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Transport is non-reciprocal when not only the sign, but also the absolute value of the current depends on the polarity of the applied voltage. It requires simultaneously broken inversion and time-reversal symmetries, for example, by an interplay of spin–orbit coupling and magnetic field. Hitherto, observation of nonreciprocity was tied to resistivity, and dissipationless non-reciprocal circuit elements were elusive. Here we engineer fully superconducting non-reciprocal devices based on highly transparent Josephson junctions fabricated on InAs quantum wells. We demonstrate supercurrent rectification far below the transition temperature. By measuring Josephson inductance, we can link the non-reciprocal supercurrent to an asymmetry of the current–phase relation, and directly derive the supercurrent magnetochiral anisotropy coefficient. A semiquantitative model explains well the main features of our experimental data. Non-reciprocal Josephson junctions have the potential to become for superconducting circuits what pn junctions are for traditional electronics, enabling new non-dissipative circuit elements. Diodes exhibit non-reciprocal current–voltage relations, that is, the resistivity depends on the direction of the current flow. Now an array of Josephson junctions with large spin–orbit interaction acts as the superconducting version of a diode, where dissipation-free supercurrent flows in one direction, but not the other.

The study substantiated a scheme for cleaning seeds of high categories from an intractable impurity (Tatar buckwheat). In the theoretical study of the technological process of seed cleaning, the method of mathematical modeling was used.

In the scope of this thesis, four different topological materials grown by molecular beam epitaxy are investigated. The MBE growth conditions have to be optimized for each of the materials. The structural properties of the grown films are studied using x-ray diffraction (XRD), atomic force microscopy (AFM) and energy-dispersive x-ray spectroscopy (EDX). Low-temperature magneto-transport measurements are performed to characterize the magnetic and electrical properties of the samples. The topological insulator (Bi1-xSbx)2Te3 (BST) serves as a basic building block for several applications in this work. The morphology of the grown BST film is improved by optimizing the growth conditions such as growth temperatures, elemental material fluxes, growth duration, etc., and by employing special growth methods like a two-step deposition technique. The optimal growth conditions are found by measuring the film morphology by atomic force microscopy and using x-ray diffraction to confirm the epitaxial and single-phase growth mode. Bulk-insulating BST films with surface carrier dominated conduction and carrier concentrations as low as 10^12cm^-2 are obtained by varying the bismuth-antimony ratio. Furthermore, the necessity of protecting the top surface of the MBE grown film with a capping layer is established. A 3nm-thin Al2O3 which is deposited by atomic layer deposition (ALD), can protect the BST film from oxidization. A robust ferromagnetic order is successfully achieved by doping (Bi1-xSbx)2Te3 with vanadium. The magnetic-field dependence of the Hall resistance is investigated and both, the ordinary Hall effect and the anomalous Hall effect are revealed by the observed hysteresis loops. By extracting the critical temperature and anomalous Hall amplitude from the transport measurements at 2K, the amount of vanadium dopants and the optimal growth temperature are specified. Most significantly, the quantum anomalous Hall effect is observed in the vanadium-doped BST films. As expected, the longitudinal resistance vanishes and the Hall resistance reveals the quantized value of 25.81kOhm when measuring the magnetic-field dependence at 30mK. The quantized resistance is still preserved at 100mK. Furthermore, the current-induced breakdown and the temperature stability of the quantum anomalous Hall effect are investigated. Apart from topological insulators, their superconducting relatives recently attracted a significant interest. Sn1-xInxTe is predicted to be candidate topological superconductor. Thin Sn1-xInxTe films are grown on a Bi2Te3 buffer layer by molecular beam epitaxy. It is found that the films only can be turned into a superconducting state after annealing them in-situ in the MBE chamber right after growth. The critical temperature depends on the annealing time and temperature suggesting a thermal activation process for the superconducting properties. Tunnel-junction devices are fabricated on the films and surface-sensitive tunneling spectra are measured. Even though measurements on regular Hall bar devices suggest conventional superconductivity a peculiar behavior is revealed by the tunneling spectroscopy. Here, a two-gap structure is observed which points to the coexistence of bulk and surface superconductivity. Because of the special properties of the material the surface superconductivity is bound to be topological. Finally, nano-scaled (Bi1-xSbx)2Te3 films are grown by the selective area growth (SAG) method. A Si3N4 film is deposited on a sapphire substrate and afterwards patterned into nanostructures by electron-beam lithography and etched by reactive-ion etching. The pre-patterned substrate is used to selectively grow (Bi1-xSbx)2Te3 nanostructures in the molecular beam epitaxy chamber. The selective growth of the BST films is achieved by optimizing the growth temperature such that film growth takes place only inside the pre-patterned structures and not on top of the Si3N4 layer. Again, by tuning the bismuth-antimony ratio a bulk-insulating film growth is confirmed by magneto-transport measurements on selectively grown Hall bar devices. Furthermore, the efficiency of two different kind of gates fabricated on a BST nanowire is compared. It is found that the gating efficiency of an after growth fabricated top gate is higher than the efficiency of selectively grown BST-side gates.

