Physics Theses and Dissertations

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https://hdl.handle.net/1794/2048

This collection contains some of the theses and dissertations produced by students in the University of Oregon Physics Graduate Program. Paper copies of these and other dissertations and theses are available through the UO Libraries.

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Open Access
Leptons as a Window to Dark Matter
(University of Oregon, 2024-12-19) Radick, Aria; Cohen, Tim
There is a huge amount of evidence the dark matter exists, however we still do not know what kind of particle it is. Many experiments have been performed to test different models for dark matter, but its nature still remains elusive. In this work we study two different ways of looking for dark matter by using leptons. First, we look at low threshold experiments in the form of dark matter-electron scattering. We know that the rate of dark matter-electron scattering depends on the underlying velocity distribution of the dark matter halo. In particular, dark matter electron scattering is more sensitive to the high velocity tail which can be significantly different depending on the dark matter halo model. This work quantifies the effects of different dark matter halo models and parameter choices on these rates, finding an $\mathcal{O}(0.01\%)$ to $\mathcal{O}(100\%)$ change in the rate predictions in silicon targets. Secondly, we use a different lepton, the muon, to search for dark matter at colliders. In particular, we simulate a particular class of dark matter model, known as flavored dark matter, at a theoretical future muon collider to predict the capability of such a machine to detect or place bounds on this model, if it were to be built. We focus on the less-explored regime of feeble dark matter interactions, which suppresses the dangerous lepton-flavor violating processes, gives rise to dark matter freeze-in production, and leads to long-lived particle signatures at colliders. We find that the interplay of dark matter freeze-in and its mediator freeze-out gives rise to an upper bound of around TeV scales on the dark matter mass. The signatures of this model depend on the lifetime of the mediator, and can range from generic prompt decays to more exotic long-lived particle signals. In the prompt region, we calculate the signal yield, study useful kinematics cuts, and report tolerable systematics that would allow for a $5\sigma$ discovery. In the long-lived region, we calculate the number of charged tracks and displaced lepton signals of our model in different parts of the detector, and uncover kinematic features that can be used for background rejection. We show that, unlike in hadron colliders, multiple production channels contribute significantly which leads to sharply distinct kinematics for electroweakly-charged long-lived particle signals. This dissertation includes previously published co-authored material.
Open Access
Novel entangling gates and scalable trap designs for trapped-ion quantum computing
(University of Oregon, 2024-12-19) Quinn, Alexander; Wang, Hailin
Trapped ions have received much attention as a platform for quantum computing, a purpose they may be well-suited for on account of their natural features: Ions of the same species are inherently identical; they can be manipulated using electric and magnetic fields via both their net charge, which allows external fields to couple to ions’ center-of-mass motion, and via internal electronic transitions; and when they are isolated from the environment, their states can remain coherent for long time spans, at least by the standards of quantum information experiments. Despite these features, the ability to carry out useful quantum computing in trapped ions is limited, with two major practical constraints being 1) the difficulty of coherently controlling ions’ states and 2) the limited physical scale of existing trapped-ion quantum computers, which typically hold no more than tens of ions. This document present a set of projects meant to help address these limitations through different tracks, including the development and testing of control techniques for trapped ions and the design of new types of traps. Firstly, we present an entangling gate carried out on quantum bits (qubits) encoded in a set of electronic energy levels that have been relatively unexplored until recently and whose viability for quantum information processing may enable more efficient architectures for trapped ion quantum computing. Specifically, we entangled a pair of qubits encoded in two Zeeman sublevels of the D5/2 metastable excited state of a pair of trapped 40Ca+ ions using Raman laser beams 10s of THz detuned from resonance to limit scattering rates (a fundamental error source in Raman gates). We demonstrate that high-fidelity gates (98.6(1)% subtracting state preparation and measurement error, and 99.1(1)% subtracting both SPAM and erasure) can be performed with this encoding scheme, show that the main source of error is technical noise, and employ a leakage detection scheme that allows decay or deshelving from the metastable level to be heralded, potentially making correcting this class of errors easier. After this, we shift focus from control techniques and discuss scalable trap design, focusing on a project to fabricate ion traps using 3D printing, a technique that could potentially enable microfabricated traps with high harmonicity, power efficiency, and depth of confinement relative to the 2D (planar) microfabricated traps widely used in efforts to scale up trapped-ion quantum computing. We design, simulate, fabricate, and carry out preliminary electrical testing on metallized trap prints, demonstrating some minimum viability of the technique, and show computationally that traps produced this way could have trapping characteristics similar to those of other 3D microfabricated designs. Finally, we consider scalable trap design for continuousvariable quantum computing (CVQC), a quantum computing scheme where, in trapped ions, information is encoded in the states of the vibrational modes of an ion crystal. A key requirement for universal CVQC is the ability to perform non-Gaussian operations, which can be difficult to carry out electronically. In this work, we present a 2D ion trap design that can perform non-Gaussian motional operations in an all-electronic way, with the design process accounting for the geometric limitations imposed by 2D traps. We consider some of the motional operations possible with this trap and estimate their associated coupling rates, finding that under a reasonable set of assumptions about operating parameters, coupling rates for non-Gaussian operations could be achieved that are comparable to those previously achieved with all-electronic Gaussian operations could likely be achieved with this trap design. This dissertation includes co-authored, published material.
