08 Fakultät Mathematik und Physik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9

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Open Access
Akustische Ortung kollabierender Kavitationsblasen in Wasser und in Gewebephantomen
(1993) Staudenraus, Joachim; Köhler, Michael; Lünstroth, Uwe; Eisenmenger, Wolfgang
Für moderne Ultraschall- und Stoßwellenquellen, wie sie z.B. medizintechnisch in der Sonographie und in der Lithotripsie eingesetzt werden, beobachtet man in Wasser intensive Kavitation. Unter Einwirkung des äußeren Drucks implodieren die als Folge hoher Unterdruckpulsanteile gebildeten Kavitationshohlräume, ein Vorgang der als Blasenkollaps bezeichnet wird. Mittels einer geeigneten Anordnung mehrerer Hydrophone gelingt es, Kavitationsblasen durch Rekonstruktion des Ausgangspunktes der bei Blasenkollaps abgestrahlten Stoßwellen zu orten.
Open Access
Akustische Schichtmikroskopie durch Pulsmessungen im ns-Bereich
(1982) Eisenmenger, Wolfgang
Im folgenden soll über Untersuchungen von Ladungs- und Polarisationsschichtungen in dielektrischen Folienmaterialien berichtet werden, sowie über deren zeitliche Entwicklung in der Polarisationsphase. Ein erster Vorschlag, akustische Sprungfunktionen oder Druckpulse für eine Analyse von Ladungsverteilungen einzusetzen, stammt von Laurengeau und Mitarbeitern. Zum Zeitpunkt des Vorschlages existierten noch nicht ausreichend gute elektronische und akustische Meßverfahren, um die erforderliche hohe Auflösung realisieren zu können. Inzwischen ist durch die Verfügbarkeit von Verstärkern im GHz-Bereich, aber auch durch neue Methoden der Anregung von akustischen Druckstufen bzw. von Druckpulsen eine Analyse von Schichtstrukturen im Auflösungsbereich von μm möglich.
Open Access
Analysis of magnetic excitations in molecular nanomagnets
(2006) Kirchner, Nadejda; Dressel, Martin (Prof. Dr.)
This PhD-thesis is about methods of modeling- and analysis of experimental data on molecular nanomagnets. It has two important results. We present the computer code developed for simulation of Frequency Domain Magnetic Resonance Spectra (FDMRS) on molecular nanomagnets in terms of the single-spin Hamiltonian model. The program enables an automatic and high precision determination of the zero-field splitting parameters of mono- and many-nuclear complexes with high spin ground state. It was successfully applied to the ZFS studies by FDMRS on various molecular magnets. Another result of this work is the new development of the generalized effective spin Hamiltonian model. Interactions of non-Heisenberg type (single-ion crystal fields and antisymmetric exchange) were introduced into the model for the first time being expressed through non-collinear tensors. The model gives reasonable results by explanation of the origin of magnetic anisotropy of a tetrameric Ni(II) cluster possessing S4 symmetry. It indicates the pronounced role of the non-compensated orbital moment in the system accompanied by collective action of the single-ion crystal fields.
Open Access
Analysis of the acoustlc transients in the pulse response of the linear electro-optic effect
(1976) Veeser, Hermann; Bogner, Udo; Eisenmenger, Wolfgang
The pulse response of the linear electro-optic effect is investigated with high temporal and spatial resolution in KDP, KD*P, and biaxial LiCOOH · H2O. Experimental results, which are explained theoretically, show that the piezo-optic transients induced by the piezoelectric effect, are generated by stress release waves. Starting from the surfaces, these excite by reflection, diffraction, and superposition, due to the finite crystal dimensions, normal vibration modes too. It is shown that the direct electro-optic effect can be measured for all crystal classes at the beginning of the pulse response. The pulse method also yields static electro-optic, piezo-optic, and elastic constants. The values of r63, r63, p66 and c66 measured in KDP, agree well with those of other authors. Measurement of the direct electro-optic effect of LiCOOH · H2O in some directions yields values within 1 to 4 × 10-12 m/V.
Open Access
Analysis of the phonon spectrum emitted by superconducting Al-tunneling junctions at high quasiparticle injection rates
(1981) Welte, Michael; Eisenmenger, Wolfgang
The 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.
