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
A new perspective on the jamming transition: geometry reveals hidden symmetries
(University of Oregon, 2016-10-27) Morse, Peter; Corwin, Eric
Jamming is a physical process which is both easy to describe and incredibly difficult to understand. One such difficulty is that mechanical treatments of jamming focus on pressure, force, stress, and strain, which are identically zero below jamming, making it hard to differentiate systems whcih which are near or far from the transition. Instead, I introduce a geometric framework based on the Voronoi tesselation which treats all of phase space on an equal footing. This work will show that the jamming transition can be seen entirely through the geometry of the local environment of particles encoded in the Voronoi tesselation, and it will build the framework for an as yet undefined field theory for jamming.
Open Access
A Reservoir of Timescales in Random Neural Network
(University of Oregon, 2022-10-04) Istrate, Nicolae; Mazzucato, Luca
The temporal activity of many biological systems, including neural circuits, exhibits fluctuations simultaneously varying over a large range of timescales. The mechanisms leading to this temporal heterogeneity are yet unknown. Here we show that random neural networks endowed with a distribution of self-couplings, representing functional neural clusters of different sizes, generate multiple timescales of activity spanning several orders of magnitude. When driven by a time-dependent broadband input, slow and fast neural clusters preferentially entrain slow and fast spectral components of the input, respectively, suggesting a potential mechanism for spectral demixing in cortical circuits.
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
A Search for Emerging Jets in the Dijet Invariant Mass Spectrum Using 139 ifb of Proton-Proton Collision Data at a Center-Of-Mass Energy of 13 TeV With the ATLAS Detector
(University of Oregon, 2022-10-26) Kilgallon, Aaron; Strom, David
A search for emerging jets in the dijet topology is presented here using 139 ifb of √s = 13 TeV Run 2 ATLAS proton-proton collision data. Emerging jets constitute a class of dark jet models that have long-lived hadronization components, resulting in unique signatures within particle detectors. These jet signatures are the result of phenomenological considerations of self-interacting dark matter. These models provide an explanation for the baryon-antibaryon asymmetry as well as a well-motivated dark matter candidate particle, which make them particularly compelling. Due to the unusual nature of these jets containing high displaced track and displaced vertex multiplicities that vary significantly on the dark sector parameters, machine learning techniques such as unsupervised classification are ideal in the search for these types of models. A Classification Without Labels method known as the CWoLa method was used to extract limits on heavy Beyond the Standard Model vector boson Z’ particles that produce pairs of emerging jets in the large-R dijet topology. Limits were set on the cross-sections of these signatures and exclude Z’ particles decaying to emerging jets from 10 fb to 2 fb between masses of 1.3 TeV and 4.0 TeV. Production of Z’ particles with masses up to 3.1 TeV and a fixed 20 GeV width were excluded for dark sector couplings down to 0.015. Future considerations for emerging jets analyses are shown in the context of dedicated emerging jets triggers that were designed for use at ATLAS in Run 3.
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
Accurate and Precise Calibration of Advanced LIGO Detectors in the Era of Gravitational Wave Astronomy
(University of Oregon, 2019-04-30) Karki, Sudarshan; Frey, Raymond
The first direct detection of gravitational waves in 2015, and the multiple detections that followed ushered in the era of gravitational-wave astronomy. With these developments, the focus of the gravitational-wave community shifted from detection to precision measurement, requiring a factor of ten improvement in calibration accuracy to maximize the astrophysical information that can be extracted from these detected signals. This dissertation discusses the implementation and characterization of a radiation-pressure-based calibration system called the Photon calibrator that is used as the primary calibration reference for the Advanced LIGO detectors. It also discusses the techniques and procedures used to realize sub-percent accuracy calibration of absolute displacement fiducials introduced using the Photon calibrator system during Advanced LIGO’s first and second observing runs. Using the Photon calibrator systems, frequency dependent calibration of the interferometer responses was achieved at the level of 2-3% in magnitude and 3- 5 degrees in phase across the LIGO detection band. This level of calibration accuracy has already played a significant role in extracting astrophysical parameters from LIGO’s detections. With the LIGO and Virgo detectors operating at design sensitivity, updated rate estimates indicate that measurement of the Hubble constant with gravitational waves with 1% accuracy will be possible within the next decade. This will require absolute amplitude calibration of the detectors at the sub-1% level. This dissertation also discusses the improvements that have been implemented in the Photon calibrator systems that will reduce the uncertainty in absolute displacement to below 0.5%. The gravitational waves from the post-merger phase of binary neutron stars are expected to contain interesting features at frequencies up to few kHz, carrying rich information about neutron-star astrophysics. This dissertation discusses the calibration errors introduced by test mass deformations caused by calibration forces at frequencies above 1 kHz. The errors, estimated using Finite Element Analysis, is in reasonable agreement with measurement results in the 1 to 5 kHz band. These investigations have enabled the reduction of calibration uncertainty at these frequencies, which should enhance our ability to decipher the neutron star astrophysics encoded in the gravitational wave signals from the post-merger phase. This dissertation includes previously published co-authored material.
