Browsing by Author "Neubauer, Kelly J."

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    Quantum simulation of an extended Dicke model with a magnetic solid
    (Springer Nature, 2024) Marquez Peraca, Nicolas; Li, Xinwei; Moya, Jaime M.; Hayashida, Kenji; Kim, Dasom; Ma, Xiaoxuan; Neubauer, Kelly J.; Fallas Padilla, Diego; Huang, Chien-Lung; Dai, Pengcheng; Nevidomskyy, Andriy H.; Pu, Han; Morosan, Emilia; Cao, Shixun; Bamba, Motoaki; Kono, Junichiro
    The Dicke model describes the cooperative interaction of an ensemble of two-level atoms with a single-mode photonic field and exhibits a quantum phase transition as a function of light–matter coupling strength. Extending this model by incorporating short-range atom–atom interactions makes the problem intractable but is expected to produce new physical phenomena and phases. Here, we simulate such an extended Dicke model using a crystal of ErFeO3, where the role of atoms (photons) is played by Er3+ spins (Fe3+ magnons). Through terahertz spectroscopy and magnetocaloric effect measurements as a function of temperature and magnetic field, we demonstrated the existence of a novel atomically ordered phase in addition to the superradiant and normal phases that are expected from the standard Dicke model. Further, we elucidated the nature of the phase boundaries in the temperature–magnetic-field phase diagram, identifying both first-order and second-order phase transitions. These results lay the foundation for studying multiatomic quantum optics models using well-characterized many-body solid-state systems.
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    Spin structure and dynamics of the topological semimetal Co3Sn2-xInxS2
    (Springer Nature, 2022) Neubauer, Kelly J.; Ye, Feng; Shi, Yue; Malinowski, Paul; Gao, Bin; Taddei, Keith M.; Bourges, Philippe; Ivanov, Alexandre; Chu, Jiun-Haw; Dai, Pengcheng
    The anomalous Hall effect (AHE), typically observed in ferromagnetic (FM) metals with broken time-reversal symmetry, depends on electronic and magnetic properties. In Co3Sn2-xInxS2, a giant AHE has been attributed to Berry curvature associated with the FM Weyl semimetal phase, yet recent studies report complicated magnetism. We use neutron scattering to determine the spin dynamics and structures as a function of x and provide a microscopic understanding of the AHE and magnetism interplay. Spin gap and stiffness indicate a contribution from Weyl fermions consistent with the AHE. The magnetic structure evolves from c-axis ferromagnetism at $$x = 0$$to a canted antiferromagnetic (AFM) structure with reduced c-axis moment and in-plane AFM order at $$x = 0.12$$and further reduced c-axis FM moment at $$x = 0.3$$. Since noncollinear spins can induce non-zero Berry curvature in real space acting as a fictitious magnetic field, our results revealed another AHE contribution, establishing the impact of magnetism on transport.
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    Studies of the Interplay of Magnetic and Electronic Phenomena in Kagome Lattices and Other Systems
    (2024-04-10) Neubauer, Kelly J.; Dai, Pengcheng
    Novel magnetic materials form a vibrant research field investigating the interplay between topology, magnetism, and electronic correlation. The crystal geometry and magnetic state of a material can significantly influence the emergent electronic properties. For example, the kagome lattice has natural geometric frustration known to host flat electronic bands, non-trivial topological properties, and unusual spin textures. This dissertation reports studies of magnetic materials, including several kagome systems, that exhibit interesting electronic responses, including large anomalous and topological Hall effects and magnetoresistances. The electronic phenomena were studied via magneto-transport measurements. The magnetic phenomena were studied via neutron scattering measurements, which can directly probe spin structures and dynamics. First, this dissertation describes the large, anisotropic magnetoresistance observed in BaFe2As2. It was determined that this effect could not be described by only the anisotropic Fermi surface but must consider the field-dependent magnetic structure. Second, the neutron scattering studies of the kagome lattice material YMn6Sn6 are described. Skyrmions, the typical signature of the topological Hall effect, were absent in YMn6Sn6. Instead, YMn6Sn6 was one of the first examples of a material that hosts a non-coplanar spin texture that provides the non-zero spin chirality to support the effect without requiring skyrmions. Next, this dissertation presents the doping dependence of the magnetism and electronic behavior of another kagome lattice material, Co3Sn2-xInxS2. In this study, neutron scattering was used to determine the spin dynamics and structures, which indicated upon In doping that the ferromagnetic spin structure cants to form a noncollinear spin structure. Such a spin texture could induce non-zero Berry curvature and support an additional contribution to the anomalous Hall response. Finally, this dissertation details a study of the kagome B35 phase of FeGe. Previous studies observed an enhanced anomalous Hall effect below the canting temperature. The neutron scattering measurements in this thesis ruled out skyrmions or defects as the mechanism for the effect. They also provided an understanding of the magnetic order in the spin-flop phase. Overall, this dissertation contributes to a deeper understanding of the interplay between non-trivial magnetism and electronic behavior and supports the potential of novel magnetic materials in spintronics applications.