Universität Stuttgart
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Item Open Access Reflection of high-frequency phonons at silicon-solid interfaces(1981) Marx, Dieter; Eisenmenger, WolfgangIn reflection experiments with phonons of frequencies above 280 GHz propagating along (110) directions we observed large deviations from the acoustic mismatch theory for silicon-metal, silicon-condensed gas, and silicon-liquid helium interfaces.Item Open Access Diffusive scattering of high-frequency phonons at free silicon surfaces(1983) Marx, Dieter; Eisenmenger, WolfgangComparing measurements of high-frequency phonon reflection at the uncovered and optically polished (100)-silicon surface, with calculations considering phonon focusing, reveal complete diffusive scattering with at most 4% specular reflection contribution. Two possible mechanisms causing diffusive scattering are discussed.Item Open Access Analysis of the phonon spectrum emitted by superconducting Al-tunneling junctions at high quasiparticle injection rates(1981) Welte, Michael; Eisenmenger, WolfgangThe phonon spectrum emitted by superconducting Al-tunneling junctions is analyzed by experiments, in which two junctions of different energy gaps are used as phonon generator and detector. The energy gap of the Al-films is varied by the evaporation conditions. The experiments show clearly that the phonon spectrum and the quasiparticle distributions in the generator are strongly nonthermal and depend markedly on the primary tunneling injection. At high injection rates also significant consequences of gap reductions and instabilities are observed.Item Open Access Physikalisch-medizinische Aspekte selbstfokussierter elektromagnetisch erzeugter Stoßwellen(1988) Eisenmenger, WolfgangGegenüber schon länger bekannten Methoden der Stoßwellenerzeugung in Flüssigkeiten wie Unterwasserfunkenentladung, chemische Detonation, mechanischer Schlag oder der Fokussierung von piezoelektrisch erzeugten Druckwellen, wurde die elektromagnetische Stoßwellenerzeugung erst im Jahre 1959 eingeführt. Sie diente seinerzeit zur Erzeugung von ebenen Stoßwellen für physikalische Stoßfrontuntersuchungen. Da mit diesem Generator bereits ebene Stoßwellen mit Drucken bis zu 700 bar über einem Querschnitt von 50 mm Durchmesser erzeugt werden konnten, ist verständlich, daß bei geeigneter Fokussierung leicht höhere Fokusdrucke bei wesentlich geringeren Drucken am Stoßwellengenerator selbst zu realisieren sind. Diese Situation führte zur Entwicklung des Stoßwellenlithotriptors "Lithostar" mit Linsenfokussierung sowie zum Vorschlag eines selbstfokussierenden elektromagnetischen Stoßwellengenerators.Item Open Access Electric field-induced gas emission from PVDF films(1987) Bihler, Eckardt; Holdik, Karl; Eisenmenger, WolfgangCharge injection, conduction and trapping are important processes for stabilizing the electric polarization in the piezoelectric polymer PVDF. In order to study the nature of the pertinent charges in PVDF we measured the gas emission from PVDF using a permeable electrode under an applied electric field up to 0.7 MV/ cm. The films were covered on one side with evaporated copper, on the other side a copper wire gauze was used as a permeable electrode. The polymer films were mounted in an UHV-system with a built-in quadrupole mass spectrometer for residual gas analysis. Charging the permeable electrode negatively, gas emission was found mainly consisting of hydrogen, hydrogen fluoride, and fluorine. For comparison FEP-and PET-films were examined.Item Open Access Polarization distributions in isotropic, stretched or annealed PVDF films(1989) Bihler, Eckardt; Holdik, Karl; Eisenmenger, WolfgangThe time development of the polarization distribution across the film thickness in polyvinylidene fluoride (PVDF) was observed using the pressure step response technique. The crystallite phase composition of the samples was changed by annealing and stretching at elevated temperatures. It is shown that the crystallite phase composition, e.g., the β crystallite content, determines the spatial distribution of the permanent polarization in PVDF.Item Open Access Dynamics of polarization growth and reversal in PVDF films(1989) Womes, Manfred; Bihler, Eckardt; Eisenmenger, WolfgangMeasurements are presented of the time development of the dielectric displacement and the remanent polarization in polyvinylidene fluoride (PVDF) for poling times ranging from 1 μs to 1000 s and poling fields between 0.8 and 2.0 MV/cm. For longer times (0.1 to 1000 s), the time dependence of the polarization distribution across the film thickness is also determined. After application of a steep rectangular high-voltage pulse, the sample is shorted to zero voltage. The remanent polarization under the short-circuit conditions is compared to the maximum dielectric displacement under the external poling field. A significant time delay of the buildup of the remanent polarization was observed as compared to the dielectric displacement under field. This time delay depends significantly on the applied field strength and the crystallinity of the films. In the case of polarization reversal, a flipping back' of the polarization was observed for shorter poling times of up to 200 μs. Under these conditions, a large part of the polarization is reversed under the field, but after the removal of the field, most of the polarization returns to the original direction. The results can be explained by the ferroelectric cooperative coupling of oriented crystallite dipoles to charges trapped at the surface of polarized crystallites.Item Open Access Down-conversion of high-frequency acoustic phonons(1987) Galkina, Tatjana I.; Blinov, A. Y.; Bonch-Osmolovskii, M. M.; Koblinger, Otto; Lassmann, Kurt; Eisenmenger, WolfgangMeasurements of phonon transport in amorphous media can give valuable information on the structural properties of these materials and may be of practical interest for its own concerning the question of thermalization in electronic devices. The existence of two-level systems in a-Si:H as one of these technically important materials has been concluded from measurements of dispersion and attenuation of acoustic surface waves.Item Open Access Enhancement of the Kapitza conductance at 0.32 meV phonon energy(1983) Heim, Ulrich; Schweizer, Rainer J.; Koblinger, Otto; Welte, Michael; Eisenmenger, WolfgangUsing quasimonochromatic relaxation phonons emitted by superconducting tunneling functions, the phonon transfer through real solid-liquid helium interfaces was tested. We found an enhancement of the energy transmission if the incident phonons exceed the threshold energy Eo = 0.32 meV. This sharp threshold energy shifts to 0.42 meV if the lighter isotope 3 He is used.Item Open Access Electric field profiles in electron-beam-charged polymer electrets(1983) Gerhard-Multhaupt, Reimund; Haardt, Martin; Eisenmenger, Wolfgang; Sessler, Gerhard M.A recently developed method, which uses piezoelectrically generated pressure steps for the determination of electric-field profiles in dielectrics, has been applied to electron-beam-charged polyfluoroethylenepropylene (FEP) and polyethyleneterephthalate (PETP) electrets. The results indicate that the technique can be employed to study volume charge effects in thin dielectrics. If properly calibrated, the method provides a quantitative measure of charge-integral functions or electric-field distributions in polymer foils.