Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
Browse
307 results
Search Results
Item Studies in alternative theories of gravity and advanced data analysis(Montana State University - Bozeman, College of Letters & Science, 2024) Gupta, Toral; Chairperson, Graduate Committee: Neil J. Cornish; This is a manuscript style paper that includes co-authored chapters.The field of gravitational wave astronomy is generating groundbreaking findings, yielding unique insights on some of the most extraordinary phenomena in the universe and providing invaluable information on testing the principles of general relativity. All gravitational wave signals detected so far appear to come from compact binaries - black holes and neutron stars. We use information from these sources to probe strong fields of gravity and to constrain modified theories of gravity. However, solely relying on template- based searches for known astrophysical sources biases our gravitational wave signal search towards well-modeled systems, potentially overlooking unpredicted sources with limited theoretical models, hindering the extraction of new physics. Further work in this thesis focuses on building improved signal and noise models to enhance our capability of detecting gravitational signals of all within and beyond the constraints of theoretical predictions. This includes introduction of new basis functions with added modifications to develop a signal-agnostic waveform reconstruction model using Bayesian inference. Additionally, this study discusses improvements in the speed and performance of the BayesWave trans-dimensional Bayesian spectral estimation algorithm, which includes implementing a low-latency analysis and various enhancements to the algorithm itself. In essence, this study is centered on developing a comprehensive understanding, both theoretical and observational, of astrophysical objects along with the spacetime that governs their dynamics.Item Thermalization and exciton localization in 2D semiconductors(Montana State University - Bozeman, College of Letters & Science, 2023) Strasbourg, Matthew Christopher; Chairperson, Graduate Committee: Nick Borys; This is a manuscript style paper that includes co-authored chapters.2D semiconductors are a promising class of materials to investigate for applications in the next generation of photonic devices. They can be used to generate quantum light and also exhibit correlated many-body phenomena. Many of the novel optoelectronic properties of 2D semiconductors are associated with strongly-bound hydrogen-like states known as excitons. Excitons in 2D semiconductors have binding energies on the order of 100s of meV and are stable at room temperature. At low temperatures, higher-order excitonic states such as charged excitons and biexcitons--multiple-bound excitons that are like hydrogen molecules-- and localized excitons that emit quantum light are also observed. Whether excited optically or electronically, a diversity of high-energy excitons and free carriers are produced directly after excitation. The relaxation and thermalization of these initial states influence the formation of excitons, biexcitons, and localized excitons. Here, I present work that (i) investigates the thermalization of excited states in a prototypical 2D semiconductor, monolayer (1L-) WSe2, and reports the discovery that the generation of charged biexcitons is enhanced with increasing photoexcitation energy, (ii) shows the emergence of quantum emitters (QEs) in a new 2D QE platform: 1L-WSe2 nanowrinkle arrays induced by Au nano stressors, and (iii) uses a novel method to classify the excited-state dynamics of 2D QEs and differentiate emitter populations. A suite of low-temperature energy- and time- resolved optical spectroscopies are used to conduct this work. This work shows how excited state thermalization affects the formation of exciton and biexcitons and investigates the optical properties of an emergent class of 2D quantum light emitters.Item Broken-symmetry phases of matter and their effects on electronic and magnetic properties(Montana State University - Bozeman, College of Letters & Science, 2023) Peterson, Sean Fahlman; Chairperson, Graduate Committee: Yves U. Idzerda; This is a manuscript style paper that includes co-authored chapters.Physical symmetries inherent to a material are often reflected in its electronic and magnetic properties. The in-plane four-fold rotational symmetry of thin-film ferromagnets inherent to their tetragonal lattice is also exhibited by their cubic anisotropy. The magnetization as a function of applied magnetic field can be calculated via the Stoner- Wohlfarth model. These calculated hysteresis loops were fit to measured hysteresis loops to determine anisotropy constants consistent with known values. An electronic nematic state reduces the in-plane four-fold rotational symmetry of materials by inducing a structural transition from tetragonal to orthorhombic/monoclinic, with two-fold symmetry. This reduced symmetry persists in the electronic thermal transport. Nematicity enhances nearest-neighbor hopping along one axis and reduces it along the other. This results in a deformed Fermi