Open Access
Non-Hermitian Structures in Soft Matter
(University of Oregon, 2024-12-19) Melkani, Abhijeet; Paulose, Jayson
Among the major advances in theoretical condensed matter physics in the past twenty years was to characterize topological insulators using the symmetry classes of Hermitian operators. These advancements were applied to various soft matter systems such as mechanical networks where they revealed the presence of topologically protected zero-frequency edge modes. They were also extended to Floquet operators (which occur in non-equilibrium settings) and non-Hermitian operators (occurring in systems with non-reciprocal couplings or subject to external gain/loss). In classical settings, such as in soft matter, non-Hermitian operators are ubiquitous and have revealed rich behavior such as odd elasticity/viscosity, skin effect, and nonreciprocal transitions across a variety of phenomenological systems. This dissertation deals with using non-Hermitian physics to understand collective behavior in soft matter systems. First, we consider a localization-to-delocalization phase transition when shear is applied to thermally fluctuating directed polymer chains. These chains cannot cross each other and are placed on a substrate consisting of a periodic arrangement of vertical grooves. We will characterize this phase transition using the properties of the diffusion operator governing the polymer configurations---this operator becomes non-Hermitian at nonzero shear. Second, we consider networks of classical mechanical oscillators with spring stiffnesses that are modulated in a time-periodic manner. We find the conditions for parametric resonance and one-way amplification to arise in these networks using the symmetries of the non-Hermitian Floquet operator governing the equations of motion. Specifically, we shall show how a clockwise moving wave in a ring of oscillators can be amplified while the counter-clockwise moving mode remains unamplified. In investigating these physical systems, we also developed some techniques which are widely applicable. Specifically, we developed a formulation to study systems that are invariant after a combined translation in both space and time. Compared to conventional Floquet techniques, this formulation involves integration of the system dynamics for shorter periods avoiding extraneous degeneracies of eigenvalues. We also characterized the real-to-complex eigenvalue transition in parametrized pseudo-Hermitian matrices which is typically accompanied by a drastic change in the behavior of the underlying system. This dissertation contains previously published as well as unpublished co-authored materials.
Open Access
The Sound of Ions: Using Trapped Atomic Ion Motion for Quantum Computation and Sensing
(University of Oregon, 2024-08-07) Metzner, Jeremy; Allcock, David
Encoding qubits in the electronic states of atoms has enabled the ability toperform computations, sense the environment, and gain a deeper understanding of other physical systems through quantum simulation. In the trapped ion platform, each of these applications is made possible, or enhanced by coupling the qubit to the motion of the trapped ions. The motion can be used to do more than just mediate interactions with qubits and is itself a resource for computation, sensing and simulation. The work presented here focuses on methods for manipulation and entanglement of trapped ion motional states in a spin-independent way while retaining the spin to enable measurement of the quantum state of the motion. We have shown the ability to use a set of spin-independent operations including displacement, beam splitter, squeezing and two-mode squeezing, to sense the phase of the oscillator states with precision exceeding the standard quantum limit (SQL) by up to 5.9 dB and approaching the ultimate Cram´er-Rao bound. With qubits still generally required for utilizing the motional states for quantum information experiments, qubit measurement makes it difficult to preserve any motional state during state detection. We have developed the technique of encoding both states in a metastable manifold which could enable methods to preserve particular motional states of longer-chains during state detection. We present preliminary results demonstrating this preservation of motional states using a mixed atomic-species trap at Lincoln Lab. The metastable encoding also provides the unique ability to engineer and explore non-Hermitian qubit Hamiltonians, where we have shown the ability to break the quantum speed limit.
