Universität Stuttgart
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Item Open Access Investigating superconductivity by tunneling spectroscopy using oxide heterostructures(2017) Fillis-Tsirakis, Evangelos; Mannhart, Jochen (Prof. Dr.)Item Open Access From Hermitian to non-Hermitian topological phases of matter(2019) Rui, Wenbin; Metzner, Walter (Prof. Dr.)The focus of this thesis lies on extending the theory of topological phases of matter from the Hermitian to the non-Hermitian regime. This includes not only the extension of conventional concepts such as topological invariants and topological boundary states in the theory of Hermitian topological phases, but also the exploration and characterization of entirely new topological phases unique to non-Hermitian systems.Item Open Access Structure and electronic properties of epitaxial monolayer WSe2(2019) Mohammed, Avaise; Takagi, Hidenori (Prof. Dr.)Item Open Access Ultrafast spectroscopy of single quantum dots(2012) Wolpert, Christian; Lippitz, Markus (Juniorprofessor Dr.)In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.Item Open Access Organic solar cells : correlation between molecular structure, morphology and device performance(2010) Bruder, Ingmar; Weis, Jürgen (Prof. Dr.)The development of efficient organic solar cells could be one approach to provide mankind with cheap, sustainable and ecofriendly energy. The introduction of bulk heterojunction and tandem device architectures led recently to devices with power conversion efficiencies close or even higher than n = 6%, showing the potential of organic photovoltaics. Nevertheless, to compete for the foreseeable future to inorganic solar cell technologies, the power conversion efficiencies of organic solar cells have to rise further in the range of 10 % into 15 %. Since the functioning of organic photovoltaics is based on a complex interplay of the electronic properties of its molecular components, it is desirable for an efficient evolution, to identify structural and energetical key characteristics of the molecular components that can lead to efficiency gains. Furthermore, there are virtually no limits for the synthesis of new photoactive materials for the use in organic photovoltaics. Therefore, it is crucial for the device fabrication as well as under a chemical point of view, to narrow potentially promissing classes of molecules and their derivatives under certain physical criteria. One aim of this study was to find and identify so far unknown design criteria for molecules providing high efficiencies in organic solar cells. Thus, the question was raised: What is the physical cause for the differing performance of various metal-phthalocyanines (MPc's with M = Zn, Cu, Ni, Fe) in organic solar cells. Therefore, MPc/C60 based bilayer heterojunction solar cells were fabricated showing a clear dependence of the optimal layer thickness and overall performance on the employed MPc material. Initially, the origin of these differences were explored through structural analysises by AFM and high resolution XRPD measurements on powder and evaporated thin films. The optical properties of the metal phthalocyanines were investigated by solidstate fluorescence and absorption measurements. The lowest excited states of the MPc series were explored by correlated multi-reference ab inito calculations. A high open circuit voltage Voc of a solar cell is a prerequisite for high efficiencies. Unfortunately, the Voc of small molecule based organic solar cells is usually considerably lower than the HOMO-LUMO offset of the device, which determines the theoretical maximum of the Voc in a first approximation. Thus, the question was investigated: What causes the difference between the possible open-circuit voltage and the actual measured voltage and how can this difference be reduced? To answer this question, heterojunction solar cells were produced containing ZnPc or one of the novel synthesized Phenyl-ZnPc, Naphtyl-ZnPc or Anthracenyl-ZnPc as p-conducting and C60 as n-conducting organic layers. By adding the respective aryl substituents to the ZnPc core, the polarizability of the molecules was successively increased. Concurrently, an increase of the Voc from 550 mV to 790 mV by using the highly polarizable Anthracenyl-ZnPc instead of ZnPc was achieved. Quantum mechanical calculations, simulating the charge separation mechanism at the DA-interface of Phenyl-ZnPc/C60 and Naphtyl-ZnPc/C60 showed, that the interplay between characteristic packing and polarization effects could lead to considerably different Coulomb interactions of the electron-hole pairs at the DA-interface. The control of the conduction type and Fermi-level of semiconductors is crucial for the realization of all optoelectronic devices. In inorganic as well as in organic devices this can be achieved by defined doping of appropriate