https://etd.auburn.edu/handle/10415/1 Wed, 15 Apr 2026 06:24:30 GMT 2026-04-15T06:24:30Z https://etd.auburn.edu/handle/10415/10220 Collective Excitations in Low-Dimensional Magnetic Materials Tang, Chunli Low-dimensional quantum materials provide a versatile platform for exploring collective excitations such as phonons, magnons, and topological spin textures. Advances in synthesis techniques, including molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and mechanical exfoliation (ME), have enabled the discovery and investigation of atomically thin materials and oxide heterostructures that exhibit rich quantum phenomena. These low-dimensional systems can host collective excitations, facilitate the interplay among lattice, charge, and spin degrees of freedom, and enable the study of fundamental quantum phenomena that govern energy dissipation, information transport, and phase transitions in condensed matter. Understanding and controlling these collective excitations is essential for advancing quantum information technologies, spintronics, and energy-efficient computing. However, the manipulation of such excitations through external stimuli, including optical excitation, magnetic interactions, and interfacial strain, remains incompletely understood. This dissertation investigates collective excitations in three forms, lattice vibrations (phonons), topological spin textures, and spin dynamics (magnons), in two-dimensional transition-metal dichalcogenides (TMDCs) and epitaxial oxide thin films. First, the optical excitation of chiral phonons is investigated in monolayer MoS2 as a route to probing phonon-driven magnetic phenomena. TMDCs host chiral phonons at high-symmetry points of the Brillouin zone, providing an ideal platform to study their interaction with the valley degree of freedom. Helicity-resolved magneto-Raman spectroscopy reveals a doubly degenerate chiral phonon mode at the Brillouin zone center. Wavelength- and temperature-dependent measurements show that the chiral phonon is activated through resonant excitation of the A exciton. The observed linear Zeeman splitting is reproduced by theoretical calculations based on morphic effects in nonmagnetic crystals. Second, topological spin textures are explored in proximity-coupled Fe-doped monolayer WSe2 (Fe:WSe2) synthesized by CVD on a Pt Hall-bar device. Electrical transport measurements reveal antisymmetric humps in the Hall resistivity. Temperature-dependent measurements further identify additional antisymmetric peak features that are distinct from the intrinsic anomalous Hall effect. These features are attributed to the topological Hall effect arising from non-coplanar spin configurations, indicating the presence of topologically nontrivial magnetic textures. Third, magnon--phonon coupling is investigated in epitaxial La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) heterostructures using ferromagnetic resonance (FMR) spectroscopy. The FMR spectra reveal strong coupling between the Kittel magnon mode and transverse acoustic phonons, evidenced by anticrossing gaps in the Kittel dispersion. Vibrating sample magnetometry (VSM) and second-harmonic generation (SHG) measurements show that the STO substrate undergoes a cubic-to-tetragonal structural phase transition, which introduces interfacial strain. This strain lifts the degeneracy of the Kittel mode and splits it into multiple bands. An analytical model incorporating magnon--phonon coupling reproduces the strain-induced magnon splitting, establishing a platform for a tunable hybrid magnon--phonon cavity system. To support these studies, QuDAP (Quantum Materials Data Acquisition and Processing), an open-source Python-based software package for automated instrument control and real-time data visualization based on Quantum Design physical property measurement systems (PPMS), was developed and implemented. Together, these results demonstrate multiple pathways for manipulating collective excitations in low-dimensional quantum materials while advancing experimental capabilities for quantum materials research. Tue, 14 Apr 2026 00:00:00 GMT https://etd.auburn.edu/handle/10415/10220 2026-04-14T00:00:00Z https://etd.auburn.edu/handle/10415/10219 Dietary barley inclusion to replace wheat, with or without carbohydrases supplementation in Pacific white shrimp (Litopenaeus vannamei) diets: Effects on growth performance, nutrient utilization, and physiological gene expression Gonzalez, Jairo Supplementation of exogenous enzymes in barley-based diets could aid in hydrolyzing the non-starch polysaccharides (NSP). To investigate the effects of enzyme supplements in barley- based diets replacing wheat, a 5-week growth trial was conducted on Pacific white shrimp (Litopenaeus vannamei). Six isonitrogenous (35%) and isolipidic (7%) diets were formulated, namely, whole wheat-based (17.42 g/100 g diet) basal diet (BW); barley-based basal diet (BB); and four enzyme-supplemented diets were BB supplemented with either xylanase (XYL) or a multi-enzymatic solution (MES) at manufacturers’ recommended dose (low, L) and double the recommended dose (high, H) and denoted as BB:XYL-L, BB:XYL-H, BB:MES-L, and BB:MES- H, respectively. Shrimp (0.72 ± 0.03 g) were stocked in 108-L aquaria. Diets were randomly assigned, with six tanks replicates per treatment. At the completion of the trial, growth performance parameters, apparent digestibility coefficients (ADC), and physiological gene expression in hepatopancreas were analyzed. Results showed no significant differences in growth performance across treatments (p > 0.05). However, significant differences in