Open Access
A Search for Dark Photons with the FASER Detector at the LHC
(University of Oregon, 2024-08-07) Fellers, Deion; Torrence, Eric
The FASER experiment at the LHC is designed to search for light, weakly-interacting particles produced in proton-proton collisions at the ATLAS interaction point that travel in the far-forward direction. FASER is sensitive to probing previously unconstrained dark photon models, which is a theoretical particle that could provide a portal between the standard model of particle physics and a dark sector that contains a dark matter particle. This dissertation presents the first results from a search for dark photons decaying to an electron-positron pair in FASER, using a dataset corresponding to an integrated luminosity of $27.0$\,$\mathrm{fb}^{-1}$ collected at center-of-mass energy $\sqrt{s} = 13.6$\,TeV in 2022 in LHC Run 3. No events are seen in an almost background-free analysis, yielding world-leading constraints on dark photons with couplings $\epsilon \sim 2 \times 10^{-5} - 1 \times 10^{-4}$ and masses $\sim 17~\mev - 70~\mev$. This dissertation contains previously published as well as unpublished co-authored materials.
Open Access
Characterization of Temporal-Mode Transformations via Spectral Interferometry
(University of Oregon, 2024-08-07) El Demery, Mostafa; Smith, Brian
The use of temporal-mode encoding for quantum information science has gained interest due to its robustness to environmental perturbation and suitability for integrated photonics. Temporal-mode transformations, analogous to interferometric networks for spatial-mode encoding, form the basis of many quantum information protocols utilizing temporal-mode encoding. Accurate and efficient characterization of temporal-mode transformations is essential to ensure precise manipulation of quantum information encoded in the temporal modes of light. This dissertation presents a method to determine the temporal-mode transformation of a device by means of spectral interferometry. We demonstrate the feasibility of the method to extract the temporal mode transformation from a suitable set of measurements and set constraints on experimental parameters for achieving characterization. We anticipate that this approach to assess temporal-mode transformations will be applicable to a broad range of systems being pursued in quantum information applications.
Open Access
A Tailored Approach to Engineering Solid State Single Photon Sources
(University of Oregon, 2024-01-09) Klaiss, Rachael; Aleman, Benjamin
Integrated quantum information technologies such as photonic circuits, quantum transducers, and magnetic sensors require robust single-photon sources in precise locations. Solid-state single photon emitters (SPEs) hosted by mid-bandgap defects in 2D material hexagonal boron nitride (hBN) are bright and stable at room temperature and demonstrate strong coupling to external fields, making them desirable candidates for quantum device applications. However, the specific atomic structure of hBN SPEs remains unidentified, making deterministic engineering a challenge. While recent studies have narrowed the range of possible defect candidates by demonstrating the role of carbon in hBN SPEs, the methods to engineer carbon-based defects in hBN either produce randomly located emitters or require bottom-up crystal growth on structured substrates. We achieved patterned arrays of SPEs via focused ion beam (FIB) milling followed by chemical vapor deposition (CVD) of nanocrystalline graphite source for carbon diffusion, and found that both techniques are necessary for significant and repeatable creation of SPEs. This technique creates localized emitters with ten times the yield of carbon annealing alone. Furthermore, by adjusting the parameters of FIB exposure time and carbon annealing time, we found multiple different parameter combinations that successfully created SPEs, demonstrating the adjustability of this technique based on device application requirements. Additionally, we performed atomic force microscopy to characterize the surface morphology of hBN regions patterned by Ga+ FIB to create SPEs at a range of ion doses and found that material swelling is strongly correlated to successful SPE creation. Furthermore, we simulated vacancy and impurity profiles to elucidate how Ga+ FIB patterning induces lattice damage in the form of vacancies, structural voids, and amorphous layers, creating a diffusion barrier to control the introduction of carbon impurities to engineer isolated SPEs with high resolution of process control. Our results provide novel insight into the formation of hBN SPEs created by high-energy, heavy-ion FIB that can be leveraged for monolithic hBN photonic devices and a wide range of low-dimensional solid-state SPE hosts. This dissertation includes previously published and unpublished coauthored material.
