Frequency Dependent Investigation of the Vortex Dynamics in High Temperature Superconductors

[摘要] The high transition temperature and short coherence length of the high temperature superconducting cuprates lead to new vortex dynamics not found in conventional low temperature superconductors. The existence of a vortex-liquid state and a second-order phase transition between this vortex-liquid and the vortex-solid states has raised important questions about the vortex dynamics near this phase transition and about the interaction between the vortices and sample defects. In this thesis, new frequency dependent measurements systems are developed to measure the ac impedance and ac magnetic susceptibility of YBa2Cu3O7 single crystals over a broad frequency range from 10(2)Hz to 10(7)Hz. In addition, a miniature Hall probe magnetometer is used to measure the third harmonic susceptibility of a YBa2Cu3O7 single crystal. New critical scaling relations and analysis techniques are developed for ac magnetic susceptibility and the third harmonic transmissivity which for the first time enable critical scaling analysis to be applied to the measurements of these physical quantities. In twinned YBa2Cu3O7 single crystals with only point defects, we provide the most conclusive evidence yet for the existence of a second-order vortex-solid to vortex-liquid phase transition by demonstrating consistent critical scaling for three different experimental techniques: dc voltage versus current, ac impedance versus frequency, and ac susceptibility versus frequency. We also use the new frequency dependent techniques to investigate the effects of symmetry breaking in the vortex system by introducing columnar defects with heavy ion irradiation. In the case of parallel columnar defects, the symmetry is broken parallel and perpendicular to the columns, and the vortex behavior is consistent with a Bose-glass to vortex-liquid phase transition. If the symmetry is broken further by introducing columnar defects at two different orientations, a new splayed-glass phase is found. The presence of canted columnar defects leads to stronger disorder and possible entanglement of the vortices resulting in slower critical dynamics near the vortex phase transition. The experimental techniques developed in this thesis provide valuable new tools for probing the vortex dynamics of these systems and future work on other vortex and spin systems.

[发布机构] University:California Institute of Technology;Department:Physics, Mathematics and Astronomy

[关键词] Physics