surface compressed (elongated) along the axis of stronger (weaker) electron hopping. This drags van Hove singularities through the Fermi level, affecting quasiparticle lifetimes. Calculating conductivity from the Boltzmann kinetic equation, nematicity enhances thermal transport along one axis and diminishes it along the other. Additionally, s-wave superconductivity coexisting with nematicity creates a feedback on the superconducting gap with a d-wave instability, which can lead to gapless excitations. In the case of weak feedback, nematic superconductors behave like fully-gapped superconductors along both axes, where transport decreases exponentially with temperature. Once gapless excitations form, transport along both axes becomes T -linear at low-T . Similarly, striped antiferromagnetism (AFM2 and AFM3) reduces the rotational symmetry of a square unit cell to a larger two-fold symmetric magnetic cell. Modeling the band structure with a tight- binding model and considering a smaller periodicity in momentum-space, gaps the Fermi surface along one axis. Calculating conductivity reveals diminished transport along one axis and enhanced thermal transport along the other. Considering d-wave superconductivity in this model results in two cases. One has highly anisotropic transport with greatly enhanced T -linear transport along one axis and diminished transport decreasing exponentially with temperature along the other. The second has weakly anisotropic transport with diminished T -linear conductivity along both axes. The symmetry of a material's properties, such as magnetic anisotropy and thermal transport, are intrinsically linked to their crystalline, electronic, and magnetic symmetries.Item The interplay between the central engine and the circumnuclear environment in Compton-thin AGN(Montana State University - Bozeman, College of Letters & Science, 2022) Chalise, Sulov; Chairperson, Graduate Committee: Anne Lohfink; This is a manuscript style paper that includes co-authored chapters.All massive galaxies harbor a supermassive black hole (SMBH) at their galactic center. If these SMBH are actively feeding then they are called Active galactic nuclei (AGN). Their accretion system contains a corona, an accretion disk and an axisymmetric dusty torus. The torus can be connected physically and dynamically to the circumnuclear disk of the galaxy which acts as a molecular gas reservoir for material to be accreted onto the SMBH. Further, AGN can emit radiation from radio up to the gamma rays. The AGN accretion disk emits photons mostly in the optical/UV band which are Compton up-scattered in the corona to generate X-rays. If present, a jet can produce additional high-energy and Synchrotron emission. In some AGN, a huge amount of material can be stripped away from the accretion disk creating an outowing wind. These --radiation pressure, jet, wind etc.--inject energy back into the host galaxy, regulating the SMBH growth. There exist a complex interplay between the AGN feeding and feedback. Understanding this interaction between the central engine and its circumnuclear environment is vital in context of galaxy evolution. My work aims to study this interaction in low to moderately obscured (or Compton-thin) AGN using their broadband multi-epoch X-ray spectra plus other emission bands whenever appropriate. From the spectral analysis of broad-line radio galaxy 3C 109, I was able to constrain its high-energy cutoff for the first time. In another Seyfert galaxy Mrk 926, I was able to explore the origin of its soft excess, and found that a warm coronal origin was slightly preferred. Finally, I performed a joint multi-wavelength analysis with a physical torus model of a sample of Polar-scattered Seyfert 1 galaxies. I utilized their multi-epoch broadband X-ray spectra along with their infrared spectral/photometric data, and was able to constrain their torus properties. Despite being a sample of similar moderately-inclined Compton-thin AGN, I found a complex and varied distribution of gas and dust in their torus.Item A multi-wavelength study of dwarf galaxies with active massive black holes(Montana State University - Bozeman, College of Letters & Science, 2023) Kimbrell, Seth Jordan; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.Dwarf galaxies which host massive black holes with M less than or equivalent to 106M circled dot give us an opportunity to better understand the formation mechanism behind the supermassive black holes that live in the center of galaxies. Studying how common massive black holes in dwarfs are is an important step in constraining the channels that led to those supermassive black holes. An important part of that study is understanding in what types of dwarf galaxies we can expect to find massive black holes. I present a multi-wavelength study of dwarf galaxies which attempts to find any trends in the morphologies of the hosts of active massive black holes. I begin by modeling the structures of a sample of galaxies which have been identified as black hole hosts; I then perform an identical modeling on a sample of galaxies which show no signs of hosting a massive black hole. I finish by describing an