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 Epitaxy and scanning tunneling microscopy image contrast of copper-phthalocyanine on graphite and MoS2(1994) Ludwig, Christoph; Strohmaier, Rainer; Petersen, Jörg; Gompf, Bruno; Eisenmenger, WolfgangMonolayers of copper–phthalocyanine (Cu–Pc) on highly oriented pyrolytic graphite (HOPG) and MoS2 prepared by organic molecular beam epitaxy have been investigated by scanning tunneling microscopy. On both substrates there exist well defined preparation conditions leading to ordered two-dimensional arrays of flat lying molecules. On HOPG they form a close-packed structure with a nearly quadratic unit cell, whereas on MoS2 we found two phases, one close-packed and one rowlike phase. This rowlike phase can be explained by a long range interaction due to an adsorbate induced superstructure of the substrate, which also can be seen in the scanning tunneling microscopy images. In images with submolecular resolution, the molecules appear different on the two substrates. On MoS2 they look like a four-leaved clover, on graphite they show a more detailed inner structure.Item Open Access Continuous high resolution phonon spectroscopy up to 12meV : measurement of the A+ binding energies in silicon(1986) Burger, Wilfried; Lassmann, KurtWe 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.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 Tuning charge order in (TMTTF)2X by partial anion substitution(2021) Pustogow, Andrej; Dizdarevic, Daniel; Erfort, Sebastian; Iakutkina, Olga; Merkl, Valentino; Untereiner, Gabriele; Dressel, MartinIn the quasi-one-dimensional (TMTTF)2X compounds with effectively quarter-filled bands, electronic charge order is stabilized from the delicate interplay of Coulomb repulsion and electronic bandwidth. The correlation strength is commonly tuned by physical pressure or chemical substitution with stoichiometric ratios of anions and cations. Here, we investigate the charge-ordered state through partial substitution of the anions in (TMTTF)2[AsF6]1-x[SbF6]x with x≈0.3, determined from the intensity of infrared vibrations, which is sufficient to suppress the spin-Peierls state. Our dc transport experiments reveal a transition temperature TCO = 120 K and charge gap ΔCO=430 K between the values of the two parent compounds (TMTTF)2AsF6 and (TMTTF)2SbF6. Upon plotting the two parameters for different (TMTTF)2X, we find a universal relationship between TCO and ΔCO yielding that the energy gap vanishes for transition temperatures TCO≤60 K. While these quantities indicate that the macroscopic correlation strength is continuously tuned, our vibrational spectroscopy results probing the local charge disproportionation suggest that 2δ is modulated on a microscopic level.Item Open Access Thermal motion of one-dimensional domain walls in monolayers of a polar polymer observed by Video-STM(1993) Ludwig, Christoph; Eberle, Gernot; Gompf, Bruno; Petersen, Jörg; Eisenmenger, WolfgangScanning tunneling microscopy (STM) has been used to investigate monolayers of the ferroelectric copolymer polyvinylidenefluoride/trifluoroethylene P(VDF/TrFE) showing images of ordered polymer monolayers. By scanning with video frame rate, direct observation of the motion of onedimensional domain walls was also possible for the first time. The images clearly show domain walls normal to the polymer chains. From measurements of the temperature dependence of the domain wall velocities the activation energy for the thermally generated kink motion was estimated. These results are compared with theoretical models describing domain wall motion in ferroelectric PVDF.Item Open Access Low-energy optical conductivity of TaP : comparison of theory and experiment(2021) Yaresko, Alexander; Pronin, Artem V.The ab-plane optical conductivity of the Weyl semimetal TaP is calculated from the band structure and compared to the experimental data. The overall agreement between theory and experiment is found to be best when the Fermi level is slightly (20 to 60 meV) shifted upwards in the calculations. This confirms a small unintentional doping of TaP, reported earlier, and allows a natural explanation of the strong low-energy (50 meV) peak seen in the experimental ab-plane optical conductivity: this peak originates from transitions between the almost parallel non-degenerate electronic bands split by spin-orbit coupling. The temperature evolution of the peak can be reasonably well reproduce by calculations using an analog of the Mott formula.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 Phonon spectroscopy of the low energy vibrations of interstitial oxygen in germanium(1993) Gienger, Martin; Glaser, Markus; Lassmann, KurtIn oxygen doped Ge we find by phonon spectroscopy with superconducting tunnelling junctions a series of lines between 0.18 meV and 4.08 meV which can be interpreted as due to low lying states of the interstitial oxygen (Oi) as a rigid rotator around a <111>-axis slightly perturbed by the lattice potential. The sequence of transitions can be fit assuming a binding angle of (106 ± 1)° which is much smaller than the value of (162 ± 1)° for Oi in Si. Line shifts and splittings with uniaxial stress along <100>, <110> and <111> are in qualitative agreement with this interpretation.