Theses and Dissertations from UMD
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Item UNDERSTANDING NANOSCALE PHYSICS BY ADVANCEMENT OF NOVEL NITROGEN-VACANCY CENTERS BASED SCANNING PROBES AND SIMULATIONS(2024) Zhang, Huayu; Ouyang, Min; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis consists of two parts centering on understanding nanoscale material physics by technology development and simulation. In the first part, we briefly introduce the properties and applications of NV centers, then we present a cost-effective method for fabricating and characterizing a new class of durable, highly integrated scanning quantum sensors using both fiber-based and cantilever-based NV center scanning probes using nanodiamonds. The nitrogen-vacancy (NV) center is a photostable fluorescent atomic defect in diamond, exhibiting unique magneto-optic effects. Its Hamiltonian is sensitive to the local magnetic, electric field, temperature, and strain, making NV centers ideal for atomic-scale quantum sensing and various scientific and technological applications.The fiber-based NV center scanning probe is compatible with any tuning fork-based scanning probe microscope and supports multiple operational modes, including near-field excitation with far-field detection, far-field excitation with near-field detection, near-field excitation with near-field detection and far-field excitation with far-field detection. These diverse functional modes enable high-sensitivity and high-resolution quantum imaging and sensing applications, such as magnetic field imaging, electric field imaging, and thermal imaging at the nanoscale. The cantilever-based NV center scanning probe is suitable for use with any commercial cantilever-based scanning probe microscopes and it allows highly integrated microwave manipulation through metal coating. Additionally, we have developed a methodology to accurately determine the orientation of NV centers beneath the probe, establishing a foundation for utilizing these unique NV scanning probes for quantum sensing. Compared to existing fabrication methods, our method is reproducible, low-cost, and robust, offering significant advancements in NV center scanning probe technology. For the second part of thesis, we also investigated the phonon modes and manipulation methods of phonons in nanoparticles. The phonons are quasiparticles representing quantized sound waves in materials, they play a crucial role in defining the thermal, electrical, optical, and mechanical properties of materials. At the nanoscale, phonons exhibit unique behaviors due to quantum confinement effects and interactions with other quasiparticles. We investigated phonons in dumbbell-shaped Au-CdSe nanoparticles using time domain, frequency domain and eigenfrequency analyses, demonstrated an all-optical phonon mode manipulation method and explored potential chiral phonon modes in chiral gold nanocubes through FEM simulations. This research opens new avenues for manipulating material properties and enhancing device performance.Item HIGH-THROUGHPUT COMBINATORIAL EXPLORATION OF QUANTUM MATERIALS AND DEVICES FOR SPINTRONIC AND TOPOLOGICAL COMPUTING APPLICATIONS(2024) Park, Jihun; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This doctoral dissertation aims to explore via high-throughput methodologies heavy-element-based quantum materials and devices for spintronic and topological computing applications. It is organized into three parts: (1) the development of spin wave devices based on magnetic insulators for magnon spintronics, (2) the search for spin-triplet superconductors based on Bi alloys (Bi–Ni and Bi–Pd) for superconducting spintronics, and (3) fabricating Josephson junctions based on topological insulators for topological quantum computing.The first part of this dissertation is to develop spin wave devices based on acoustically driven ferromagnetic resonance (ADFMR) using magnetic materials, including yttrium iron garnet (YIG). Spintronic devices based on ferromagnetic metals entail Joule heating and energy loss due to the moving of charge carriers. On the other hand, spin waves can be used without resistive losses. ADFMR is an efficient platform for generating and detecting spin waves via magneto-elastic coupling. While numerous ADFMR studies in ferromagnetic metals have been reported, there is no such report on magnetic insulators. This is due to (1) thermal degradation of piezoelectric substrates (e.g., LiNbO3) during the film crystallization (T > 800°C for YIG), (2) reaction between substrate and film materials, and (3) low ADFMR signals due to intrinsically low magnetostriction. The first part of this thesis attempts to address these issues to achieve YIG ADFMR devices by utilizing rapid thermal annealing to minimize thermal damage, a SiO2 buffer layer to avoid unwanted chemical reactions during crystallization, and a time-gating method for enhanced signal-to-noise ratio. YIG thin films deposited via pulsed laser deposition and crystallized by rapid thermal annealing show decent ferromagnetic behavior. YIG devices show exotic angle- and field-dependent absorption features, indicative of ADFMR. The observed ADFMR pattern is consistent with simulations. This result indicates the first