Wu, Judy ZWang, Xiang2009-05-082009-05-082008-01-012008http://dissertations.umi.com/ku:10069https://hdl.handle.net/1808/4543The most promising characteristic of a High Temperature Superconductor (HTS) is its ability to carry larger electrical current at liquid nitrogen boiling temperature and strong applied magnetic field with minimal dissipation. Numerous large scale applications such as HTS transmission cables, HTS magnets and HTS motors have been developed using HTS materials. The major limitation that prevents its wide commercialization is its high cost-to-performance ratio. However, the effort to further improve HTS current carrying capability is jeopardized by a mysterious thickness dependence of the critical current density (Jc) --- Jc monotonically decreases with increasing thickness (t) at 77 K and self-field (SF). This poses a great challenge for both HTS applications and the understanding of vortex dynamics. What further complicates this issue is the complex defect structure in HTS films as well as the creep nature of magnetic vortices at a finite temperature. After a systematic study of the temperature and magnetic field effects on Jc-t, we conclude that Jc-t is most likely the result of a collective pinning effect dictated by a random pinning potential. Besides that, thermal fluctuations also alter Jc-t in a predictable way. Therefore, by either modifying the vortex structure or pinning structure, Jc-t can be eliminated. Indeed, a thin film Jc has been restored in a HTS/insulator/HTS trilayer while the magnetic coupling is weakened. Moreover, Jc-t has been removed when the random distributed point pins are overpowered by strong linear defects.104 pagesENThis item is protected by copyright and unless otherwise specified the copyright of this thesis/dissertation is held by the author.Condensed matter physicsCritical current densityDefectsFilmHigh tc superconductorThicknessDissertationopenAccess