Open Access
Binary Black Hole Astrophysics with Gravitational Waves
(University of Oregon, 2024-01-09) Edelman, Bruce; Farr, Ben
Gravitational Waves (GWs) have quickly emerged as powerful, indispensabletools for studying gravity in the strong field regime and high-energy astrophysical phenomena since they were first directly detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) on September 14, 2015. Over the course of this dissertation work gravitational-wave astronomy has begun to mature, going from 11 GW observations when I began to 90 at the time of writing, just before the next observing run begins. As the network of GW observatories continues to grow and these observations become a regular occurrence, the entire population of merging compact objects observed with GWs will provide a unique probe of the astrophysics of their formation and evolution along with the cosmic expansion of the universe. In this dissertation I present four studies that I have led using GWs to better understand the astrophysics of the currently most detected GW source, binary black holes (BBHs). We first present a novel data-driven technique to look for deviations from modeled gravitational waveforms in the data, coherent across the network of observatories, along with an analysis of the first gravitational- wave transient catalog (GWTC-1). The following three studies present the three different approaches to modeling populations of BBHs, using parametric, semi- parametric and non-parametric models. The first of these studies uses a parametric model that imposes a gap in the mass distribution of black holes, looking for evidence of effects caused by pair-instability supernovae. The second study introduces a semi-parametric model that aims to take advantage of the benefits of both parametric and non-parametric methods, by imposing a flexible perturbation to an underlying simpler parametric description. This study was among the first data-driven studies revealing possible structure in the mass distribution of BBHs using GWTC-2, namely an additional peak at 10M⊙ . The final study introduces a novel non-parametric model for hierarchically inferring population properties of GW sources, and performs the most comprehensive data-driven study of the BBH population to date. This study is also the first that uses non-parametric models to simultaneously infer the distributions of BBH masses, spins and redshifts. This dissertation contains previously published and unpublished material.
Open Access
Seek and Ye Shall Find: Machine Learning and Searches for New Physics
(University of Oregon, 2024-01-09) Bradshaw, Layne; Chang, Spencer
The discovery of the Higgs boson confirmed that the Standard Model is the correct description of nature below some high energy scale. However, we know the Standard Model is incomplete and have yet to find significant deviations from it. Without well-motivated directions to guide new physics searches, we need to reconsider where and how we search. We explore this in 3 parts here. We start by identifying 3- and 4-point on-shell amplitudes involving top quarks that are most susceptible to new physics. Using the Hilbert series as a cross-check, we are able to create an independent set of amplitudes for four-fermion and two-fermion, two-boson interactions. After translating these amplitudes to the lowest-dimension SMEFT-like operator, we use pertubative unitarity to place an upper bound on the coupling, under the assumption that the new physics appears around the TeV scale. With this, we find a number of top quark decay modes that could be probed at the HL-LHC. Next, we compare the efficacy of a number of methods to decorrelate the output of a machine learned classifier from the invariant jet mass. This decorrelation preserves the background dominated sidebands in the invariant mass distribution as tighter cuts are made on the network’s output. This increases the potential discovery significance of the new physics. We compare 4 techniques which broadly fall into one of 2 categories—data augmentation or training augmentation. We find that the simpler and computationally cheaper data augmentation techniques perform comparably to the training augmentation techniques across a variety of qualitatively different signals. Finally, we turn to machine learning based anomaly detection, with the aim of explaining the physics learned by an image-based autoencoder. Adapting techniques from the literature, we make use of two strategies to mimic the autoencoder. Despite fundamental differences, we find that both techniques, when compared to the autoencoder, order background events similarly and perform comparably as anomaly detectors across a wide swath of signals. The mimicker networks independently use the same high-level observables, giving us confidence that these features are indeed those learned by the autoencoder. This dissertation includes previously published co-authored material.
