We study the metastability of magnetic nanoparticles with size distributions. We simulate an array of magnetic nanoparticles with a spin S = 1 ferromagnetic Blume-Capel model on a square lattice. Studying decays of the metastable state in the Blume-Capel model at low temperatures requires an extremely long computational time in kinetic Monte Carlo simulations. Therefore, we use an advanced algorithm adapted from the Monte Carlo with absorbing Markov chain algorithm for the Ising model in order to study the Blume-Capel model with size distributions. We modeled the particle size distributions as distributions of magnetic anisotropy. We compute the low-temperature average lifetime of the magnetization relaxation using kinetic Monte Carlo simulations with the advanced algorithms. We also calculate the lifetime using the absorbing Markov chains method for analytical results. Our results show that the lifetime of the metastable state follows a modified-Arrhenius law where the energy barrier has a dependency on temperature and standard deviation of the distributions in addition to magnetic field and magnetic anisotropy. The magnetic anisotropy barrier is determined by the smallest particle within a given distribution. We also study magnetization relaxation in different single critical droplet regions using different dynamics: Glauber and phonon-assisted dynamics. We find that the lifetime follows the modified-Arrhenius law for both dynamics, and an explicit form of the lifetime differs in different regions for different dynamics. For the Glauber dynamics, the Arrhenius prefactor does not depend on the standard deviation of the distribution of the magnetic anisotropy. For the phonon-assisted dynamics, however, even the prefactor of the lifetime depends on the standard deviation and is significantly reduced for a wide distribution of magnetic anisotropy. Furthermore, the phonon-assisted dynamics forbids transitions between degenerate energy states and results in an increase of the energy barrier at the single critical droplet region boundary compared to that for the Glauber dynamics. We find that the spin system with a distribution of magnetic anisotropy finds lower-energy relaxation pathways to avoid degenerate state, and the energy barrier becomes the same for both dynamics. / Ph. D.

Anisotropy of magnetic susceptibility (AMS) have been used with great success for determining the internal structure and fabrics of Jurassic and Cretaceous plutons of felsic-intermediate compositions in the White-Inyo Range of eastern California. However, application of the AMS techniques to the Mount Barcroft pluton, located in the northern White Mountains, has yielded anomalous scalar and directional AMS data indicative of unprecedented heterogeneity on the meter-kilometer scale. The 165 Ma Mount Barcroft pluton is primarily of granodiorite composition and was intruded into the Barcroft Structural Break, a northeast striking, steeply dipping structure that juxtaposes Mesozoic metavolcanic rocks to the north against Proterozoic-Paleozoic metasedimentary rocks to the south. Two oriented hand samples (A and B) were collected at each of 78 sites distributed on a 1 kilometer grid pattern across the 5 by 15 kilometer Mount Barcroft pluton and oriented cores were prepared from these hand samples for AMS analysis. Microstructure identification of single thin sections prepared for each sample site yielded primarily magmatic with minor solid-state structures. A highly heterogeneous distribution of scalar parameters (Km, P%, F%, L%, T) was documented both between sample sites and between the A and B cores at individual sites. The heterogeneity may be the result of complex mineral assemblages and the interaction between different magnetic mineral species ranging from single domain to pseudo-single domain to multidomain magnetite. More problematic are the directional parameters between A and B cores in orientation and fabric type (e.g. prolate and oblate susceptibility ellipsoids) occur which cannot be readily explained by a complex mineral assemblage. Different fabric types in A and B cores at individual sample sites could be the result of discrete, temporally unrelated, magma pulses of variable composition and viscosity. Heterogeneity of scalar and directional AMS parameters in the Mount Barcroft pluton, and its contrast with the homogeneous AMS signatures within similar age plutons to the south, may provide evidence for a previously unrecognized magma source beneath the northern White Mountains. / Master of Science

<p dir="ltr">The Alpine Fault, located in the South Island, New Zealand, is a subject of intense geological study due to its potential to trigger large earthquakes. It encompasses a complex system with the interplay of mechanics, thermodynamics, and fluid. Gaining insights into these systems not only enhances our understanding of the fault but also holds the potential to guide risk mitigation efforts.</p><p dir="ltr">The damage extent and fracture networks within the metamorphic rock mass adjacent to the fault can be effectively characterized by seismic anisotropy, an elastic property of rock, where seismic waves travel at different speeds with variation directions. This thesis presents a comprehensive exploration of seismic anisotropy in the hanging wall immediately adjacent to the principal slip zone of the Alpine Fault in New Zealand. Leveraging the borehole seismic data from a unique scientific drilling project and advanced numerical modeling techniques, the ultimate goal is to invert and parameterize the bulk seismic anisotropy.</p><p dir="ltr">Motivated by these challenges, the thesis undertakes several key initiatives: The first effort focuses on gaining a comprehensive understanding of an innovative method for seismic measurement: Distributed Acoustic Sensing (DAS) – examining its operational principles, factors influencing observed wavelets, and how it contrasts with traditional point sensors for accurate interpretation. Subsequently, the research introduces the implementation of an open-source seismic wave solver designed for modeling elastic wave propagation in complicated anisotropic media. This solver is further optimized for computational efficiency with its performance rigorously benchmarked.</p><p dir="ltr">With this preparedness, the inversion is further facilitated by high-performance computing (HPC) and a deep-learning algorithm specifically designed for automatically picking transit times. The inverted bulk elastic constants, compared to the intact rock, reveal 28% to 35% reductions in qP-wave velocity, characterizing the damage due to mesoscale fracture. Further analysis sheds light on the existence of orthogonal fracture sets and an intricate geometrical arrangement that agree with the previous borehole image log. This represents an advancement in our ability to characterize and understand the geologic processes with seismic anisotropy.</p>

