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High Critical Current Density Superconductivity

This project was focused on a basic scientific problem, one that is at the heart of application and development of superconductivity for efficient energy usage. This is improvement in high current density which can only be achieved by learning to pin vortices to the physical structure of its underlying superconducting material. Quantized vortices are the tornado of electrical supercurrents, ubiquitously present in a type II superconductor similar to the water whirling out of a bathtub drain; the magnetic flux of these superconducting vortices is quantized. The promise of superconductivity for more efficient energy transport depends critically on developing a basic understanding of vortex pinning in the crystalline solid. Central to this agenda is the basic understanding of interactions between vortices, the quantum state of the vortex core, and the potential for pinning from materials defects. We address the former two items in our program. Our approach is to study the vortex-vortex interactions and the stability of resulting vortex structures, as well as to investigate spectroscopically the vortex core electronic structure using small angle neutron scattering (SANS) and spatially resolved NMR methods. Our work has been on a class of superconductors called unconventional superconductors a class, which includes all high temperature superconductors and some heavy fermion superconductors. The former are where the potential applications will lie. The latter are a paradigm system where basic scientific questions can be addressed. We first completed a NMR study of vortex structures in the high-temperature, superconducting compound Bi 2 Sr 2 Ca 1 Cu 2 O 8.2 (Bi-2212) which we obtained in the form of very high quality single crystals from Professor S. Uchida's group in Tokyo. We processed these crystals to exchange the more common isotope 16 O with the rare isotope 17 O which has a sensitive NMR signal. Our NMR spectrum and spin-lattice relaxation measurements support our model for a structural instability which we discovered in the vortex system at magnetic fields in the range of 4 to 6 T depending on chemical doping. We developed a model to explain this instability based on electrical charge of ≈10-3 e trapped on the vortex core. The results are now in press in Nature Physics to appear in November 2010. The second part of this project was to perform SANS measurements of the vortex structures on the compound UPt 3. The SANS measurements were performed at PSI