Physics and Astronomy electronic theses and dissertations (MU)
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The items in this collection are the theses and dissertations written by students of the Department of Physics and Astronomy. Some items may be viewed only by members of the University of Missouri System and/or University of Missouri-Columbia. Click on one of the browse buttons above for a complete listing of the works.
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Item Neutron scattering investigation of the modulated magnetic phase in monoclinic Cr₅Te₈ and how it relates to the large magnetoresistance(University of Missouri--Columbia, 2025) Vaninger, Mitchel B; Miceli, Paul F.[EMBARGOED UNTIL 12/01/2026] There is extraordinary interest in developing metallic 2D van der Waals materials that exhibit room-temperature ferromagnetism (FM) with perpendicular magnetic anisotropy (PMA). Such systems could be incorporated into 2D van der Waals materials engineering with the potential for spin control and spintronic applications. Cr₅Te₈ is among a family of such materials that displays a rich variety of magnetic and electronic phenomena. In this work, neutron scattering measurements reveal the emergence of a modulated magnetic phase in Cr₅Te₈ immediately above the FM transition temperature (Tc = 155 K). This modulation develops along the vdW stacking axis and persists over a short temperature range (155 K - 180 K). This coincides with the onset and disappearance of the giant magnetoresistance (GMR) effect observed in [2], demonstrating a relationship between the two. The application of a magnetic field along the stacking direction suppresses the modulation and restores the uniform FM phase, thereby acting as a switch that reduces the resistivity in a manner analogous to the transition from the paramagnetic (PM) to FM state. At low temperatures (TN = 70 K), the antiferromagnetic (AFM) phase of Cr₅Te₈ is modeled for the first time. Refinement of the magnetic structure reveals a small, highly disordered moment on an interstitial Cr site (0.82 μb). In addition, clear evidence of the two-dimensionality of the Ferromagnetism in the molecular CrTe₂ layers is revealed by magnetic diffuse scattering. Investigating both an in-plane and an out-of-plane reflection, allows for the separation of contributions from different components of the magnetic moment. These give rise to markedly different diffuse scattering behavior, revealing two distinct types of coexisting short range order. Diffuse scattering from the out-of-plane components display correlations of ≈11Å centered on the Bragg reflection and persists up to the highest measured temperature of 250 K disappearing at the onset on the FM ordering. Diffuse scattering from the in-plane component originates from the magnetic modulation and continues to a much lower temperature (< 80K), shows correlations of ≈40 Å, and is centered at the modulated phase's satellite reflections.Item Spintronic properties in 3D and 2D magnets : uncompensated magnetism and emergent properties(University of Missouri--Columbia, 2025) Ghosh, Pousali; Singh, Deepak K.[EMBARGOED UNTIL 12/01/2026] This thesis investigates spintronic phenomena in both bulk and two-dimensional geometrically frustrated magnetic systems, unified by the theme of exploring unconventional magnetism, static and dynamic magnetic correlations, and emergent spin textures. The first part of the work focuses on the bulk intermetallic compound nickel monosilicide (NiSi), which has never yielded magnetic ground state in literature. Using neutron scattering techniques, we reveal a non-collinear uncompensated antiferromagnetic ground state with a weak ferromagnetic component with very high transition temperature of TN ≥ 700 K. This distinct magnetic structure is responsible for intriguing magnetic and transport properties that persist up to room temperature, making NiSi a promising candidate for spintronic applications. In the second part, we transition to two-dimensional frustrated magnets, where artificial honeycomb lattices were fabricated using a self-assembled diblock copolymer approach, enabling access to nanoscopic element sizes beyond photolithographic limits. We use extensive neutron scattering techniques to investigate static and dynamic behavior of a magnetic (Py) 2D honeycomb lattice. We observe here that this system hosts a highly degenerate ground state that persists against high magnetic fields. We also observe strong dynamic behavior in this system where the kinetics is mediated by magnetic charge defects arising at the vertices of the Py honeycomb lattice. An important finding from this work is that while the relaxation time of the magnetic charge defects for a thicker lattice (8.5 nm Py) increases with the increase in temperature, that for a thinner, 6 nm Py honeycomb lattice is temperature indenpendent down to the lowest temperature. This suggests quantum mechanical nature of the kinetics observed. Finally, substituting Permalloy with Neodymium surprisingly results in similar dynamic behavior, despite bulk Neodymium being an antiferromagnet below TN = 20 K and paramagnetic beyond this temperature. The kinetics could no longer be explained by migration of magnetic charge defects, it can rather be explained by formation of vortex-type micro-spin profile quasiparticles. These vortex-like magnetic quasiparticles resemble meron-like excitations and align with emerging concepts in topological spin textures. Their presence in non-magnetic hosts and realization of magnetization in special confined geometries suggests potential for low-power spintronic applications. The final chapter of this thesis further explores the static correlations in these quasiparticles by utilizing Small Angle Neutron Spectroscopy which confirms the existence of these particles in the confined geometry of the 2D Nd honeycomb lattice.Item Automated analysis of AFM images of membrane proteins via conventional and deep learning methods(University of Missouri--Columbia, 2025) Lisowski, Creighton