areas within the device. Thus, it has been investigated, how the molecular structure of a dopant should be in order to reduce its diffusivity and increase the evaporation temperature to allow a more efficient processing of the compound. As a result, the novel p-dopant 2,3-di(N-phthalimido)-5,6-dicyano-1,4-benzoquinone (BAPD) was synthesized and compared to the state-of-the-art dopant F4TCNQ. In addition to basic and applied physical questions, I worked on the development of new, efficient solar cell architectures during my PhD thesis. In the course of this work it could be shown, that an efficient organic tandem cell can be prepared from a solid state dye-sensitized solar cell combined with a vacuum-deposited bulk heterojunction solar cell. The complementary absorption of the dyes, as well as an adequate serial connection of both subcells, leads to a high power conversion efficiency of n = (6.0±0.1)% under simulated 100 mW/cm2 AM 1.5 illumination.Item Open Access Spectroscopic study of CaMnO3/CaRuO3 superlattices and YTiO3 single crystals(2009) Yordanov, Petar; Keimer, Bernhard (Prof. Dr.)The first two sections of Chapter 1 give a general overview of the research topics and experimental methods discussed in the thesis. Further on, in Chapter 2, some of the most important characteristics and mechanisms underlying the physics of transition metal oxides are presented. As the experimental part of the thesis includes studies on manganites and titanates, these two classes of compounds are exemplified in the exposition of Chapter 2. Several recent works in the emerging research field of transition metal oxide interfaces and superlattices are also discussed along with a brief introduction in x-ray spectroscopic methods with synchrotron radiation. Chapter 3 introduces the principles of optical spectroscopy and the simplest models for dielectric function, i.e., Lorentz oscillator and Drude dielectric function. The following Chapter 4 introduces two of the experimental techniques in optical spectroscopy, reflectance and spectroscopic ellipsometry. Further on, we describe the design of a new home-built apparatus for near-normal reflectance with high magnetic fields. Several critical technical details and findings during the assembling process are also discussed. Chapter 5 represents a comprehensive experimental spectroscopic study of a prototypical superlattice system made from an antiferromagnetic insulator CaMnO3 and a paramagnetic metal CaRuO3. The resulting interface ferromagnetic state was closely investigated by means of optical spectroscopy as well as by soft x-ray scattering and absorption methods. This study led us to the conclusion that magnetic bound states, i.e. magnetic polarons, have to be considered in the description of this SL system. Chapter 6 describes a polarized far infrared reflectance study with high magnetic field on the ferromagnetic Mott insulator YTiO3, single crystals. All 25 infrared-active phonon modes were observed. The temperature and magnetic-field dependence of the phonon modes revealed a weak spin-phonon coupling in YTiO3 and largely extended temperature range (up to TM ~ 80 - 100K), for the field-induced effects on the oscillator parameters. This later observation, uncovered short-range magnetic order state which remains even at temperatures as high as three times the temperature of the actual ferromagnetic transition of Tc ~ 30K. While a quantitative theoretical description of these data is thus far not available, they point to a complex interplay between spin, orbital, and lattice degrees of freedom due to the near-degeneracy of the Ti t2g orbitals in YTiO3.Item Open Access Probing the electronic structure of new 3D Dirac semimetals(2019) Topp, Andreas; Ast, Christian R. (Dr. habil.)In this thesis, ARPES was used to measure the band structure of novel 3D Dirac semimetals, many of which were previously unknown concerning their electronic structure. The main results were obtained characterizing materials of space group (SG) no. 129 and more specifically ZrSiS and related compounds.Item Open Access Automated parametric Rietveld refinement and its application to two dimensional X-ray powder diffraction experiments(2011) Rajiv, Paneerselvam; Joswig, Manfred (Prof. Dr.)Parametric Rietveld refinement has opened new possibilities to simultaneously refine multiple powder diffraction patterns collected in in situ 2D experiments; in that way the models of crystallographic variables that changes with external variables can be introduced into the refinement. The substitution of a variable with its model during the refinement has several advantages, including the improved precision of variables, direct extraction/refinement of some parameters from powder data which is otherwise impractical (e.g., activation energy), etc. The basic requirement for the realization of