## Electronic Theses and Dissertations

8-2020

#### Document Type

Doctoral Dissertation

Ph. D.

#### Department

Physics and Astronomy

Physics, PhD

Freelon, Byron

#### Committee Member

Sumanasekera, Gamini

Yu, Ming

#### Committee Member

Ramezanipour, Farshid

#### Author's Keywords

FeSC; mott insulators; 2D materials; XRD; NPD; topological materials

#### Abstract

Current theories of high-temperature superconductivity suggest that electrons must organize into Cooper pairs in order for a material to exhibit a superconducting phase. Electrons in insulators experience significant repulsive interactions that tend to keep electrons localized at atomic positions. In contrast, electrons in metals are delocalized, interact weakly, and are free to conduct electricity. Therefore, the formation of Cooper pairs should have different mechanisms for metals compared to insulators. This contrast raises the debate about the origin of high-temperature superconductivity in iron-based material, whether it depends on the strong or weak coupling. Many iron-based materials are metallic in the normal phase; however, before entering the superconducting phase, iron-based superconductors are believed to harbor insulating characteristics in close proximity to a Mott insulator. Furthermore, because superconductivity in iron-based materials occurs the border of correlation-induced electronic order, it is crucial to understand the nature of the ordered states. The newly reported iron oxychalcogenide Ca2O2Fe2.6OS2 is an antiferromagnetic (AFM) insulator at room temperature. Oxychalcogenides are structurally similar to the iron-based superconductors and it is possible to tune the Fe-Fe ion distance to drive the material from an insulating to a metallic phase. It is unexpected that a decrease in the Fe-Fe ion distance for Ca2O2Fe2.6OS2 results in enhanced insulating properties instead of making the material more metallic. This violates the predictions of the well-established electron band theory. The first aim in this work was to examine the novel Mott insulator Ca2O2Fe2.6OS2, crystal structure, and the effect of selenium doping on the material. X-ray powder diffraction (XRD) and Rietveld analysis were used to study the crystal structure. Also, neutron powder diffraction was used to study the magnetic peak intensity behavior with changes in temperature. Transport measurements were performed on both samples and activation energies (E$_a$) was calculated as 0.0694 eV and 0.06098 eV for Ca2O2Fe2.6OS2 and Ca2O2Fe2.6OS1.75Se0.25 respectively. The Rietveld fits confirmed that the material had tetragonal crystal system with space group $P_4/mmm$ for both samples. The calculated $\beta$ showed that this Mott insulator has the two dimensional Ising model. The volume of crystal increased with decreasing temperature while the atomic site occupancy increased. Also, the doping did not affect the crystal structure, however it suppressed the magnetic behaviour in Ca2O2Fe2.6OS1.75Se0.25. The second aim of this work was to study the structure of iron oxychalcogenides La2O2Fe2O(S, Se)2, compare the short-range to the average structure and understand the short-range behavior in this type of Mott insulators. Neutron powder diffraction (NPD) was used to study the short and average structure of La2O2Fe2O(S, Se)2. The obtained NPD data were analyzed using Rietveld analysis and pair distribution function (PDF) method. We observe the presence of fluctuating nematic ordering from low temperature to room temperature. High values of the isotropic thermal parameter (U33) for the oxygen atom O2 along c-axis was observed. The results also suggested the presence of short-range local distortion in both La2O2Fe2O(S, Se)2 materials which can be evidence of nematic behavior in the short-range of these materials. The last part of this research investigated the transition metal dichalcogenides (TMDs). TMDs are receiving a large amount of attention because they have been posited as a potential successor to silicon in the future of electronics manufacturing. The fabrication of large TMD crystals is currently an active area of industrial work. Most TMDs are low dimensional materials in the sense that their structures consist of stacks of hexagonally atomic layers of transition metal atoms or chalcogens sandwiched by transition metal (TM) layers. In addition to this, TMD electronic properties are primarily contained in the 2D TM planes. The last aim of this research was to study the transition metal dichalcogenide MoTe2 in two of its three stable phases (2H-MoTe2 and 1T'-MoTe2). Also, we studied the Td phase, which results from the phase transition of 1T'-MoTe2 at low temperatures. Powder diffraction techniques were implemented to study the detail of the crystal structure of bulk phases. Rietveld analysis was used to analyze the powder diffraction data of all MoTe2 phases. Electron dispersion x-ray spectroscopy (EDX) and Raman spectroscopy were performed on both samples to check the purity of the samples. The EDX and Raman spectroscopy showed that the samples were clear from impurities. The Rietveld analysis determined the crystal structure of 2H-MoTe2, 1T'-MoTe2 and Td-MoTe2 were hexagonal P63/mmc, monoclinic P21/m, and orthorhombic Pmn21, respectively. Our results showed that temperature affected the atomic site occupancy in the studied MoTe2 phases, which might be related to the stacking faults in MoTe2 layers.

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