Open Access
Studies of evolution in populations with long-range dispersal
(University of Oregon, 2024-01-09) Villiger, Nathan; Paulose, Jayson
Long-range dispersal of offspring is ubiquitous in nature, from seeds that disperse random distances thanks to being carried by animals, to pollen that gets carried long distances by the wind, and even viruses that spread around the world with the help of infected travelers on intercontinental airplane journeys. Long-range dispersal can lead to founder events throughout a landscape, as the first individual to colonize a new region benefits from abundant resources and a lack of competition, which can result in that individual's genes making a disproportionately large contribution to future generations near the territory it colonized. Long-range dispersal can drive range expansions when individuals disperse beyond the bounds of the population's current range. Range expansions driven by long-range dispersal can have dramatic consequences, for example as invasive species take over habitats with no ecological architecture to keep them in check or pandemics rapidly spread around the world. Range expansions driven by long-range dispersal accelerate as they progress and have remarkably different dynamics than the constant-speed expansions carried out by populations with exclusively short-range dispersal. These jump-driven expansions can be challenging to model in part because the dynamics are dominated by the rare longest dispersal events. Recent theoretical advances have enabled predictions about such quantities as population growth rates and the evolution of neutral diversity during range expansions driven by power law dispersal kernels. However, these theories rely on various simplifying assumptions which are not always met by natural populations, and their applicability to more complex but realistic population dynamics remains an open question. Another open question is how to connect theoretical results with real-world biological populations. This dissertation addresses these open questions by developing methods of simulating range expansions with more realistic population dynamics and extracting dispersal parameters from genomic data. In Chapter II, we use simulations to explore the consequences of departing from assumptions of the simplified models that led to the aforementioned predictions about population growth and the evolution of neutral diversity. We show that qualitative trends are preserved but reveal quantitative signals of the more realistic local dynamics. In Chapter III, we use simulations to investigate what determines the fate of fitness-affecting mutations that appear during range expansions driven by long-range dispersal, a situation for which there is no existing theory. We find that mutation outcomes are independent of the fitness effect they confer across a wide range of effect sizes. In Chapter IV, we show that convolutional neural networks can learn dispersal parameters from genomic samples taken from individuals in populations with long-range dispersal, bringing the growing body of theoretical work in this field closer to samples that could be taken from actual biological populations. This dissertation contains previously published and unpublished coauthored material.
Open Access
Building and Characterizing Graphene Nanomechanical Resonator Networks
(University of Oregon, 2024-01-09) Carter, Brittany; Alemán, Benjamín
Networks of nanoelectromechanical (NEMS) resonators are useful analogs for a variety of many- body systems and enable impactful applications in sensing, phononics, and mechanical information processing. Two main challenges are currently limiting progress toward realizing practical NEMS networks. The first is building a platform of interconnected resonators that is scalable in both size and tunability. The second is spatially quantifying the mechanical parameters of each resonator in the network and their coupling. In this work, we address these two main challenges with a novel scalable platform to build the network and a compatible method to characterize mechanical parameters. Together, this work fills in a vital gap for the experimental realization of programmable NEMS networks.We first present a novel platform of suspended graphene resonators that hosts strong coupling and is scalable in 2D. In this platform, we suspended graphene over pillar arrays, in which large areas of suspended graphene act as drumhead resonators and shared membrane between adjacent resonators allows for direct coupling through strain. We demonstrate the versatility advantages of our graphene-based resonator network by providing evidence of strong coupling through two different tuning methods. We demonstrate the 2D scalability potential of this platform with evidence of coupling between three resonators. Finally, we show noteworthy coupling dynamics of inter-resonator higher order mode coupling that is enabled by our versatile platform. We then demonstrate a scalable optical technique to spatially characterize graphene NEMS network. In this technique, we read out the fixed-frequency collective response as a single vector. Using just two response vectors, we solve for the site-specific elasticity, mass, damping, and coupling parameters of network clusters. Compared to multiple regression, our algebraic fully characterizes the network parameters without requiring a priori parameter estimates or iterative computation. We apply this technique to single-resonator and coupled-pair clusters and find excellent agreement with expected parameter values and spectral response. Our approach offers a direct means to accurately characterize both classical and quantum resonator systems.
