Exploring the Unusual Flux Pinning behavior in Ba-122

High-temperature superconductors offer a considerable potential for applications. They may replace conventional superconductors in magnet applications thereby enabling higher magnetic fields or operation temperatures, which would save energy required for cooling and make these devices (e.g. medical MRI systems) more economical. In addition, new application for the generation and distribution of inexhaustible energy can be anticipated. Magnetic fields penetrate high-temperature superconductors in the form of so called vortices, which have to be pinned to enable loss free currents. Insufficient pinning of vortices is still one main hurdle for the application of high-temperature superconductors. While significant improvements have been obtained by optimizing the size, density, and morphology of pinning centers (crystallographic defects) during the past years, the proposed project aims at boosting the critical (i.e. loss free) current densities by charge doping. Recent results indicate that flux pinning is significantly enhanced in the vicinity of a quantum critical point, which is positioned at a certain doping level. These observations fuel the hope that quantum fluctuations can enhance flux pinning significantly in technical relevant superconductors. This hypothesis will be explored using Ba-122 samples with different doping. It is noteworthy, however, that quantum fluctuations also occur in other non- conventional superconductors such as the cuprates, which are currently most promising for applications of high-temperature superconductors. High quality single crystals of Co-, P-, and K-doped Ba-122 with a wide range of doping concentrations will be grown at the National Institute of Advanced Industrial Science and Technology (AIST, Japan). We will use two approaches to separate the influence of the defect structure from effects of quantum fluctuations. First, a benchmark defect structure will be introduced into the crystals by means of fast neutron irradiation at TU Wien, which overwrites the defect structure in the pristine crystals and thus can be considered as equivalent at all doping concentrations. Second, magnetization measurements will be performed under high pressure at AIST, which leave the defect structure of the pristine crystals unchanged, while shifting the doping concentration at which the quantum critical points occur. Finally the application potential of our findings will be explored by a comprehensive analysis of the field and temperature dependence of the critical current densities, which will be optimized by artificially introduced pinning centers (e.g. BZO-nanoparticles).

The most important property for applications of superconductivity is in most cases the loss free current transport. However, loss free currents are possible only up to the so-called critical current, which depends on both, the intrinsic properties of a certain superconductor and the prevailing defect structure in the crystal lattice. The critical currents in particular compounds of the iron-based superconductors (BaFe2As2) were investigated in this study. These materials become superconducting only after adding doping atoms and the critical current density has an unexpectedly sharp maximum at a certain doping concentration. The reason of the maximum was unclear but very important for the theoretical understanding as well as applications. It was explored in close collaboration with a group at AIST (National Institute of Advanced Industrial Science and Technology) in Tsukuba. Introducing defects by means of high-energy neutrons was a central method and performed at the research reactor in Vienna (Atominstitut). The introduced defects enhanced the critical currents significantly and the sharp maximum disappeared. This means that the sharp maximum does not arise from the intrinsic properties of the materials (which hardly change by the neutron irradiation) but from the particular defect structure at this doping level, where also a structural and magnetic phase transition occurs. The neutron-induced defect structure resulted in a universal dependence of the critical currents on the superconducting transition temperature (which defines the phase boundary of superconductivity), namely in form of a power law. Similar power laws have been reported for other intrinsic properties of unconventional superconductors. These power laws cannot be explained theoretically yet, as the mechanism of high-temperature superconductivity; thus, our findings contribute to the fundamental understanding of unconventional superconductivity and will be important for confirming future theories of high temperature superconductivity. In summary, the project resulted in valuable results for the fundamental understanding of unconventional superconductivity as well as the optimization of the properties of these materials for application in medicine and power engineering.