Universität Stuttgart
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Item Open Access Electronic properties of [3.3]TTF-phan TCNQ-crystals(1982) Schweitzer, Dieter; Taopen, Chu; Krieger, Claus; Staab, Heinz A.-Item Open Access Electrons and holes in InSb under crossed magnetic and stress fields. 4, Stimulated Raman scattering in the valence bands(1988) Wolfstädter, Bernd; Trebin, Hans-Rainer; Pascher, Harald; Häfele, Hans-GeorgIn a series of four papers magnetooptical transitions are presented for InSb crystals, which are subjected to uniaxial stress perpendicular to the magnetic field. Here, in the first paper, we establish an 8×8 k⋅p Hamiltonian matrix for stress Tǁ[100] and field Bǁ[001] and diagonalize it exactly. The dependence of valence and conduction states on stress and longitudinal momentum is discussed and compared with the geometry of parallel fields TǁBǁ[001]. Characteristic features are extracted for inter- and intraband transitions. Under crossed fields, the levels are separated much stronger with stress, yielding more insight than in the parallel configuration.Item Open Access Opto-electronic pattern recognition system(1970) Tiziani, Hans J.; Beyeler, Bernhard H.; Witz, WernerAn opto-electronic technique for the classification of grains of photographic emulsions by shape is described. Fortunately, the shapes to be distinguished differ only in their two-dimensional rotational symmetry group. A special optical filtering procedure has been developed to take account of the implied positional and rotational invariance. The transfer from the optical to the electrical signal is obtained in a simple manner. The method works satisfactorily even for a rather complicated cluster. Some results are discussed.Item Open Access Speckling in diffraction patterns and optical images formed with the laser(1970) Hopkins, Harold H.; Tiziani, Hans J.It has been shown that the known characteristics of speckling in both diffraction patterns and optical images can be explained on the basis of a simple physical model. This explains not only the occurence of a predominant size but also the high contrast in speckle patterns.Item Open Access Temperature and pressure dependence of the resistivity of β-(BEDT-TTF)2X (X=I3, I2Au) and αt-(BEDT-TTF)2I3(1988) Weger, Meir; Bender, Klaus; Klutz, Thomas; Schweitzer, Dieter; Gross, Frieder; Heidmann, Claus Peter; Probst, Christian; Andres, KlausThe temperature dependence of the resistivity of the organic metals and superconductors {β-(BEDT-TTF)2I3, β-{3-(BEDT-TTF)2I2Au and αt(BEDT-TTF)2I3 was measured at ambient and at several isotropic pressures (up to 2.5 kbar). By the application of a relatively low pressure, it is possible to measure the term in the resistivity that is linear in temperature. This term is masked by the larger T2 term at ambient pressure. It is shown that in the temperature range below 120 K, the external modes are mainly responsible for the resistivity, while above this temperature the flipping of the CH2 groups becomes important. The linear electron-phonon coupling corresponds to a value of λ between 1 and 1.5.Item Open Access Wavelength dependent triplet state populating mechanisms of naphthalene in heavy atom hosts(1973) Schweitzer, Dieter; Zuclich, Joseph A.; Maki, August H.We have made zero-field optically-detected magnetic resonance measurements on the triplet state of naphthalene-d8 incorporated in the heavy-atom host crystals p-dichlorobenzene, p-dibromobenzene, and s-tetrachlorobenzene. The dependence of the relative triplet sublevel populating rates on the exciting wavelength is interpreted in terms of the electronic excitation transfer mechanisms. We find that upon excitation of the host S1 ← S0 transition in p-dichlorobenzene, energy transfer to the guest T1 takes place via the guest S1, whereas in the other host crystals the intermediate is mainly the host T1 band. Upon host T1 ← S0 excitaton, trapping takes place from a spin-aligned excitor band in s-tetrachlorobenzene, whereas the initial alignment is lost prior to trapping in the dihalobenzene hosts.Item Open Access Two superconducting phases in the organic conductor: β-(BEDT-TTF)2I3(1986) Creuzet, Francois; Bourbonnais, Claude; Creuzet, Gerard; Jerome, Denis; Schweitzer, Dieter; Keller, Heimo J.We have investigated the nature and the stability of the high-Tc(∼8 K) and low-Tc (∼1.5 K)superconducting phases (respectively called β-H and β-L) of the organic conductor β-(BEDT-TTF)2I3. First, by resistivity measurements and following a well-defined temperature-pressure cycling process, we show that the β-H phase is stabilized at ambient pressure and low temperature, exhibiting a very sharp and complete superconducting transition at 8.1 K and 1 bar. Secondly, an AC-susceptibility study reveals that this state is stable and exhibits bulk superconductivity at high Tc and ambient P as long as the sample is not warmed above 125 K. Finally, we present NMR results on the 1H spin-lattice relaxation rate behaviour in both the β-H phase and the β-L phase.Item Open Access Satellite phonon absorption lines above the 875 GHz resonance of interstitial oxygen in silicon(1987) Dittrich, Ehrenfried; Scheitler, Wolfgang; Eisenmenger, WolfgangAcoustic phonon spectroscopy with superconducting tunneling junctions as phonon generator and detector revealed a large number of sharp absorption lines between 875 GHz (oxygen resonance) and 1.35 THz for silicon doped with interstitial oxygen (Si:0i). The strength of these lines scales with the square of the oxygen concentration ranging from 1017 to 1018 cm-3. Under mechanical stress the lines show a frequency shift almost identical to the main oxygen resonance at 875 GHz as well as to the also observable isotope resonance. These satellite absorption lines are therefore discussed as 0i–0i neighbour interaction. This is supported by the influence of annealing.Item Open Access Effects of radiation damping on particle motion in pulsar vacuum fields(1989) Finkbeiner, Bernd; Herold, Heinz; Ertl, Thomas; Ruder, HannsThe effects of radiation reaction on the motion of charged particles are studied in strong electric and magnetic fields with special attention to the vacuum near-field region of an oblique rotator. For strong radiation damping a local velocity field is derived from the Lorentz-Dirac equation, which efficiently describes the motion of electrons and positrons in the whole range of typical pulsar parameters. The velocity field makes it possible to define regions in the inner magnetosphere, where particle trapping occurs due to the radiation losses. By numerical integration of particle trajectories from the pulsar surface, regions around the magnetic poles are found which are defined by particle emission into the wave zone. The shapes of the escape regions on the pulsar surface are determined to a considerable extent by the presence of the accumulation regions.Item Open Access 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.