Open Access
Searching for Gravitational Waves Associated with Flaring Galactic Magnetars
(University of Oregon, 2024-01-09) Merfeld, Kara; Frey, Raymond
The third observing run of Advanced LIGO and Virgo (O3) took place fromApril 1st, 2019 to September 30th, 2019, and from November 1st, 2019 to March 27th, 2020. The multi-messenger astronomy efforts during O3 included conducting gravitational-wave follow-up searches to electromagnetic burst sources, specifically Gamma-Ray Bursts, Fast Radio Bursts (FRBs), and magnetar x-ray bursts. The overarching goal of the research described in this dissertation is to improve the sensitivity of the LIGO burst searches in the third observing run, and to expand on our data analysis methods for the next observing run. Magnetars are highly magnetized neutron stars with intermittent x-raybursting behavior. We present a gravitational-wave follow-up search on the magnetar bursts from O3. This is an expansion on a similar search that was done in the second observing run (O2), and we present the differences in search methods and their effects. We place the most stringent upper limits on gravitational wave energy of any gravitational-wave search to date, and while these upper limits are still not low enough to be astrophysically meaningful, they do provide a framework for future searches. FRBs are short-duration, bright bursts of radio signal from far outside MilkyWay galaxy. We conduct the first-ever search for unmodeled gravitational-wave transients coincident with FRBs detected by the Canadian Hydrogen Intensity Mapping Experiment, the largest population of FRBs detected so far. We search over both repeating and non-repeating FRBs. Although we find no evidence for a signal, the study does lay the groundwork for future FRB searches from sources within our detection radius. A stacked search in which multiple triggers are analyzed simultaneously ismotivated by a number of very marginal triggers in the O3 magnetar search. We develop a version of an existing LIGO burst pipeline that can perform a stacked analysis. We describe the methods, and demonstrate a reduction in the root-sum- squared strain that an unstacked event would need to have if it were to be detected in a stacked analysis with a specific p-value. We also present sensitivity studies to determine how to optimize our pipeline.
Open Access
Boosted Analysis of Higgs Pair Production in the bbτ + τ − Lephad Final State
(University of Oregon, 2024-01-09) Luongo, Nicholas; Torrence, Eric
This dissertation presents the development of a boosted analysis in the searchfor the resonant production of a new heavy scalar X decaying to two Higgs bosons, which is predicted by some Beyond the Standard Model theories. The bbτ + τ − semi-hadronic decay channel of the Higgs bosons is considered. The analysis is developed using Monte Carlo simulated data and validated with 0.11 fb−1 of proton-proton collisions at √s = 13 TeV from the ATLAS detector at the Large Hadron Collider (LHC). Scalar X masses of 1, 1.6, and 2 TeV are considered and expected limits of 5.29 × 103 , 23.42, and 18.60 fb, respectively, are placed on the pp → X → HH cross section at 95% confidence level. These results are compared to existing resolved and boosted ATLAS bbτ + τ − analyses. A new method for di-τ identification and a kinematic neural network for event selection are also described. This dissertation contains previously published and unpublished material.
Open Access
Easy on the Ions: Photon Scattering Errors from Far-Detuned Raman Beams in Trapped-Ion Qubits
(University of Oregon, 2024-01-09) Moore, Isam; Allcock, David
The viability of quantum computers depends on the development of scalable platforms with low error rates. Our "Oregon Ions" group has studied one such scalable architecture proposal including the limitations placed on logic gate fidelity by photon scattering. We studied spontaneous Raman scattering-induced errors in stimulated Raman laser beam-driven logic gates in metastable- and ground-manifold-encoded qubits. For certain parameter regimes, we found that previous, simplified models of the process significantly overestimated the gate error rate due to spontaneous photon scattering. We developed an improved model, which shows that there is no fundamental lower limit on gate error due to spontaneous photon scattering for electronic ground state qubits in commonly-used trapped-ion species when the Raman laser beams are red detuned from the main optical transition. Additionally, spontaneous photon scattering errors are studied for qubits encoded in a metastable D5/2 manifold, showing that gate errors below 10^-4 are achievable for all commonly used trapped ions. Furthermore, we extended this theory from hyperfine to Zeeman qubits and we measured scattering rates from far-detuned Raman beams in a metastable D5/2 Zeeman qubits in 40Ca+, obtaining results that matched theoretical expectations. Finally, we present progress towards implementing a two-qubit Mølmer-Sørensen gate with these Raman beams in trapped 40Ca+ ions.