Dissertation, RWTH Aachen University, 2019; Aachen 1 Online-Ressource (III, 104 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, RWTH Aachen University, 2019, LiMn$_{2}$O$_{4}$ (LMO) is a candidate for the cathode material in the next generation of lithium ion batteries. To specifically tailor and optimize the material, knowledge of the defect chemistry, microstructure, diffusion and the effect of doping is required. In this study oxygen and lithium diffusion of dense, undoped and Al-doped LMO-pellets is investigated using isotope tracer diffusion experiments. The analysis is carried out by secondary ion mass spectrometry (SIMS). The pellet samples are prepared by spray drying and subsequent field assisted sintering technique (FAST). To investigate the oxygen diffusion in undoped and Al-doped LMO pellets oxygen exchange experiments in 18O2-enriched atmosphere are performed and analyzed by means of SIMS. Due to an occurring phase transition at the surface during the experiments in the temperature range of 400 °C to 700 °C and a pO2 of 200 mbar no results are obtained without the knowledge of the transition kinetics. The lithium diffusion in the undoped and Al-doped LMO pellets is investigated using an amorphous 6Li enriched thin-film, deposited on the pellets by pulsed laser deposition (PLD). Diffusion anneals in air in a temperature range of 20 °C to 210 °C were carried out to introduce tracer diffusion profiles between thin-film and pellet. The resulting diffusion profiles show several features assigned to film, bulk and grain boundary diffusion. To obtain tracer diffusion coefficients from the experimental profiles two two-dimensional numeric models are used and compared. The first consisting of infinite parallel grain boundaries and grains and the second being the brick-layer model. The modelling yields lithium tracer diffusion coefficients for the amorphous film, the bulk material, and the grain boundaries. The results show a fast grain boundary diffusion being orders of magnitude faster than bulk diffusion. Results obtained from the numerical models are compared to Harrison’s type A and B regimes of classic diffusion kinetics in polycrystalline samples. Activation energies of the corresponding diffusion processes are obtained by temperature dependent experiments. In this study, bulk and grain boundary diffusion in LMO are d for the first time using one experiment only., Published by Aachen

Superconductivity is one of the most mesmerising phenomena in condensed matter physics research, and the most recent material class, where it was discovered in, are iron-based superconductors. These compounds display a prodigious interplay between their lattice/orbital degrees of freedom, magnetic order, superconductivity and electronic nematicity, which gives rise to a rich phase diagram. Today there is a broad consensus that superconductivity is driven by spin fluctuations (paramagnons), whose fingerprint in the excitation spectra (observable by inelastic neutron scattering) is the so-called spin resonance mode. It is directly related to the details of the superconducting pairing mechanism, and its connection to the lattice structure and magnetic order is studied by X-ray, elastic and inelastic neutron scattering experiments in this thesis. The impact of antiferromagnetic order with large moments on the superconducting state is investigated by polarised inelastic neutron scattering in underdoped Ba(Fe1-xCox)2As2, where both orders coexist on a microscopic scale. Since superconductivity emerges in the presence of broad magnetic anisotropy gaps, it is shown that the corresponding spin resonance mode appears only in the two transversal channels, at two distinct energies, and longitudinal contributions are gapped. This situation is in contrast to the one in optimally and overdoped BaFe2As2, where isotropic spin resonance modes are observed. Concluding, this anisotropy is attributed to a band- and orbital-selective pairing mechanism. In Ba1−xNaxFe2As2 the interplay between structure, magnetic order and superconductivity is more intricate. The spins in the already orthorhombic and magnetically ordered phase undergo a second magnetic transition at a lower temperature, which rotates the moments from an alignment within the FeAs-layers to a perpendicular arrangement, by concomitantly suppressing the orthorhombic distortion. This spin reorientation transition highlights the importance of spin-orbit coupling in this material class. Moreover, in a naive picture, low-energy spin fluctuations, which promote superconductivity, rotate concomitantly with the associated static moments. However, polarised inelastic neutron scattering experiments revealed that this is not the case, and thus clearly demonstrate that the polarisation of low-energy spin excitations is uniform in iron-based superconductors. Furthermore, the spin resonance mode in Ba1−xNaxFe2As2 consists of two contributions which display a remarkably different doping dependence. On the one hand, the intensity of the high-energy part traces the shape of the superconducting dome in the phase diagram. While, on the other hand, it is shown by absolute unit calculation, that the intensity of the low-energy part is the accumulated spectral weight, which is removed from the magnetic Bragg peaks, as magnetic order and superconductivity compete for the same electronic states below Tc. For this reason, the low-energy part is most intense close to the magnetic end-point in the associated phase diagram and is strongly reduced in intensity beyond that point. In particular, the low-energy part in Ba0.61Na0.39Fe2As2 displays the strongest resonance mode ever observed in iron-based superconductors, while the high-energy contribution is similar intense as the one in optimally Co-doped BaFe2As2. Consequently, this observation provides a simple explanation on the origin of split spin resonance modes, observed in underdoped iron-based superconductors.

Once more, neutron scattering using large high-quality single crystals is proven to be a powerful technique to study magnetism on a microscopic level. Several layered Ruthenate single-crystals could be grown in better quality or larger dimensions than the existing crystals at the starting of this work, enabling the neutron scattering studies [10, 35, 25, 36, 26]. Thus important progress in the research of layered Ruthenates could be achieved: The incommensurable magnetic fluctuations in Sr2RuO4 , which are induced by the Fermi-surface nesting of the quasi one-dimensional bands, were studied in the superconducting and non-superconducting state up to very low energy transfers, far below the expected cooper-pair binding energies. Therefore, it is strongly suggested that the quasi one-dimensional bands are not the active bands driving the superconductivity, giving an important contribution to the question of the symmetry of the superconducting order parameter. The magnon dispersion of Ca2RuO4 was determined throughout the whole first Brillouin zone giving an important contribution to the discussion of the ground state of this compound and revealing the enhanced strength of spin-orbit coupling in that 4d system. The oxygen octahedron rotation of SrRuO3 was determined with high precision very efficiently. Thus, it became possible to shed light on the six-fold twinning of this compound and the changing twinning fraction induced by magnetic fields. A reorientation of the crystal structure in the magnetic phase induced by magnetic fields and a memory effect of the reorientation, which goes in hand with a magnetic shape-memory effect, was discovered in that oxide. The spin density distribution in the unit cell of SrRuO3 could be reconstructed using polarized neutron diffraction data. A spin-density wave has been confirmed in Ca0.5Sr1.5RuO4 . The propagation vector amounts to (0.305,0.305,1) and the spins freeze in like in spin- or cluster- glasses becoming static at about 4K [10].

