Universität Stuttgart
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Item Open Access Molecular solids containing BEDT-TTF and structurally related transition metal complexes(1986) Keller, Heimo J.; Niebl, Roland; Nöthe, Dietrich; Schweitzer, Dieter; Weber, AngelikaThe special physical properties of BEDT-TTF salts, e.g. their two-dimensional behaviour, are caused by the typical geometry of this donor and the arrangement of the cations in the crystal lattice of its salts. In order to extend the range of physical properties produced by BEDT-TTF like compounds, we synthesized sulfur containing transition metal complexes with a molecular structure very similar to BEDT-TTF. The bis(5.6-dihydro-1.4-dithin-2.3-dithiolate)metallates of the nickel triad can be prepared in different oxidation states as anions and in neutral form. The anionic complexes can be reacted with BEDT-TTF cations.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 1,3,5-Tris(dimethylamino)benzol mit Iod und 1,3,5-Tricyanobenzol : eine ungewöhnliche Dimerisierung und ein "normaler" Donor/Akzeptor-Komplex(1988) Keller, Heimo J.; Niebl, Roland; Renner, Gerd; Ruhr, Dorothea von der; Schweitzer, DieterWir sind auf der Suche nach "organischen" Ferromagneten. Als Zielsubstanz haben wir kristallisierte Radikal-Ionen mit bahnentarteten Grundzuständen ausgewählt. Molekulare Festkörper aus planaren organischen Radikalionen zeigen ein breites Spektrum teilweise recht interessanter physikalischer Eigenschaften, die durch kollektives Elektronenverhalten zu erklären sind. In den letzten Jahren wurde vor allem das elektrische Verhalten dieser Substanzen untersucht. Dies führte zur Entdeckung einer Vielzahl "organischer Metalle", von denen einige bei tiefer Temperatur auch supraleitend werden.Item Open Access Zero bias anomalies in point-contact characteristics of αt-(BEDT-TTF)2I3(1994) Ernst, Gabriele; Nowack, Andreas; Weger, Meir; Schweitzer, DieterThe zero-bias anomaly in point-contact characteristics of the organic superconductor α-(BEDT-TTF)2I3 is investigated as a function of temperature and magnetic field. It is found that the zero-bias anomaly is insensitive to magnetic fields up to 5 T. In contrast, a structure at 5 meV, conventionally designated as the superconducting gap - but which is 4 times larger than the expected BCS gap - is strongly affected by magnetic fields above 1 T.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 Temperature and angular dependence of the upper critical field in β-(BEDT-TTF)2I3 and its deuterated analogue(1986) Heidmann, Claus Peter; Andres, Klaus; Schweitzer, DieterWe present measurements of the temperature and angular dependence of the upper critical fields of β-ET2I3 and d-β-ET2I3. The data are analyzed in terms of an anisotropic effective mass model. The results of cooling rate experiments are reported and the effect of deuteration on superconductivity is discussed.Item Open Access A study of the use of laser speckle to measure small tilts of optically rough surfaces accurately(1972) Tiziani, Hans J.A new method is proposed where small tilts can be measured with high accuracy even in the presence of a lateral shift. For this purpose the speckling of the object, an optically rough surface, is recorded in the Fourier plane before and after a tilt. By illuminating the developed plate with a laser, fringeswith spacings inversely proportional to the tilt angle can be observed.Item Open Access Lattice dynamics of 3-dimensional tilings modelling icosahedral quasicrystals(1993) Los, Joop; Janssen, Ted; Gähler, FranzA study of the lattice dynamics of three-dimensional tilings modelling icosahedral quasicrystals is presented. The phonon density of states is calculated, and the character of the eigenstates is determined. Three different types of commensurate approximants are considered, namely symmetrized, perfect and randomized approximants. It appears that the density of states is smoothed by randomization. The participation ratio, which measures the rate of localization of an eigenmode, is given as a function of frequency. Only the states at the very upper end of the frequency spectrum appear to be localized, whereas all other states are extended. The density of states at low frequencies is analyzed in more detail, by applying a Brillouin zone integration over the lowest branches. It is found that these lowest branches scale for successive approximants.Item Open Access Speckleverfahren zur absoluten Abstandsmessung(1987) Hege, Günter; Tiziani, Hans J.Eine ausführliche Untersuchung möglicher Speckleverfahren zur absoluten Abstandsmessung von rauhen Oberflächen hat gezeigt, daß eine axiale Auflösung im Submikrometerbereich nur durch Auswerten der lateralen Specklegeschwindigkeit möglich ist. Hierzu wird bei ruhendem Objekt ein fokussierter Laserstrahl akustooptisch um 0,2 mm periodisch ausgelenkt. Die entstehende Specklebewegung wird von einem Gittersensor erfaßt und von einem Zählprozessor bewertet.