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PhD Thesis (Muroran Institute of Technology)

Abstract: Superconductors are of high interest in many fields besides physics, due to their potential energy saving nature. Hence, they exhibit zero resistance below a criti- cal temperature TC. Possible applications for superconductors are strong electro magnets, SQUIDs, loss-less electric wires, Quantum Computers (e.g. dwave), etc. The biggest problem in applying superconductors more widely lies in the still very low TC. However, increasing TC strongly requires researching superconduct- ing materials on the energy and nanoscale regime. Currently, the superconduc- tors with the highest TC under ambient pressure are Mercury (Hg)-based cuprate superconductors, whose TC is about 150 Kelvin. The Bi-based cuprate supercon- ductors researched at our lab, with a similar crystal structure to the Hg-based cuprates also show a relatively high TC (∼ 94K) and are environment-friendly due to the absence of Hg. Bi-based cuprate superconductors have intensively been studied concerning the origin of high TC superconductivity (SC) and in relation to improving TC. Impurity effects have the potential to reveal important informa- tion about the characteristics of the electronic properties, leading to clarifying the origin of superconductivity. Recently, impurity effects of Iron (Fe), which is sub- stituted for Copper (Cu) inside the Cu-O layer, attract much attention since they cause a large suppression of TC with an enhancement of the magnetic correlation similar to that of the so-called stripe order. In the present study, I have studied the effects of Fe on superconducting quasipar- ticles in Bi-based high-Tc cuprates by using Scanning Tunnelling Microscopy and Spectroscopy (STM/S), where the single crystals of which the samples were cut from have been grown using the floating zone method. The STM/S data of the Fe- doped and the Fe-free (pure) Bi-based cuprate superconductors were measured at around 8K. The spatial distribution of the LDOS spectra were measured over a field of view, covering more than ten thousand measurement points with a spa- tial resolution well below the atomic nearest neighbour distance. The STS spectra were analysed using a self-developed reduction framework. Using the indepen- dent algorithms, I have revealed and characterised distortions, produced by the convolution of instrumental and intrinsic perturbations and recovered the atomic, and electronic position information as well as impurity-sites in three dimensional image data. Based on the recovered STM/S data, I evaluated the influence of the quasiparticle lifetime broadening Γ on the energy gap properties such as ampli- tude, shape and spatial distribution. It was found that the energy gap amplitude ∆ and Γ largely depend on the spatial position and the local concentration of Fe, not only on the surface but also in the bulge of the sample. Further, by decoupling ∆ from the influence of Γ, it was shown that ∆ more strongly correlates with the topographic structure of the BiO layer than Γ, which correlates only weakly. (read more)

Master Thesis (TLS-Tautenburg, Jena University)

Abstract: The aim of the master thesis was to give insights into the radio properties and the internal dynamics of the galaxy cluster Abell 1240. The study focuses on the two shock fronts, seen as relics, originating from the cluster and propagating to the north and south. More specifically, the research aims at determining the spectral index and obtaining information about the polarisation properties of the relics. The presence of double relics on the periphery of the galaxy cluster Abell 1240 was confirmed in my thesis. The symmetry and the directional properties suggest a common origin for the southern and the northern relic. The obtained radio maps show signs of discrete sources inside the diffused emission. The existence of these discrete sources cannot be confirmed since they might be artifacts from the cleaning process, which assumes unresolved sources and might have build up those sources from the diffused emission. Indications have been found that the polarization of the northern relic is approximately ten percent.The remaining objects of the observed field (approx. 6° × 6°) in which Abell 1240 is located were also investigated. The spectral index as well as the flux of 439 mapped objects were determined. This flux (and therefore the spectral index and the degree of polarisation) of the sources are strongly direction dependent. This means the variations of the flux and phase values becomes larger as one moves farther away from the phase center of the Field. This is due to the decrease in the telescope’s sensitivity moving away from the phase center. The process of data reduction takes a big part in radio astronomy. Before starting the data reduction and analysis I wrote a small python script (py interferometer) to simulate the data gathering process. It helped me understand the workings of a radio interferometer. It made me aware of how physical quantities would behave and which possible difficulties could arise during the evaluation. The data was taken with the Westerborck Synthesis Radio Telescope (WSRT), which is located in the Netherlands. The WSRT is maintained and hosted by ASTRON. It also provides a data reduction pipeline called NEWSTAR to help produce useful information from the observations of the WSRT. NEWSTAR contains software especially designed to reduce data obtained only by the WSRT. I choose to use the more independent pipeline called CASA (the Common Astronomy Software Applications package), which is provided by the National Radio Astronomy Observatory (NRAO). The CASA software was primarily designed to work with VLA data but it also works with many other instruments such as the WSRT and LOFAR. However, not every function of the software can currently be used with the WSRT data, therefore I had to write a few scripts for some parts of the calibration and data analysis. The Thüringer Landessternwarte hosts its own Low Frequency Array (LOFAR) station. This is also a reason for the use of the CASA and it’s software packages. A data reduction and imaging pipeline, based on CASA and python, was created to reduce WSRT datasets. It was confirmed that the polarisation calibration can be done with CASA. (read more)

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