Open Access
Principles of Traffic Organization in Ant Transportation Networks
(University of Oregon, 2024-01-09) Kittell, Justin; Schofield, Robert
Collectively, a colony of ants can execute complex and highly organized behaviors, not least of which is the formation of ant ‘paths’ - the steady bidirectional flow of individuals and resources that provides the colony with nutrients. This bidirectionality necessitates the organization of opposing fluxes, with the choice of organizational scheme impacting the energetic efficiency of the colony. In this work, we perform an experimental investigation into the organizational principles employed by the leaf-cutting ant Atta cephalotes under varying levels of lateral confinement. We first extract the statistical properties of the unconfined path via automated imaging and analysis. This characterization is a critical first step in understanding the steady state organization resulting from ant behavior alone. We then explore how the behavior and resulting path properties evolve for the same path under different levels of confinement. This analysis reveals direct quantitative evidence of a three-lane structures, as well as simple examples of energetic optimization at critical widths. Finally, we verify the origin of these structures through simulation of a 'null' model for insect behavior, revealing that the organization demonstrated by Atta cephalotes foragers under confinement results from changes in individual behavior, not solely from ant-wall or ant-ant interaction. This analysis provides a framework for understanding the behavioral trends of a natural transportation system in terms of energetic optimization, with potential impacts on the development of autonomous networks in human engineered systems.
Open Access
On Local Mechanical Properties of Thin Pressurized Shells with Combined Geometric and Material Anisotropies
(University of Oregon, 2024-01-09) Sun, Wenqian; Paulose, Jayson
Thin elastic shells are ubiquitous in nature. Indentation measurements (i.e., poking) provide a useful way for probing mechanical properties of these shell structures. While spherical and cylindrical shells made of isotropic materials are well studied, many shells in nature have geometric anisotropy (e.g., ellipsoidal pollen grains) and/or material anisotropy (e.g., cells that have special growth directions), and mechanics of these shells are relatively less understood. I will present some new insights on indentation responses and buckling pressure of shells with geometric and material anisotropy using the shallow-shell theory. First, I will describe the indentation stiffness of pressurized ellipsoidal and cylindrical elastic shells that are made of isotropic materials. We are able to derive a closed form for the indentation stiffness of shells with arbitrary asphericity and internal pressure. Our results provide theoretical support for previous scaling and numerical results on the stiffness of ellipsoids and allow us to isolate the distinct contributions of geometry and pressure-induced stresses on shell elasticity. I will then add the effects of material orthotropy, which assigns different elastic properties along orthogonal directions. For a commonly used model of orthotropy, we find a simple rescaling transformation that can effectively map a rectilinearly orthotropic shallow shell to an isotropic one with a different local geometry. With the rescaling transformation, we obtain new analytical insights for indentation responses and buckling of orthotropic shells. Our results provide a new perspective on how isotropic and orthotropic materials are related, isolate the effect of material orthotropy on shell elasticity, and can provide experimentalists with a means to analyze the internal pressure of biological structures that are made of orthotropic materials using atomic force microscopes.
Open Access
Growth and Guidance: A Study of Neuron Morphology and How it is Modified by Fractal and Euclidean Electrodes In Vitro.
(University of Oregon, 2023-07-06) Rowland, Conor; Parthasarathy, Raghuveer
For well over a century, neuroscientists have been studying the inherent ties between neuronal morphology and functionality. Santiago Ramón y Cajal, in his work that ultimately awarded him a Nobel Prize in 1906, established that neurons function as the fundamental unit of the nervous system. Ramón y Cajal himself recognized the relationship between neuronal form and function by proposing the wiring economy principle, which states that the nervous system’s complex network of neurons is efficiently wired in a way that minimizes wiring length. The research within this dissertation works towards the goal of optimizing the design of the electrode-neuron interface of medical implants by building upon Ramón y Cajal’s foundational ideas and integrating them with the techniques of fractal analysis.The dissertation begins by addressing the question of how electrode geometry impacts the morphology of the networks of neurons and glia interfacing with the electrode. This was done by interacting dissociated mouse retinal cell cultures in vitro with vertically aligned carbon nanotube (VACNT) electrodes grown on a silicon dioxide (SiO2) substrate and patterned into Euclidean and fractal geometries. The VACNT-SiO2 material system was shown to perform exceptionally well at guiding neurons onto the VACNTs and glia onto the surrounding SiO2. Furthermore, the electrode geometries that performed the best at supporting a healthy network of neurons and glia were those that balanced providing a large VACNT electrode area with maintaining connectedness in the surrounding SiO2 surface and allowing it to interpenetrate the VACNT electrode. Following these in vitro experiments, three-dimensional models of pyramidal neurons from the CA1 region of the rat hippocampus were reconstructed using confocal microscopy. The fractal properties of the neurons and how these relate to their functionality were then analyzed. It was then demonstrated that the natural, fractal behavior of the neurons, though limited in its scaling range, was sufficient to provide the neurons with an optimal balance between connectivity and building and operating costs. The dissertation concludes by reviewing the results of these studies, providing directions for future work, and discussing the implications regarding electrode design. This dissertation includes previously published co-authored material.