demonstration of ADFMR in magnetic insulators. The second part of this work performs combinatorial synthesis of Bi–Ni and Bi–Pd alloys, which possibly show spin-triplet superconductivity. Such spin-triplet Cooper pairing would allow field-controllable spin polarization in superconductors, enabling superconducting spintronic applications. Furthermore, this type of device possibly provides evidence of superconducting pairing symmetries. In Bi–Ni spread study, Bi3Ni acts as a superconducting host material, where the superconductivity is identified to be varied according to two competing mechanisms: carrier doping and impurity scattering. These results can provide useful guidance in studying superconducting materials with stoichiometric defects. In the Bi–Pd spread films, two superconducting phases are identified with maximum Tc of 3.1 and 3.7 K, corresponding to BiPd and Bi2Pd phases, respectively. With Bi2Pd thin films, spin injection devices are fabricated and characterized. The Bi2Pd spin injection device showed unusual pair-breaking behavior where the superconductivity of Bi2Pd is destroyed significantly by unpolarized current injection. These superconducting spintronic studies demonstrate prompt device exploration via combinatorial methods, efficiently providing insight into spin-triplet superconductivity and its applications. Lastly, this dissertation aims to fabricate topological Josephson junctions based on Yb6/SmB6/Yb6 trilayers. SmB6 is a topological insulator characterized by a robust insulating bulk state and topological surface states. Superconducting proximity effects on the topological surface states can generate topological superconductivity, which can be utilized for fault-tolerant topological quantum computing. This dissertation addresses challenges in fabricating topological Josephson devices. With statistical analysis, device failure mechanisms are identified and addressed, allowing for improved design and fabrication. The improved devices showed Josephson junction-like behavior. The junction characterization revealed that 100% of measured samples showed Josephson features with prominent statistical reproducibility, possibly induced by the Klein effect. The dependence of SmB6 dimensions on the junction behavior is also investigated, along with possible proposed scenarios. These results demonstrate that the combinatorial approaches allow for efficient and prompt investigation of novel quantum materials and devices, facilitating phase diagram studies, materials screening, and stoichiometric controls.Item Proton Energization during Magnetic Reconnection in Macroscale Systems(2024) Yin, Zhiyu; Drake, James; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic reconnection is a widespread process in plasma physics that is crucial for the rapid release of magnetic energy and is believed to be a key factor in generating non-thermal particles in space and various astrophysical systems. In this dissertation, a set of equations are developed that extend the macroscale magnetic reconnection simulation model kglobal to include particle ions. The extension from earlier versions of kglobal, which included only particle electrons, requires the inclusion of the inertia of particle ions in the fluid momentum equation. The new equations will facilitate the exploration of the simultaneous non-thermal energization of ions and electrons during magnetic reconnection in macroscale systems. Numerical tests of the propagation of Alfvén waves and the growth of firehose modes in a plasma with anisotropic electron and ion pressure are presented to benchmark the new model. The results of simulations of magnetic reconnection accompanied by electron and proton heating and energization in a macroscale system are presented. Both species form extended powerlaw distributions that extend nearly three decades in energy. The primary drive mechanism for the production of these nonthermal particles is Fermi reflection within evolving and coalescing magnetic flux ropes. While the powerlaw indices of the two species are comparable, the protons overall gain more energy than electrons and their powerlaw extends to higher energy. The power laws roll into a hot thermal distribution at low energy with the transition energy occurring at lower energy for electrons compared with protons. A strong guide field diminishes the production of non-thermal particles by reducing the Fermi drive mechanism. In solar flares, proton power laws should extend down to 10's of keV, far below the energies that can be directly probed via gamma-ray emission. Thus, protons should carry much more of the released magnetic energy than expected from direct observations. In Encounter 14 (E14), the Parker Solar Probe encountered a reconnection event in the heliospheric current sheet (HCS) that revealed strong ion energization with power law distributions of protons extending to 500keV. Because the energetic particles were streaming sunward from an x-line that was anti-sunward of PSP, the reconnection source of the energetic ions was unambiguous. Using upstream parameters based on the data observed by PSP, we simulate the dynamics of reconnection applying kglobal and analyze the resulting spectra of energetic electrons and