Purpose This study aims to investigate the effect of gusts on aircraft wake vortices. Aircraft wake vortices present a potential risk to following aircraft, particularly during final approach and landing, as wake vortices may remain in the flight corridor for a long time. Wind and turbulence are key factors that influence the wake vortex evolution and the wake vortex generation in the aircraft. Flying through a gust influences the wake vortex roll-up process and its evolution. Note that vertical and lateral gusts may affect counter-rotating wake vortices differently. Both vortices influence each other by inducing a downward velocity. Disturbances may therefore lead to local vortex tilting and later to a complex three-dimensional deformation. This work uses two different hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation (RANS-LES) approaches to investigate the effect of gusts on wake vortex evolution. In a one-way coupling, a pre-calculated RANS velocity field of the aircraft’s near-field is being swept through an LES domain. The effect of a sine gust on the turbulent wake is modeled by manipulating the RANS-field accordingly. As a more sophisticated approach, the concept of a two-way coupling is being presented. Here an LES solver is bi-directionally coupled with an unsteady RANS (URANS) solver, exchanging values at every physical time step of the simulation. Design/methodology/approach A one-way coupling approach of the LES code MGLET and the RANS code TAU is presented to simulate the gust effect on aircraft wake vortices. Additionally, the concept of the two-way coupling of these two codes incorporating a coupling module. Findings The gust effect of wake vortices subjected to a crosswind can be simulated. The vortex physics is analyzed. Unexpected behavior like fast upwind vortex decay is revealed. Practical implications The understanding of the aircraft wake vortex physics during landing provides valuable information for wake vortex advisory systems. Originality/value The effect of gust on wake vortices during and after landing has not been studied so far. The hybrid one-way coupling approach, as well as the concept of the two-way coupling, are relatively new.

This paper investigates the evolution of a streamwise aligned columnar vortex with vorticity ω in an axial background shear of magnitude Ω by means of linear stability analysis and numerical simulations. A long wave mode of vorticity normal to the plane spanned by the background shear vector Ω and the vorticity of the vortex are excited by an instability. The stationary wave modes of the vertical and lateral vorticity are amplified. In order to form a helical vortex, the lateral and vertical vorticity can be phase shifted by half a wavelength. The linear and nonlinear evolutions of the vortex in the shear flow are studied numerically. Linearized simulations confirm the results of the stability analysis. The nonlinear simulations reveal further evolution of the helix in the shear flow. The linearly excited mode persists in co-existence with evolving smaller scale instabilities until the flow becomes fully turbulent at the time of O(100 Ω−1). Turbulent mixing dampens the amplifying mode. The described phenomenon of vortex meandering may serve as an alternative explanation for the excitation of wind turbine wake meandering in the atmospheric boundary layer.

This paper is a short introduction to C. Ginzburg's article on a circumstantial paradigm.

Franz Neumann

As an unavoidable consequence of lift aircraft generate a pair of counter-rotating and long-lived wake vortices that pose a potential risk to following aircraft. The prescribed aircraft separations during landing to avoid wake vortex hazards contribute significantly to capacity restrictions of large airports. Severe encounters of wake vortices have also been reported during cruise. Wake vortex behavior is largely controlled by the prevailing meteorological conditions and the interaction with the ground. The most important meteorological parameters are ambient wind, wind shear, turbulence, and temperature stratification. The Deutsches Zentrum für Luft- und Raumfahrt (DLR) develops wake vortex advisory systems for airports and en route which aim at optimizing the air traffic with respect to the measured and predicted wake vortex behavior. As part of such systems simple probabilistic wake vortex prediction models are required that predict wake vortex behavior accurately, robust, and fast. Highly resolving large eddy simulations (LES) conducted on the SuperMUC supercomputer provide valuable insights in the physics of wake vortex behavior under various atmospheric conditions. These LES contribute indispensable guidance for the development of the real-time/fasttime wake vortex models.

We report on a comprehensive de Haas-van Alphen (dHvA) study of the iron pnictide LaFe2P2. Our extensive density-functional band-structure calculations can well explain the measured angular-dependent dHvA frequencies. As salient feature, we observe only one quasi-two-dimensional Fermi-surface sheet; i.e., a hole-like Fermi-surface cylinder around Gamma, essential for s+/- pairing, is missing. In spite of considerable mass enhancements due to many-body effects, LaFe2P2 shows no superconductivity. This is likely caused by the absence of any nesting between electron and hole bands.

Considering metallic films at room temperature, we present the first theoretical study of the spin Nernst and thermal Edelstein effects which takes into account dynamical spin-orbit coupling, i.e., direct spin-orbit coupling with the vibrating lattice (phonons) and impurities. This gives rise to two novel processes, namely a dynamical Elliott-Yafet spin relaxation and a dynamical side-jump mechanism. Both are the high-temperature counterparts of the well-known $T = 0$ Elliott-Yafet and side-jump, central to the current understanding of the spin Hall, spin Nernst and Edelstein effects at low $T$. We consider the experimentally relevant regime $T > T_D$, with $T_D$ the Debye temperature, as the latter is lower than room temperature in transition metals such as Pt, Au and Ta typically employed in spin injection/extraction experiments. We show that the interplay between intrinsic (Bychkov-Rashba type) and extrinsic (dynamical) spin-orbit coupling yields a nonlinear $T$- dependence of the spin Nernst and spin Hall conductivities., 9 pages, 4 figures

