VictorJuiz/thesis huggingface dataset:
Superconductivity in UPt3

This thesis presents a theoretical study of superconductivity in UPt3 Hexagonal UPt3 is observed to have two superconducting transitions in zero applied field as temperature is reduced. This has established UPt3 as an unconventional super conductor (a superconductor in which the order parameter is not invariant under spatial operations). An established model (the original E-REP model) proposes that the superconducting order parameter has two components that are degenerate in hexagonal symmetry. This degeneracy is split by the presence of an antiferromagnetic moment (the moment direction is in the basal plane). This thesis demonstrates that the original E-REP model predicts a phase diagram that disagrees with the topology of a recent experimentally determined phase diagram. Two new formulations of the E-REP model are presented that can give rise to the observed phase diagram. A study of the observed six-fold anisotropy of the upper critical field with the field in the basal plane is also presented. It is shown that the E-REP models can account for the observed behavior provided that the antiferromagnetic structure in a magnetic field becomes single domain. Recent experimental results indicate this not to be the case, so that the E-REP models cannot account for the modulation. Surface superconductivity has been observed in UPt3 and has been found to behave differently than in conventional superconductors. This thesis initially presents an extension of the original analysis of surface superconductivity to include more general boundary conditions. A detailed microscopic calculation of the boundary conditions for general order parameter symmetries at specular and diffusive boundaries is then presented. These calculations are incorporated into a phenomenological model for UPt3 and the results are shown to account for the experimental observations. Further experiments on surface superconductivity are proposed that can potentially eliminate some phenomenological models of superconductivity in UPt3.

Advisor: Walker, Michael B.
In Vivo Neutron Activation Analysis and Associated Dosimetry

The use of neutron activation analysis in-vivo for the measurement of calcium in the human body has proved to be a more accurate and less hazardous technique than many other methods currently used. The health hazard associated with this technique is determined in this study by a method of neutron dosimetry based on the radiation damage in plastics caused by neutron induced fission fragments. The lower legs of a phantom were irradiated for 10 minutes with four 23Q 49 Pu-Be neutron sources immersed in water. The 3.1 MeV Ca gamma rays from the legs were counted by two 8" dia. by 4" Nal(Tl) scintillation detectors in the Whole Body Counter at the Toronto General Hospital. The activity was such that there was a statistical error of 2%. The reproducibility of the results was determined from a series of 10 irradiations and the standard deviation was found to be +/- 2.3%. The expected interference from the 3.1 MeV peak in 37S produced by the 37Cl(n,p)37S reaction was estimated to be about 2% and an experimental assessment of this interference showed that the contribution made by 37S to the 49Ca peak was about 4% . For the neutron dosimetry, the plastic damaged by the fission fragments was etched in a 6N solution of NaOH, and the resulting holes counted by an electrical sparking method. Experiments were carried out to find optimum processing procedures and to check the range of linearity of the system. It was found that 232th foils were adequate for determining the significant neutron dose, which was about 0.8 rem during the irradiation of the legs. If required, 235U foils may also be used to monitor thermal fluences. A comparison of fast neutron doses measured by this technique with a completely independent method at Brookhaven National Laboratories showed very good agreement.

Keywords: Physics
Advisor: McNeill, K. G.
Magnetic and transport properties of FeGe2

We have measured the electrical resistivity and the elastic and inelastic neutron scattering cross-sections of the itinerant antiferromagnets FeGe2 and Fe(Ge.96Ga.04)2. FeGe2 has a tetragonal structure with a collinear antiferromagnetic phase below TC-IC = 263 K, and an incommensurate spin density wave (SDW) phase between TC-IC and the Néel temperature (289 K). The ordering wavevector is along the a-axis and the moments are ferromagnetically aligned along the c-axis. We determined the phase diagram of FeGe2 for basal plane fields up to 16 T using resistivity measurements. The incommensurate phase is much more stable (up to ~30 T) in a magnetic field than theory predicts (1 T). Quantum oscillations appeared in the resistivity below 4 K for a c-axis magnetic field above 7 T. The resulting effective carrier mass is 0.45 ± .05 me, and the frequency is 190 ± 10 T. We used elastic neutron scattering to study the magnetic phases of Fe(Ge .96Ga.04)2. TC -IC = 183 K and this transition is clearly first-order. The low-temperature phase consists of a commensurate SDW with a long-wavelength modulation along the a-axis with weaker magnetic scattering reflections which are not displaced along the a-axis. Resistivity measurements show that the low-field commensurate phase is destroyed at temperatures below 20 K by a 12 T basal-plane magnetic field. Inelastic neutron scattering on FeGe2 at 11 K up to energy transfer (E = 540 meV shows magnon dispersion that is well described by a classical Heisenberg spin-wave model. The exchange constant along the c-axis, SJFM = 68 ± 5 meV, is 15 times higher than along the a-axis, ' SJAFM' = 4.4 ± 1.0 meV, making FeGe2 a unique example of a quasi-one-dimensional itinerant magnet. Up to 350 meV the c-axis magnon linewidth can be described by Γ( E) = (2.7 ± .6) * 10-3 MeV -1E2 (E2 dependence of a hydrodynamic model). We measure Γ values several orders of magnitude larger than localised models predict, presumably due to itinerant effects which also lead to a continuum of scattering above ~350 meV. At higher temperatures the peak height and lifetime of the c-axis magnons decrease, changing rapidly between TC-IC and TN. Magnon-like excitations persist to 600 or 700K as might be expected for a 1-D system without long-range order.

