Browsing by Subject "superconductivity"
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Item Dielectric-Loaded Microwave Cavity for High-Gradient Testing of Superconducting Materials(2011-08-08) Pogue, Nathaniel JohnstonA superconducting microwave cavity has been designed to test advanced materials for use in the accelerating structures contained within linear colliders. The electromagnetic design of this cavity produces surface magnetic fields on the sample wafer exceeding the critical limit of Niobium. The ability of this cavity to push up to 4 times the critical field provides, for the first time, a short sample method to reproducibly test these thin films to their ultimate limit. In order for this Wafer Test cavity to function appropriately, the large sapphire at the heart of the cavity must have specific inherent qualities. A second cavity was constructed to test these parameters: dielectric constant, loss tangent, and heat capacity. Several tests were performed and consistent values were obtained. The consequences of these measurements were then applied to the Wafer Cavity, and its performance was evaluated for different power inputs. The Q_0 of the cavity could be as low as 10^7 because of the sapphire heating, therefore removing the ability to measure nano-resistances. However, with additional measurements in a less complex environment, such as the Wafer Test Cavity, the Q_0 could be higher than 10^9.Item Enhanced Superconducting Properties of Iron Chalcogenide Thin Films(2013-07-26) Chen, LiAmong the newly discovered iron-based superconductor, FeSe with the simplest structure and a transition temperature (T_c) around 8 K arouses much research interest. Although its Tc is much lower than that of the cuprates, iron chalcogenide has low anisotropy, slow decrease of the critical current density (J_c) with increasing magnetic field and high upper critical field H_c2 as well as easy composition control, which makes it a promising candidate to substitute NbSn/NbTi for high field applications. Compared with its bulk counterpart, iron-based superconductor thin film has a great potential in developing the ordered quasi-2D structure and is suitable for coating technology which has already been applied in YBa_2Cu_3O_7-x coated conductors. In this thesis, we first optimized pure FeSe thin films by different growth conditions using pulsed laser deposition (PLD) and post-annealing procedures. The microstructure properties of the films including the epitaxial quality, interface structure and secondary phase have been studied and correlated with the superconducting properties. Second, we reported our initial attempt on introducing the flux pinning centers into FeSe_0.5Te_0.5 thin films either under a controlled oxygen atmosphere or with a thin CeO_2 interlayer. The microstructure of the FeSe_0.5Te_0.5 films including the epitaxial quality, the interface structure and the secondary phase have been studied and correlated with the in-field performance of the superconducting thin films to explore the pinning properties of these nanoscale defects. Very recently, ion beam assisted deposition (IBAD) substrates have been used to grow high quality FeSe_0.5Te_0.5 tape with excellent in-field performance. The film on IBAD substrate involves multiple steps of seed layer and buffer layer deposition to establish the epitaxial growth template. Therefore a simplified and cost effective iron-based coated conductor is more desirable. Towards the practical application, we demonstrated the growth of superconducting FeSe_0.5Te_0.5 film on amorphous glass substrates for the first time. The film is highly textured with excellent superconducting properties, e.g., T_c of 10 K and J_c under self-field as high as 1.2?10^4 A/cm^2 at 4 K. Further optimization of the film growth with various nanoscale interlayers has been carried out. In addition the Te rich iron chalcogenide thin film with composition close to the composition with antiferromagnetic (AFM) transition has been demonstrated. Compared to the FeSe_0.5Te_0.5 which claimed to be the optimum composition from the literature report, the FeSe_0.1Te_0.9 is even more promising for the high field application with its coexistence of super high upper critical field and high critical current density.Item Quantum tunneling, quantum computing, and high temperature superconductivity(Texas A&M University, 2005-02-17) Wang, QianIn this dissertation, I have studied four theoretical problems in quantum tunneling, quantum computing, and high-temperature superconductivity. I have developed a generally-useful numerical tool for analyzing impurity-induced resonant-state images observed with scanning tunneling microscope (STM) in high temperature superconductors. The integrated tunneling intensities on all predominant sites have been estimated. The results can be used to test the predictions of any tight-binding model calculation. I have numerically simulated two-dimensional time-dependent tunneling of a Gaussian wave packet through a barrier, which contains charged ions. We have found that a negative ion in the barrier directly below the tunneling tip can deflect the tunneling electrons and drastically reduce the probability for them to reach the point in the target plane directly below the tunneling tip. I have studied an infinite family of sure-success quantum algorithms, which are introduced by C.-R. Hu [Phys. Rev. A {\bf 66}, 042301 (2002)], for solving a generalized Grover search problem. Rigorous proofs are found for several conjectures made by Hu and explicit equations are obtained for finding the values of two phase parameters which make the algorithms sure success. Using self-consistent Hartree-Fock theory, I have studied an extended Hubbard model which includes quasi-long-range Coulomb interaction between the holes (characterized by parameter V). I have found that for sufficiently large V/t, doubly-charged-antiphase-island do become energetically favored localized objects in this system for moderate values of U/t, thus supporting a recent conjecture by C.-R. Hu [Int. J. Mod. Phys. B {\bf 17}, 3284 (2003)].