This thesis deals with thin films of the ferromagnetic semiconductor EuO. These are prepared by molecular beam epitaxy under ultra high vacuum conditions and their properties are investigated in-situ by means of low energy electron diffraction, magneto-optical Kerr effect (MOKE), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The experiments are carried out in a low temperature STM apparatus with a base temperature of 5.3K. The development of a highly sensitive MOKE instrument is in the technical focus of this work. EuO is grown in (100) orientation on a ferromagnetic Ni(100) single crystal as well as on single crystalline graphene on Ir(111). An antiferromagnetic coupling between the magnetic moments of the Ni(100) and the EuO(100) film turns out. The 3.3nm thin stoichiometric films of EuO on graphene show a Curie temperature Tc=75K, which is increased with respect to films of the same thickness grown on other substrates and even with respect to bulk EuO. Point defects in EuO films are analyzed with help of atomically resolved STM images. The defect concentration can be quantified using highly-resolved STM topography images. The mobility of the oxygen vacancies is shown by a series of topography STM images and a mobility of the vacancies along the EuO directions is found. Investigations of the electronic structure of the EuO(100) surface are performed by STM and STS, which require a high sample quality, i.e, a topographically very well ordered and flat surface with a minimum of defects. The ultra thin films of EuO on graphene described in thesis are of such high quality. A local n-doping and an upward band bending is found at the oxygen vacancy sites. Tunneling spectroscopy maps of the ferromagnetic EuO(100) surface show standing wave patterns, which are the characteristic fingerprint of the surface state in EuO(100). The energy vs. momentum space of the surface state is explored by evaluating the surface state scattering patterns. A dispersion relation determined this way of the surface state is free electron like, with an effective mass of the mass of a free electron. The same measurements are performed in the paramagnetic phase of EuO at 81K. Thereby the surface state is shifted in energy. The shift indicates indirectly the spin degeneracy of the surface state in the paramagnetic phase, which is lifted in the ferromagnetic phase of EuO. This behavior is analogous to the well known spin splitting of the EuO conduction band. The spin splitting of the surface state (measured with this work) amounts to 0.66eV.

We discuss different scenarios which allow for inducing unconventional superconducting pairing correlations in heterostructures. Specifically, we investigate superconductor-ferromagnet hybrids using a recent extension of the Quasiclassical Theory of Superconductivity and discuss both the long range triplet Josephson effect and point contact spectra. In the second part, we investigate a microscopic model of the superconducting proximity effect in topological insulators.

In this work the local and nonlocal conductance of mesoscopic normal-metal/superconductor hybrid structures has been measured. Subgap anomalies in both local and nonlocal conductance were observed. For the nonlocal conductance both positive and negative signs were found as a function of energy, hinting at a competition of crossed Andreev reflection and elastic cotunneling. The measured data suggests that the signals are caused by quantum mechanical interference rather than Coulomb interaction.

The present paper studies the dependance of F-centre diffusion on the Ca++ impurity concentration in KCl crystals in the temperature range 300 - 400°C. An expression is obtained for the dependance of the F-centre diffusion coefficient on impurity concentration.

Based on the complete set of equations describing nonequilibrium states of S1 - I - S2 tunnel junctions due to the quasi particle current, distributions of electronic excitations on the S films of symmetric junction (similar films composing a tunnel junction) are calculated for arbitrary tunnel barrier transparencies for the case T < Delta, eV - 2Delta < 2Delta (eV is the applied voltage, Delta is the energy gap). Superconductivity suppression in the nonequlibrium state is considered (eV > 2Delta). A theory of generation of phonons with frequencies omega > 2Delta which accompanies the quasi particle tunneling, is suggested. The available experimental results are interpreted and new features in the distribution of generated recombination phonons are predicted.

The work describes the setup of an ultra low temperature Scanning Tunneling Microscope (STM), which works in a conventional ³He-cryostat with a base temperature of 240 mK and a vector magnet with a magnetic field up to 1 T. The STM is rigid and compact and therefore it is insensitive to vibrations and can be used without elaborate damping. The STM is well shielded against radio-frequency radiation and all electrical lines are filtered at low temperatures. The measurement of the differential conductance dI/dU of superconducting aluminum yields a temperature of the electronic system of 270 mK and a energy resolution of 100 µeV. The STM is nonmagnetic, what has been demonstrated on the successful observation of superconductivity of aluminum, which has a critical field of only 10 mT. Furthermore the characteristic hysteresis in the phase transition between the normal- and the superconducting state could be observed on a aluminum tip. The atomic resolution of graphite at 240 mK in a magnetic field of 1 T shows the capability to work in high fields as well.The investigation of the Proximity-Effect has been performed with the developed STM using tunneling spectroscopy and recording the differential conductance dI/dU on the System aluminum/gold. The temperature dependence as well as the dependence on the magnetic field was investigated. The thickness of the superconductor aluminum was 320 nm and the thickness of the normal metal gold has been varying from 20 nm up to 190 nm. The induced minigap decreases slower with increasing thickness of the normal metal, than predicted by the theory. Based on experimental values of the mean free path, the differential conductance is calculated in the frame work of the quasiclassical theory. In contrast to the experimental curves the theoretical curves show a significantly smaller minigap. The Variation of parameters like the interface resistance and the coupling parameter between superconductor and normal metal do not result in a better description of the experimental data. Even the assumption of a reduced normal metal thickness yields no agreement regarding the temperature dependence. Only the assumption of a ten times larger thickness of the superconductor results in a satisfied description of the dI/dU-curves. Also the temperature dependence can be described within this assumption.The Experiments and the required increase of the superconducting layer indicate that the superconductivity in aluminum is less suppressed by the normal metal gold, than predicted by the theory in the case of a thin film.

Magnetic domain walls are found to exhibit quasiparticle behavior when subjected to geometrical variations. Because of the spin torque effect such a quasiparticle in a potential well is excited by an ac current leading to a dip in the depinning field at resonance for current densities as low as 2 x 10 power 10 A/m power 2. Independently the resonance frequencies of transverse walls and vortex walls are determined from the dc voltage that develops due to a rectifying effect of the resonant domain wall oscillation. The dependence on the injected current density reveals a strongly nonharmonic oscillation.

We consider the production of spatially separated entangled electrons in transport between a superconductor and ferromagnets: Spin singlet Cooper pairs are filtered into different leads by spin dependent tunneling. We find the full counting statistics depending on the magnetizations and identify the probabilities for productions of entangled electrons taking into account the superconducting proximity effect. Positive contribution to cross correlations are due to coherent transport of two electrons from the superconductor and negative contribution caused by Fermi statistics. We finally show the conditions for a violation of a Bell inequality as experimental test of entanglement.