Keywords: Physics
Advisor: Mason, Thom
Interpretation of EM Sounding Data for a Stratified Earth by Means of Non-Linear Regression

Direct inversion methods for electromagnetic soundings have not been used widely for routine interpretation of field data, even though they provide more useful information to the interpreter than do indirect methods, because existing inversion routines require large amounts of computer time and are relatively difficult to use. In an attempt to make direct inverse methods accessible for routine use, a study was made of the various available methods and a new computer program was written. This program solves for conductivity on a layering specified by the user, and uses the Fast Hankel transform with Anderson’s (1979) filter coefficients, a quintic spline approximation, and improvements in the Marquardt-Fletcher nonlinear regression algorithm to reduce the computer time required for convergence. The routine was tested with numerically generated sounding data which includes some random noise, as well as with several sets of field data. The results indicate that the routine solves easily for simple structures with moderate (≤ 10:1) conductivity contrasts, while larger contrasts in the conductivity structure require some care in setting up the starting model. Resistive structures are not resolved well, but the depth to conductive features in the stratigraphy can be determined accurately. Results from the inversion of five sets of field data are given: they were found to compare well with geophysical information from well logs or with interpretations made by other means.

Keywords: Geophysics
Advisor: West, G. F.
A Short Baseline Transient Electromagnetic Method for Use on the Sea Floor

A towed electromagnetic system capable of mapping the electrical conductivity of the sea floor over a large area has many possible applications, including mapping Quaternary geology, and understanding the physical properties of midocean ridge hydrothermal systems in association with massive sulphide deposits. However, the electrical conductivity of seawater is usually much greater than that of the sea floor, rendering the majority of electromagnetic systems presently employed on land unsuitable for marine use. A theoretical study of the transient step-on responses of some common controlled source, electromagnetic systems to adjoining conductive half-spaces shows that for two systems, the horizontal, in-line, electric dipole-dipole (ERER) and horizontal, coaxial, magnetic dipole-dipole (HRHR), the position in time of the initial portion of the transient indicates the conductivity of the sea floor, while at distinctly later time, a second characteristic of the transient is a measure of the larger seawater conductivity. The diagnostic separation between the two parts of the transient response does not occur for many other systems, including several commonly used for exploration on land. The concept of apparent conductivity is defined for the transient system in terms of the time of the first arrival of the transient signal. This apparent conductivity is used to produce characteristic curves of the HRHR system response for different layered earth models. A prototype HRHR system operating on a 100 m scale has been designed and constructed. A successful test of the system in shallow water was conducted in the coastal waters of Vancouver Island. The survey yielded 37 conductivity measurements along three lines. The instruments were towed by a ship along the sea floor. The tow cable carried both current to the transmitter coil and the received signal back to the ship for processing. Both the shape and amplitude of the received signal are indicative of the conductivity of the bottom sediments. Inversion of the data suggest that a varying thickness of 1.2 S·m-1mud overlies rock or sediment with a conductivity of about .1 S·m-1 . Improvements in equipment design might allow the production in real time of a continuous map of the conductivity structure of the sea floor.

Keywords: Geophysics
Advisor: Edwards, R. Nigel
A Novel Method of Computing the EM Response of a Conductive Plate in a Conductive Medium

