Browsing by Author "Moya, Jaime M"

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    Effects of synthesis conditions on the transition metal dichalcogenide TiSe2
    (2020-04-24) Moya, Jaime M; Morosan, Emilia
    TiSe2 is part of a family of materials known as the transition metal dichalcogenides. Their quasi-two dimensional crystal structure sometimes gives rise to interesting phenomena, spanning a vast array physical and electronic properties including charge order or superconductivity when various intercalants or dopants are added. TiSe2 was shown to have charge ordering at a temperature of 200 K almost 45 years ago. Despite the time that has elapsed between this discovery and now, TiSe2 continues to be an intensely studied material because the nature of its charge ordering is still under debate. Some of the contradicting results are fueled by sample dependency related to growth method and conditions. Because of the small band gap or band overlap in TiSe2, it is not surprising that dilute impurities and growth conditions can drastically aect the transport properties of TiSe2. In this work, I systematically study the effect of variable growth conditions including post synthesis cooling rate, anneal time, and temperature, on the electrical resistivity of TiSe2. I find that slow cooling polycrystalline TiSe2 post synthesis drastically increases the low temperature resistivity, which is in stark contrast to the metallic low temperature resistivity observed in single crystalline TiSe2 grown by iodine vapor transport, where the iodine charge dopes the sample. Together, the logarithmic divergence of the resistivity and signatures in low temperature magnetoresistance point to signatures of the weak-localization effect. Annealing samples at low temperatures post synthesis also increase the low temperature resistivity, but with a less profound aect. Finally, quenching samples from high temperature, freezes in disorder, and decreases the low temperature resistivity.
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    Quantum criticality and complex spin textures in the extremes of magnetism: from the itinerant moment to local moment limits
    (2022-12-02) Moya, Jaime M; Morosan, Emilia
    Magnetism exists on a spectrum spanning the local moment to itinerant moment limits. In the local moment limit, magnetic moments arise from partially filled electronic shells and the magnetic moments are localized in real space. Moments in itinerant magnets come from strong electron-electron interactions of itinerant electrons. Consequently, moments in the itinerant limit are delocalized in real space. While no current theory can successfully interpolate between the regimes, both are areas of vibrant research. This dissertation is composed of two projects, one at each end of the magnetic spectrum. The first project details the single crystal growth and discovery of a new itinerant antiferromagnet, Ti3Cu4, which magnetically orders below the Neel temperature TN = 11.3 K. TN can be continuously suppressed towards a quantum critical point (QCP) at T = 0 with a magnetic field Hc II c = 4.89 T. A Fermi liquid-non-Fermi liquid crossover is measured convergent at the QCP together with the divergence of the Gruneisen parameter. Density functional theory (DFT) calculations reveal that Fermi surface nesting cannot explain the experimentally determined magnetic ordering wavevector, making Ti3Cu4 unique among the itinerant antiferromagnets. From a quantum criticality perspective, Ti3Cu4 is the first field-induced QCP in a d-electron system, likely reflecting an anomalously small energy scale compared to other d-electron magnets. The second project describes the search for topological spin textures in the centrosymmetric square-net Eu(Ga1-xAlx)4 system. In non-centrosymmetric topological spin-texture hosts, the Dzyaloshinskii-Moriya (DM) interaction has been deemed crucial in stabilizing the topological spin textures. By contrast, the mechanism for stabilizing topological spin-textures in centrosymmetric magnets is debated since the DM interaction is absent. Here, the magnetic field – temperature phase diagrams of Eu(Ga1-xAlx)4 (0.15