Very small pinning centers in superconducting thin films were produced applying a patterning technique based on self-organization of salt-loaded spherical micelles made of diblock copolymers. In a subsequent plasma treatment, the polymer matrix is removed completely, while the metal salts are reduced to metallic nanoparticles used as etching mask to transfer the resulting pattern into silicon substrates. The pinning sites produced by growth of a niobium thin film on top of such substrates are much smaller than those produced by conventional methods like optical, electron-beam or focused ion beam lithography. To the best of our knowledge, this method delivers the smallest periodic artificial pinning centers studied yet. Due to their nanoscaled size, the artificial pinning sites guarantee single-vortex pinning even at temperatures well below the critical temperature. This is shown by the fact that Little-Parks oscillations can be observed only up to the first matching field, where each of the pinning sites contains one vortex, and not up to higher orders as observed for wire networks or perforated films with larger holes. Several samples containing such artificial pinning centers were characterized using a SQUID-magnetometer to obtain their magnetic moment, by high resolution electron microscopy and by transport measurements at low temperature. Although the order of the resulting artificial pinning centers is not perfect, pronounced integer and fractional matching features induced by commensurability effects when the vortex lattice matches the lattice of artificial pinning sites are observed within a wide temperature range. Experimental data of a niobium layer on top of silicon nanopillars shows an unexpected temperature dependence of matching features at the first matching field. This effect can be understood by applying a modified concept of collective pinning in a triangular array with non-perfect order.

(de)PET, preform, blow moulding, heating, microwave, processing, construction, Ill., graph. Darst

In this thesis, I present the first experimental implementation of a Josephson junction for Bose-Einstein condensates. The weak link between two BECs constitutes the nonlinear generalization of the well known Josephson junction of weakly-coupled superconductors that are separated by a thin insulating barrier. In our experiment, the required overlap of two macroscopic wavefunctions is provided by loading a BEC into an optical double well potential. It is realized by a superposition of a one-dimensional optical lattice with a focused laser beam optical dipole trap. The tunneling dynamics between the two potential wells exhibits two distinct dynamical regimes. For small initial population imbalances of the two wells we observe nearly sinusoidal Josephson tunneling oscillations, which are characterized by an oscillating population and relative phase. The situation changes drastically, if the initial population imbalance is chosen above a critical value. In this case, resonant tunneling between the two wells is prohibited because the difference between the on-site particle interaction energies in the two wells exceeds the tunneling energy splitting. As a consequence, the atomic distribution becomes self-locked and the relative phase evolves unbound in time. This regime of prohibited tunneling, which has no analogon in superconducting Josephson junctions, is called “macroscopic quantum self-trapping”.

(de)Acoustic dissipation, combustition instabilities, vortex shedding, perforated plates, bias flow, Ill., graph. Darst

(de)Acoustic dissipation, combustition instabilities, vortex shedding, perforated plates, bias flow, Ill., graph. Darst

(de)Clean routing, emission-referred choice of route, reduction of the emission, on-board-measurement, Ill., graph. Darst

We present a detailed semiclassical study of a clean disk--shaped insulator--normal-metal--superconductor hybrid system in a magnetic field. It is based on an exact secular equation that we derived within the microscopic Bogoliubov--de Gennes (BdG) formalism. Results obtained from a classification of electron and hole orbits are in excellent agreement with those from an exact numerical diagonalization of the BdG equation. Our analysis opens up new possibilities for determining thermodynamic properties of mesoscopic hybrid systems., Comment: 5 pages, 8 figures

(de)Neutron scattering, high temperature superconductivity, magnetic excitations

(de)Active optical fibers, fiber lasers, rare-earth-doping, vapor-phase deposition, Ill., graph. Darst

(de)Acoustic dissipation, combustition instabilities, vortex shedding, perforated plates, bias flow, Ill., graph. Darst

(de)Superconductivity, current limiter, short-circuit, Ill., graph. Darst

This is a theoretical investigation of the magnetic, structural and dynamical properties of the ferromagnetic Heusler compound Ni-Mn-Ga. Its physical properties have been studied by means of the Density Functional Theory. The challenge of this work is to explain in terms of the electronic structure and phonon spectra the entire variety of different crystal structures of Ni-Mn-Ga and their stability. It was shown that this crystal exibits anomalies both in the electronic structure and the phonons. Important for Ni-Mn-Ga features include modulation shuffling of the structure, phonon softening, phonon acoustic-optical coupling, electron-phonon coupling and the charge transfer. A combination of these properties allowed to draw a solid physical view on the nature of the Heusler alloys and the alloy Ni-Mn-Ga.

(de)Photonic crystals, silicia, superprism, vertical straylight method

A numerical approach for assessing the potential of 4-vortex wakes concerning an alleviation of the wake vortex hazard is presented. The first step of the numerical approach comprises the systematic investigation of 4-vortex topologies representing generic farfield vortex wakes. Once a configuration with minimum induced rolling moment is determined in a second step an airliner model configuration is modified using simple design methods in order to produce a 4-vortex wake similar to the afore found optimal 4-vortex topology. Its realizability is proven by simulation of the nearfield and extended nearfield wake of the modified airliner configuration. A maximum rolling moment reduction of approximately 50% compared to a 2-vortex reference configuration is obtained for a 4-vortex system with a circulation ratio of Gamma_2/Gamma_1=-0.35 and a spacing ratio of b_2/b_1=0.35. For the DLR-F11 it is shown that it is feasible to produce this specific vortex topology in the nearfield by modifying the airliner configuration.

(de)Magnetic sensors, hall effect, high-temperature superconductor, NDE, cryo-cooler, Ill., graph. Darst.

Using a pump-probe magneto-optic technique the magnetic flux penetration into superconducting YBCO films has been investigated. After zero field cooling below the critical temperature, an external magnetic field is applied perpendicular to the film. In order to study the flux penetration on a nanosecond timescale a single pulse of a fs-laser has been used. The laser pulse is focused with a cylindrical lense onto the film from the substrate side of the sample. Hereby a line with a width of 50 µm is heated, deviding the sample into two rectangular superconducting regions with equal size. Snapshots of the flux moving into the sample are recorded with a part of the trigger pulse separated by a beam splitter. Depending on the external magnetic field and temperature two different ways of magnetic flux penetration into the superconductor have been observed: as a homogeneous flux gradient, or as dendritic flux avalanches.

