Search

The workshop titled "US Muon Workshop 2021: A road map for a future Muon Facility" was held virtually on February 1-2, 2021. The workshop aimed to bring together world experts in muon spectroscopy ($\mu$SR) and other techniques along with interested stakeholders to evaluate the scientific need to construct a new $\mu$SR facility in the United States (US). The more than 200 participants highlighted several key scientific areas for $\mu$SR research, including quantum materials, hydrogen chemistry, and battery materials, and how each area could benefit from a new, high flux pulsed muon source. Experts also discussed aspects of the $\mu$SR technique, such as low-energy $mu$SR, novel software developments, and beam and detector technologies that could enable revolutionary advances in $\mu$SR at a next-generation facility. The workshop concluded with discussion of a concept being developed for a new $\mu$SR facility at the Spallation Neutron Source (SNS) of Oak Ridge National Laboratory (ORNL). That novel design concept was first envisioned by many of the same $\mu$SR experts at a workshop held previously at ORNL in 2016. The participants expressed that the current design had the potential to be a world-leading $\mu$SR facility, and strongly encouraged the principal investigators to continue their work in order to refine the concept and determine instrument parameters that would enable new scientific opportunities.

We used transport and inelastic neutron scattering to study the optimally phosphorus-doped BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ superconductor ($T_c = 30$ K). In the normal state, we find that the previously reported linear temperature dependence of the resistivity below room temperature extends to $\sim$ 500 K. Our analysis of the temperature and energy ($E=\hbar��$) dependence of spin dynamical susceptibility at the antiferromagnetic (AF) ordering wave vector $��^{\prime\prime}({\bf Q}_{\rm AF},��)$ reveal an $��/ T$ scaling within $1.1, 4 figures

This thesis details a collection of experiments performed on two condensed matter systems, Co-doped BaFe2As2 and URu2Si2. These two materials are related by their structural type (ThCr2Si2-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)2As2 group of materials is due to the high superconducting transition temperature in these (and related) materials, while the compound URu2Si2 was studied due to the presence of a poorly-understood 'hidden order' phase. Muon spin relaxation/rotation/resonance (µSR) was used to measure several single crystals of the series Ba(Fe2-xCox)2As2 with Cobalt concentrations x = 0.038, 0.047, 0.061, 0.074, 0.107 and 0.114, and a single crystal of Sr(Fe0.87Co0.13)2As2. The two samples with the lowest doping, x = 0.038 and x = 0.047, showed strong c-axis magnetism occurring below the magnetic transition, TSDW. 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. The remaining compounds (with x = 0.061, 0.074, 0.107, 0.114 and Sr(Fe0.87Co0.13)2As2) 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, ns ≈ λ2, based on two s-wave gaps describes the data well. Below TSC, a paramagnetic frequency shift was observed indicative of field-induced magnetism along the c crystallographic direction. Measurements of URu2Si2 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. Under hydrostatic pressure of 10.1 kbar, URu2Si2 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. Due to the large difference in their properties, and hence the motivation for studying Ba(Fe1-xCox)2As2 and URu2Si2, they will be introduced and presented separately. Chapter 1 will provide the necessary background material on Ba(Fe,Co)2As2, while Chapter 2 will provide the background for the work on URu2Si2. Chapter 3 will describe the experimental techniques that were used to study these systems. Original research results on Ba(Fe,Co)2As2 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. Chapter 5 details the original research results on URu2Si2. 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, URu1.9Re0.1Si2. 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 URu2Si2 under hydrostatic pressure. Applied pressure increases the hidden order transition, but eventually leads to antiferromagnetism, the phase in which the experiment was performed. Doctor of Philosophy (PhD)

This thesis comprises studies on two ferromagnetic superconductors, URhGe (Tcurie=9.SK and Tsc=2S0mK) and UCoGe (Tcurie=2.SK and Tsc=800mK). These properties are interesting because the current theory to explain superconductivity predicts that ferromagnetism should destroy superconductivity. Not only is that not true in these materials, but ferromagnetism and superconductivity are thought to arise from a common mechanism. The studies conducted on these materials arise from that possibility, in an attempt to understand the unconventional nature of these materials.Original work is contained in chapters 4, Sand 6. All of this work is currently not published in sources other than this thesis. Chapter 1 will give an introduction to these materials, and the work that has been done on them by other groups, and work done on related materials. Chapter 2 will give details of the various experimental methods used in measuring the structure and properties of the materials studied. This work was conducted by the author at McMaster University, with the assistance of individuals from the Brockhouse Institute for Materials Research, and the Center for Electron Microscopy at McMaster University. Chapter 3 will provide an introduction to the technique of muon Spin Resonance/Relaxation (μSR). This work was done at the TRIUMF facility in Vancouver, British Columbia, with the assistance of several TRIUMF staff. The data was collected by the author, and other members of Dr. Luke's research group as well as collaborators from TRlUMF and from Columbia University. Chapter 4 will present the measurements made on UCoGe, while Chapter 5 presents the measurements of URhGe. Details of the crystal growth and structure characterization measurements are included in these chapters, along with resistivity, bulk magnetization and μSR measurements. Both zero- field (ZF) and transverse field (TF) μSR has been performed. This work focuses on studying the magnetic moment size, and the magnetic volume fraction around the ferromagnetic transition, and to temperatures as low as 20mK. Consideration is also given to the magnetic and superconducting properties in the low-temperature region. In the Introduction, URhGe is presented first, followed by UCoGe, since this was the order in which they were discovered. The results obtained from UCoGe are presented first, since work on that compound was started before the work on URhGe. Chapter 6 focuses on the conclusions drawn from this work, comparing the measurements of both materials. Master of Science (MS)