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Item Open Access Excitonic Fano resonances in Ta2NiSe5 and Ta2NiS5(2016) Larkin, Timofei I.; Keimer, Bernhard (Prof. Dr.)Item Open Access Structural and electronic properties of nickelate heterostructures(2016) Wrobel, Friederike; Keimer, Bernhard (Prof. Dr.)The fabrication of thin films and multilayers has led to the discovery of novel functional properties which are widely used in electronic devices nowadays. The limit of such a material design is atomic layer-by-layer deposition which was made possible through shuttered molecular beam epitaxy (MBE) growth. In the course of this thesis project a newly developed oxide MBE system was used to grow two different types of nickelate heterostructures, namely superlattices (SLs) consisting of metallic and paramagnetic LaNiO3 sandwiched between a large band-gap insulator and a combination of lanthanum nickelate and cuprate layers into a single hybrid structure. The former type was intensively studied in the last years and a transition to a weakly insulating, antiferromagnetically ordered state was observed in samples where the LaNiO3 thickness had been reduced to only two unit cells. So far little was known about the influence of the growth method on the defects in the samples and consequently on their physical properties. The use of oxide MBE enabled us to improve the overall sample quality of nickelate SLs and to design a novel material. We first optimized the growth of LaNiO3 and thoroughly analyzed the heterostructures by synchrotron-based x-ray diffraction, transmission electron microscopy, and temperature-dependent electrical resistivity. Furthermore we conducted in-depth studies, including x-ray absorption and magneto-transport measurements. The knowledge gained thereby was used grow new, layered nickelate-cuprate hybrid structures with novel electronic and magnetic properties.Item Open Access Ordering phenomena in nickelate heterostructures studied by elastic and inelastic photon scattering(2016) Hepting, Matthias; Keimer, Bernhard (Prof. Dr.)Item Open Access Magnetic and transport properties of YBa2Cu3O7 - La0.7Ca0.3MnO3 heterostructures(2016) Mustafa, Luqman; Keimer, Bernhard (Prof. Dr.)The exploration of interface properties in complex oxide heterostructures and superlattices is one of the new exciting fields in condensed matter sciences. This is particularly originating from the technological advances in synthesizing heterostructures with atomic scale precision by advanced thin film deposition techniques. There is a plethora of novel achievements culminating in unexpected results, such as generating artificial multifunctional materials with the prominent example of the appearance of interface electrical conductivity and even superconductivity in between insulating films (SrTiO3 - LaAlO3). In this thesis a special case of heterostructures is treated. Here, heterostructures composed of superconducting YBa2Cu3O7 and half-metallic ferromagnetic La2/3Ca1/3MnO3 are investigated and the interplay of the two long-range antagonistic ordering principles - superconductivity and ferromagnetism - is intended to be studied. Whereas the physics of such structures with the CuO2 planes of the superconducting YBa2Cu3O7 oriented parallel to the substrate plane ( i.e. the short coherence length of the superconductor, ξc ~ 0.1 nm is facing the interface perpendicular) has been explored in great detail, little is known in the case of the CuO2 planes oriented perpendicular to the substrate plane and thus ab ~ 1.6 nm is pointing perpendicular to the interface. In the former case, the properties of the heterostructures and superlattices are determined by an interplay of charge transfer and orbital reconstruction, but the mechanisms occurring in the latter case are unknown so far. Prior to elaborated experiments to study the interface properties at an atomistic scale, the technology of fabricating such structures has to be accomplished and their macroscopic properties (structure, transport and magnetic properties) have to be investigated. It is the goal of this thesis to prepare the ground for the atomistic studies by developing the technological prerequisites for the growth of (110)-oriented YBCO-LCMO heterostructures and characterize their structural, electric and magnetic macroscopic properties. Due to the sensitivity of the macroscopic properties of such structures to the crystallographic perfection of the interfaces a substantial part of this thesis is devoted to the corresponding enabling technology. Advanced PLD techniques are used to fabricate single layer (110)-oriented YBCO and LCMO films, bilayers as well as twin-free (103)-oriented LCMO-YBCO-LCMO trilayers and (110)-oriented YBCO-LCMO-YBCO structures with ultrathin LCMO films (nominally 1-2 nm ) in between 50 nm YBCO. These (110)-oriented trilayers serve as a precursor for a prototype planar Josephson junction technology. A comparison of the experimental results for (001) and (110) - oriented heterostructures reveals distinct changes in the ordering temperatures Tc and TCurie giving a fingerprint of different microscopic mechanisms taking place at the interfaces. Furthermore, in the twin-free (103)-oriented trilayer samples a novel positive Meissner effect has been observed which is ascribed to the magnetic domain arrangement of the LCMO.Item Open Access Oxidische FETs mit sub-100 nm Gatelänge(2016) Woltmann, Carsten; Mannhart, Jochen (Prof. Dr.)Item Open Access Experimental investigation of VO2-based field-effect devices(2016) Lanzano, Marco; Mannhart, Jochen (Prof. Dr.)Field-effect transistors based on Mott insulators could overcome the limits encountered with modern electronic devices. In this work I investigate the construction of an all-oxide device amiming at characterizing the electric-field-induced metal-insulator transition in vanadium oxide (VO2) in order to define the potential of this material for next-generation electronic devices.Item Open Access Critical dynamics in classical antiferromagnets(2016) Tseng, Kuo Feng; Keimer, Bernhard (Prof. Dr.)This thesis reports on a neutron spin-echo study of the critical dynamics in the S = 5/2 antiferromagnets MnF2 and Rb2MnF4 with three-dimensional (3D) and two-dimensional (2D) spin systems, respectively, in zero external field. Both compounds are Heisenberg antiferromagnets with a small uniaxial anisotropy resulting from dipolar spin-spin interactions, which leads to a crossover in the critical dynamics close to the Néel temperature, TN. By taking advantage of the μeV energy resolution of the spin-echo spectrometer, we have determined the dynamical critical exponents z for both longitudinal and transverse fluctuations. In MnF2, both the characteristic temperature for crossover from 3D Heisenberg to 3D Ising behavior and the exponents z in both regimes are consistent with predictions from the dynamical scaling theory. The amplitude ratio of longitudinal and transverse fluctuations also agrees with predictions. In Rb2MnF4, the critical dynamics crosses over from the expected 2D Heisenberg behavior for T ≫ TN to a scaling regime with exponent z = 1.387(4) , which has not been predicted by theory and may indicate the influence of long-range dipolar interactions. This work establishes a basis for high-resolution studies of critical antiferromagnetic fluctuations by neutron spin-echo. The next step is the investigation of magnetic quantum criticality. First measurements were conducted on TlCuCl3, which exhibits a quantum critical point under moderate pressure.Item Open Access Transport in two-dimensional electron systems in ZnO under the influence of microwave radiation(2016) Kärcher, Daniel; Keimer, Bernhard (Prof. Dr.)This thesis covers the influence of microwave radiation on the two-dimensional electron gas in ZnO heterostructures. We investigated samples with different electron densities and found microwave-induced resistance oscillations (MIRO) on samples with densities higher than 3.9·10^11 cm^-2. This is the first observation of MIRO in an oxidic system and the third condensed matter system that shows MIRO at all. Moreover we also found microwave-induced oscillations in the photovoltage and photocurrent signals. The frequency dependence as well as power and temperature dependence were investigated in detail. They are found to be similar to MIRO in GaAs and SiGe systems.