The following treatise is a collection of three experimental reports, detailing measurements made over the last several years on the magnetic properties of specific correlated electron systems. Each of these systems is an unconventional superconductor at low temperatures, but in each the metallic state from which the superconductivity condenses is poorly understood. The experiments presented will focus on temperatures greater than the superconducting transition temperature, and in particular on magnetic properties of the normal state, which are thought to be important. Original work is contained in Chapters 3, 4 and 5. Chapter 3 describes our search for the presence of time-reversal symmetry breaking in the pseudo-gap state of La2-xSrxCuO4 with zero-field μSR, and is largely based on previously published data. Additional data on the related systems La(1.875)Ba(0.125)CuO(4) and HgBa(2)CuO(4+δ) are also presented. Based on this data, we put strict upper limits on any time-reversal symmetry breaking field which can be associated with the pseudo-gap, and show that the current interpretation of recent neutron scattering results in the literature cannot be correct. Chapter 4 summarizes our explorations of overdoped La(2)-(x)Sr(x)CuO(4) in applied magnetic field with transverse-field μSR. We see an unconventional broadening of the local magnetic field distribution in response to applied field, and discuss possible interpretations. This chapter has also been prepared for publication. Chapter 5 describes measurements of the non-linear magnetic susceptibility of URu(2)Si(2) as a function of temperature and hydrostatic pressure. By examining the temperature dependence, we draw conclusions about the existence of the anti-ferromagnetism and 'hidden order' at each pressure, and construct a preliminary pressure-temperature phase diagram. / Thesis / Doctor of Philosophy (PhD)

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<p>Yb2Ti2O7 is one member of a series of magnetic compounds with the pyrochlore lattice structure. For specific types of single-ion anisotropy and exchange interactions, the geometry of the pyrochlore lattice frustrates near-neighbor interactions and coaxes a wide variety of unusual magnetic ground states from such compounds. Yb2Ti2O7 is unique among these compounds in that the source of the frustration is not immediately obvious when one considers the combination of single-ion anisotropy (XY-like) and the nature of the exchange interactions (ferromagnetic) present therein. A conventional magnetic transition was indeed initially expected based on the observation of specific heat anomaly near 200mK. However, many studies produced no signs of long-range magnetic order below this temperature. Intriguingly, above the transition, evidence for unusual two-dimensional correlations came in the form of rods of magnetic diffuse neutron scattering. This thesis contains four articles that detail the results of several neutron scattering studies on Yb2Ti2O7. The goal of these studies was to determine the nature of the static and dynamic spin correlations throughout the magnetic field vs. temperature phase diagram of Yb2Ti2O7.</p> <p>We first performed a time-of-flight neutron scattering experiment on a single crystal of Yb2Ti2O7, which we prepared using the optical floating zone method. This initial study provided a comprehensive survey of the phase diagram, including the previously unexplored response to a magnetic field. We found that the rods of diffuse scattering change qualitatively upon cooling below the temperature of the reported specific heat anomaly, showing signs for the development of short-range three-dimensional correlations. Additionally, we discovered that a relatively small magnetic field applied along the [110] direction could remove the diffuse scattering entirely, and produce sharp spin wave excitations in the inelastic channel, indicating long range spin correlations.</p> <p>We further quantified the temperature dependence of the diffuse scattering in zero-field using a triple-axis neutron spectrometer. The crossover from two-dimensional correlations to short-range three-dimensional correlations was found to begin at 400mK and reach completion near the temperature of the specific heat anomaly, ∼200mK. Our measurements of the low temperature specific heat of several single crystal samples, as well as a powder sample, revealed that significant sample-dependence of the magnetic properties exists. The single crystal samples were shown to have broader features in the specific heat at relatively low temperatures compared to the powder samples, pointing to some amount of structural disorder in the single crystals.</p> <p>To understand the nature of the structural defects in the single crystals, we compared the structure of a crushed single crystal of Yb2Ti2O7 to that of a powder sample using neutron powder diffraction. The major conclusion of that work was that the single crystal is non-stoichiometric, containing 2.3% excess ytterbium on the (non-magnetic) titanium sublattice. The introduction of additional magnetic moments into the system is expected to be the cause of the sample-dependence of the specific heat anomaly.</p> <p>Finally, we fit the spin wave dispersions in the field-polarized state, as measured by time-of-flight inelastic neutron scattering, to an effective spin-1/2 anisotropic exchange Hamiltonian. The microscopic parameters extracted from these fits place Yb2Ti2O7 close to exotic Quantum Spin Liquid phases predicted for the anisotropic spin-1/2 pryochlore model. The exchange parameters also reveal that the source of the frustration in Yb2Ti2O7 comes from the “quantum spin ice” nature of its exchange interactions.</p> / Doctor of Philosophy (PhD)

