6}$ cm/s. The highest experimental ${\ensuremath{\mu}}_{\mathrm{aa}}^{+}$ (obtained at the lowest E which allowed the observation of a distinct hole transit pulse) was 400 ${\mathrm{cm}}^{2}$/V s at 10 K and 3 kV/cm. It will be shown that the low-temperature results can be understood in terms of a standard band-model description, whereas the continuation of the experimental temperature-dependence law (\ensuremath{\mu}\ensuremath{\propto}${T}^{n}$) (for both holes and electrons) into the high-temperature regime remains a problem for future theoretical work. Electron transits were obtained down to 22.5 K [${\ensuremath{\mu}}_{\mathrm{aa}}^{\mathrm{\ensuremath{-}}}$ (22.5 K) = 24.5 ${\mathrm{cm}}^{2}$/V s]. No field dependence of the electron mobility was detected.

Knowledge-based help systems

Abstract: Our research goals are to understand the nature of, construct and evaluate intelligent interfaces as knowledge-based systems In this paper we demonstrate the need for help systems as an essential part of human-computer communication Help strategies are based on a model of the task (to understand what the user is doing or which goals he/she1 wants to achieve) and a model of the user (to guarantee that these systems are non-intrusive and that they pay attention to the needs of individual users)We illustrate that passive and active help systems have to be constructed as knowledge-based systems Two operational systems (PASSIVIST and ACTIVIST) are described to show the usefulness of this approach

Quantum tunneling rates for asymmetric double-well systems with Ohmic dissipation.

Abstract: The dynamics of an asymmetric double-well system coupled to a heat bath is studied at low temperatures where transitions between the wells involve quantum tunneling. The time evolution of the occupation probabilities of the wells is calculated by means of instanton techniques. For a wide range of parameters incoherent relaxation is found at a rate whose dependence both on temperature and bias is nonanalytic, in general.

Effect of hydrostatic pressure, temperature, and doping on self-diffusion in germanium.

Abstract: The tracer-diffusion coefficient for $^{71}\mathrm{Ge}$ has been measured in Ge single crystals as a function of pressure, temperature, and doping Ion-beam sputtering was used for microsectioning The activation volume in intrinsic Ge increases slightly with temperature from 024\ensuremath{\Omega} at 876 K to 041\ensuremath{\Omega} at 1086 K (\ensuremath{\Omega} is the atomic volume) The fairly small values of the activation volume show that the defect or defects which act as diffusion vehicles must be either strongly relaxed and/or spread out Measurements of the doping dependence performed at 973 K show that the diffusivity increases with n doping and decreases with p doping This supports the view that self-diffusion in Ge proceeds by a vacancy mechanism and that the vacancy acts as an acceptor As a consequence the contribution of negatively charged vacancies, which is about 77% for intrinsic material, increases (decreases) with n doping (p doping) The measurements of the pressure dependence of the diffusivity in doped materials, also performed at 973 K, show that the activation volume is larger for pth volume for the neutral vacancy and 028\ensuremath{\Omega} for the negatively charged vacancy

Testing aspheric surfaces with computer-generated holograms: analysis of adjustment and shape errors.

Abstract: Various interferometric methods can be used for testing aspheric surfaces with high accuracy. Using a partially compensating lens in a Twyman-Green arrangement together with a computer-generated hologram instead of a null corrector gives great flexibility. For quantitative analysis a high-resolution automatic fringe analysis is necessary. The interferometric arrangement and the setup for the interferogram data acquisition are described along with different algorithms for calculating the wave front data. Tilt and decentering of the aspheric surface and the synthetic hologram as well as the actual aspherical parameters can be derived from the calculated wave front using Zernike polynomials to communicate with a ray tracing program. For small adjustment and shape errors the linearity will hold, leading to fast correction and numerical compensation of their effects on the surface error. The method is verified by several examples. Experimental results agree with the proposed model.

Density Functionals for Correlation Energies of Atoms and Molecules

Abstract: The correlation energy, Ec, is usually defined as the difference of the exact (non-relativistic) energy, E, and the Hartree-Fock (HF) energy, EHF .1 Ec is a very small part of E only (1.4% for the He atom, 0.3% for Ne, 0.1% for Ar), but it is non-negligible in absolute value: for valence-shell removal, ...Ec is 1.1 eV for He, 9.5 eV for Ne, and 9.3 eV for Ar. Inclusion of Ec is important in cases where the number of (strongly interacting) electron pairs is changed, for dissociation energies (De), ionization potentials and excitation energies, e.g.. Correlation is responsible for 23% of Dein the case of H2, and for 84% of De in the case of Li2; Na2 and K2 are unbound at the HF level.

Some properties of silane-grafted moisture-crosslinked polyethylene

Abstract: Silane-grafted polyethylene materials are processed in conventional thermoplastic fabrication machines. The shaped articles are then crosslinked in water by the formation of Si-O-Si crosslinks. This paper represents studies on the crosslinking progress in different environments at various temperatures. Molecular orientation is shown to become permanent and mostly irrecoverable even at 150°C, in the silane-grafted solid state crosslinked specimens (the crosslinking temperature in water is well below the polymer melting temperature). These frozen molecular orientations have a significant effect on the tensile properties of the crosslinked materials causing higher yield stresses and lower elongations at break. The thermal and tensile properties of some silane-grafted crosslinked polyethylene samples and peroxide-crosslinked materials are compared and analyzed.