Open Access
Anisotropy of phonon emission from hot electrons in germanium
(1976) Reupert, Wolfram; Lassmann, Kurt; Groot, Peter de
We have measured quantitatively the anisotropy of the electron-acoustical phonon scattering in n-germanium in five directions of the (lTO) plane utilizing tunneling junctions for calibrated detection of the phonon radiation emitted from a small avalanche breakdown region in the germanium surface. For comparison and evaluation of phonon focusing a constantan heater and a tunneling junction were also used as phonon sources. A similar experiment has been reported by A. Zylbersztejn, but he could only compare the ratio of the amplitudes of longitudinal and transverse phonon pulses in a given crystal direction to the calculation.
Open Access
Anomalous recovery of the remanent polarization after heating polarized PVDF films up to 180°C
(1992) Eberle, Gernot; Eisenmenger, Wolfgang
Biaxially stretched 38-μm-thick PVDF (polyvinylidene fluoride) samples with about 80% β content were poled at room temperature at E=200 MV/m for 5 s. The remanent polarization was measured with the PPS method first at 20°C, then after heating to 180°C, and a third time after cooling the sample to room temperature again, to determine its temperature-dependent stability. To distinguish between irreversible and reversible polarization loss due to heating of polarized PVDF films, the polarization was measured at elevated temperatures and after cooling down to room temperature again. The reversible polarization loss causes the pyroelectricity of PVDF.
Open Access
Ausbreitung freifeldfokussierter Hochenergiedruckimpulse in Wasser
(1991) Staudenraus, Joachim; Eisenmenger, Wolfgang
Im Zuge der klinischen Einführung von Stoßwellengeneratoren zur Zertrümmerung von Harnsteinen sind in jüngerer Zeit physikalische Fragestellungen zur Fokussierung hochenergetischer Druck- und Stoßwellenimpulse in Flüssigkelten auf starkes Interesse gestoßen. Schalldruckimpulse hoher Amplitude erfahren dabei als Folge nichtlinearer Ausbreitung eine zunehmende Aufstellung der Wellenfront. Für die Fokussierungsexperimente steht eine elektromagnetische Impulsschallquelle in Kalottenform zur Verfügung.
Open Access
Binding energies of trapped charges in PVDF and P(VDF/TrFE)
(1994) Küssner, Bodo; Dehlen, Bernhard; Eberle, Gernot; Eisenmenger, Wolfgang
The remanent polarization in PVDF and P(VDF/TrFE) is stabilized by trapped charges. We measured the remanent polarization P as function of temperature during heating from room temperature to 180°C. The discharge current is then computed as the first derivative -dP/dT. Assuming a Debye relaxation and a continuous distribution of binding energies g(E) of the trapped charges we can show that the discharge current I(T) is proportional to g(E=mT), with m=const. The constant m can either be calculated numerically or by analytical approximation, With this method the distribution g(E) can be determined. These data are consistent with the extraordinary long life time of the remanent polarisation in PVDF and P(VDF/TrFE) at room temperature.
Open Access
Biological effects and physical characterization of shock waves by an XL-1 experimental lithotripter
(1989) Brümmer, Franz; Staudenraus, Joachim; Nesper, Martina; Suhr, Dierk; Eisenmenger, Wolfgang; Hülser, Dieter F.
Extracorporeal shock wave lithotripsy (ESWL) has become the clinical standard method for non-invasive disintegration not only of concrements in kidney and urinary tract but also of gallstones. Despite the widespread clinical use of ESWL, the mechanism of stone destruction is not yet really understood, but several possibilities (cavitation, shock wave reflection) are discussed. The final cause of various side effects is still under discussion. Nevertheless, during the last few years much effort was put into possible extensions of ESWL applications On the other hand , physical characterizations of shock waves are rarely reported and combined measurements of biological effects in vitro and physical characterization of the applied shock waves are not available. We, therefore, examined the influence of water temperature and gas content on the shock wave efficency in biological systems and determined several physical characteristics (pressure amplitudes, rise time etc.) of the shock waves under the same experimental conditions.
Open Access
Breitbandige Mikrowellenspektroskopie an UNi2Al3
(2012) Steinberg, Katrin; Dressel, Martin (Prof. Dr.)