Open Access
Direct Experimental Observation of 3D Vortex States in Multilayer Fe/Gd using Scanning Electron Microscopy with Polarization Analysis (SEMPA)
(University of Oregon, 2023-07-06) Moraski, Rich; McMorran, Benjamin
The global market for power solely for data center usage is estimated to be $12.4 billion by 2027[1]. In 2021, data center electricity consumption was ∼400 TW h, representing almost 2% of the global energy demand[2]. Ongoing efforts in spintronics, which use spin currents instead of traditional charge currents at a fraction of the power[3], are paving the way for significant savings, both financially and environmentally. There has been considerable research into alternatives for memory[4–6] including a magnetic structure known as a skyrmion, a self-supporting magnetic texture characterized by a non-trivial topology. Recent advances in creating room-temperature stable skyrmions has reignited interest in these objects. Building on previous work in the McMorran group, this research set out to build a more complete understanding of the 3D structure of metastable magnetic skyrmions, specifically in Fe/Gd thin films. This was done using traditional trans- mission electron microscope (TEM) techniques along with a unique scanning elec- tron microscope with polarization analysis at the University, the SEMPA. Data col- lected using a TEM in Lorentz mode, providing information integrated through the bulk of the material, was combined with data from SEMPA, providing surface- sensitive information about the top of the material. Analysis of the data suggests atopologically complex winding nature for the magnetization of skyrmions in this material. Presented herein is a brief introduction of the magnetic structures found in Fe/Gd multilayer thin films; an analysis using new analytical tools built for this purpose of the data collected; and a user’s manual for SEMPA, including mainte- nance and troubleshooting guidance.
Open Access
Two Searches for Signals of Dark Matter with the ATLAS Detector in 139 ifb of LHC $\sqrt{s}$ = 13 TeV Proton-Proton Collision Data
(University of Oregon, 2023-03-24) Gledhill, Galen; Majewski, Stephanie
This dissertation presents two searches for signals of dark matter in an integrated luminosity of 139 ifb of proton-proton collision data collected at a center of mass energy of $\sqrt{s}$ = 13 TeV with the ATLAS detector at the Large Hadron Collider (LHC). The search for direct pair production of the supersymmetric partner to the top quark (the stop) in the all-hadronic $t\bar{t}$ plus missing transverse momentum final state yields no significant excess over the expected Standard Model background and was able to exclude stop masses up to 1.25 TeV for dark matter candidate masses below 200 GeV. The search for dark mesons decaying into top and bottom quarks is sensitive to a proposed strongly coupled dark sector which contains a viable dark matter candidate scalar baryon. This analysis considers the all-hadronic channel of a final state of all top and bottom quarks ($tttb$ or $tbtb$) with no additional missing transverse momentum. No previous LHC searches have considered this dark meson model and we expect to provide new constraints on dark pion masses of up to 500 GeV.
Open Access
Developing a Platform for cQED Studies of Silicon Vacancy Centers in Diamond within the Good-Cavity Limit
(University of Oregon, 2023-03-24) Pauls, Abigail; Wang, Hailin
Silicon vacancy centers (SiVs) in diamond are local defects in the diamond lattice that behave as atomic-like systems with electronic energy levels and optical transitions. The SiV's optical properties and long spin decoherence times ($> \! 10$ ms @ 100 mK), along with its ability to be integrated into nano-engineered devices while maintaining its optical coherence, make it an attractive option as a solid state spin qubit for applications in quantum information.\cite{ref23,ref24,ref25} Here I present my work to develop a composite platform for cavity quantum electrodynamics (cQED) studies of SiVs in diamond in the good-cavity limit, $\kappa