15 Fakultätsübergreifend / Sonstige Einrichtung

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/16

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End date: 1969Subject: search.filters.subject.530

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    Quantum generation and detection of incoherent phonons in superconductors
    (1967) Eisenmenger, Wolfgang; Dayem, Aly H.
    The energy gaps in the quasiparticle excitation spectrum of superconductors cover a corresponding frequency range from about 1 to 1000 GHz. The relaxation and recombination of excited quasiparticles occur mostly via electron-phonon interaction. Thus a superconductor may be utilized as a quantum phonon generator and detector in a frequency range where conventional means of generation become prohibitively difficult. We report on experiments designed to investigate the possibility of using super conducting tunnel diodes for phonon generation and detection.
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    Energy relaxation and intervalley relaxation of hot electrons in n-type germanium
    (1966) Seeger, Karlheinz; Schweitzer, Dieter
    The average current density in n-type germanium subjected to superimposed strong ac and weak dc electric fields was both measured and calculated for frequencies of 9 and 35 GHz. Repopulation relaxation of the conduction band valleys and energy relaxation were taken into account. Experimental and theoretical result agree qualitatively.
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    The anisotropy of conductivity of n-type germanium in strong d.c. fields
    (1965) Schweitzer, Dieter; Seeger, Karlheinz
    The electric conductivity of n-type germanium at large d.c. field intensities has been measured in three directions of symmetry of the cubic lattice. From these data the repopulation of valleys of the conduction band has been determined. Measurements were done with three materials of different purity. The variation of repopulation between these materials can be explained qualitatively by the influence of Coulomb scattering at ionized impurities. The data are compared with those of Nathan and theoretical data of Franz and Reik et al. The relations of these data to those of the current component perpendicular to the field direction (Sasaki et al.) are also given.
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    Dynamic nuclear polarisation in p-doped silicon by microwave ultrasonics
    (1969) Hausser, Karl H.; Schweitzer, Dieter
    We have investigated the dynamic nuclear polarisation (DNP) of 29 Si nuclei in highly doped silicon single crystals. The saturation of the electronic Zeeman levels in a magnetic field of 3.3 kG at 1.6°K was obtained using microwave ultrasonics of about 9 GHz. The ultrasonic waves were generated in a cavity by means of a transducer consisting of a thin film of CdS evaporated on the polished surface of the silicon. Preliminary results obtained with a silicon-rod of 2 x 2 x 12 mm cut with the long axis parallel to the (110) direction rendered a reduction of the NMR signal to about ⅓ in agreement with the expected negative sign of the DNP.
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    Messung der Hyperschalldämpfung in Quarz
    (1965) Eisenmenger, Wolfgang; Kinder, Helmut; Lassmann, Kurt
    Zur Bestimmung der Hyperschalldämpfung in zylindrischen Quarzstäben für Longitudinalwellen in x-Richtung werden zwei Meßanordnungen für den Frequenzbereich 0,5 GHz bis 2.5GHz sowie für die Frequenz von 1O GHz bei Temperaturen von 4° K bis 273° K beschrieben. Im niedrigeren Frequenzbereich erfolgte eine Rauschbefreiung des Empfangssignals durch elektronische Abtastung und Summation. Neben der Temperatur- und Frequenzabhängigkeit der Dämpfung wurden Geometrieeinflüsse bei der Hyperschallanregung und Ausbreitung näher untersucht. Dabei traten Modeninterferenz und Keilwinkelfehler (Parallelitätsabweichungen der Stabendflächen) besonders deutlich in Erscheinung.
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    A theoretical and experimental study of lens centring errors and their influence on optical image quality
    (1966) Hopkins, Harold H.; Tiziani, Hans J.
    Any small decentring of a surface of an optical system may be resolved into an angular tilt of the surface about its intersection with the optical axis and a second-order displacement along the axis. A simple method is described for calculating the lateral shift and coma of the axial image resulting from a surface tilt, and also the contribution of this axial coma to the variance of the wavefront aberration. This leads to a system of tolerances for the tilt and eccentricity of any simple or compound component, and for cementing errors in the latter case. The applications of the resulting formulae are illustrated with examples. An optical unit has been constructed, with which the tilt error of each surface of a compound system may be measured. The axial coma of a decentred system may be measured using a wavefront-reversing interferometer which is also described. The decentring errors of a badly mounted microscope objective have been measured, and the axial coma expected from the theory determined interferometrically. Good confirmation of the theory was obtained, and star images further confirm the order of magnitude of the axial coma.
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    Phonon generation and detection by single particle tunneling in superconductors
    (1969) Eisenmenger, Wolfgang
    In reviewing the experiments we should summarize the arguments indicating that the observed phonon signals are due to non thermal phonon radiation emitted in the decay and recombination of injected quasiparticles. It appears very difficult to explain the observed structure in the dependence of the signal amplitude as a function of the generator voltage in terms of heat pulses, e.g., phonons with Planck's distribution. An estimate of the possible heat effect arrives at much smaller signal amplitudes than observed. In addition, only little temperature dependence of the signal amplitude is to be expected. In contrast to this, quantum processes provide a straightforward understanding of the experimental findings.
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    Anwendung der optischen Filtriertechnik
    (1969) Tiziani, Hans J.; Beyeler, Bernhard H.; Witz, Werner
    Die vorliegende Arbeit gibt die Grundlagen zu einem Pattern-Recognition-System, welches sich ganz spezifisch zur Erkennung einfacher geometrischer Figuren (KristalIform-Analyse) eignet. Sämtliche gewünschten Invarianzeigenschaften ergeben sich hier auf natürliche Weise, im Gegensatz zu den in Abschnitt 2 diskutierten Methoden der optischen Filtrierung. Eine Verallgemeinerung des Systems (z. B. im Hinblick auf die Erkennung von Schriftzeichen) ist hingegen nicht denkbar, da sich Buchstaben nicht in eindeutiger Weise durch ihr Verhalten unter der zweidimensionalen Drehgruppe unterscheiden. Zur Lösung solcher Probleme wird man auf die optischen Filtriertechniken gemäss Abschnitt 2 zurückgreifen. Möglichkeiten, die bestehende Apparatur zu verbessern, wurden untersucht, Es wäre zum Beispiel vorteilhaft, das Raumfrequenzspektrum auf elektro-optischem Wege abzutasten (mit einer Bildaufnahmeröhre oder dergleichen); Dadurch könnte die Umlaufgeschwindigkeit beträchtlich gesteigert werden. Weitere Verbesserungen betreffen die Stabilität des mechanischen Aufbaus, die Auswertung der Fourierkoeffizienten a n und anderes mehr.