Date of Award

8-2001

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Dr. Lisa M . Paulius

Second Advisor

Dr. Clement A. Bums

Third Advisor

Dr. Alvin Rosenthal

Fourth Advisor

Dr. Robert Shamu

Abstract

The topics covered in this thesis are related to two objectives: one refers to the search for methods of improving die critical current density of high-temperature superconductors, a subject which remains of continuing interest for its importance in technological applications. The other direction is aimed at clarifying the phase diagram of high-Tc materials.

The interaction between the structural defects and the vortex system plays a significant role in the capability of these materials to carry large electrical transport currents. Through proton irradiation induced defects we follow the evolution of the critical current density, and its enhancement with increasing point-like defects induced in detwinned, single crystals of YBa2Cu30 7_6. The nature, mechanism, and implications of the peak effect are studied through electrical transport, and magnetization measurements in order to obtain a comprehensive temperature dependence. Our study reveals that the point defects, rather than the oxygen defect clusters are primarily responsible for the suppression of the peak effect.

The presence of different types and densities of defects in the high-temperature materials yields a very reach and diverse magnetic phase diagram. We studied and compared through ac-, and dc- electrical transport measurements the first-order vortex melting transition from the vortex lattice to the vortex liquid. The investigation was performed on detwinned, heavy-ion irradiated, proton irradiated, and twinned crystals with an emphasis on resolving the angular dependence of die melting line, and of the lower critical point of the melting transition. While a minor shift in the temperature dependence of the melting line is yielded by low-doses of heavy-ion irradiation, the angular dependence of the lower critical point is dramatically affected by the defects. An increase in the lower critical point in die whole angular range is obtained after proton irradiation, while heavy-ion irradiation yields a pronounced increase around 0° where vortices align with the columnar defects.

Comments

Fifth Advisor: Dr. Pnina Arigur

Access Setting

Dissertation-Open Access

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