<p>This thesis details a collection of experiments performed on two condensed matter systems, Co-doped BaFe<sub>2</sub>As<sub>2</sub> and URu<sub>2</sub>Si<sub>2</sub>. These two materials are related by their structural type (<em>ThCr<sub>2</sub>Si<sub>2</sub></em>-type) serving as great examples of the diversity of material properties present in this family. They are also both superconducting materials and belong to the collection of strongly-correlated electron systems. The interest in studying the Ba(Fe,Co)<sub>2</sub>As<sub>2</sub> group of materials is due to the high superconducting transition temperature in these (and related) materials, while the compound URu<sub>2</sub>Si<sub>2</sub> was studied due to the presence of a poorly-understood 'hidden order' phase.</p> <p>Muon spin relaxation/rotation/resonance (µSR) was used to measure several single crystals of the series Ba(Fe<sub>2-<em>x</em></sub>Co<em><sub>x</sub></em>)<sub>2</sub>As<sub>2</sub> with Cobalt concentrations <em>x </em>= 0.038, 0.047, 0.061, 0.074, 0.107 and 0.114, and a single crystal of Sr(Fe<sub>0.87</sub>Co<sub>0.13</sub>)<sub>2</sub>As<sub>2</sub>. The two samples with the lowest doping, <em>x </em>= 0.038 and <em>x </em>= 0.047, showed strong c-axis magnetism occurring below the magnetic transition, T<sub>SDW</sub>. The measurements suggest that the local magnetic field is increasingly disordered as the concentration of Co increases. These samples were shown to exhibit both superconductivity and magnetism, but that the entire sample contains non-zero local magnetic fields, meaning that superconductivity exists in or near regions of strong magnetic order.</p> <p>The remaining compounds (with <em>x </em>= 0.061, 0.074, 0.107, 0.114 and Sr(Fe<sub>0.87</sub>Co<sub>0.13</sub>)<sub>2</sub>As<sub>2</sub>) were measured with zero-field (ZF)-µSR and no magnetic ordering was found down to T = 1.65 K. An analytic Ginzburg-Landau model was used to fit the data and obtain absolute values for the penetration depth, λ. A model for the temperature dependence of the density of superconducting carriers, n<sub>s</sub> ≈ λ<sup>2</sup>, based on two <em>s</em>-wave gaps describes the data well. Below T<sub>SC</sub>, a paramagnetic frequency shift was observed indicative of field-induced magnetism along the c crystallographic direction.</p> <p>Measurements of URu<sub>2</sub>Si<sub>2</sub> under chemical and hydrostatic pressure have focused on measuring the spin correlations that are present in the hidden order phase. The chemical pressure that is induced by 5% Re doping perturbs, but does not destroy, the commensurate spin excitations. The spin gap that is present in the parent material is also present under this chemical doping. The hidden order phase survives at least halfway to the quantum critical point to ferromagnetism, but is weakened by the Re substitution.</p> <p>Under hydrostatic pressure of 10.1 kbar, URu<sub>2</sub>Si<sub>2</sub> becomes antiferromagnetic, but the spin correlations are found to be qualitatively similar to those of the hidden order phase. The width in reciprocal space (Q-width) of the excitations and their gapped nature remains unchanged upon entering the antiferromagnetic phase. Quantitatively, there is an increase in the magnitude of the gap at Q = (1.4 0 0). This may be a result of the increase in the transition temperature preceding the entry to the antiferromagnetic phase.</p> <p>Due to the large difference in their properties, and hence the motivation for studying Ba(Fe<sub>1-<em>x</em></sub>Co<em><sub>x</sub></em>)<sub>2</sub>As<sub>2</sub> and URu<sub>2</sub>Si<sub>2</sub>, they will be introduced and presented separately. Chapter 1 will provide the necessary background material on Ba(Fe,Co)<sub>2</sub>As<sub>2</sub>, while Chapter 2 will provide the background for the work on URu<sub>2</sub>Si<sub>2</sub>. Chapter 3 will describe the experimental techniques that were used to study these systems.</p> <p>Original research results on Ba(Fe,Co)<sub>2</sub>As<sub>2</sub> are presented in Chapter 4. This is mainly focused on µSR measurements of dopings that display superconductivity. Samples that did not order magnetically were measured in the mixed state to measure the vortex lattice to extract the various properties, including the superconducting pairing symmetry. Samples that did order magnetically were measured to analyze the amount of magnetic disorder and discover the extent of coexistence or phase separation between magnetism and superconductivity.</p> <p>Chapter 5 details the original research results on URu<sub>2</sub>Si<sub>2</sub>. This involved crystal growth of these compounds, and two neutron scattering experiments to measure the spin correlations while perturbing the hidden order state. The first experiment was done on a Re-doped crystal, URu<sub>1.9</sub>Re<sub>0.1</sub>Si<sub>2</sub>. Doping with Re suppresses the hidden order, eventually leading to ferromagnetism at higher dopings. This work showed that the spin correlations are also suppressed, but not as quickly as the hidden order. The second experiment was on pure URu<sub>2</sub>Si<sub>2</sub> under hydrostatic pressure. Applied pressure increases the hidden order transition, but eventually leads to antiferromagnetism, the phase in which the experiment was performed.</p> / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/13621 |
| Date | 04 1900 |
| Creators | Williams, Travis J. |
| Contributors | Luke, Graeme M., Physics and Astronomy |
| Source Sets | McMaster University |
| Detected Language | English |
| Type | thesis |
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