<p dir="ltr">The Cascadia subduction zone features many along-strike variations in geophysical signatures that appear independent of properties in the subducting Juan de Fuca plate. Past studies have hypothesized that controls on these variations, namely subcretion, seem linked to overriding plate characteristics but may be influenced by characteristics of the downgoing slab as well. Nowhere is this more apparent than in southern Cascadia, which features the highest seismogenesis, broadest forearc topography, and lowest Bouguer gravity along the Cascadia margin. Additionally, the northward migration of deformation related to the San Andreas fault’s evolution and potential subslab buoyancies introduce further complexities making it difficult to parse contributions of tectonic processes to individual geophysical observations. To better understand contributions from Cascadia subduction and San Andreas evolution on tectonic processes, 60 Magseis Fairview nodal seismometers were deployed in southern Cascadia (Klamath Mountains) between April and May of 2020. We perform ambient noise tomography using Rayleigh and Love waves to constrain radial anisotropy and reveal seismic characteristics in the forearc. We find low VSV (<3.4 km/s) in the lower crust of the forearc consistent with previous studies. This is paired with high (>10%) positive radial anisotropy suggesting these materials are dominated by (sub)horizontal fabrics. We also observe relatively high VSV and VSH and negative radial anisotropy (~ -10%) in the upper crust of the forearc to ~10 km depth. These results suggest that the upper crust, which is dominated by the Klamath terrane, is characterized by (sub-vertical) deformational fabrics, likely related to brittle deformation superimposed on the accretionary history of the Klamath terrane, while the lower crust shows fabrics consistent with what would be expected due to basal accretion of oceanic crust (e.g, sedimentary rocks with or without basaltic slivers). The correlation of positive radial anisotropy with low shear-wave velocities (~3.4 km/s), low Bouguer gravity, high conductivity, and high rates of seismogenic activity (LFEs, tremor distribution, and episodic slow slip events) suggest that this basally accreted material may be infiltrated by fluids derived from the downgoing oceanic lithosphere.</p>

Collective phenomena can give quantum materials unusual properties not found in common materials. Electronic correlations are responsible for intriguing emergent effects like superconductivity, metal-to-insulator transitions, magnetism, etc. Also, anisotropic excitations of polar quantum matter can lead to hyperbolicity, when one crystal axis is metallic and another dielectric. Polaritons, half-light half-matter quasiparticles, have exotic properties in hyperbolic media and are influenced by electronic correlations. In this dissertation, we use infrared near-field optical nanoscopy to interrogate various quantum materials both with strong anisotropy and electronic correlations and study their interplay and tunability. We first understand how near-field microscopes read out optical anisotropy and use our theory to study the metal-to-insulator transition in polycrystalline VO₂. Next, we demonstrate extreme tunability of hyperbolic phonon polaritons in α-MoO₃ by interfacing graphene. Finally, we introduce two novel hyperbolic systems: CrSBr and MoOCl₂, which host magnetically-enhanced hyperbolic exciton polaritons and ultra-low-loss hyperbolic plasmon polaritons, respectively.

Understanding the Earth’s interior requires exploring minerals under extreme pressures and temperatures, conditions often unattainable by experimental methods. Atomistic simulations provide a powerful tool to investigate these extreme environments, offering insights into minerals' physical and chemical behavior deep within the Earth. However, complex phase relations and pronounced anharmonic effects pose significant challenges to these simulations. To address these challenges, we developed advanced methodologies and employed cutting-edge atomistic simulation techniques. Our work focused on modeling phonon behavior, simulating X-ray, IR, and Raman spectroscopy, and evaluating key properties such as thermodynamics, compressive strength, and thermoelasticity. We extended the quasiharmonic approximation for thermoelasticity and introduced a new formalism for third-order elasticity to tackle the complexities inherent in these systems. Our research sheds light on phenomena like hydrogen bond disordering, tunneling, diffusion, and hydrogen bond-induced elastic anisotropy under extreme pressure. These advancements significantly enhance our understanding of the thermal and chemical structures of the Earth’s deep interior.

