Preparation and characterization of planar hybrid Josephson junctions based on Ba-122 pnictide single crystals

In this work, it has been realized to fabricate and investigate of the Josephson junctions based on iron pnictide Ba(Fe_1-xCo_x)₂As₂ single crystals (Co:Ba-122) using established thin-film process. The designed layout was achieved for the first time worldwide which is a planar hybrid Josephson junctions. The c-axis planar junctions were achieved, in which were tested different barriers such as the intrinsic barrier, or metal barrier (gold) then were further developed to use the insulating barrier (titanium oxide, aluminum oxide). The high surface quality is required since it utilizes the procedure of the photolithographic preparation process used for thin film technologies. However, the single crystals materials have rough surface structure; therefore it was necessary to develop a new polishing method which made it possible to reduce the surface roughness of crystals to less than 5 nm. Other difficulties in the work were connected to the small dimensions of the sample 2 x 2 x 0.25 mm or even less after cleaving the single crystal, which allows preparing only one single junction on a crystal. Another problem is an influence of degradation can be caused by using water, chemical removal (like acetone), and oxidation in the air as well. The solutions for all these issues are mentioned, it can be found in chapter 4. There are other difficulties with the insulating layer between the pnictide electrodes and the lead counter electrodes, which are probably due to problems with the chemical removals and lithography planar mask. To avoid that it was attempted to making several insulating layers by using subsequent with different masks. Despite these difficulties, the planar Josephson junction on Ba-122 single crystals could be successfully achieved. The planar Josephson junction design allows measuring the electrical properties of these junctions like resistance on temperature on each electrode and the junctions itself as well as temperature dependent current-voltage characteristics and differential conductance vs. voltage. The planar hybrid Josephson junction design has been tested first on undoped single crystals. SN-tunneling junctions are formed for spectroscopic studies of these contacts with a gold barrier. Josephson effects observed in planar junctions between Co: Ba-122 pnictide materials and conventional s-wave superconductor counter electrodes. The investigation of Josephson junctions showed that the RCSJ-model can be used to describe the experimental I-V characteristics. However, there were excess currents at several sample types, which were included in the model. The determination of the I_cR_N-product gives information on the characteristic quality of the Josephson junctions. The I_cR_N-product of the Interface-TiO_x barrier could be realized up to 1.15 mV which is the highest value reported in literature up to now. The advantage of high values of those I_cR_N-products increased the possibilities for applications like superconducting quantum interference devices (SQUID). The quantitative study of the microwave response has been performed within this work. The measured microwave modulation could be described by the simulations, which suggest a Josephson nature of the excess current. The modulation of the critical Josephson current in the external magnetic field was investigated. However, this differs significantly from the expected Fraunhofer pattern, may be caused by the inhomogeneity of the barrier. Additionally, results from tunneling and Andreev spectroscopy i.e. temperature dependent dI/dV-V spectra are shown. Through the measured spectra of differential resistance, and calculated differential conductance, the energy gaps values were determined to about Δ₁= 2.4 meV, and Δ₂ = 8.4 meV in agreement with others results published in the literature. Summarizing, a new thin film technology was realized to use pnictide single crystals as an electrode of tunneling junctions. The measurements show a good quality of the junctions and will be helpful to explain the physics of the superconducting in the iron-based materials.