Breitbandige Mikrowellenspektroskopie ist eine effektive Methode, die komplexe Leitfähigkeit von dünnen Metallfilmen in einem großen Frequenzbereich zu bestimmen. Grundlage dieser Methode ist die Messung des komplexen Reflexionskoeffizienten S11 im Bereich von 45 MHz bis 50 GHz und von 1K bis 300K. Das Mikrowellensignal wird durch die Probe, die das Ende eines Koaxialkabels abschließt, reflektiert, vom Netzwerkanalysator detektiert und mit dem Eingangssignal verglichen. Eine umfangreiche Kalibrierung ist notwendig um die Eigenschaften des Koaxialkabels den gesamten Frequenz- und Temperaturbereich zu bestimmen. Ziel dieser Arbeit ist es gewesen, die optischen Eigenschaften des Schwere Fermion Systems UNi2Al3 zu untersuchen. Dieses Material besitzt eine anisotrope Gleichstromleitfähigkeit und wird supraleitend bei TC = 1K. Um die Anisotropie auch im Mikrowellenbereich untersuchen zu können, wurde die Proben als Streifen in Corbino-Geometrie gemessen. Die Streifengeometrie verursacht eine zusätzliche Frequenzabhängigkeit der Probenimpedanz. Diese wird mit einem RCL-Modell beschrieben und ist unabhängig von der Leitfähigkeit der Probe. Es wurde eine neue Auswertemethode entwickelt, die diesen Geometrieeffekt berücksichtigt. Ein weiterer Effekt der Streifengeometrie ist ein erhöhtes Auflösungsvermögen des Spektrometers. Somit konnte erstmals die anisotrope Leitfähigkeit von UNi2Al3 im Mikrowellenbereich gemessen werden. Die komplexe Leitfähigkeit lässt sich durch das Drude-Modell beschreiben. Die ermittelte Streurate liegt im GHz-Bereich vergleichbar zu UPd2Al3. Auch hier ist eine Anisotropie zu beobachten. Ein neu aufgebautes Spektrometer im einen ³He-Kryostaten ermöglicht breitbandige Messungen an UNi2Al3 im supraleitenden Zustand. Erste Messungen zeigen im Realteil der Leitfähigkeit keinen Kohärenzpeak, was für eine unkonventionelle Supraleitung spricht.
Open Access
Broadband microwave spectroscopy on correlated electrons
(2004) Scheffler, Marc; Dressel, Martin (Prof. Dr. rer. nat.)
Heavy-fermion compounds are prime examples of strongly correlated electron systems. To study their electrodynamic properties, optical spectroscopy is an established tool and has shown that the characteristic energy scales of heavy fermions are much lower than those of normal metals. However, conventional spectrometers do not allow for investigations of the particularly interesting excitations of the mobile charge carriers, i.e. the heavy fermions, because the corresponding frequencies are in the microwave range: experiments using geometrical optics are not possible any more for these low frequencies, but the electromagnetic waves have to be guided, for example using coaxial cables. In the present study these problems were solved, and a broadband microwave spectrometer was developed that is particularly suited to study metals at low temperatures. It employs the Corbino geometry, i.e. the flat sample terminates the open end of a coaxial cable, thus reflecting the microwave signal propagating in the cable. The reflection coefficient then yields the electrodynamic properties of the sample. Unfortunately the microwave signal is also influenced by the transmission properties of the coaxial cable, and therefore one has to calibrate the spectrometer carefully. Here we measure up to three different calibration samples at the same low temperatures as the sample, i.e. we perform a full low-temperature calibration. Following this procedure, the precision of the obtained reflection coefficient is typically 0.001. The frequency range of the spectrometer is 45 MHz to 20 GHz and its temperature range 1.65 K to 300 K. Using this spectrometer, the microwave properties of the heavy-fermion system UPd2Al3 were studied, in particular the complex conductivity at low temperatures. Here a frequency dependence was observed that follows closely the prediction of the Drude model, but exhibits an extremely low scattering rate. This was expected for heavy fermions in general and for this particular material UPd2Al3, but here for the first time this could be unambiguously verified. The temperature dependence of the directly observable scattering rate scales with the temperature dependence of the resistivity, allowing a determination of the otherwise hardly accessible charge carrier density.