ADC of energy (p = 0.010) and dry matter (p = 0.001) were observed. The BB:XYL-L diet significantly increased energy digestibility compared to the BB diet without enzyme supplementation. Nonetheless, dietary barley inclusion with or without enzyme supplementation in shrimp diets did not significantly (p > 0.05) impact the expression of tor, ghsr1l, cat, tgfβ, propo, lgbp, and hsp70 in the hepatopancreas. However, hierarchical clustering of multigene responses showed that BB:MES-L and BB:MES-H had gene expression profiles similar to BW. Based on these findings, barley could be used as a wheat replacement without compromising growth performance, and the addition of carbohydrase could enhance energy digestibility and physiological responses. Tue, 14 Apr 2026 00:00:00 GMT https://etd.auburn.edu/handle/10415/10219 2026-04-14T00:00:00Z https://etd.auburn.edu/handle/10415/10218 Falls and Violent Mechanisms of Traumatic Brain Injury Reduce Very Long-Term Locomotor Functional Independence Beth, Matthew Traumatic Brain Injury (TBI) is a significant global public health concern and is a leading cause of long-term disability. In 2021, approximately 37.9 million people worldwide were living with TBI-related disabilities (Liu et al., 2026). While research on short-term post-TBI outcomes is well documented, much less is known about how TBI mechanisms shape functional independence across the lifespan. As such, the objective of this dissertation was to determine whether TBI mechanisms predict very long-term locomotor and stair-climbing functional abilities post-TBI. The dissertation comprises three separate investigations. The first involved a systematic review synthesizing existing research on the relationship between TBI mechanisms and long-term functional independence outcomes. In accordance with PRISMA standards, studies were identified, screened, and evaluated for risk of bias to clarify how TBI mechanisms from falls, motor vehicle accidents (MVAs), violence-related injuries, and sports-related injuries affect long-term functional recovery trajectories. Findings from different studies commonly indicate strong evidence that TBI mechanisms are associated with variability in long-term functional outcomes and disability. Secondly, a longitudinal analysis examined whether TBI mechanisms predict very long-term locomotor functional independence. Data were obtained from the Traumatic Brain Injury Model Systems (TBIMS) national database. Ordinal Functional Independence Measure (FIM) locomotion scores were converted to interval-level logit measures via Rasch analysis, thereby enhancing measurement accuracy and interpretability. After converting the data, linear mixed-effects regression models were applied to evaluate locomotor functional trajectories across follow-up data collections (collected up to 30 years post-TBI). Third, there is another longitudinal analysis focusing on a more complex mobility activity requiring coordination, balance, and lower-body strength: stair-climbing for functional independence. Using the same Rasch analysis procedure and subsequent mixed-effects modeling, longitudinal changes in stair-climbing ability were examined over 30 years post-TBI. In both longitudinal analyses, TBI mechanisms were significant predictors of the respective mobility-related functional independence domains. Individuals with TBIs caused by either falls or violence-related mechanisms exhibited lower very long-term locomotor and stair-climbing functional independence relative to their peers with TBIs from MVAs or sports-related mechanisms. The results indicate that TBI mechanisms function as an early prognostic indicator of very long-term functional recovery. Collectively, this dissertation highlights the longitudinal impact of injury mechanism on mobility trajectories post-TBI and emphasizes the necessity of integrating etiological context into long-term rehabilitation planning. By synthesizing a systematic review with longitudinal mixed-effects modeling using Rasch-transformed functional measures, this work expands current knowledge of how TBI mechanisms shape functional independence decades post-TBI and provides evidence to support clinical prognostication and rehabilitation strategy development. Tue, 14 Apr 2026 00:00:00 GMT https://etd.auburn.edu/handle/10415/10218 2026-04-14T00:00:00Z https://etd.auburn.edu/handle/10415/10217 Three Essays on Applied Economics: Analyses of Market Dynamics and Consumer Behavior in Food Industries Deb Palash, Prokash This dissertation examines how food systems are shaped by market integration, price volatility transmission, and heterogeneous consumer adoption. The first essay analyzes volatility spillovers between capture fisheries and aquaculture products in Bangladesh, showing that the rapid expansion of aquaculture has important implications for captured fish markets and food-security outcomes. The second essay investigates animal protein price volatility in the European Union and finds that volatility is largely homegrown, transmitted through dense intra-EU trade linkages rather than external markets. The third essay develops a Hurdle Structure Latent Class Mixed Model to study household adoption of plant-based meat alternatives in the United States, identifying distinct consumer trajectories and the demographic and economic factors associated with sustained adoption. Together, these essays contribute new empirical evidence and methodological tools for understanding market dynamics and consumer behavior in modern food industries. Tue, 14 Apr 2026 00:00:00 GMT https://etd.auburn.edu/handle/10415/10217 2026-04-14T00:00:00Z