Doctoral Dissertations

Date of Award

12-2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

James R Thompson

Committee Members

Hanno H. Weitering, Adriana Moreo, Janice L. Musfeldt

Abstract

Pinning landscapes in modern second generation coated conductors are excellent candidates for studies of vortex pinning. The ability to produce engineered defects in thin films of high temperature superconductors allows one to investigate representative distinct pinning sites, with the objective of understanding how different pinning centers contribute, compete and evolve under varying conditions of magnetic field strength and orientation, and temperature.New contact-free methods were developed specifically to investigate this system in new ways, especially the dependence of the critical current density Jc on orientation of the magnetic field. A superconducting quantum interference device (SQUID)-based magnetometer was used to determine angular critical current density profiles. The induced currents produced in contact-free methods allow one to investigate a range of temperatures that is difficult to access by traditional transport measurements.Materials with three distinctive pinning landscapes were investigated: Specifically, samples were studied that were dominated by columnar defects, by isotropic defects, or a mixture of these two types of pins. These studies over an expanded temperature-field-orientation phase-space clearly show competition between not only strong and weak pinning centers, but also between the angularly selective and nonselective pinning. This evidence is seen in critical current density profiles, the α [alpha]-values describing the falloff of Jc with magnetic field, and matching field effects in the three samples studied.

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