Universität Stuttgart
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Item Open Access Energy gap reduction in superconducting tin films by quasiparticle injection(1977) Fuchs, Jürgen; Epperlein, Peter W.; Welte, Michael; Eisenmenger, WolfgangIn Sn-/-Sn-/-Pb tunneling structures the energy gap ΔSn of Sn is reduced by quasiparticle injection via single-particle tunneling between the Sn films. ΔSn as function of the quasiparticle density is probed by the Pb contact and found in agreement with the theory of Owen and Scalapino. An instability of the energy gap of Sn is observed at the critical gap reduction ratio predicted by this theory for a first-order phase transition.Item Open Access Quasiparticle recombination and 2Δ-phonon-trapping in superconducting tunnelling junctions(1976) Eisenmenger, Wolfgang; Lassmann, Kurt; Trumpp, Hans-Joachim; Krauß, RichardThe experimental recombination lifetime τeff of quasiparticles in superconducting films in general exceeds the intrinsic recombination lifetime τR by phonon trapping. On the basis of geometric acoustic propagation and reabsorption of phonons emitted in quasiparticle recombination, τeff is calculated as a function of film thickness d taking into account longitudinal and transverse phonon reabsorption, bulk loss processes and acoustical phonon transmission into the substrate. With increasing thickness d three characteristic ranges are found: range 1 with film thickness d small compared to the phonon reabsorption mean free path Λw, range 2 with d larger than Λw and dominating boundary losses, and range 3, also with d larger than Λw but with dominating bulk losses. For very small d the relation between τeff and τR, the intrinsic recombination lifetime, contains only the limiting angle of total reflection of phonons within the superconducting film. Therefore, τR can be directly obtained by τeff measurements and from the sound velocities of the film-substrate system. Range 2 is characterized by a linear dependence of τeff on d. In this range it is not possible to obtain τR from τeff measurements, however, τeff allows a determination of the phonon boundary transmission. Range 3 shows no thickness dependence of τeff on d in the limit of large d values. In this range a further method for obtaining τR from τeff values is suggested.Item Open Access Reflection of high frequency phonons at free silicon surfaces(1978) Marx, Dieter; Buck, Jochen; Lassmann, Kurt; Eisenmenger, WolfgangIn reflection experiments at free silicon [100]-surfaces we could distinguish between specularly and diffusely reflected transverse phonons propagated along <100>-directions. With increasing phonon frequency the number of diffusely scattered phonons increase relative to that of specularly reflected phonons.Item Open Access Effective quasiparticle recombination times and electronic density of states at the Fermi level in superconducting films(1978) Epperlein, Peter W.; Lassmann, Kurt; Eisenmenger, Wolfgang-Item Open Access Intrinsic and experimental quasiparticle recombination times in superconducting films(1977) Eisenmenger, Wolfgang; Lassmann, Kurt; Trumpp, Hans-Joachim; Krauß, RichardExperimental quasiparticle recombination lifetime data for superconducting Al, Sn, and Pb films are compared with calculations based on a ray acoustic model taking account of the film thickness dependence of the reabsorption of recombination phonons. Information on the true or intrinsic quasiparticle recombination lifetime obtained from these and other data is discussed.Item Open Access Resonance and relaxation attenuation by neutral acceptors in a magnetic field(1977) Schad, Hanspeter; Lassmann, Kurt; Zeile, HeinrichFine structure, level splitting, and relaxation times of the acceptor ground state in cubic semiconductors can be deduced from ultrasonic attenuation. By the application of a magnetic field it is possible to vary the coupling strength and the number of acceptors on speaking terms with the ultrasonic wave. Making use of this possibility, we have determined directly effective coupling constants for relaxation attenuation and the temperature dependence of the critical intensity for saturation of the resonant interaction. Results obtained for p-type GaAs and Si are discussed in comparison with analogous results for glasses.Item Open Access Emission, absorption and propagation of acoustic THz-waves in solids(1979) Eisenmenger, WolfgangRecent experimental developments in generation and detection of THz acoustical phonons have led to studies of phonon emission, propagation and absorption in solids. New results of this acoustical phonon spectroscopy concern phonon interactions with collective excitations (phonons, photons, magnons), localized excitations (resonant scattering of impurities, radiationless transitions) and electronic excitations in metals, superconductors and semiconductors. The different experimental methods and their applications to phonon interaction studies are discussed.Item Open Access Anisotropy of phonon emission from hot electrons in germanium(1976) Reupert, Wolfram; Lassmann, Kurt; Groot, Peter deWe 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.Item Open Access Ultrasonic spectroscopy in p-type silicon(1979) Zeile, Heinrich; Mathuni, Otto; Lassmann, KurtFrom ultrasonic resonant absorption over a wide frequency range we have determined the distribution of the energy splittings of the acceptor ground state in very pure Si(B) crystals. The measured distributions with maxima of the order of 10μeV fit well to the expected electric field distribution from the~ 10 12 cm-3 residual donors. The critical intensity for saturating the resonance attenuation has been measured in crystals of various acceptor concentrations and as a function of temperature. Although the average distance of acceptor atoms is much greater than the Bohr-radius of the bound defect-electrons, relaxation times are found to be shortened by the acceptor-acceptor interaction.Item Open Access Ultrasonic attenuation due to the neutral acceptor indium in silicon(1976) Schad, Hanspeter; Lassmann, Kurt-