<p>Biological membranes play host to a number of processes essential for cellular function and are the most important biological interface. The structurally complex and highly dynamic nature of the membrane poses significant measurement challenges, requiring an experimental technique capable of accessing very short, nanometer length scales, and fast, micro-pico second time scales.</p> <p>The experimental work presented in this thesis uses a variety of neutron scattering techniques to study the structure and dynamics of biologically relevant model membrane systems. The main body of this work can be sub-divided into two distinct topics: (1) lateral diffusion of lipid molecules in a bilayer; and (2) the measurement of domains in the membrane.</p> <p>Diffusion is the fundamental mechanism for lipids and proteins to move throughout the lipid matrix of a biological membrane. Despite a strong effort to model lipid diffusion, there is still no coherent model which describes the motion of lipid molecules from less than a lipid-lipid distance to macroscopic length scales. The experiments presented on this topic attempt to extend the range over which diffusion is typically measured by neutron scattering, to initiate the development of a more complete lipid diffusion model.</p> <p>Lipid domains and rafts are thought be platforms for many cellular functions; however, their small size and transient nature makes them notoriously difficult to observe. The penultimate chapter of this thesis provides evidence supporting the existence of domains in a model lipid/cholesterol system by probing of the dynamics of the system. The challenge of observing these structures directly was addressed by modifying the traditional neutron triple-axis spectrometry setup to increase its sensitivity to systems with short-range order. This technique was employed to examine the coexistence of fluid and gel domains in a single-component lipid bilayer system, as well as the presence of highly ordered lipid domains in a model membrane containing cholesterol.</p> / Doctor of Philosophy (PhD)

The rare earth pyrochlore oxides, with formula R2B2O7, are a chemically versatile family of materials that exhibit a diverse array of magnetic phenomena. In this structure the R and B site cations each form a corner-sharing tetrahedral network, a motif that is prone to intense geometric magnetic frustration. As a consequence of their magnetic frustration, rare earth pyrochlores are observed to host a number of remarkable states such as spin ice and spin liquid states. In this thesis we endeavor to explore the phase diagrams of the rare earth pyrochlores through the lens of chemical pressure. Chemical pressure is applied by varying the ionic radius of the non-magnetic B site cation, which either expands or contracts the lattice, in analogy to externally applied pressure. We apply positive chemical pressure by substituting germanium at the B site and negative chemical pressure by substituting lead at the B site. We also consider the effect of platinum substitution, which has nominally negligible chemical pressure effects. In the ytterbium pyrochlores, we find that positive chemical pressure tunes the magnetic ground state from ferromagnetic to antiferromagnetic. Remarkably, we also find that the ytterbium pyrochlores share a ubiquitous form to their low temperature spin dynamics despite their disparate ordered states. In the terbium pyrochlores, we find that positive chemical pressure promotes ferromagnetic correlations - the opposite effect of externally applied pressure. Our studies of platinum pyrochlores reveal that platinum, while non-magnetic, is able to facilitate superexchange pathways. Thus, the magnetic ground states of the platinum pyrochlores are significantly altered from their titanate analogs. The work in this thesis highlights the delicate balance of interactions inherent to rare earth pyrochlore magnetism and shows that chemical pressure is a powerful tool for navigating their phase spaces. / Thesis / Doctor of Philosophy (PhD) / Rare earth pyrochlores have the chemical formula R2B2O7, where R is a magnetic rare earth element and B is a non-magnetic element. Materials of this type are widely studied because they have a propensity to exhibit exotic magnetic properties. In this thesis, we study the effect of varying the size of the non-magnetic B site atom, which is termed chemical pressure. As B is made larger or smaller, the crystal lattice expands or contracts, mimicking the effect of externally applied pressure. High-pressure synthesis techniques were used to prepare R2B2O7 compounds with B site cations that are typically too small (germanium), too large (lead), or too unstable (platinum) under ambient pressure conditions. Our characterizations of these high-pressure materials have revealed that their magnetism is remarkably sensitive to the application of chemical pressure.

Selective dissolution also known as dealloying is a corrosion process in which one component of a binary alloy system is selectively removed through an electrochemically controlled process which leads to the formation of a porous metal "sponge" with a porosity that is completely interconnected and random in direction. Nanoporous metals are desirable since they have larger surface areas than an equal volume of non-porous material. Because of their enormous surface area per volume, these highly porous metal electrodes are superior materials for high surface area applications such as in biomedical devices, microfilters and catalysts. Understanding the kinetic processes governing the development of porosity during dealloying and having ability to change the electrochemical conditions will allow us to better control over the average ligament size and distribution in porosity. The basic kinetic processes involved in the formation of these structures are related to such issues as environmental effects and electrochemical conditions on diffusion, microscopic coarsening phenomenon at room temperature and elevated temperatures, alloy passivation, and Gibbs-Thomson effects. The average pore size and distribution was found to depend on the electrolyte composition, dealloying rate, applied potential and time. The porosity was found to significantly coarsen at room temperature during the dealloying process and this coarsening was highly dependent on the applied potential. It is showed that the commonly accepted measurement of the critical potential for alloy dissolution calculated based on extrapolation of anodic polarization data results in an overestimation of this quantity. A series of constant applied potential experiments prove to be a more accurate method for critical potential determination. / Ph. D.