Open Access
Characterization of harmonic modes and parasitic resonances in multi-mode superconducting coplanar resonators
(2023) Beydeda, Cenk; Nikolaou, Konstantin; Tochtermann, Marius; Ebensperger, Nikolaj G.; Untereiner, Gabriele; Farag, Ahmed; Karl, Philipp; Ubl, Monika; Giessen, Harald; Dressel, Martin; Scheffler, Marc
Open Access
Charakterisierung verschiedener Hydrophone unter Stosswellenbedingungen in Wasser
(1991) Staudenraus, Joachim; Köhler, Michael; Eisenmenger, Wolfgang
Als Drucksonde wird eine Glasfaser in das akustische Wellenfeld eingeführt und die durch zeitliche Druckänderung hervorgerufene zeitliche Variation der Lichtreflexion als Hydrophonsignal verwendet. Im gut reproduzierbaren Stoßwellensignal des optischen Sondenhydrophons kann Kavitation auf oder in nächster Nähe der Faserstirnfläche, aufgrund des wesentlich höheren Reflexionskoeffizienten gegen Wasserdampf, ausgeschlossen werden. Im Unterschied zu den anderen Hydrophonen ist somit hier von einer vollständigen Unterdruckübertragung auszugehen. Im übrigen stimmen die Stoßwellensignale des optischen Sondenhydrophons und des Membranhydrophons solange überein, bis im Signal des letzteren kavitationsbedingt der Unterdruckanteil verschwindet.
Open Access
Charge and polarization dynamics in polymer films
(1985) Holdik, Karl; Eisenmenger, Wolfgang
The PPS-method has been shown to be a powerful instrument to reveal the polarization evolution within a dielectric under a forming field. Because the final polarization found in a material is not generally a monotonic function of the formation time this can be used to control the polarization pattern by variation of external parameters during the process. The reason for inhomogenious polarization patterns and thus the cause of inhomogenious piezo- (and pyroelectric) reply of the examined dielectric materials was found to be predominantly charge carrier transport.
Open Access
Charge degrees of freedom in quasi-two-dimentional organic conductors
(2022) Iakutkina, Olga; Dressel, Martin (Prof. Dr.)
Open Access
Charge-order phase transition in the quasi one-dimensional organic conductor (TMTTF)2NO3
(2020) Majer, Lena N.; Miksch, Björn; Lesseux, Guilherme Gorgen; Untereiner, Gabriele; Dressel, Martin
Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the (TMTTF)2X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor (TMTTF)2NO3. We found that the (TMTTF)2NO3 compound develops a peculiar anisotropy with a doubled periodicity (ab′-plane) of the ESR linewidth below about TCO=(250±10) K. This behavior is similar to observations in the related compounds (TMTTF)2X (X=PF6, SbF6 and AsF6), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous (TMTTF)2ClO4, known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below TCO≈250 K.
Open Access
Continuous high resolution phonon spectroscopy up to 12meV : measurement of the A+ binding energies in silicon
(1986) Burger, Wilfried; Lassmann, Kurt
We have measured the binding energies of Ga+, Al+, and In+ centers in silicon with energy-resolved phonon-induced electrical conductivity. For Ga+ and Al+ we obtain the value of about 2 meV as earlier found for B+, whereas the binding energy of In+ is 6 meV. Spectral structures attributed to impurity interactions found for higher concentrations of In at energies up to about 12 meV demonstrate that acoustic phonons up to this energy are transmitted from the tunnel junction to the substrate.
Open Access
Dependence of the phonoionization of A+-states in Si on uniaxial pressure
(1987) Groß, Peter; Gienger, Martin; Lassmann, Kurt
By the new technique of phonon induced conductance we have investigated the dependence on pressure of the phonoionization response of shallow A+-states in Si with superconducting Al-junctions as monochromatic phonon generators. In the case of B+ and Al+ we obtain a much more complicated behaviour than previously found for B+ with FIR-photoconductivity which may be connected with differences in coupling for short wavelength phonons. In the case of in+ on the other hand a shift to lower energies is observed for uniaxial pressure in [100]-direction whereas for pressure in [111]-direction only the signal intensity varies but not the position of the threshold.
Open Access
Depopulation effects in the phonoconduction response of A+ -states in silicon and germanium
(1990) Groß, Peter; Gienger, Martin; Lassmann, Kurt
We show here that deviations from these assumptions are effective in the experiment and responsible in particular for the observed illumination dependence of the phonoconduction signal. The phonon spectrum of an Al - STJ at bias V = eU > 2ΔAl consists of a continuum with a sharp threshold at Ώm = V - 2ΔAl. In the differentiated spectrum Ώm becomes the most prominent bias-tunable feature, namely the quasimonochromatic line of phonon spectroscopy used for the approximate analysis of sharp phonon scattering resonances such as the OI line of Fig. 1a (which shows inversion of the center due to strong scattering).