Open Access
Amorphous Structure Controls Mechanical Properties of Jammed Solids
(University of Oregon, 2021-11-23) Arceri, Francesco; Corwin, Eric
A vast variety of physical systems falls within the description of amorphous solids. From glasses to grains, all of these materials share a disordered structure of their constituents. Understanding the nature of the mechanical properties of such systems is a conundrum which still poses challenging open questions. Recent experimental advances have led to the conclusion that the preparation of the system controls its stability against mechanical perturbations. In particular, amorphous solids can be classified as marginally stable or highly stable with respect to external perturbations. In this work I show that the amorphous structure, whether marginally or highly stable, uniquely controls the mechanical response of amorphous solids. First, I show that thermal glasses under very high pressure share the same mechanical and vibrational propertiesof athermal granular packings near the onset of rigidity. Secondly, I investigate the role of mechanical stability in the context of rheology, in particular with respect to cyclic shear training, and show that jammed solids are able to store an information of the repeated shear deformation only if the system, or a portion of it, is marginally stable. This dissertation includes previously published and unpublished coauthored material.
Open Access
Analysis of radiative decays of charged B mesons to baryonic final states
(University of Oregon, 2008-09) Strube, Jan, 1978-
The abundance of B mesons at B factories opens the door to the search in rare decays for physics outside of the Standard Model. Flavor-changing neutral current transitions proceed only via higher order in the Standard Model, resulting in a b [arrow right] sÂ³y branching fraction of about 3 x 10 -4 , but hypothesized particles could alter the rate significantly. Decays of B mesons that proceed via this electroweak penguin diagram are an interesting example of flavor-changing neutral currents, due to the large number of accessible final states with observables that are sensitive to new processes. This dissertation describes the analyses of such decays B - [arrow right] p Â³, B - [arrow right] Â£ 0 p Â³, B - [arrow right] p and B - [arrow right] p using about 350 million B meson pairs recorded by the BABAR detector in the years 2001 through 2006. In addition to the decay rate, the distribution of the invariant mass of the baryon pair is presented, using a method for statistical unfolding. The analysis is the first of these decays at the BABAR experiment and lays the groundwork for future analyses of the branching fractions and angular correlations of b [arrow right] s decays containing A hyperons and other baryons at BABAR or higher luminosity B factories.
Open Access
Astrophysics with Gravitational Wave Signals from Core-Collapse Supernovae
(University of Oregon, 2019-09-18) Roma, Vincent; Frey, Ray
The next generation of gravitational wave detectors will improve the detection prospects for gravitational waves from core-collapse supernovae. The complex astrophysics involved in core-collapse supernovae pose a significant challenge to modeling such phenomena. The Supernova Model Evidence Extractor (SMEE) attempts to capture the main features of gravitational wave signals from core-collapse supernovae by using numerical relativity waveforms to create approximate models. These models can then be used to perform Bayesian model selection to determine if the targeted astrophysical feature is present in the gravitational wave signal. In this dissertation, SMEE's model selection capabilities are extended to include features in the gravitational wave signal that are associated with g-modes and the standing accretion shock instability. For the first time, SMEE's performance is tested using simulated data for planned future detectors, such as the Einstein Telescope, Cosmic Explorer, and LIGO Voyager. SMEE's performance is improved by creating models from the spectrograms of supernova waveforms instead of their time-series waveforms that contain stochastic features. In third generation detector configurations, about 50% of neutrino-driven simulations were detectable at 100 kpc, and 10% at 275 kpc. The explosion mechanism was correctly determined for all detected signals. This dissertation contains previously published co-authored material.
