15 Fakultätsübergreifend / Sonstige Einrichtung
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/16
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Item Open Access Optical evidence of Anderson-Mott localization in FeSi(1994) Degiorgi, Leonardo; Hunt, M. B.; Ott, Hans Rudolf; Dressel, Martin; Feenstra, Bokke Johannes; Grüner, George; Fisk, Zachary; Canfield, P.We have investigated the electrodynamic response of single-crystalline FeSi over a frequency range from d.c. to 10 hoch 5 cm-1. At low frequencies, we found evidence for an Anderson-Mott localization behaviour of the low-temperature optical conductivity, while at high frequencies the excitation spectrum resembles that of a conventional semiconductor. The missing spectral weight below the gap energy is redistributed around the gap edge, in disagreement with previous claims based on optical measurements.Item Open Access Optical investigations of the electrodynamics of UPd2Al3(1994) Dressel, Martin; Degiorgi, Leonardo E.; Grüner, George; Wachter, Peter; Sato, Noriaki; Komatsubara, Takemi; Uemura, YasutomoWe have investigated the electrodynamic response of UPd2Al3. At low temperatures, we observe the formation of a low-frequency narrow resonance, which indicates the development of the many-body coherent state. We do not find any evidence of a gap absorption associated to a spin density wave state below the antiferromagnetic phase transition at TN = 14 K, which would develop as a consequence of a Fermi surface instability.Item Open Access AC conductivity of deformed germanium single crystals at T = 4.2 K(1986) Dressel, Martin; Helberg, Hans WilhelmDislocations introduced in semiconductor single crystals generate deep electronic states in the gap which form one-dimensional energy bands along the dislocation lines. Because dangling bonds can be charged by trapping electrons or holes, quasi-metallic conduction along the dislocations is expected. Plastic deformation produces a network of dislocations with only small unconnected segments of ideal behaviour. Therefore the conductivity of the dislocation core can only be obtained by special dc measurements or from high frequency conductivity.Item Open Access The correlation of lattice phonon frequencies with the superconducting transition temperatures of some (BEDT-TTF) salts(1992) Dressel, Martin; Eldridge, John E.; Williams, Jack M.; Wang, H. HauThe far-infrared powder absorption spectra, between 10 cm-1 and 350 cm-1, of seven BEDT-TTF salts that exhibit superconductivity at or near ambient pressure, have been measured as a function of temperature, between 5 K and 300 K. The temperature dependences of the peak frequencies, along with the measured deuterium isotopic frequency shifts, separated the lattice from the internal modes. The frequencies of the observed lattice modes were found to increase with Tc in the three κ-phase salts in agreement with the reduced volume of the unit cell, and to have a mixed response in the three β-phase compounds. Furthermore, the general range and magnitude of the frequencies in the higher-Tc κ-phase compounds were found to be larger than in the lower Tc β-phase compounds. Consequently, no evidence was found for a correlation between higher values of Tc and a softer lattice in these organic superconductors.Item Open Access Transmission of XeCl-excimer laser pulses through optical fibers : dependence on fiber and laser parameters(1992) Hillrichs, Georg; Dressel, Martin; Hack, Hrabanus; Kunstmann, Rudolf; Neu, WalterMeasurements of the transmission of XeCl excimer laser pulses (308 nm) through optical all-silica fibers are reported. The observed photodegradation effects during long time irradiation are dependent on the laser energy density and the laser repetition rate. Also the influence of the fiber drawing parameters and fiber geometry have been investigated. Absorption spectra of the fibers in the wavelength range from 200nm to 400nm before and after the irradiation with the excimer laser give some insight in the physical reasons for the changes in transmission properties. A spectral analysis of the fiber fluorescence has been performed.Item Open Access Temperature and frequency dependence of microwave conductivity of isotropic reticulate doped polymers(1990) Ulanski, Jacek; Lüpke, Gunter; Dressel, Martin; Helberg, Hans WilhelmFrequency dependence of conductivity for reticulate doped systems is observed at room temperature only around 1 GHz and even so it is relatively weak. The temperature dependence of conductivity is characteristic of the CT complex used and not of the polymer matrix. For the system containing TTF-TCNQ, for which the d.c. conductivity has a maximum at c. 230 K (i.e. metal-like behaviour at higher temperatures), this maximum becomes more pronounced and shifts towards lower temperatures with increasing frequency in the GHz range. The temperature dependence of the microwave conductivity is weaker than that of the d.c. conductivity. Such behaviour can be described by a modified Maxwell-Wagner model if an appropriate shape factor for the conducting inclusions is introduced, and if a relatively high conductivity of the continuous phase is assumed. We conclude that charge-carrier transport in reticulate doped polymers is not controlled by insulating barriers. The disorder within microcrystals plays a fundamental role, while the CT complex crystalline network is continuous in spite of very low concentration.Item Open Access Microwave cavity perturbation technique. Part 3, Applications(1993) Dressel, Martin; Klein, Olivier; Donavan, Steve; Grüner, GeorgeThe resonant cavity perturbation method as described in the preceding two parts of this series is applied to study the electrodynamical properties of different materials in the microwave and millimeter wave spectral range. We briefly discuss the relevant uncertainties which are asociated with the different measurement techniques and we find that employing the amplitude technique it is possible to measure both the width and frequency to nearly the same precision. We then demonstrate the broad range of applicability of this technique by showing results obtained on several different materials, ranging from an insulator to a superconductor. The performance limitations of this technique are discussed in detail.Item Open Access Optical investigation of the electrodynamic response of UPd2Al3(1994) Degiorgi, Leonardo E.; Dressel, Martin; Grüner, George; Wachter, Peter; Sato, Noriaki; Komatsubara, TakemiWe have investigated the electrodynamic response of UPd2Al3 both above and below the antiferromagnetic phase transition at TN = 14 K. At low temperatures, we observe the formation of a low-frequency narrow resonance, which indicates the development of the many-body coherent state. However, we do not fmd any evidence of a gap absorption associated with a spin-density-wave state, which would develop if the state arised as a consequence of a Fermi surface instability.Item Open Access Microwave conductivity investigations in plastically deformed silicon(1990) Brohl, Michael; Dressel, Martin; Helberg, Hans Wilhelm; Alexander, HelmutWe present new results on microwave continuous-wave (c.w.) conductivity investigations of plastically deformed floating-zone (FZ) silicon single crystals. N-type samples with various doping and deformation levels were investigated by the cavity perturbation technique. The anisotropy and absolute values turn out to be very sensitive to experimental conditions but on the whole the one-dimensional nature of dislocation conduction is confirmed. The C.W. anistropy follows the respective dislocation structure anisotropy closely. For clear results, care has to be taken of the number of deep point-defect centres also introduced by deformation. Furthermore the existence of a low-temperature conducting state is shown for a Czchrochalski (CZ) sample annealed at 650°C containing rod-like defects.Item Open Access Low-temperature microwave surface impedance of the conventional organic superconductor κ-(BEDT-TTF)2Cu(NCS)2(1993) Dressel, Martin; Bruder, S.; Grüner, George; Carlson, K. Douglas; Wang, H. Hau; Williams, Jack M.-
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