Quantum theory of activated events in presence of long-time memory

Abstract: The temperature dependence of kinetic processes involving activation over a potential barrier is studied in the temperature range where quantum effects are important. Deviations form the Arrhenius law are determined and a simple formula for the dominant quantum corrections is obtained. The dissipative coupling to the environment is explicitly taken into account and the role of long-time memory is emphasized.

Ignition of lean methane-air mixtures by high pressure glow and ARC discharges

Abstract: Glow and arc discharges have been used to study the spark-ignition of lean quiescent methane-air mixtures at room temperature and a pressure of p =2 bar. The regions of stable long-duration sparks with aluminum, cadmium, copper, platinum and tungsten electrodes as well as the fall voltages and voltage gradients of the positive column have been determined. For both discharge modes the minimum ignition energies and the corresponding quenching distance have been obtained for Cu electrodes at equivalence ratios of 0.59, 0.67 and 0.77. Minimum ignition energies for various electrode materials have been measured at the quenching distance and an equivalence ratio of =0.67. The experimental results indicate that the energy dissipated in the anode and cathode fall is completely lost to the electrodes, whereas the energy of the positive column determines the flame initiation process. These findings are substantiated by spark-flashlight schlieren photographs of the incipient spark and flame kernel.

Zeeman analysis of the ytterbium luminescence in indium phosphide.

Abstract: Ytterbium-doped InP emits a characteristic luminescence band near 1.0 \ensuremath{\mu}m, arising from intra-4f-shell transitions of ${\mathrm{Yb}}^{3+}$(4${\mathrm{f}}^{13}$). A Zeeman analysis of the complex set of emission lines, in magnetic fields up to 22 T, reveals that the spectrum arises almost entirely from only one type of cubic Yb center, presumably on a substitutional cation site.

General Purpose Vehicle System Dynamics Software Based on Multibody Formalisms

Abstract: SUMMARY This paper pursues two objectives: Firstly, to review the state-of-the-art of general purpose vehicle system dynamics software and secondly, to describe two representatives, the program MEDYNA and the program NEWEUL. The general modeling requirements for vehicle dynamics software, the multibody system approach and a comparative discussion of multibody software are given. The two programs NEWEUL and MEDYNA are described with respect to modeling options, computational methods, software engineering as well as their interfaces to other software. The applicability of these programs is demonstrated on two selected examples, one from road vehicle problems and the other from wheel/rail dynamics. It is concluded that general purpose software based on multibody formalisms will play the same role for mechanical systems, especially vehicle systems, as finite element methods play for elastic structures.

Spectral reflectance measurements using an integrating sphere in the infrared.

Abstract: Determination of the reflectance for directional irradiation of diffusely reflecting samples requires measurement of the radiation reflected into the whole hemisphere. This can be done with integrating spheres. An arrangement based on a rapid-scan Fourier transform spectrometer equipped with a gold-coated integrating sphere allows measurement of the spectral reflectance in the 1.5μm–∼15μm wavelength range. Problems of calibration and accuracy are discussed. Results are compared with those of conventional techniques: measurement of the directional emittance of heated samples and measurement of the specular reflectance.

Immunofluorescence for the quantitative determination of nitrifying bacteria: interference of the test in biofilm reactors

Abstract: The direct and the indirect fluroescent-antibody techniques (FA-technique) were applied to determine the number of specific cells in the biofilm of a fixed-bed reactor which is used for the secondary treatment of wastewater. The immune-reaction between fluorescing antibodies and the antigenic surface of the specific cells was prevented by slime covering the bacteria in thick layers. The masking effect could not be removed by physical (heat, ultrasonication, washing) and chemical (detergents, chelating, agents, salts) treatment of the cells.

Imaging with Optically Generated Thermal Waves

Abstract: (Carslaw I959), with temperature T, coordinate z, mass density p, specific heat c, thermal conductivity k, and time t. This type of equation is known from diffusion. As the equation is not symmetrical with respect to time inversion, it describes an irreversible process which steadily reduces existing gradients. On this basis it is difficult to imagine what a thermal wave could be. A wave is characterized by a periodicity in time and space. The question is, therefore, whether a temperature periodicity in time is correlated with a periodicity along the z-coordinate. If temperature is modulated at the plane z = 0 at the angular frequency cl and with amplitude ATO, then the modulation at z > 0 is (Carslaw & Jaeger 1959)

The Vocal Repertoire of the Adult Senegal Bushbaby (Galago Senegalensis Senegalensis)

Abstract: Acoustic structures, situations and proposed messages of vocalisations used by the nocturnal arboreal prosimian species Galago senegalensis senegalensis are described. Eighteen different sounds are identified which could be related to one of the following functional categories: contact, aggressive and defensive and attention and alarm behaviour. The call types within these behavioural categories show striking similarities in their acoustic structures. Vocalisations used during inter- and intragroup contact situations demonstrate harmonic spectra with low frequency fundamentals and formants and slight frequency modulations. Noisy spectra as well as an irregular repetition rate characterize sounds uttered by the aggressor during agonistic encounters. They can be intensified by visual threat displays. Harmonic spectra, high frequency fundamentals and formants, and rapid frequency modulations are typical for calls given by submissives during aggressive intraspecific interactions. Sounds which indicate social or external disturbance factors show noisy and harmonic spectra with broad dominant frequency range as well as a regular repetition rate. Factors that might have favoured the evolution of this call system are discussed.