Open Access
Baryons in the Intergalactic Medium: A Hubble Spectroscopic Legacy Archive Search
(University of Oregon, 2022-10-04) Brunnenmeyer, Trevor; Bothun, Greg
Galaxies are not closed systems but are constantly interacting with their environment which generally contains a) other nearby galaxies and b) some form of warm gaseous medium existing between galaxies (known as the Intergalactic Medium or IGM). As a result, a number of physical mechanisms exist to liberate baryons (stars and gas) from within a galaxy and transfer them to the IGM. Currently there is a well-documented “missing baryon” problem in Cosmology as known galaxies do not contain enough baryons to be consistent with Big Bang Cosmology. This implies a substantial amount of Baryons must exist in the IGM, but most of this remains undetected. Using absorption in Quasar (QSO) spectra from the Hubble Spectroscopic Legacy Archive (HSLA) we can detect individual species present in the IGM. In this work, I build up and refine a pipeline to analyze 688 QSO spectra from the HSLA. At the end of the pipeline, there are multiple possible species identifications for each absorption feature. Looking at models for the extremes of the possible absorptions puts bounds that between 1.6% and 48% of the total baryon density from Big Bang Cosmology can be found in extragalactic clouds. These results do not solve the Missing Baryon Problem, but do provide evidence that baryons liberated from galaxies could be part of the complete solution. This dissertation includes previously unpublished co-authored material.
Open Access
Beyond the Standard Model: Dark Mesons and Custodial Symmetry
(University of Oregon, 2020-09-24) Tong, Tom; Kribs, Graham
We describe our investigations on possible new physics beyond the Standard Model that reveal their connections with custodial symmetry. First, we consider several strongly-coupled dark sectors with fermions that transform under the electroweak group. We construct the non-linear sigma model describing the dark pions and match the ultraviolet theory onto a low energy effective theory that provides the leading interactions of the lightest dark pions with the Standard Model. We uncover two distinct classes of effective theories: “Gaugephilic” and “Gaugephobic”. Second, we demonstrate such a dark sector could be accessible to current searches by studying the production and decay of dark mesons at the LHC. Dark pions can be pair-produced and decay in one of two distinct ways: “gaugephilic” or “gaugephobic”. We recast a vast set of existing LHC searches to determine the current constraints on the dark meson. We find the relative insensitivity of LHC searches, especially at 13 TeV, can be blamed mainly on their penchant for high mass objects or large missing energy. Future dedicated searches would undoubtedly improve sensitivity. Finally, we consider custodially-symmetric UV physics, mapping their effects onto higher-dimensional operators in a custodial basis. This basis explicitly identifies the global SU(2)R symmetries of the Higgs and flavor sector with custodial preserving and violating operators. Custodially symmetric UV physics that contributes purely to oblique corrections at leading matching order leads to the electroweak \rho-parameter = 1 at tree-level. Nevertheless, such UV physics can also generate non-oblique corrections, and thus \rho = 1 is insufficient to claim custodial violation. We therefore identify a set of observables that are able to capture the leading tree-level effects of integrating out a custodially-symmetric UV sector. We illustrate our results with four examples: a heavy singlet scalar; a heavy Z' transforming under U(1)B-L; heavy W's and Z's transforming under SU(2)L X SU(2)R X U(1)B-L; and a heavy W'L coupling purely to left-handed fields. These examples demonstrate that our observables could be used to “fingerprint” custodial symmetry of UV physics. This dissertation consists of previously published and unpublished co-authored 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
Bio-Inspired Fractal Electrodes Interfacing with Retinal Cells
(University of Oregon, 2020-09-24) Moslehi, Saba; Taylor, Richard
Neurostimulation implantable devices are used extensively in treating a variety of neurodegenerative conditions such as Parkinson’s, Alzheimer’s and age-related macular degeneration. Current devices fail to provide high enough resolutions due to the lack of understanding of the neuron-implant interface connections and fundamental structural and mechanical differences between the electrodes’ material and geometry to those of the targeted tissue. These differences trigger the immune responses of the nervous system that engulf the implant and push away the targeted neurons from the electrodes’ surface, therefore causing a further drop in the resolution of the device. As long as this issue is unresolved, other approaches for increasing the resolution, such as providing smaller electrode sizes combined with materials with enhanced electrical stimulation/recording properties are not sufficient. In this thesis, an excellent electrode material candidate combined with geometric patterning approach is tested to guide the immune response against the implant into regions away from the surface of the electrodes as well as enhance neuronal adhesion and outgrowth on the electrodes’ surface. First, by introducing a simple Euclidean geometry of rows of vertically aligned carbon nanotube forests separated by rows of silicon, fundamental behavioral trends of retinal neurons and glial cells in encountering two materials with substantial mechanical and topographical differences in neighboring regions are studied. It is shown that the immune response of the glial cells and adhesion and outgrowth of neurons can be controlled and guided by changing the roughness and stiffness of the electrode material vs the substrate. Next, by adopting fractal electrode geometries while using the same materials, it is shown that the driven responses of neurons and glial cells can further be enhanced through fine tuning fractal characteristics of the electrode’s geometry. Furthermore, preliminary results from future work on comparison between fractal and several Euclidean geometries are discussed. By adopting the appropriate materials patterned in an optimal geometry, the immune response of the nervous system towards implants can be controlled and guided to reduce the distance between the implant’s electrodes surface and targeted tissue and hence increase the resolution.
