Ph.D. (Doctor of Philosophy)
Department of Physics
Since its discovery more than a century ago, the fascinating phenomenon of superconductivity remains as mysterious as ever with the greatest challenge facing the condensed matter community being the elusive understanding of the microscopic origin leading to its inception. Many outstanding questions remain of which the role played by nematic fluctuations and structural disorder for the stabilization of superconductivity is not well understood. In this respect, superconducting YBa2Cu3O6+x (YBCO)-type cuprates present an ideal framework in which charge doping, structural disorder and various site manipulations can be explored in efforts to address some of these questions. Extensive synthesis and characterization efforts of the properties of polycrystalline and single crystal cuprates were performed. Advanced x-ray scattering experiments were conducted at state-of-the-art beamlines at the Advanced Photon Source (APS). This work presents the results of a comprehensive study of many new compositions designed to explore the effects on superconductivity of a large number of chemical substitutions at the diverse sites of YBCO.
Superconducting YBCO-type cuprates exhibit many imperfections at their crystallographic sites in addition to a highly tunable oxygen content at several vacant sites available inthe so-called chain copper layer, thus, supporting various short-range oxygen ordering configurations that may or may not materialize depending on the overall composition of the investigated material. To this end, I investigated the structural properties of five ErBa2Cu3O6+x (EBCO) single crystals tuned to produce superconducting transition temperatures (TC) of 0, 22, 55, 75 and 92 K. Systematic three-dimensional reciprocal space mapping was performed at temperatures between 30 and 300 K. Analysis of the data showed modulated diffuse scattering intensities that arise from short-range local correlations between the Cu and O layers. The modulated intensities weaken with increased oxygen doping until they disappear for the optimally doped crystal with the highest TC. Our diffuse scattering data provide evidence for short range nematic order persisting in all the investigated compositions. Our data provide no evidence for charge density waves in EBCO in contrast to those observed in YBCO, LSCO or other related cuprates.
On the other hand, we identified some combinations of elements substituting at the Y and Ba sites that can be manipulated to produce not only the YBCO structure but also other structures with different tetragonal symmetries and unit cell dimensions. These (La, Sr, Ba)CuO3−δ materials were demonstrated to exhibit a great potential for exploitation in the form of air electrodes in building toxic-free solid oxide fuel cells (SOFC). A comparison with commonly used SOFC materials, results obtained with our samples demonstrate enhanced thermomechanical properties, high conductivity, and a better thermal and chemical stability leading to best performance observed for any copper-based cathode material with impressive power density capabilities.
Poudel, Bisham, "Superconducting Cuprates: Synthesis, Characterization and Diffuse Scattering Properties" (2021). Graduate Research Theses & Dissertations. 7561.
Northern Illinois University
Rights Statement 2
NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.