protons. Power law distributions extending nearly three decades in energy develop with proton energies extending to 500keV, consistent with observations. The significance of these results for particle energization in the HCS will be discussed.Item UNDERSTANDING AND CONTROLLING NANOSCALE CHIRALITY: MATERIALS SYNTHESIS, CHARACTERIZATIONS, MODELING AND APPLICATIONS(2024) Liu, Hanyu; Ouyang, Min; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chirality, the property of objects possessing non-superimposable mirror images, initially identified and explored in organic and biological molecules, has gained growing interests in the realm of inorganic nanomaterials due to its foreseeable applications in the fields such as Enantiochemistry, Nanophotonics, Spintronics. In the first segment of this dissertation, we demonstrate a bottom-up synthetic strategy to induce chirality in plasmonic nanoparticles and hybrid plasmonic-semiconductor nanostructures. Subsequently, we detail a simplified analytical coupled-oscillators model to facilitate the understanding of plasmonic-chiral coupling and predict various chiroptical responses based on different coupling strengths, validated through finite element method simulations. Furthermore, advancements in characterizing nanoscale chirality with high spatial resolution at the single nanoparticle level are explored using a novel polarization-dependent optical atomic force microscopy technique, overcoming resolution limits in far field measurements. Finally, we demonstrate the employment of nanoscale chirality to induce spin polarization and enable unique nanoscale chiral Floquet engineering.Item TOPOLOGICAL PHOTONICS: NESTED FREQUENCY COMBS AND EDGE MODE TAPERING(2024) Flower, Christopher James; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Topological photonics has emerged in recent years as a powerful paradigm for the designof photonic devices with novel functionalities. These systems exhibit chiral or helical edge states that are confined to the boundary and are remarkably robust against certain defects and imperfections. While several applications of topological photonics have been demonstrated, such as robust optical delay lines, quantum optical interfaces, lasers, waveguides, and routers, these have largely been proof-of-principle demonstrations. In this dissertation, we present the design and generation of the first topological frequency comb. While on-chip generation of optical frequency combs using nonlinear ring resonators has led to numerous applications of combs in recent years, they have predominantly relied on the use of single-ring resonators. Here, we combine the fields of linear topological photonics and frequency microcombs and experimentally demonstrate the first frequency comb of a new class in an array of hundreds of ring resonators. Through high-resolution spectrum analysis and out-of- plane imaging we confirm the unique nested spectral structure of the comb, as well as the confinement of the parametrically generated light. Additionally, we present a theoretical study of a new kind of valley-Hall topological photonic crystal that utilizes a position dependent perturbation (or “mass-term”) to manipulate the width of the topological edge modes. We show that this approach, due to the inherent topological robustness of the system, can result in dramatic changes in mode width over short distances with minimal losses. Additionally, by using a topological edge mode as a waveguide mode, we decouple the number of supported modes from the waveguide width, circumventing challenges faced by more conventional waveguide tapers.Item Unifying Searches for New Physics with Precision Measurements of the W Boson Mass(2024) Sathyan, Deepak; Agashe, Kaustubh; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Standard Model (SM) of particle physics has been extremely successful in describing the interactions of electromagnetic, weak nuclear, and strong nuclear forces. Yet, there are both unexplained phenomena and experimentally observed tensions with the SM, motivating searches for new physics (NP). Collider experiments typically perform two kinds of analyses: direct searches for new physics and precision measurements of SM observables. For example, experimental collaborations use collider data to search for NP particles like the heavy superpartners of the SM particles, whose observation would be clear evidence of supersymmetry (SUSY). These direct searches often consider kinematic regions where the SM background is small. This strategy is unable to probe regions of the NP parameter space where the SM background is dominant. The same collaborations also measure the masses of SM particles, which not only serve as consistency tests of the SM, but can also probe effects of NP. In 2022, the Collider Detector at Fermilab (CDF) collaboration published the most precise measurement of the $W$ boson mass: $m_W$ = 80433.5 $\pm$ 9.4 MeV. This measurement is in $7\sigma$ significance tension with the SM prediction via the electroweak (EW) fit, $m_W^{\rm pred.