X-ray search for massive black holes among irregular/disturbed galaxies, including the discovery of a very bright X-ray source which is extremely likely to be a massive black hole in a dwarf-dwarf merger. This is one of the first active massive black holes discovered in such a late-stage merger, and it is also notable for radiating at nearly its upper limit.Item A deeper look into X-ray-selected AGN candidates in dwarf galaxies with Chandra(Montana State University - Bozeman, College of Letters & Science, 2023) Sanchez, Adonis Arismendy; Chairperson, Graduate Committee: Amy E. Reines; This is a manuscript style paper that includes co-authored chapters.The ability to accurately discern active massive black holes (BHs) in local dwarf galaxies is paramount to understanding the origins and processes of "seed" BHs in the early universe. We present Chandra X-ray Observatory observations of a sample of three dwarf galaxies (M* < or = 3 x 109 M circled dot, z < or = 0.15) pre-selected by Latimer et al. (2021a) as candidates for hosting active galactic nuclei (AGN). The galaxies were selected from the NASA-Sloan Atlas (NSA) with spatially coincident X-ray detections in the eROSITA Final Equatorial Depth Survey (eFEDS). Our new Chandra data reveal X-ray point sources in two of the three galaxies with luminosities of log(L 2-10keV/[erg s -1]) = 39.1 and 40.4. In the target galaxy with the nondetection, we calculate an upper limit on the luminosity for a potential source. We observed notably higher fluxes and luminosities from the two detected X-ray sources compared to their original eFEDS observations, pointing to possible X-ray variability on the scale of a few years. We plot and fit the spectra of the X-ray sources with a power-law model, finding the likely presence of intrinsic absorption. The X-ray luminosities are above that expected from XRBs, but we cannot definitively rule out stellar-mass compact objects with the data on hand. Assuming the X-ray sources are accreting massive BHs with masses that scale with the stellar mass of the host galaxies, the Eddington ratios are on the order of a few x 10 -3.Item The evolution of the chemical abundance gradients in the merging Magellanic Cloud dwarf galaxies(Montana State University - Bozeman, College of Letters & Science, 2023) Povick, Joshua Tyler; Chairperson, Graduate Committee: David L. Nidever; This is a manuscript style paper that includes co-authored chapters.Dwarf galaxies are some of the most abundant objects in the Universe, but most of them are very distant and very faint. While observing these galaxies does pose some challenges, they are important to study because it is believed that larger galaxies, such as the Milky Way (MW), form from a series of dwarf mergers in a process called hierarchical merging. As if by chance, the Magellanic Clouds (MCs) are both bright enough and close enough to resolve individual stars. These two dwarf satellites of the MW are also in the process of merging together, presenting a great opportunity to examine how the abundance gradients of galaxies are impacted by intergalactic interactions. A great tool to study the MCs is the Apache Point Galactic Evolution Experiment (APOGEE). APOGEE is an H-band near infrared survey commissioned to measure chemical abundances and accurate radial velocities of the MW and its neighborhood. In the MCs, APOGEE was able to observe 6130 red giant branch (RGB) stars in the Large Magellanic Cloud (LMC) and 2062 RGB stars in the Small Magellanic Cloud (SMC). Individual stellar ages are derived using multiband photometry and spectroscopic parameters to compare to stellar isochrones. Using the abundance measurements of 20+ elements and the derived stellar ages, abundance gradients and their evolutions are extracted from radial abundance trends. The stellar ages in the LMC reveal that recent star formation has been concentrated in the center of that galaxy. The fields that overlay a spiral arm in the north of the LMC reveal median ages of ?2 Gyr. The age-metallicity relation (AMR) remains mostly flat with the exception of an increase in overall metallicity ?2 Gyr ago. Looking at the evolution of many abundance gradients in the LMC there is a U-shaped trend with an extremum around the same time as the increase in metallicity. Additionally, the SMC also shows a U-shaped trend in its abundance gradient evolutions albeit a few billion years earlier than the LMC. These results all correspond to a conjectured close interaction between the LMC and SMC in the recent past.Item Exploring exchange and transport dynamics in complex systems through nuclear magnetic resonance(Montana State University - Bozeman, College of Letters & Science, 2021) Nelson, Madison Lee; Chairperson, Graduate Committee: Joseph D. Seymour and Sarah L. Codd (co-chair); This is a manuscript style paper that includes co-authored chapters.Nuclear magnetic resonance (NMR) is uniquely qualified for non-invasive studies of systems providing insights into macro-, meso-, and microscale structures. NMR relaxation and diffusion methods are applied to characterize transport and magnetization exchange dynamics in various complex systems. These