Ph.D. (Doctor of Philosophy)
Department of Physics
Superconductors--Magnetic properties; Metallic films--Magnetic properties; Niobium--Magnetic properties
Commensurate effect is one of the intriguing properties observed in superconducting films containing periodic hole-arrays. It represents itself as minima in the magnetic field dependence of the resistance or maxima in the field dependence of the critical current when an integer number of flux quantum is commensurate with the unit-cell of the artificial hole-array in a superconducting film. Two mechanisms, vortices pinning enhancement by the hole-array or critical temperature oscillations of a wire network, can account for this effect at temperatures close to the zero-field critical temperature. This thesis investigates the commensurate effect of niobium superconducting films with periodic hole-arrays near the zero-field critical temperature. Experimental approaches have been developed to distinguish the possible mechanisms. The field dependence of the resistance in various field directions and the angular dependences of the critical temperature and resistance reveal that at temperatures close to the zero-field critical temperature superconducting niobium films with periodic hole-arrays behave like superconducting wire networks and the observed commensurate effect originates from hole-induced critical temperature suppression at noninteger flux quantum fields rather than the widely believed pinning enhancement at integer flux quantum fields. These conclusions are supported by the results of patterned films with various thicknesses, hole-hole separations and the symmetries of the hole-arrays. For comparison, a continuous thin film and a thin strip are also investigated. This research will significantly advance the understanding on superconductors with artificial defects and the results can be directly applied to different types of the defects such as arrays of magnetic dots.
Hua, Jiong, "Commensurate effect in superconducting niobium films containing arrays of nanoscale holes" (2008). Graduate Research Theses & Dissertations. 1846.
ix, 109 pages (some color pages)
Northern Illinois University
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