The circuit theory of mesoscopic transport provides a unified framework of spin-dependent or superconductivity-related phenomena. We extend this theory to hybrid systems of normal metals, ferromagnets and superconductors. Our main results is an expression of the current through an arbitrary contact between two general isotropic nodes. In certain cases (weak ferromagnet and magnetic tunnel junction) we derive transparent and simple results for transport properties.

In contrast to the classical superconductors there is actually no consensus on the theory of high-temperature superconductivity. Especially the symmetry of the order parameter and the coupling mechanism which leads to the superconducting state below a critical temperature Tc are still open questions. In addition, the energy gap Δ as a function of temperature T and a possible existence of the so-called pseudogap above Tc are unsolved problems. Tunneling experiments play an important role to solve this challenge, because they can measure the density of states directly and therefore examine the energy gap and the symmetry of the order parameter. The tunneling experiments described in this thesis used the break junction technique. The main advantages of this technique are a very high quality of the junction and an in situ adjustable contact area. The investigated materials were mainly Bi2Sr2Can-1CunO2n+4+δ(n = 1 - 3). The small single crystals with typical sizes of 2 x 1 x 0.01 mm³ were cracked in liquid helium while the current voltage characteristic could be measured simultaneously. With a lock-in technique the computer controlled setup could also measure the first and the second derivative. The tunneling current was always parallel to the c-direction. The energy gap Δ was determined from tunnel- and point-contact characteristics. The former ones were compared with numeric calculations based on the extended Dynes model for various symmetries of the order parameter. Most of the characteristics have been described much better by a s-wave than a d-wave symmetry. The energy gap of optimally doped samples at 4.2 K was Δ = 12.7 ± 0.5 meV (n = 1), Δ = 26 ± 1 meV (n = 2) and Δ = 36 ± 1.5 meV (n = 3). This linear dependence of the gap upon the number n of CuO2 planes has been observed not only for Bi-compounds, but also for Tl- and Hg-based superconductors. The steeper slope dΔ =dn for superconductors containing Ba instead of Sr and first measurements on a new superconductor HgSr2Ca2Cu3O8 have given an evidence that the value of the gap is controlled by the nature of the blocks including the CuO2- and BaO- or SrO-planes. For Bi-2201 and Bi-2212 the gap Δ as a function of the hole concentration p scaled in the same way as Tc(p) and led to a constant value 2Δ/kBTc = 12.5 ± 2 (6.9 ± 0.5) for the single (double) layer. In the whole doping range there was no hint for a pseudogap above Tc. Due to the adjustable contact area it was possible to measure high-resistance intrinsic junctions (stacks) with a small number of contacts (n =< 28) and often negligible small Josephson current. These criteria were important to avoid heating effects. However, stacks always have much higher quality compared with single contacts. The very sharp gap feature in the SIS-like characteristics (Bi-2212) ruled out any anisotropic contribution to the symmetry of the order parameter. The characteristics of SNS-stacks (n = 4) could be interpreted as a new effect called Intrinsic Multiple Andreev Reflection Effect (IMARE), which has not been theoretically understood yet. A reproducible fine structure at subgap voltages in the differential conductivity of BSCCO Josephson junctions prooved the strong coupling between the AC-Josephson current and the optical Raman-active phonons. The characteristic voltages Ures have been found to be independent of T, Tc and Δ. Applying an external magnetic field, Ures remained constant but the amplitudes of the resonances showed the same Fraunhofer-like patterning as the critical Josephson current. The temperature dependence of the amplitudes have shown the typical Riedel singularity. The overall form of the fine structure was in good agreement with Raman scattering spectra of the optical phonon modes in this material. These results were verified by a new theoretical model by E.G. Maksimov et al.. Moreover, A.A. Abrikosov used this experimental results as a part to formulate a new theory of high-temperature superconducting cuprates.

The first part of this thesis describes the development of a new high resolution x-ray single crystal diffractometer (RED) and its multiple applications. It is shown that the application of the presently available technologies allows very flexible and even completely new possibilities of structural investigations of single crystals with x-rays in laboratories. The quick exchange between different beam optics allows flexible and efficient scientific work. Available are standard optics for Bragg data collection (Cu-K_alpha- and Mo-K_alpha radiation), an additional secondary monochromator unit for pure elastic scattering experiments and a Ge-fourfold monochromator for high resolution measurements using pure Cu-K_alpha_1 radiation. The standard optics allow in combination with the large dynamical ranges (max. 500.000 cps linear) of the available detectors detailed investigations of structures with extreme differences between integrated intensities. Furthermore, detailed temperature dependent measurements can be performed down to 20 K by using an additional closed-cycle cryostat, e.g. for structural investigations at phase transitions. The Ge-fourfold monochromator yields high resolutions comparable to those of similar instruments at synchrotron sources (e.g. Si-(111) reflection with a halfwidth of < 0.0030°, Rocking scan). This allows a very precise analysis of reflection profiles for characterization of sample quality and the observation of smallest structural distortions. On principle the available radiation intensity of the RED is significantly smaller than that of a synchrotron. Nevertheless measurements of several samples show that the flexibility of the RED yields scientific possibilities for structural investigations at the home laboratory which are normaly available only at synchrotron facilities. The second part of this work describes structural investigations with neutron diffraction of strontium doped lanthanium cuprate La(2-x)Sr(x)CuO4 (LSCO), a high-temperature-superconductor (HTSC). LSCO is a layered perovskite which undergoes a structural transition from a high-temperature tetragonal phase (HTT) to a low temperature orthorhombic phase (LTO) up to x=0.21. In the LTO-phase the CuO6 octahedrons are tilted in an ordered and well defined manner against the CuO4-planes. Up to now, the question of the possible coupling between superconductivity and lattice structure of the HTSC could not be finally answered. As a result of this experts are highly interested in the recent conclusions of different working groups using local methods on LSCO with x=0.15 (max. Tc=38 K). They deviate from their results partially strong deviations of the local structure from the mean structure. Diffraction with fast neutrons enables the accurate distinction of the mean square displacements (msd) of the atoms. This allows clear statements about the upper limits of the local deviations from the mean structure, which gives the opportunity to check the different postulated models.Therefore, a set of neutron diffraction experiments was performed during this thesis on LSCO samples (x=0.15) at different temperatures above and below their structural phase transitions. The results show an enlargement of the msd of the oxygens into the tilt directions of the CuO6-octahedrons which is independent from temperature. This yields small (