techniques are highly sensitive to molecular mobility restrictions which correlate to the ability to monitor thermodynamic phase transitions and changes in molecular environment. NMR diffusion and relaxation measurements are applied to characterize the effect of xylose on transport within zeolite beads. The ability for NMR to explore the transport phenomenon on multiple length and time scales is exploited to characterize how the introduction of xylose effects the transport structure of the bead. Eigenvector simulations of magnetization evolution within a coupled pore system during multidimensional NMR measurements, T1-T2 relaxation correlation experiments, allowed for insights into complex diffusion and exchange occurring within multiple systems. Additionally, multidimensional relaxation NMR measurements, in the form of varying echo-time spin-spin relaxation dispersion T2(tau) and spin-spin relaxation exchange T2-T2 experiments, are demonstrated to successfully characterize thermodynamic structural rearrangements of two natural straight-chained hydrocarbons and a natural wax. Temperature dependent magnetization exchange was found in both the longitudinal and transverse magnetization. The results indicate the ability of NMR relaxometry to detect magnetization exchange without mass or molecular exchange, also known as spin diffusion, including in the transverse magnetization. Spatial domain extent can be inferred from the exchange timescale and an estimate of the spin diffusion coefficient. NMR relaxometry methods were extended to glycerol behenate, a common pharmaceutical component. Glycerol behenate was decomposed into its three base components to explore how polymorphic structure and exchange depend on temperature within each pure lipid through T2(tau) and T2-T2 NMR relaxation experiments. These methods allowed for in-situ monitoring of thermodynamic dependent exchange across domains in addition to decoupling of transverse and longitudinal exchange. The results allow for calculation of exchange length scales across the micro- and mesoscales within the lipids. Ultimately, multidimensional NMR relaxometry is successfully demonstrated to be an effective technique for characterizing and monitoring structural changes in lipids across various phase transition temperatures and time and length scales.Item Thermal transport in superconductors with coexisting spin density wave order(Montana State University - Bozeman, College of Letters & Science, 2021) Choudhury, Sourav Sen; Chairperson, Graduate Committee: Anton Vorontsov; This is a manuscript style paper that includes co-authored chapters.In this thesis we study thermal transport in a two-dimensional system with coexisting s- or d-wave Superconducting (SC) and Spin Density Wave (SDW) orders. We analyse the nature of coexistence phase in a tight-binding square lattice with Q = (pi, pi) SDW ordering. The electronic thermal conductivity is computed within the framework of the Boltzmann kinetic theory, using Born approximation for the impurity scattering collision integral. We describe the influence of the Fermi surface (FS) topology, the competition between the SC and SDW order parameters, the presence or absence of zero energy excitations in the coexistence phase, on the low temperature behavior of thermal conductivity of the various paring states. We present qualitative analytical, and fully numerical results that show that the heat transport signatures of various SC states emerging from collinear SDW order are quite distinct, and depend on the symmetry properties of the SC order parameter under translation by the SDW nesting vector Q. A combination of (pi, pi)-SDW and the d x 2-y 2 pairing state results in fully gapped excitations, whereas (pi, pi)-SDW co-existing with either d xy or s-wave pairing states may always have gapless excitations. There appear special stable Dirac nodal points that are not gapped by the SC order in the coexistence phase, resulting in finite residual heat conductivity.Item Identifying RR Lyrae variable stars in the NoirLab Source Catalog with template fitting(Montana State University - Bozeman, College of Letters & Science, 2022) Matt, Kyle Louis; Chairperson, Graduate Committee: David L. NideverRR Lyrae are periodic variable stars generally with periods between 5 hours and 1 day. They can be used as standard candles for accurate distance measurements and thus are useful for studying the structure of the Milky Way and its stellar clusters. The second data release of the NoirLab Source Catalog is a large collection of 68 billion time-series measurements of 3.9 billion objects. To process this large volume of data, we designed a computer software package in Python called Leavitt to automate the detection process and measure their properties including period, magnitude, epoch of maximum brightness and amplitude of their pulsations by fitting their light curves to templates. In addition to identifying RR Lyrae, it is expected that Leavitt can be extended to identify similar variable stars such as Cepheids in the same dataset. Distances were calculated for the initial catalog of RR Lyrae candidates using parameters measured with this script.