M; Kosztin, IoanThis dissertation develops computational approaches to automate the analysis of membraneassociated proteins imaged by atomic force microscopy (AFM). Cellular membranes interact with numerous proteins whose folding, assembly, and conformational dynamics are essential to cellular function. Here, we focus on two systems: Candidalysin (CL), the poreforming virulence factor of Candida albicans, and SecYEG, a core protein in the general secretory system of E. coli. We introduce PsPolypy, an open-source Python toolkit that detects polymeric particles in AFM images and automatically performs skeletonization, topology classification, and persistence length calculation to quantify Candidalysin polymerization and bending mechanics. We then compare traditional image classification methods with convolutional neural networks for supervised classification of the protruding sides of SecYEG. Finally, we develop an unsupervised deep learning workflow that clusters protein conformations and applies localization atomic force microscopy to enhance in-plane AFM resolution, validated on synthetic AFM images generated from all-atom molecular dynamics simulations. Together, these studies establish a framework that bridges experimental and computational techniques to quantitatively characterize protein structure and dynamics from AFM data.Item A study of magneto-optical and magnetic properties of band-engineered 2D Dirac semimetals(University of Missouri--Columbia, 2025) Chakraborty, Amarnath; Vignale, GiovanniThis thesis investigates the electronic and magnetic properties of low-dimensional quantum materials using analytical modeling, continuum theories, and numerical simulations. The study focuses on nonsymmorphic Dirac semimetals, Bernal bilayer graphene, and twisted bilayer graphene, exploring how lattice symmetry and external perturbations, such as magnetic fields, influence the band structure and stabilize novel quantum phases, including gapped, semi-metallic, and topological states. The work begins by developing a framework to describe the magneto-optical properties of nonsymmorphic semimetals, including optical absorbance and polarization rotations (Faraday and Kerr) under magnetic fields or intrinsic magnetization coupling with the out-of-plane spin. The focus then shifts to Bernal-stacked bilayer graphene, where we analyze the insulator–metal phase transition driven by an in-plane magnetic field and an out-of-plane displacement field. This transition occurs at high magnetic fields, leading us to propose the role of trigonal warping in reducing the critical field to experimentally accessible values. Also, we explore an investigation of twisted systems, where the reduced size of the Moiré Brillouin zone further enhances tunability. While the insulator–metal transition is not observed in twisted bilayer graphene, twisted multilayer graphene could provide an excellent platform to visualize similar effects, potentially revealing unique signatures in its magnetic response.Item Ultrafast and nonlinear optical studies of chemical vapor deposited two-dimensional hybrid halide perovskite films(University of Missouri--Columbia, 2025) Babaian, Dallar Diana; Guha, Suchismita; Yu, PingTwo-dimensional (2D) Ruddlesden–Popper halide perovskites have emerged as promising materials for next-generation optoelectronic applications due to their structural tunability, high exciton binding energies, and enhanced environmental stability. In this thesis, we investigate the ultrafast excited-state dynamics and carrier relaxation processes in Chemical Vapor Deposition (CVD)-grown 2D Hybrid Halide Perovskite (HHP) thin films, focusing primarily on the prototypical lead based systems butylammonium lead iodide (BA₂PbI₄), phenylethylammonium lead iodide (PEA₂PbI₄), and the tin based perovskite phenylethylammonium tin chloride iodide (PEA₂SnClₓI₄₋ₓ). The first part of this work entailed building a transient absorption spectroscopy setup based on an amplified femtosecond laser system. In addition to a full scale optical setup involving steering optics, delay line, and a spectrometer, we developed custom computer algorithms to interface with the optical table components for automating data acquisition as well as analysis. This system along with optical techniques were used to investigate the steady state, temperature dependent, time resolved, and nonlinear optical properties of HHP thin films. We characterize the interplay between excitons, quasi-free carriers, and defect-mediated recombination pathways. We report a rich evolution of the ground state bleaching and photoinduced absorption features, with multiple decay regimes governed by three rate constants (monomolecular, bimolecular, and Auger recombination). At excited state densities below ∼ 4 × 10¹² cm⁻², the decay of the GSB in PEA₂PbI₄ requires all three rate constants, highlighting the coexistence of free carriers and excitons even in a regime where the Saha equation would predict nearly complete excitonic dominance. These results suggest that even a small fraction of quasi-free carriers can trigger exciton ionization, an insight with significant implications for photovoltaic operation under continuous illumination. We also explore how organic cation chemistry, dimensionality, and defects impact carrier thermalization and recombination. The hot carrier Fermi temperature is higher in PEA₂PbI₄ compared to BA₂PbI₄, attributed to fewer defect states. Despite these differences, both materials exhibit comparable ultrafast cooling times (∼150 fs), consistent with strong Fr¨ohlich coupling. In contrast, the tin based perovskite PEA₂SnClₓI₄₋ₓ displays a slower cooling time of ∼1 ps, indicating its potential as a candidate for hot carrier extraction. This work demonstrates that CVD-grown 2D perovskites offer a powerful platform for tuning excited-state dynamics through chemical and structural control. Our results provide both mechanistic understanding and design strategies for optimizing these materials for use in light-harvesting and light-emitting applications.