This thesis describes the development of a novel, simple, and robust integral equation technique to compute the electromagnetic response of a three dimensional plate-like conductor embedded in a conductive host. Thin sheet or plate-like conductors are mathematical models of many base metal mineral deposits, including mineralized veins, shears, and dipping strat. Solutions for the thin sheet models are usually formulated using the integral equation technique with the electric field as the variable. The appeal in this approach is its simplicity in derivation and the ease in interpretation of solutions. However, many of the approximate computer algorithms derived directly from the exact theory have suffered to some extent from the defect that the solutions fail to generate strong vortex and tend to become unstable when the host conductivity becomes too small. The determination of the scattered electric field within the plate requires the solution of an integral equation in which inductive and channeling terms couple together. The magnitudes of the terms vary with the selection of parameters. They can differ by many orders of magnitude particularly for very resistive hosts. The accuracy with which the terms can be derived numerically depends on the selection of appropriate basis functions. A small set of functions which can describe accurately both the current flows is difficult to find. There are several powerful techniques to circumvent the problem, but these methods usually result in complicated and costly software. Our approach to resolve the problem is to deform the thin plate so as to create an equivalent network model for which the channeling and vortex currents can be described equally well by one simple set of unknowns. The new model uses a set of basis functions with a spatial representation bearing a strong resemblance to a lattice composed of many thin conductive strips, called elements, arranged to look like a bottle-box with the top and base removed. It has finite length, width, and thickness, and consists of m x n basic square loops. In order to check our formulation, the EM responses of the new model in a conductive whole space are compared with equivalent responses of a thin disk derived by West and Edwards (1985) for a wide range of induction and channeling response parameters. Other tests of the lattice plate have shown that the solutions are accurate, stable, and robust, and the algorithm is efficient. The lattice plate is used in forward modeling to study the responses of plate-like conductors in the earth and in the sea floor to various Controlled Source Electromagnetic sounding systems.

Keywords: Geophysics
Advisor: Edwards, R. N.
A Plausible Mechanism for Generating Negative Coincident-Loop Transient Electromagnetic Responses

Coincident-loop transient electromagnetic measurements are normally positive at all delay times, but occasionally they become negative at late delay times. The most likely cause of these sign reversals are polarizable conductivity structures (i.e. structures whose conductivity varies in the sense $\partial\sigma$/$\partial$s $>$ 0, where s is the Laplace transform variable representing the frequency). However, structures such as half-spaces, buried conductors or buried conductive layers are not capable of explaining the negatives unless the polarizabilities are implausibly large. The negatives can be explained with realistic polarizabilities if the structure is such that the following three 'favourable coupling conditions' are satisfied at the positions where negatives occur: (1) the transmitter couples well to the body, (2) at late delay times the electromagnetically induced current couples poorly to the receiver, and (3) the polarization couples well to the receiver. Polarizable structures which satisfy the favourable coupling conditions are: a buried conductor in a very resistive host, a thin conductive overburden, two interacting conductors, an overburden which comes to an edge and a thin dipping dyke. The field examples of negative transients presented in this thesis are all associated with these structures and the polarizabilities required to model the negatives are small, so the favourable coupling conditions provide a plausible general explanation of the negative transients.

Keywords: Geophysics
A Robust Integral Equation Solution for EM Scattering by a Plate in Conductive Media

A new integral solution formulated with the Galerkin method is derived and applied to model the electromagnetic response of a thin conducting plate in a stratified medium. The solution is expressed as two coupled integral equations, one for the scalar potential of the equivalent current density and the other for the corresponding magnetic field. The solution adopts the form of a single equation plus a constraint on the current density in two limiting cases. Where the host medium is resistive, the scalar potential equation forces the current to be divergence free. For static field excitation, the magnetic field equation forces the current to be curl free. This property of the solution is responsible for the robustness of the method for a wide range of model parameters. The Galerkin method requires multiple integration which can be extremely time-consuming when the required Green function is not analytical. To overcome this problem, new integration techniques are introduced in which the Green function is represented as a product of geometrical and electrical factors. The geometrical factors are time-consuming to calculate but can be pre-calculated and used in many model evaluations. The electrical factor can be computed relatively easily, but cannot be saved. This separation permits the response of the plate in a layered environment to be calculated extremely efficiently. Parametric scaling arguments are used to determine when it is necessary to compute the EM response with a solution accounting for both galvanic and inductive effects. Estimators are developed and tested which allow one to predict if the complete solution to be replaced by simpler solutions such as those which treat the inductive and galvanic components separately. These estimators may be used to predict the mode of current excitation itself, or the mode of current excitation which dominates the sensed fields.

Keywords: Geophysics
Advisor: West, G. F.
The Effects of Inhomogeneities on Magnetotellurics