The enhancement of the emission rate of charge carriers from deep-level defects in electric field is routinely used to determine the charge state of the defects. However, only a limited number of defects can be satisfactorily described by the Poole-Frenkel theory. An electric field dependence different from that expected from the Poole-Frenkel theory has been repeatedly reported in the literature, and no unambiguous identification of the charge state of the defect could be made. In this article, the electric field dependencies of emission of carriers from DX centers in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}:\mathrm{T}\mathrm{e},$ Cu pairs in silicon, and Ge:Hg have been studied applying static and terahertz electric fields, and analyzed by using the models of Poole-Frenkel and phonon assisted tunneling. It is shown that phonon assisted tunneling and Poole-Frenkel emission are two competitive mechanisms of enhancement of emission of carriers, and their relative contribution is determined by the charge state of the defect and by the electric-field strength. At high-electric field strengths carrier emission is dominated by tunneling independently of the charge state of the impurity. For neutral impurities, where Poole-Frenkel lowering of the emission barrier does not occur, the phonon assisted tunneling model describes well the experimental data also in the low-field region. For charged impurities the transition from phonon assisted tunneling at high fields to Poole-Frenkel effect at low fields can be traced back. It is suggested that the Poole-Frenkel and tunneling models can be distinguished by plotting logarithm of the emission rate against the square root or against the square of the electric field, respectively. This analysis enables one to unambiguously determine the charge state of a deep-level defect.

Topic of this work is the analysis and improvement of the sensitivity of high-Tcsuperconducting rf-SQUIDs for operation in unshielded environment. The increase ofthe low-frequency noise due vortex motion in an applied magnetic field leads to adrastic reduction of sensibility of the sensors, which affects possible application ofSQUIDs. Aim of this work was the reduction of the vortex motion by simplemeasures, i. e. specially positioned micrometer holes (antidots), in order to achieve afield independent noise performance of the SQUIDs. This work can be divided intothree major parts: 1. The penetration and motion of vortices in rf-Washer SQUIDs with applied magneticfield has been examined and compared with existing models of the surface barrierand the thermal activated vortex motion. 2. Concepts for the reduction of the low frequency noise due to vortex motion havebeen developed. These are based on the pinning of vortices by artificial defectsespecially micrometer holes, so called antidots. 3. The noise reduction due to vortex pinning by antidots in rf-Washer SQUIDs in anapplied magnetic field has been experimentally demonstrated in field-cooled andzero-field cooled measurements. In the frame of this work the connections between the penetration and motion ofvortices into and in the washer of the SQUID on one hand and the low frequencynoise on the other hand are proved and illuminated by different measurements. Thepenetration and motion of single vortices into and in the washer of the SQUIDs hasbeen detected. The reduction of the low frequency noise in applied magnetic fields tothe zero field level is realized by special arrangements of antidots. A shift of the onsetof the increase of the low frequency noise from 8$\mu$T for the SQUIDs without antidotsto 40$\mu$T in field-cooled measurements and to 25$\mu$T in zero-field cooledmeasurements with SQUIDs with antidots is achieved. The developed measurementtechniques are well suited for the further examination of the properties of vortices inrf-Washer SQUIDs and the vortex antidot interaction.

An analysis is done of the ionization of deep impurity centers by high-intensity terahertz radiation, with photon energies tens of times lower than the impurity ionization energy. Under these conditions, ionization can be described as direct tunneling and phonon-assisted tunneling in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field of the radiation. Within a broad range of intensity, frequency, and temperature, the terahertz electric field of the radiation acts like a static field. For very high frequencies and low temperatures an enhancement of tunneling as compared to static fields was observed. The transition between the quasi-static and the high frequency regime is determined by the tunneling time. For the case of deep impurities this is the time of redistribution of the defect vibrational system which depends strongly on temperature and the impurity structure.

In this work a model of the vortex-matter in strongly anisotropic cuprate high temperaturesuperconductors based on their specific structural and electronic properties is presented.After a brief introduction of the crystal growth techniques and the characterisation methods ofthe investigated Bi_2Sr_2CaCu_2O_(8+delta) (Bi-2212) single crystals, the major part of thiswork is focussed on vortex-matter investigations and the corresponding experimental results.Information on the vortex dynamics and pinning properties of the vortex-state has beenobtained from magnetisation measurements. Four different vortex-phases have beenidentified as a function of temperature and external field. From muon-spin rotation experiments,the underlying vortex-structures (flux-line lattice, quasi-2D vortex-glass, flux-line liquidand pancake liquid) have been successfully determined for the first time. Furthermore, anuniversal H-T phase diagram of the vortex-matter in Bi-2212 has been obtained by scaling themagnetic field with a characteristic field H*. Such a scaling behaviour strongly suggests that thestatic and dynamic properties of the vortex-matter are mainly driven by the anisotropy of thecuprate high temperature superconductors and the pinning strength of the pancake vortices.Finally, the characteristics of magnetic flux-front penetration has been investigated as afunction of temperature by magnetio-optic experiments. The various vortex-phases heredisplay strongly different flux-front mobility.

New Insulating Materials for High-Tc Superconductor Device Applications Most of high-Tc superconductor devices contain at least one interface between YBa2Cu307-x and a non-superconducting material. Due to the short coherence lengt:h of high-Tc compounds the electrica1 properties of these devices are sensitive to the quality of the interface. In order to avoid a suppression of the order parameter near the interface the selected non-superconducting material should have a high degree of structural and chemical...