Small-angle neutron scattering (SANS) is a very powerful laboratory technique for micro structure research which is similar to the small angle X-ray scattering (SAXS) and light scattering for microstructure investigations in various materials. In small-angle neutron scattering (SANS) technique, the neutrons are elastically scattered by changes of refractive index on a nanometer scale inside the sample through the interaction with the nuclei of the atoms present in the sample. Because the nuclei of all atoms are compact and of comparable size, neutrons are capable of interacting strongly with all atoms. This is in contrast to X-ray techniques where the X-rays interact weakly with hydrogen, the most abundant element in most samples. The SANS refractive index is directly related to the scattering length density and is a measure of the strength of the interaction of a neutron wave with a given nucleus. It can probe inhomogeneities in the nanometer scale from 1nm to 1000nm. Since the SANS technique probes the length scale in a very useful range, this technique provides valuable information over a wide variety of scientific and technological applications, including chemical aggregation, defects in materials, surfactants, colloids, ferromagnetic correlations in magnetism, alloy segregation, polymers, proteins, biological membranes, viruses, ribosome and macromolecules. Quoting the Nobel committee, when awarding the prize to C. Shull and B. Brockhouse in 1994: “Neutrons tell you where the atoms are and what the atoms do”. At NIST, there is a single beam of neutrons generated from either reactor or pulsed neutron source and selected by velocity selector. The beam passes through a neutron guide then scattered by the sample. After the sample chamber, there are 2D gas detectors to collect the elastic scattering information. SANS usually uses collimation of the neutron beam to determine the scattering angle of a neutron, which results in an even lower signal-to-noise ratio for data that contains information on the properties of a sample. We can analyze the data acquisition from the detectors and get the information on size, shape, etc. This is why we choose SANS as our research tool. The world’s top energy problems are security concerns, climate concerns and environmental concerns. So far, oil (37%) is still the No.1 fuel in world energy consumption (Oil 37%, Coal 25%, Bio-fuels 0.2%, Gas 23%, Nuclear 6%, Biomass 4%, Hydro 3%, Solar heat 0.5%, Wind 0.3%, Geothermal 0.2% and Solar photovoltaic 0.04%). Even more and more alternative energy: bio-fuels, nuclear and solar energy will be used in the future, but nuclear energy has a major safety issue after the Japanese Fukushima I nuclear accidents, and other energies contribute only a small percent. Thus, it is very important to improve the efficiency and reduce the population of petroleum products. There is probably one thing that we can all agree on: the world’s energy reserves are not unlimited. Even though it is limited, only 30% of the oil reserves is conventional oil, so in order to produce, transport, and refine of heavy crude oil without wasting huge amounts of energy, we need to reduce the viscosity without using high temperature stream heating or diluent; As more and more off-shore oil is exploited at that we need reduce the viscosity without increasing temperature. The whole petroleum consumed in U.S. in 2009 was 18.7 million barrels per day and 35% of all the energy we consumed. Diesel is one of the very important fossil fuel which is about 20% of petroleum consumed. Most of the world's oils are non-conventional, 15 % of heavy oil, 25 % of extra heavy oil, 30 % of the oil sands and bitumen, and the conventional oil reserves is only 30%. The oil sand is closely related to the heavy crude oil, the main difference being that oil sands generally do not flow at all. For efficient energy production and conservation, how to lower the liquated fuel and crude oil viscosity is a very important topic. Dr. Tao with his group at Temple University, using his electro or magnetic rheological viscosity theory has developed a new technology, which utilizes electric or magnetic fields to change the rheology of complex fluids to reduce the viscosity, while keeping the temperature unchanged. After we successfully reduced the viscosity of crude oil with field and investigated the microstructure changing in various crude oil samples with SANS, we have continued to reduce the viscosity of heavy crude oil, bunker diesel, ultra low sulfur diesel, bio-diesel and crude oil and ultra low temperature with electric field treatment. Our research group developed the viscosity electrorheology theory and investigated flow rate with laboratory and field pipeline. But we never visualize this aggregation. The small angle neutron scattering experiment has confirmed the theoretical prediction that a strong electric field induces the suspended nano-particles inside crude oil to aggregate into short chains along the field direction. This aggregation breaks the symmetry, making the viscosity anisotropic: along the field direction, the viscosity is significantly reduced. The experiment enables us to determine the induced chain size and shape, verifies that the electric field works for all kinds of crude oils, paraffin-based, asphalt-based, and mix-based. The basic physics of such field induced viscosity reduction is applicable to all kinds of suspensions. / Physics