}$ = 80354 $\pm$ 7 MeV. Many extensions to the SM can affect the prediction of $m_W$ with indirect effects of heavy NP. However, in 2023, the ATLAS re-measurement of the $W$ boson mass, $m_W$ = 80360 $\pm$ 16 MeV, was found to be consistent with the SM prediction. Both collaborations found a high-precision agreement between the measured kinematic distributions and the SM prediction of the kinematic distributions for their corresponding extracted $m_W$. We propose using the precision measurements of $m_W$ to directly probe NP contributing to the same final state used to measure $m_W$: a single charged lepton $\ell$ and missing transverse energy $\met$. This strategy is independent of modifying the EW fit, which tests indirect effects of NP on the predicted value of $m_W$. Any NP producing $\ell+\met$ which modifies the kinematic distributions used to extract $m_W$ can be probed with this method. With this strategy, since these distributions are used to search for NP while measuring $m_W$, a simultaneous fit of NP and SM parameters is required, thus unifying searches and measurements. This simultaneous fitting can induce a bias in the measured $m_W$, but only to a limited extent for our considered models. We consider three categories of NP which can be probed: ($i$) modified decay of $W$ bosons; ($ii$) modified production of $W$ bosons; and ($iii$) $\ell+\met$ scenarios without an on-shell $W$ boson. We also show that models whose signals extend beyond the kinematic region used to measure $m_W$ can be probed in an intermediate kinematic region. Our results highlight that new physics can still be discovered at the LHC, including light new physics, via SM precision measurements. Additionally, anticipated improvements in precision SM measurements at the High Luminosity LHC further enables new searches for physics Beyond the Standard Model (BSM).Item EXCURSION IN THE QUANTUM LOSS LANDSCAPE: LEARNING, GENERATING AND SIMULATING IN THE QUANTUM WORLD(2024) Rad, Ali; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Statistical learning is emerging as a new paradigm in science. This has ignited interestwithin our inherently quantum world in exploring quantum machines for their advantages in learning, generating, and predicting various aspects of our universe by processing both quantum and classical data. In parallel, the pursuit of scalable science through physical simulations using both digital and analog quantum computers is rising on the horizon. In the first part, we investigate how physics can help classical Artificial Intelligence (AI) by studying hybrid classical-quantum algorithms. We focus on quantum generative models and address challenges like barren plateaus during the training of quantum machines. We further examine the generalization capabilities of quantum machine learning models, phase transitions in the over-parameterized regime using random matrix theory, and their effective behavior approximated by Gaussian processes. In the second part, we explore how AI can benefit physics. We demonstrate how classical Machine Learning (ML) models can assist in state recognition in qubit systems within solid-state devices. Additionally, we show how ML-inspired optimization methods can enhance the efficiency of digital quantum simulations with ion-trap setups Finally, in the third part, we focus on how physics can help physics by using quantum systems to simulate other quantum systems. We propose native fermionic analog quantum systems with fermion-spin systems in silicon to explore non-perturbative phenomena in quantum field theory, offering early applications for lattice gauge theory models.Item Constraining Higgs Boson Self-coupling with VHH Production and Combination, and Searching for Wgamma Resonance using the CMS Detector at the LHC(2024) Lai, Yihui; Palmer, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since the discovery of the Higgs boson (H) with a mass of 125 GeV by the ATLAS and CMS collaborations at the CERN LHC in 2012, the focus of the particle physics community has expanded to include precise measurements of its properties, and so far the measurements align with the Standard Model (SM) predictions. Of particular interest among these properties is the Higgs boson self-coupling, which can be directly probed by measuring the cross section for the production of Higgs boson pair (HH). This thesis presents three analyses using proton- proton collision data at \sqrt{s} = 13TeV with an integrated luminosity of 138 fb−1: a search for SM Higgs boson pair production with one associated vector boson (VHH), a combination of H measurements and HH searches, and a search for a new particle decaying to a W boson and a photon (\gamma). The VHH search focuses on Higgs bosons decaying to bottom quarks, and vector boson decaying to electrons, muons, neutrinos, or hadrons, with a novel background estimation approach. An observed (expected) upper limit on the VHH production cross section is set at 294 (124) times the SM predicted value. The combination of H measurements and HH searches aims to constrain the Higgs self-coupling with the best possible precision. The search for Wgamma resonance focuses on leptonic W boson decays, achieving the world’s best sensitivity for this resonance in the mass ranges considered.Item Chiral light-matter interaction in fermionic quantum Hall systems(2024) Session, Deric Weston; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Achieving