Browsing by Author "Klemm, Mason Lee"

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    Neutron Diffraction Studies of Kagome Metal FeGe
    (2025-04-25) Klemm, Mason Lee; Dai, Pengcheng; Yi, Ming
    The two-dimensional kagome lattice is the operative characteristic in a large class of materials that exhibit a wide range exotic magnetic and electronic phenomena purely as a consequence of the unique corner-sharing triangle geometry and associated symmetries of the kagome lattice. Some kagome materials possess multiple degrees of freedom, enabling the study of the interplay between competing orders like superconductivity, magnetism, charge density wave (CDW), and topological phases among others. This thesis centers on neutron scattering experiments of the antiferromagnetically ordered (AFM) kagome metal FeGe that was recently found to host CDW order — a first in a magnetically ordered kagome material. A variety of supplemental experimental techniques were performed to aid in the characterization of various properties in FeGe including Raman spectroscopy, transport, resonant inelastic x-ray scattering (RIXS), scanning tunneling electron microscopy (STEM), muon spin resonance, and angle-resolved photoemission spectroscopy (ARPES). We additionally utilize a post-growth annealing process to tune samples from long-range CDW order to no CDW order repeatedly, acting as a powerful tuning parameter in our studies. Using elastic and inelastic neutron scattering, we uncover two competing magnetic orders at low temperatures, one A-type AFM and one screw-like order, previously believed to be a single double-cone order below TCanting. Additionally, the low temperature screw-like magnetic order is suppressed and enhanced in tandem with CDW order during annealing while the A-type order only exhibits a small enhancement at TCDW in samples with long-range CDW order. Our transport measurements reveal an order of magnitude enhancement of the anomalous Hall effect (AHE) in samples with long-range CDW order and the complete absence of AHE in samples with no CDW order. We find the AHE in FeGe is of intrinsic origin stemming from a large Berry curvature and mirrors the magnitude of the giant AHE in the related superconducting kagome family AV3Sb5 (A= K, Rb, Cs). Through a combination of Raman, ARPES, and neutron Larmor diffraction, we identify lattice instabilities and electronic band shifts at TCanting and TCDW in long-range CDW ordered samples suggesting a strong coupling between spin, lattice, and electronic degrees of freedom in the system. Lastly, we describe a microscopic mechanism for the suppression and enhancement of CDW via annealing through neutron scattering and STEM imaging. We find annealing at high temperatures creates Ge vacancies that inhibit the formation of CDW when spread uniformly throughout the sample. Upon annealing at lower temperatures, the vacancies coalesce to form large stacking faults favorable to CDW formation.