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Browsing by Subject "Superconductor"

Now showing 1 - 3 of 3
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    Annealing Study of the Cuprate Superconductor HgBa2CuO4+δ
    (2024-01-10) Wendland, Blake
    The discovery of high-temperature superconductivity in cuprates in the 1980s triggered an unprecedented investigation into this exciting family of quantum materials. This work explores the electronic phase diagram of the cuprate HgBa2Cu4+δ (Hg1201). The electronic “hole” concentration of Hg1201 and other cuprates can be adjusted by changing the density of interstitial oxygen atoms. High-quality single crystals of Hg1201 with low hole concentrations are very difficult to produce without destroying samples because harsh annealing environments are required to remove interstitial oxygen. Our goal is to reliably produce Hg1201 crystals with low hole concentrations to allow for subsequent measurement of their structural and electronic properties.
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    Proximity Effects in Ferromagnet/Superconductor Layered Heterostructures with Inhomogeneous Magnetization
    (2014-07) Wu, Chien-Te
    In this thesis, we present a theoretical investigation of the proximity effects in ferromagnet/superconductor heterostructures with inhomogeneous magnetization, including ferromagnet/ferromagnet/superconductor (F1F2S) trilayers and conical-ferromagnet/superconductor bilayers. We numerically obtain the self-consistent solutions of the Bogoliubov-de Gennes (BdG) equations and use these solutions to compute the relevant physical quantities. In F1F2S trilayers, we find that the critical temperature, Tc, can be a non-monotonic function of the angle &alpha between magnetizations in F layers. The minimum Tc(&alpha) often occurs when magnetizations are mutually perpendicular (&alpha=&pi/2). In addition, we demonstrate that the Tc minimum corresponds to the maximum of the penetration of the long-range triplet amplitudes. We compare our theoretical results with experiment and find that they are in excellent agreement. We also study other aspects of proximity effects such as the local density of states, local magnetizations, and thermodynamic functions. In conical-ferromagnet/superconductor bilayers, we obtain the relation between Tc and the thickness dF of the magnetic layer, and find that the Tc(dF) curves include multiple oscillations. Moreover, for a range of dF, the superconductivity is reentrant with temperature T: as one lowers T the system turns superconducting, and when T is further lowered it returns to normal. We demonstrate that the behavior of both m=0 and m=±1 triplet amplitudes are related to the intrinsic periodicity of conical ferromagnet. Our theoretical fits of Tc(dF) are in good agreement with experimental data. The transport properties, including the tunneling conductance and the spin polarized transport, in F1F2S trilayers are investigated. To fully take into account proximity effects, we adopt a transfer matrix method incorporated with the Blonder-Tinkham-Klapwijk formalism and self-consistent solutions to the BdG equations. We show that our method ensures that conservation laws are properly satisfied. Our results indicate that the behavior of tunneling conductance depends on the misorientation angle between magnetizations, and also exhibits resonance effects. We also investigate the bias dependence of non-equilibrium spin transfer torque and its connection to both spin currents and local magnetizations.