Candidate: Valentina Tobos

Degree of: Doctor of Philosophy

Department: Physics

Title: Vortex-Defect Interactions in High Temperature Superconductors

Committee:
Dr. Lisa M. Paulius, Chair
Dr. Clement A. Burns
Dr. Alvin Rosenthal
Dr. Robert Shamu
Dr. Pnina Ari-Gur

Date: Friday, July 6, 2001 10:00 a.m. - 12:00 p.m.
2202 Everett

Abstract:
Enhancing the critical current density of high-temperature superconductors is a subject of continuous interest in the physics community for its importance in both technological applications, as well as fundamental research.

In type-II superconductors, magnetic flux pinning is caused by the inhomogeneities present in the materials in the form of impurities, or crystallographic defects, which prevent the motion of the quantized magnetic flux lines, or vortices. The interaction between the defects and the vortex system plays a significant role in the capability of a material to carry large electrical transport currents. It is expected that flux pinning in high temperature superconductors has some special characteristics due to both the short coherence length, and the large thermal fluctuations. Through proton irradiation induced disorder we follow the evolution of the critical current density, and its enhancement with increasing defect density in detwinned, single crystals of YBa2Cu3O7-d. The nature, mechanism, and implications of the peak effect are studied through both electrical transport, and magnetization measurements. A large range of temperatures is covered
by this comprehensive method.
(over)
The dynamics of the magnetic vortices and their interaction with the defects is not yet completely understood and irradiation induced defects are extensively used in order to study this interaction. The presence of the different types, and densities of defects in the crystallographic structure yield a very reach and diverse magnetic phase diagram of high temperature materials. We studied, and compared through ac, and dc-electrical transport measurements the first-order vortex melting phase transition from the vortex lattice to the vortex liquid. The investigation was performed on untwinned, heavy-ion an proton irradiated, and twinned crystals, with an emphasis on resolving the temperature, and the angular dependence of the lower critical point of the melting line. While a minor shift in the melting line is yielded by irradiation with small doses of disorder, a completely different curvature depending on the type, and density of defects is seen in the angular dependence of the end point of the first-order melting transition.

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