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
Boundary Integral Method and Applications for Chaotic Optical Microcavities
(University of Oregon, 2020-09-24) Burke, Kahli; Noeckel, Jens
Optical microcavities offer many application possibilities in addition to being model systems for studying chaotic dynamics in the wave regime. However these systems are only analytically solvable for the simplest geometries. In order to study strongly deformed geometries, numerical methods must be used. The open nature of cavities results in complex wavenumbers for their quasi-bound modes which makes finding resonances more difficult. We discuss the main methods available for understanding these resonances. We implement a numerical package for finding resonances, computing their spatial field patterns and projecting onto surface of section plots via Husimi distributions. This software has been implemented in Julia, a modern programming language with performance and ease of use in mind. This software is available as open source, and is designed to be reusable for arbitrary two dimensional geometries. Using this package we describe a novel phenomenon that can occur in strongly deformed geometries with concavities, which we name folded chaotic whispering-gallery modes. In these cavities, folded chaotic WGMs allow for high-Q modes suitable for spectroscopy or laser applications with an important innovation, the ability to attach waveguides to the cavity. The similarities to WGMs are surprising given a theorem by Mather which rules out their existence in the ray picture. High-Q resonances occur within certain wavelength windows and we investigate the peak structure in the spectrum. The periodic orbits in the corresponding billiard system are unstable and exist within the chaotic region of phase space. The fact that such high-Q modes exist based around these orbits implies a form of wave localization. Another geometry is investigated, deformed boundaries of constant width. These are smooth curves similar to Reuleaux polygons. They have no symmetry axis, and in the ray picture have a unidirectional nature. We investigate these in open optical cavities. We find that nearly degenerate modes of opposite rotational direction can be simultaneously present, but have emission at different boundary locations. This suggests a non-reciprocal process achieved through purely geometric means which may allow for the separation of chiral components of light and applications such as optical microdiodes.
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
Chaotic electron transport in semiconductor devices
(University of Oregon, 2010-06) Scannell, William Christian, 1970-
The field of quantum chaos investigates the quantum mechanical behavior of classically chaotic systems. This dissertation begins by describing an experiment conducted on an apparatus constructed to represent a three dimensional analog of a classically chaotic system. Patterns of reflected light are shown to produce fractals, and the behavior of the fractal dimension D F is shown to depend on the light's ability to escape the apparatus. The classically chaotic system is then used to investigate the conductance properties of semiconductor heterostructures engineered to produce a conducting plane relatively free of impurities and defects. Introducing walls that inhibit conduction to partition off sections considerably smaller than the mean distance between impurities defines devices called 'billiards'. Cooling to low temperatures enables the electrons traveling through the billiard to maintain quantum mechanical phase. Exposure to a changing electric or magnetic field alters the electron's phase, leading to fluctuations in the conductance through the billiard. Magnetoconductance fluctuations in billiards have previously been shown to be fractal. This behavior has been charted using an empirical parameter, Q , that is a measure of the resolution of the energy levels within the billiard. The relationship with Q is shown to extend beyond the ballistic regime into the 'quasi-ballistic' and 'diffusive' regimes, characterized by having defects within the conduction plane. A model analogous to the classically chaotic system is proposed as the origin of the fractal conductance fluctuations. This model is shown to be consistent with experiment and to account for changes of fine scale features in MCF known to occur when a billiard is brought to room temperature between low temperature measurements. An experiment is conducted in which fractal conductance fluctuations (FCF) are produced by exposing a billiard to a changing electric field. Comparison of D F values of FCF produced by electric fields is made to FCF produced by magnetic fields. FCF with high D F values are shown to de-correlate at smaller increments of field than the FCF with lower D F values. This indicates that FCF may be used as a novel sensor of external fields, so the response of FCF to high bias voltages is investigated.