sequential/parametric refinements (or Whole Powder Pattern Fit-WPPF) in 2D X-ray powder diffraction (XRPD) is a robust software that handles the data and performs fast WPPF. This concern has been primarily addressed in this thesis with the help of a software, in combination with the existing total pattern analysis software (Topas). The developed software could considerably speedup and automate the sequential/parametric quantitative analysis of large number of 2D powder data, which is in general a monotonous and time consuming task. The software also provides routines that automatically determines the reconstructive phase transitions of samples from the 2D powder data and facilitates the independent refinements (or WPPFs) of the determined phases. Two practical scientific applications of parametric Rietveld refinement method have been demonstrated with the assistance of the developed program. The first application concerns the kinetic analysis of several polymorphs and polymorphs-additives mixtures of copper phthalocyanine (CuPC). The reaction rate constant and the order of reactions involving the phase transitions of various forms of CuPC were directly extracted from the isothermal experimental data by introducing the Johnson-Mehl-Avrami-Kolmogorov relation as a model of the phase fraction during the multi phase parametric Rietveld refinement. Parametric refinements could be successfully performed for most of the CuPC data collected in the experiment, however the convergence of some of the refinements showed a strong dependence on the reaction rate. In many cases, the precision of the refined parameters could be improved considerably when the data collected between the optimal time steps alone were used in the refinement. The second application demonstrates the feasibility of the parameterization of crystallite size with respect to the annealing time/temperature. Some of the data samples used in the kinetic analysis (CuPC) and the temperature dependent nanocrystalline TiO2 data were used in this demonstration. The success of the parameterization of crystallite size depended strongly on the quality of the data used, on the uniformity of the variation of the crystallite size with time/temperature and also on the correctness of the model that describes the crystallite size variation with time/temperature. This application in its present form is general; as such it can be used for stabilizing other variables during parametric refinement.Item Open Access Growth and characterization of transition metal oxide heterostructures with a tailored work function(2018) Rastegar Alam, Iman; Mannhart, Jochen (Prof. Dr.)Item Open Access Unconventional properties of non-centrosymmetric superconductors(2010) Klam, Ludwig; Metzner, Walter (Prof. Dr.)A kinetic theory for non-centrosymmetric superconductors (NCS) is formulated for low temperatures and in the clean limit. The transport equations are solved quite generally for any kind of antisymmetric spin-orbit coupling (ASOC) in an extended momentum and frequency range. The result is a particle-hole symmetric, gauge-invariant and charge conserving description, which is used to calculate the current response, the specific heat capacity, and the Raman response function. A detailed analysis of the gauge invariance and the associated phase fluctuations of the superconducting order parameter revealed two gauge modes: the Anderson-Bogoliubov mode on the one side and a new gauge mode on the other side, which strongly depends on the symmetry of the ASOC. As application of the kinetic theory, the polarization-dependence of the T=0 electronic Raman response in NCS is studied for two important classes of ASOC with the representative systems CePt3Si and Li2PdxPt3-xB. Analytical expressions for the Raman vertices are derived, and the frequency power laws and pair-breaking peaks are calculated. A characteristic two-peak structure is predicted for NCS and might serve as an indicator for the unknown relative magnitude of the singlet and triplet contributions to the superconducting order parameter. An efficient numerical method is introduced in order to calculate the dynamical spin and charge response of CePt3Si, using an itinerant description for the electrons. With a realistic parameterization of the band structure, the nesting function, inelastic neutron scattering cross sections, and Kohn anomalies are calculated for a selected band in the normal non-magnetic state. From the spin and charge susceptibility, a superconducting pairing interaction is constructed for the weak-coupling gap equation. A sign analysis of the decoupled gap equation supports the experimental evidence of a strong triplet contribution to the order parameter in CePt3Si. In particular for this compound, it can be shown that an increasing Rashba-type of spin-orbit coupling strengthens the triplet contribution.