Broken Kramers' degeneracy in altermagnetic MnTe

Altermagnetism is a newly identified fundamental class of magnetism with vanishing net magnetization and time-reversal symmetry broken electronic structure. Probing the unusual electronic structure with nonrelativistic spin splitting would be a direct experimental verification of altermagnetic phase. By combining high-quality film growth and $in~situ$ angle-resolved photoemission spectroscopy, we report the electronic structure of an altermagnetic candidate, $\alpha$-MnTe. Temperature dependent study reveals the lifting of Kramers{\textquoteright} degeneracy accompanied by a magnetic phase transition at $T_N=267\text{ K}$ with spin splitting of up to $370\text{ meV}$, providing direct spectroscopic evidence for altermagnetism in MnTe

Physics of the Electronic Structure and Collective Excitations in Transition Metal Compounds

In condensed matter systems, novel phenomena such as superconductivity and magnetism can emerge from an intricate interplay between the materials lattice, spin, charge, and orbital degrees of freedom. Understanding how the structural and electronic degrees of freedom are coupled in transition metal compounds is crucial not only from the viewpoint of fundamental condensed matter physics, but also to establish a potential groundwork for substantial technological advancement through the discovery of exotic phases. In this dissertation, I illustrate how the electronic structure and collective excitations can be manipulated by controlling the dimensionality and interfacial structure using molecular beam epitaxy (MBE). Specifically, three studies, each based on a different transition metal system, are presented. First, I demonstrate a significant modification to the charge and orbital configurations of LaCoO3 by utilizing dimensional confinement and interfacial charge transfer in a bi-layer heterostructure. The change in the electronic structure results in a unique magnetic ground state, which is inaccessible in bulk cobaltates. In the second study, I investigate the spin excitation in dimensionally confined ferromagnetic Fe thin films. The dispersion of the spin excitation evolves anisotropically as the thickness of the Fe layers is reduced. This physics of this scaling is accurately captured by a simple Heisenberg model based on spin exchange interactions. In the last study, I investigate how the spin exchange pattern in the antiferromagnetic phase of NdNiO3 can be adjusted by changing the component layer thicknesses in heterostructures. Using the atomic layering capability of MBE, I also demonstrate control of the exchange interaction, which changes the quantum and classical dynamics of collective magnetic excitations. In all three studies, synthesis of high-quality thin films, advanced characterization techniques involving synchrotron x-rays, and theoretical calculations are harmoniously integrated to reveal the fascinating physics in transition metal systems. The results presented in this thesis provide a pathway to realize new emergent electronic and magnetic properties by manipulating the underlying electronic interactions in thin film heterostructures

The Impact of Individual Motivations and Social Capital on the Continuous Usage Intention of Mobile Social Apps

Mobile social apps have experienced enormous growth as online personal networking media. Social exchange theory (for individual motivation), theories of collective action and social capital theory (for social capital) can be applied in order to understand how an individual’s behavior may exert effects on or receive influences from other users with regard to the continuance usage intention of mobile social apps. This study examines individual motivations and social capital affecting relationship quality in terms of trust in and satisfaction with mobile social apps and how these factors influence continuance usage intentions of mobile social apps. An online survey is used to collect 320 responses from users of mobile social apps. Our results indicate that promotional motivation and innovativeness affect relationship quality levels. Maintaining relational enhancement, social homogeneity, and social identity along with service usefulness have effects on the relationship quality level, which in turn affects continuance usage intention. Given the lack of studies regarding the application of the theories of collective action and social capital to gain a better understanding of continuance usage intentions, this study provides additional insight into how individual motivations and social capital affect continuous usage

The impact of online review helpfulness and word of mouth communication on box office performance predictions

Abstract While electronic word-of-mouth (eWOM) variables, such as volume and valence have been posited in previous studies to consistently affect product sales, there is a lack of studies on the different contexts and outcomes that affect the importance of eWOM variables. In order to fill this gap, this study attempts to use the helpfulness of reviews and reviewers as moderators to predict box office revenue, comparing the prediction performances of business intelligence (BI) methods (random forest, decision trees using boosting, the k-nearest neighbor method, discriminant analysis) using eWOM between high and low review or reviewer helpfulness subsample in the Korean movie market scrawled from the Naver Movies website. The results of applying machine learning methods show that movies with more helpful reviews or those that are reviewed by more helpful reviewers show greater prediction performance, and review and reviewer helpfulness improve the prediction power of eWOM for box office revenue. The prediction performance will improve if the characteristics of eWOM are likely to be combined to contribute to box office revenue to a greater extent

The Causal Relationships among EDI Controls: A Structural Equation Model

Abstract Advances in EDI (Electronic Data Interchange) demand appropriate controls in order to realize the potential benefits from it. Formal, informal, and automated controls are basic parts of EDI controls. The state of one of three controls is suggested to affect performance indirectly through their effect on another controls in the research model. The causal relationships are tested using structural equation modeling approach with LISREL. Informal controls turn out to play an important role in the causal relationships, as they significantly affect formal and automated controls to have indirect effect on performance. The results of the study indicate that the interrelationships among controls are closely related to system performance