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
CHARGE TRANSFER IN A 3+2-BODY, REDUCED MASS FOCK-TANI REPRESENTATION: FIRST ORDER RESULTS AND AN INTRODUCTION TO HIGHER ORDER EFFECTS
(University of Oregon, 1986-06) Straton, John Carter
The Fock-Tani (unitary) transformation of the second- quantized Hamiltonian gives a representation which treats reactants and products symmetrically, and composites exactly. Each term in the Fock-Tani potential corresponds to a specific physical process and contains terms orthogonalizing continuum states to the bound states. The difficulty in carrying out this transformation can be lessened by working in a center of mass system, giving (n-1) reduced mass particles. After a general analysis of such systems, the Fock- Tani transformations in the 3→2-body case are carried out for the reactions a⁺+(b⁺c⁻)→(a⁺c⁻)+b⁺ and a⁻+(b⁺c⁻)→(a⁻b⁺)+c⁻. It is found that for (2) the transformation in the symmetrical reduced mass system can easily be carried out, but the Jacobi reduced mass system requires the more complicated d-matrix approach. This transformation has not yet been attempted in the full 3-body system but is likely to be as difficult as that for (1). First order differential and total cross sections are computed for resonant charge transfer in (1) for a proton- hydrogen initial state. The Fock-Tani T-matrix for the initial-state Jacobi system is found to be identical to that for the full 3-body system. That for the symmetrical reduced mass system gives an error of order l/mprot in the incident wave vector. A comparison of the Jacobi version and a previous special case Fock-Tani transformation, where the proton mass is taken as infinite, is also made. Cross sections for (ls→ls) positronium formation in positron-hydrogen collisions, calculated using the same program as for the proton-hydrogen case, are found to disagree with the previous Fock-Tani result, probably due to lack of convergence of the previous result. Cross sections for reactions (1) involving muons in hydrogenic isotopes (of interest in quantum electrodynamics and catalyzed fusion) are also calculated. Finally, extension of the results to higher order is considered. Polarized Schrodinger wave functions for a system containing a hydrogenic atom and a fully kinetic external charge are found to first order. These would be used in the Fock-Tani matrix elements to account for some initial- and final-state effects. Calculations of distorted second-quantized states and second and third order T-matrix elements are also outlined.
Open Access
Charge transport in mix-conducting hetero-ionic junctions of polyacetylene ionomers
(University of Oregon, 2009-06) Lin, Fuding, 1975-
Experimental studies on mix-conducting hetero-ionic junctions of anionically (PA A ) and cationically (PA C ) functionalized polyacetylene ionomers, as well as each individual ionomer, in thin-film sandwich configurations are reported for the purpose of better understanding the interaction between ionic and electronic charge transports in mixed ionic-electronic conductor (MIEC) systems. The transport of ions in both individual ionomers as well as their hetero-ionic junction was investigated via small-amplitude AC impedance spectroscopy in the absence of significant interference from the electronic charge transport. Modeling of the impedance results reveal important information about the materials such as: ion conductivity, activation energy of ion conduction, ion hopping frequency, dielectric constant, interfacial capacitance, and estimates of effective ion density. Electrochemical injection of electronic charge carriers into PA A and PA C from gold electrodes was monitored to determine the applied potentials needed to drive hole and electron injection into each ionomer. It is found that for both ionomers, the onset voltages for unipolar and bipolar charge injection are similar, and holes can be injected at close to zero bias. The responses of the complete Au|PA A |PA C |Au hetero-ionic junction, as well as each constituent ionomer layer in Au|Ionomer|Au configuration, to various stepping biases were investigated through current-voltage and impedance measurements to study the origin of the asymmetric current-voltage response observed in the hetero-ionic junction. Analysis of the results reveal a working mechanism of a mix-conducting junction that is fundamentally different from that of a purely electronic pn junction. When illuminated with light, the Au|PA A |PA C |Au junction exhibits unidirectional photovoltage and photocurrent with the PA A side at higher potential, while the Au|PA A |Au and Au|PA C |Au samples exhibit symmetric photoresponses. The efficiency of photocurrent generation in the Au|PA A |PA C |Au junction was found to be strongly dependent on the direction of illumination and on the sample thickness. These observations can be explained by the difference in the mobility of holes and electrons and the existence of a built-in ionic space charge region at the PA A |PA C interface. A mechanism of photoresponse unique to MIEC junctions was proposed, and the magnitude of built-in potential was estimated.