The magnetotelluric (MT) method is a low-frequency, electromagnetic (EM), geophysical method which is widely used for investigating the conductivity structure of the Earth's crust and upper mantle. Although MT often seeks to determine the gross conductivity structure and the variation of conductivity with depth, it suffers from the ubiquitous presence of EM scatterers of all scales. The thesis advances the understanding of the effects of multiple and small-scale inhomogeneities on the MT impedance tensor. One of the most pressing difficulties at present in MT are distortions of the EM fields by small-scale, 3-D inhomogeneities which are often found near the surface of the Earth. The thesis investigates the effects of these inhomogeneities on the observed impedance tensor. These studies show that for a great many cases the impedance tensor conforms to a particular factorization, each part of which is related either to the local inhomogeneities or to the large scale structure. Each factor is associated with a different physical effect. This factorization is used to explain the necessary requirements for a useful parametrization of the data contained within the impedance tensor. The development of such a complete, useful decomposition is begun with a method for dealing with the most significant effect of small-scale surface bodies; namely galvanic or frequency-independent distortion of the horizontal electric fields. In addition, analytic and statistical solutions are provided to a number of relevant low-frequency EM scattering problems which model the response of inhomogeneities. An analytic solution for two semi-infinite slabs over a basement is developed in the thesis. This extends existing solutions to this problem so as to include all frequencies and arbitrary basement conductivities. For the particular case of an insulating or perfectly-conducting basement, the above method is extended to provide the EM response for a model of a quasi-anisotropic layer. This model was used to study the benefits of extensive spatial sampling of the electric field in complex media and the presence of fictitious conducting layers when data due to large lateral inhomogeneities are interpreted one-dimensionally. The study showed, for this particular model, that the use of large electrode spacing could be beneficial if used with care. The anisotropy model was also employed to investigate the question of what bulk properties the MT method samples. A statistical technique is used for investigating the response of very fine-bedding in MT demonstrating that such structures can have a relatively significant effect of up to ten degrees on the impedance phase. These results are corroborated by a deterministic model.

Keywords: Geophysics
Advisor: Bailey, Richard
A Generalized Approach to the Interpretation Of Controlled Source Electromagnetic Data Collected in Sedimentary Basins

Electrical and electromagnetic (EM) methods are widely used to explore layered geological structures such as sedimentary basins. Current methods are well developed but have rather low resolution and are easily disturbed if the earth is not well layered. To improve the resolution and accuracy of sounding methods requires that a very large set of field measurements be taken over a wide range of spatial scales and frequencies. In practice, such measurements are usually made about a spatially extended EM source. However, the mathematical complexities of modeling extended source surveys tends to obscure the simple physics behind the method which, in turn, makes interpretation difficult. I have devised a new method to aid the interpretation of survey data acquired about an extended source on a quasi-stratified terrain. The method overcomes the deleterious effects of a complicated source/receiver geometry and makes it possible to account for moderate lateral structure in the earth. It is shown that EM fields can always be factored into "geometrical" operators and "electrical" kernels. The electrical kernels represent the geometrical expansion of the EM field in the earth and are independent of the survey geometry. Conversely, the geometrical operator is independent of the earth model and represents the source/receiver configuration. Field data are transformed to estimate the "average" electric kernels of the horizontal layering. The layered earth data is then interpreted using standard inversion techniques to obtain the best fitting one dimensional model. The EM fields scattered from lateral heterogeneity are defined as the residual data which cannot be interpreted in terms of layered earth models. I develop some simple linear methods of interpreting the scattered fields in terms of anomalous lateral structure in the 1D earth. To test the interpretation concepts, a field study was carried out in the sedimentary sequence of southern Ontario using a grounded bipole source. The survey site is mainly layered, but there is also strong evidence of scattering in the data. The data were successfully separated into layered earth electric kernels and the EM fields scattered from the lateral structure, and subsequently interpreted. The results of the interpretation are consistent with the known geology indicating the utility of the methods developed in this thesis.

Keywords: Geophysics
Advisor: West, G. F.
The Generalized Electrosounding Method for Sedimentary Basin Exploration

Galvanic resistivity and electromagnetic induction sounding are presently the standard techniques for mapping the electrical structure of sedimentary basins. Taken in isolation, these methods have reached the limits of their interpretational accuracy, which can be only marginally improved by refinement of instrumental accuracy or field procedure. The Generalized Electrosounding (GES) method synthesizes the galvanic resistivity and electromagnetic techniques of sounding/profiling to utilize the wealth of information present in high-precision, broadband (0.05 Hz to 5 kHz) electric and magnetic field measurements made in the vicinity of a grounded controlled source. Analysis of these measurements should permit resolution of thin layer properties and apparent aeolotropy, which are not normally resolved by conventional electroprospecting datasets. A prototype measurement system was designed and constructed to investigate the practical acquisition of GES data. Analysis of data obtained with this system during field tests indicates that the instrumentation problem is now thoroughly understood. The principal obstacle to the use of GES for mapping geological structure and formation aeolotropy is "geological noise," a consequence of spatial undersampling of the measured fields. The most promising strategy for control of geological noise requires spatial "filtering" through the use of sensing dipoles which are large with respect to the receiver station spacing. Another feature which distinguishes G ES from conventional electroprospecting methods is the degree of possible redundancy in the data. This possible redundancy, which is a result of measuring both electric and magnetic field components over a wide range of frequencies and spatial offsets, is particularly useful in identifying non-stratified structure which might otherwise be misinterpreted as variations in stratigraphy.

Keywords: Geophysics
Advisor: West, G. F.