control over light-matter interactions is crucial for developing quantum technologies. This dissertation discusses two novel demonstrations where chiral light was used to control light-matter interaction in fermionic quantum Hall systems. In the first work, we demonstrated the transfer of orbital angular momentum from vortex light to itinerant electrons in quantum Hall graphene. In the latter, we demonstrated circular-polarization-dependent strong coupling in a 2D gas in the quantum Hall regime coupled to a microcavity. Our findings demonstrate the potential of chiral light to control light-matter interactions in quantum Hall systems. In the first part of this dissertation, we review our experimental demonstration of light-matter interaction beyond the dipole-approximation between electronic quantum Hall states and vortex light where the orbital angular momentum of light was transferred to electrons. Specifically, we identified a robust contribution to the radial photocurrent, in an annular graphene sample within the quantum Hall regime, that depends on the vorticity of light. This phenomenon can be interpreted as an optical pumping scheme, where the angular momentum of photons is transferred to electrons, generating a radial current, where the current direction is determined by the vorticity of the light. Our findings offer fundamental insights into the optical probing and manipulation of quantum coherence, with wide-ranging implications for advancing quantum coherent optoelectronics. In the second part of this dissertation, we review our experimental demonstration of a selective strong light-matter interaction by harnessing a 2D gas in the quantum Hall regime coupled to a microcavity. Specifically, we demonstrated circular-polarization dependence of the vacuum Rabi splitting, as a function of magnetic field and hole density. We provide a quantitative understanding of the phenomenon by modeling the coupling of optical transitions between Landau levels to the microcavity. This method introduces a control tool over the spin degree of freedom in polaritonic semiconductor systems, paving the way for new experimental possibilities in light-matter hybrids.Item Collective dynamics of astrocyte and cytoskeletal systems(2024) Mennona, Nicholas John; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Advances in imaging and biological sample preparations now allow researchersto study collective behavior in cellular networks with unprecedented detail. Imaging the electrical signaling of neuronal networks at the cellular level has generated exciting insights into the multiscale interactions within the brain. This thesis aims at a complementary view of the general information processing of the brain, focusing on other modes of non-electrical information. The modes discussed are the collective, dynamical characteristics of non-electrically active, non-neuronal brain cells, and mechanical systems. Astrocytes are the studied non-neuronal brain cells, and the cytoskeleton is the studied dynamic, mechanical system consisting of various filamentous networks. The two filamentous networks studied herein are the actin cytoskeleton and the microtubule network. Techniques from calcium imaging and cell mechanics are adapted to measure these often overlooked information channels, which operate at length scales and timescales distinct from electrical information transmission. Structural, astrocyte actin images, microtubule structural image sequences, and the calcium signals of collections of astrocytes are analyzed using computer vision and information theory. Filamentous alignment of actin with nearby boundaries reveals that stellate astrocytes have more perpendicularly oriented actin than undifferentiated astrocytes. Harnessing the larger length scale and slower dynamical time scale of microtubule filaments relative to actin filaments led to the creation of a computer vision tool to measure lateral filamentous fluctuations. Finally, we adapt information theory to the analog calcium (Ca2+) signals within astrocyte networks classified according to subtype. We find that, despite multiple physiological differences between immature and injured astrocytes, stellate (healthy) astrocytes have the same speed of information transport as these other astrocyte subtypes. This uniformity in speed persists when either the cytoskeleton (Latrunculin B) or energy state (ATP) is perturbed. Astrocytes, regardless of physiological subtype, tend to behave similarly when active under normal conditions. However, these healthy astrocytes respond most significantly to energy perturbation, relative to immature and injured astrocytes, as viewed through cross-correlation, mutual information, and partitioned entropy. These results indicate the value of drawing information from structure and dynamics. We developed and adapted tools across scales from nanometer scale alignment of actin filaments to hundreds of microns scale information dynamics in astrocyte networks. Including all potential modalities of information within complex biological systems, such as the collective dynamics of astrocytes and the cytoskeleton in brain networks is a step toward a fuller characterization of brain functioning and cognition.