From scientific point of view as weIl as with regard to possible applications in superconducting electronics it is interesting to combine quantum effects and ballistic transport in semiconductors with the superconducting state. Due to the high mobility and long de Broglie wave length of the charge carriers, two-dimensional electron gases (2DEG) are suitable for observing the mentioned effects. Therefore, in this work basic investigations of single superconductor/2DEG contacts are presented. The high-mobility 2DEG used in this work forms at the interface of a GalnAs/InP heterostructure. As contact materials the superconductors Nb and NbN as well as the normal meta.ls Au and Co are useel. In order to prepare elirect contacts to the buried 2DEG, the heterostructure has been etcheel uncovering the 2DEG at the mesa. In this way contacts of the mentioned materials to the 2DEG have been prepared under the same optimized conditions. They are compared with respect to their normal state contact resistances. Additionally, Nb/GaInAs/2DEG contacts using GalnAs as an intermediate layer are investigated. It turns out that argon ion bombardment used in the preparation process alters the conductivity of GaInAs in a surface layer. In addition to these mainly techonlogical investigations, measurements of the normal state resistance of NbN/2DEG contacts in a magnetic field are presented. If the magnetic field is orientated perpendicular to the plane of the 2DEG, the normal state contact resistance drops dramatically with increasing magnetic field. This decrease is explained in terms of a model which assumes ballistic transport in the 2DEG. Having this model in hand, transport near the interface as well as the properties of the contact can be characterized in more detail. A new effect in the differential resistance of NbN /2DEG contacts in magnetic fields perpendicular to the plane of the 2DEG has been observed. This effect manifests itself in a splitting of the usual resistance maximum into a three-fold structure at voltages lower than the superconducting gap voltage. It qualitatively can be explained by a model which is an extension of the aforementioned model to the case of superconducting contacts. In this extension Andreev reflection as well as inelastic scattering of ballistic electrons is taken into account. The superposition of the different energy dependencies of these processes leads to the observed splitting of the subgap resistance maximum.

GaAs crystals have been grown by the vertical Bridgman technique and have been analysed by various methods, particularly by near infrared microscopy. The main objective of the present work was to study defect zones which form in doped GaAs around dislocations as a result of reactions between impurities and dislocations. A new model for the formation of defect zones is proposed and experimental results are presented which provide strong evidence for the defect model. For this purpose crystals were doped with different impurities and the zone formation was also studied for different stoichiometric conditions. The new defect model assumes that impurities occupying the As sublattice exchange their substitutional position with As interstitials. This process provides fast diffusing interstitial impurities which can be gettered (probably by forming precipitates) at the dislocation. As a result a diluted defect zone is generated around dislocations. The zone becomes visible by NIR phase contrast through the change of the refractive index in the zone. This change in turn is due to the variation of the free carrier concentration which is proportional to the impurity concentration. According to our model we expect only those impurities to contribute to the formation of the defect zone which occupy As lattice sites (e. g. B$_{AS}$, Si$_{AS}$, Ge$_{AS}$, S$_{AS}$, Se$_{AS}$, Te$_{AS}$) but not those on Ga lattice sites (Zn$_{Ga}$, Sn$_{Ga}$, Si$_{Ga}$, Ge$_{Ga}$). This is confirmed by the experimental results. Also confirmed is the expected change in the carrier concentration in the defect zone which depends on the electrical character of the impurities, i. e. $\Delta$n$_{e}$ < 0 for S$_{AS}$, Se$_{AS}$ and Te$_{AS}$ (donors) and $\Delta$n$_{e}$ > 0 for Ge$_{AS}$, B$_{AS}$ and Si$_{AS}$ (acceptors). Since the diffusion mechanism is initiated by As interstitials the formation of defect zones is found to be strongly suppressed if the crystals are grown from a Ga-rich melt (S, Se, Te and Ge-doped GaAs). The dilution of impurities occupying As lattice sites within the defect zone is also confirmed by the study of photoluminescence spectra of Ge and Si-doped GaAs. [...]

In this work the transport properties of SNS Josephson junctions fabricated by local oxygen implantation at 200 keV in YBa$_{2}$Cu$_{3}$O$_{7_x}$ are investigated. A sub-$\mu$m masking technology was used to restrict the spatial extent of the implanted region in the lateral direction to 50 nm. It is demonstrated for the first time that this technique could be used to fabricate Josephson junctions with homogeneous barrier properties. The improved junction properties are attributed to a post-implantation annealing step at 500°C for 10 min. in an oxygen atmosphere. A detailed study of the electrical transport properties and the crystal structure of oxygen implanted YBa$_{2}$Cu$_{3}$O$_{7_x}$ thin films serves as a basis for the fabrication process of the Josephson junctions. It is found that the irradiation of YBa$_{2}$Cu$_{3}$O$_{7_x}$ with oxygen at an energy of 200 keV leads to a linear decrease of the critical temperature T$_{c}$ and a quadratic increase of the specific resistance $\rho$ with increasing implantation dose $\Phi$. Annealing the implanted samples at 500°C for 10 min. in an oxygen atmosphere leads to an increase of the transition temperature and a decrease of the specific resistance but the initial values are not restored. This is due to the fact that annealing the implanted films at 500°C leads only to a partial reordering of the oxygen sublattice but does not influence defects formed on cation lattice sites. The analysis of implanted (spatial restricted by using a sub-$\mu$m mask) and subsequently annealed YBa$_{2}$Cu$_{3}$O$_{7_x}$ microbridges revealed that the implanted region has a reduced but finite transition temperature, allowing Josephson coupling in a temperature window of 0 K of the barrier. The critical current I$_{c}$ of the junctions increases quasi exponentially with decreasing temperature. It is also demonstrated that the effective length L of the barrier region is temperature dependent (L $\propto$ (T - T$_{cN'}$)) and that this dominates the R$_{N}$(T) behaviour rather than the temperature dependence of the resistivity. Taking this into account it is shown that the I$_{c}$(T) behaviour can be very well described by conventional SNS proximity effect theory.