Universität Stuttgart
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Item Open Access Optical and magneto-optical investigations on 3D Dirac- and Weyl-semimetals(2017) Neubauer, David; Dressel, Martin (Prof. Dr.)This work concentrates on optical investigations on 3D Dirac- and Weyl-semimetals with and without applied magnetic fields. Four compounds are extensively discussed, namely the 3D Dirac semimetal Cd3As2, the Weyl semimetals TaAs and NbP, and finally evidence is found for 2D Dirac states in the iron based superconductor FeSe. For the measurements in magnetic fields a novel magneto-optical installation is designed and implemented in the lab. The design principle and characterization of this setup is presented.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 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 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 Dynamics of poling PVDF between 25°C and 120°C(1991) Eberle, Gernot; Eisenmenger, WolfgangPVDF films with high β-content are poled in an electric field of 60 MV/m at temperatures between 25°C and 120°C. At 25°C the alignment of dipoles takes place in a central poling zone during several hours of poling. When the temperature is increased to 120°C the poling time necessary to align the dipoles in this narrow zone is reduced to several seconds. In addition, at temperatures higher than 90°C and increased poling times the central poling zone first increases but later decreases in its dielectric displacement. Simultaneously in a 10 μm regime adjoining the positive electrode a secondary displacement zone starts to grow. The strong reduction of this peak under short circuit conditions indicates hetero-charge accumulation in front of an anode.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 Fabrication and characterization of cryogenically grown granular aluminum films(2025) Deshpande, Aniruddha; Dressel, Martin (Prof. Dr.)The thesis investigates the fabrication and superconducting properties of cryogenically grown granular aluminum thin films, with a focus on understanding how deposition parameters influence superconductivity. Using thermal evaporation techniques across a wide substrate temperature range, including an unprecedented 25 K regime, the study systematically correlates growth conditions (such as evaporation rate and oxygen flow) with film morphology and superconducting behavior. A dome-like dependence of the superconducting transition temperature (Tc) on resistivity is observed and refined, with 25 K films exhibiting the highest Tc and sharpest dome. Additionally, contrary to conventional assumptions, a clear dependence of Tc on film thickness is found, revealing enhanced superconductivity in thinner films. The work provides a detailed phase diagram, explores paraconductivity, and examines Kondo-like effects, establishing granular aluminum as both a platform for studying disorder-driven superconductivity and a promising material for quantum technologies.Item Open Access Competing energy scales in granular superconductors: phase-governed superfluid and pseudogap determine the superconducting dome : final report(2025) Dressel, Martin; Scheffler, MarcGranular aluminum, i.e. nm-sized pure aluminum grains closely coupled through aluminum oxide barriers, exhibits peculiar superconducting properties that are not fully understood. In particular, the critical temperature Tc can be enhanced by more than a factor of two compared to pure bulk aluminum (with Tc = 1.2 K) in a phase diagram exhibiting a so-called superconducting dome. In this project we have implemented and optimized a dedicated deposition system using thermal evaporation for the growth of granular aluminum films in controlled oxygen environment and at cryogenic temperatures. We have grown numerous granular aluminum samples at substrate temperatures around 300 K (room temperature), 150 K and 100 K (nitrogen cooling) and 30 K (helium cooling). For these substrate temperatures we find superconducting domes that become higher and more pronounced for reduced substrate temperature. For the case of helium cooling, we find Tc larger than 3 K for the top of the superconducting dome. We have performed broadband microwave spectroscopy studies on these various samples, using Corbino reflectometry for temperatures down to 1.1 K. The oxide covering the granular aluminum films requires detailed modeling of the contact resistance for proper data analysis. Here we developed a new strategy based on spectra extending to very low frequency. This allowed us to determine the kinetic inductance of the Corbino samples, which is directly related to the superfluid density. Furthermore, we fabricated planar GHz resonators from granular aluminum films and studied them down to mK temperatures and in the low-photon regime. This connects more directly our fundamental study of material properties to possible applications of superconductors with high kinetic inductance. For the electrodynamics at somewhat higher frequencies, around the superconducting energy gap, we employ THz spectroscopy. To face the challenge of characteristic energies below the low-frequency and low-temperature limits of conventional THz experimentation, we have designed and installed a dedicated new cryostat for THz studies at temperatures below 1.5 K.Item Open Access Phononenphysik : Akustik bei Terahertz-Frequenzen(1985) Eisenmenger, WolfgangMechanische oder akustische Schwingungen und Wellen in Flüssigkeiten und Festkörpern sind wie elektromagnetische Wellen quantisiert und man bezeichnet die entsprechenden Energiequanten als akustische Phononen. Für Existenz und Ausbreitung dieser mechanischen Wellen besteht wegen des atomaren Aufbaus der Materie eine obere Frequenzgrenze, die je nach Substanz im Bereich bis etwa 10 THz liegt. Die Akustik bei THz-Frequenzen ist daher eine Erweiterung des Ultraschallbereichs zu den höchstmöglichen Frequenzen und dient der Untersuchung von Wellenausbreitung, Absorption und Streuung im Volumen oder an Grenzflächen von Körpern. Streuung und Absorption werden u.a. durch Resonanzübergänge bei Fremdatomen oder Gitterdefekten, z.B. Versetzungen verursacht. Solche Untersuchungen gehören zur akustischen Absorptionsspektroskopie, wobei die Nachweisempfindlichkeit um einige Größenordnungen höher sein kann als die im vergleichbaren Frequenzbereich liegende Ferninfrarotspektroskopie.Item Open Access Far-infrared spectroscopy of lanthanide-based molecular magnetic materials(2015) Haas, Sabrina; Dressel, Martin (Prof. Dr.)This thesis demonstrates the applicability of far-infrared spectroscopy for the study of the crystal-field splitting of lanthanides in single-molecular magnetic materials. The far-infrared studies of three different kinds of single-molecular-magnetic materials, a single-ion magnet, a single-chain magnet and an exchange-coupled cluster, yielded a deeper understanding of the crystal-field splitting of the lanthanides in these materials. In addition, our results offered the opportunity to gain a deeper insight into the relaxation processes of these materials.