<p dir="ltr">Hybrid metamaterials have garnered significant attention in recent years owing to their unique properties not found in natural materials. These materials are engineered by integrating two or more distinct materials at the nanoscale, forming various microstructures such as particle-in-matrix, pillar-in-matrix, and multilayers. The recent development of vertically aligned nanocomposites (VANs) offers a platform in forming pillar-in-matrix metamaterials in a self-assembled fashion. Transition metal nitrides, such as titanium nitride (TiN), are interesting materials for VAN designs due to their outstanding plasmonic properties, chemical stability, and compatibility with various functional materials. However, the current range of material selection and morphological demonstrations in two-phase nitride-based nanocomposites is limited. There is a growing need for a deeper understanding of the self-assembly growth mechanism and greater freedom in structural and property tunability of nitride-based VANs to develop the next generation of integrated photonic and electronic devices.</p><p dir="ltr">This dissertation investigates the design, growth mechanisms, and tunability of nitride-based VANs for advanced metamaterial applications. The first chapter focuses on integrating ferromagnetic CoFe₂ into a plasmonic TiN matrix to achieve anisotropic optical and magnetic properties, as well as coupling effects between the two phases. In the second chapter, a third phase, gold (Au), is introduced into TiN-CoFe₂ VANs in a core-shell configuration, demonstrating enhanced tunability in microstructure and resultant properties, such as distinct hyperbolic behavior and switchable magnetic easy axis. The third chapter extends the exploration into three-dimensional (3D) nanostructured films by combining different VAN films (e.g., TiN-CoFe₂, TaN-CoFe₂) in multilayer configurations, demonstrating highly tunable optical properties along with ferromagnetic response. This 3D nanocomposite approach highlights the potential for advanced tunability in metamaterials beyond traditional two-phase VAN designs. The fourth chapter explores the control of stoichiometry and phase composition in TiN-CuO systems. By systematically adjusting oxygen partial pressure during deposition, a gradual transition from metallic to dielectric behavior in these nanocomposite films has been observed. This investigation provides valuable insights into the comprehensive understanding of the interaction processes within hybrid nanocomposites during self-assembly. Overall, this thesis presents diverse methodologies for tuning microstructures and functionalities within nitride-based VAN systems, showing potentials for advanced applications in optics, magnetics, and beyond in metamaterial research.</p>

This study investigated potential alterations in brain white matter after spinal cord injury (SCI). Individuals with SCI underwent magnetic resonance diffusion tensor imaging (DTI) and alterations were assessed based on dynamic changes in fractional anisotropy (FA) and apparent diffusion coefficient (ADC) parameters. Three contrast analyses compared FA and ADC values between person with SCI and healthy controls for the Boston cohort, the Denver cohort, and the grouped cohort. Persons with SCI were found to have lower FA and higher ADC values than health controls across all cohorts. These results are consistent with established correlations between DTI values and alteration of brain white matter in other chronic neurodegenerative conditions. This suggests that DTI is a useful tool for measuring white matter changes in the brain following SCI.

No / Searching for and mapping the physical extent of unmarked graves using geophysical techniques has proven difficult in many cases. The success of individual geophysical techniques for detecting graves depends on a site-by-site basis. Significantly, detection of graves often results from measured contrasts that are linked to the background soils rather than the type of archaeological feature associated with the grave. It is evident that investigation of buried remains should be considered within a 3D space as the variation in burial environment can be extremely varied through the grave. Within this paper, we demonstrate the need for a multi-method survey strategy to investigate unmarked graves, as applied at a "planned" but unmarked pauper's cemetery. The outcome from this case study provides new insights into the strategy that is required at such sites. Perhaps the most significant conclusion is that unmarked graves are best understood in terms of characterization rather than identification. In this paper, we argue for a methodological approach that, while following the current trends to use multiple techniques, is fundamentally dependent on a structured approach to the analysis of the data. The ramifications of this case study illustrate the necessity of an integrated strategy to provide a more holistic understanding of unmarked graves that may help aid in management of these unseen but important aspects of our heritage. It is concluded that the search for graves is still a current debate and one that will be solved by methodological rather than technique-based arguments.

Yes / Insights into structure–mechanical property correlations in molecular and multicomponent crystals have recently attracted significant attention owing to their practical applications in the pharmaceutical and specialty fine chemicals manufacturing. In this contribution, we systematically examine the mechanical properties of dimorphic forms, Forms I and II of 1:1 caffeine-glutaric acid cocrystal on multiple faces using nanoindentation to fully understand their mechanical anisotropy and mechanical stability under applied load. Higher hardness, H, and elastic modulus, E, of stable Form II has been rationalized based on its corrugated layers, higher interlayer energy, lower interlayer separation, and presence of more intermolecular interactions in the crystal structure compared to metastable Form I. Our results show that mechanical anisotropy in both polymorphs arises due to the difference in orientation of the same 2D structural features, namely the number of possible slip systems, and strength of the intermolecular interactions with respect to the indentation direction. The mechanical properties results suggest that 1:1 caffeine-glutaric acid cocrystal, metastable form (Form I) could be a suitable candidate with desired tablet performance to that of stable Form II. The overall, it demonstrates that the multiple faces of nanoindentation is critical to determine mechanical anisotropy and structure- mechanical property correlation. Further, the structural-mechanical property correlations aids in the selection of the best solid phase for macroscopic pharmaceutical formulation.