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The cuprate family La2–xSrxCuO4+y has been a focus of condensed matter research since the discovery of high-temperature superconductivity in related compounds almost four decades ago. Despite extensive study, the basis of superconductivity in these materials is not yet fully understood. This thesis presents a broad reciprocal-space investigation of oxygen ordering and related superstructures in superoxygenated La2–xSrxCuO4+y, combining neutron and X-ray diffraction measurements from several complementary instruments.
Reciprocal-space planes and volumes were mapped for multiple superoxygenated La2–xSrxCuO4+y single crystals with strontium doping values of x = 0.00, 0.06, and 0.09 at temperatures between 2 K and room temperature. Measurements included neutron diffraction on a powder diffractometer, reciprocal-space mapping using neutron time-of-flight and laboratory X-ray diffraction instruments, and triple-axis neutron scattering.
The measurements revealed a richer superstructure landscape than is usually captured by focusing on single modulations at a time. Five distinct categories of superstructures were identified in the x = 0.00 sample - staging, side-spots, chains, crosses, and flowers - and were observed consistently across complementary neutron and X-ray measurements. The staging, chains, crosses, and flowers appeared persistent across the full measured temperature range.
The superstructure content was found to be strongly doping-dependent: features became progressively weaker and more diffuse with increasing strontium content, with chains absent entirely for x = 0.06 and x = 0.09. The chains superstructure, observed exclusively in the x = 0.00 sample, does not correspond to previously reported superstructures in this family of materials, and its origin remains unclear. In addition, integration over extended reciprocal-space volumes further demonstrated that the cross superstructures possess a significant three-dimensional character. Finally, triple-axis neutron measurements confirmed that the x = 0.06 crystal remains in the Bmab structural phase down to 5 K, ruling out a low-temperature structural transition of the type observed in related compounds such as La2–xBaxCuO4.
Taken together, the results are consistent with a picture of oxygen ordering in superoxygenated La2–xSrxCuO4+y as a multi-scale phenomenon with coexisting spatially distinct ordering patterns. Such a picture has previously been suggested in the literature. The results further indicate that the interplay between these ordering patterns may be connected to the remarkably robust superconducting transition temperature of Tc ≈ 40 K observed across all investigated doping levels.
In parallel with the work on superstructures, the project also contributed to neutron scattering studies of magnetic correlations and spin fluctuations in the same material system, as well as to the development of online e-learning tools and virtual neutron scattering environments for student training.