Hope College Physics Department Research Experiences for Undergraduates Summer 2012 Project Summary

Superconductors made from Tl₂Ba₂CaCu2O_8-x exhibit nonlinear behavior of the complex surface impedance, Z_S=R_S+iXs, with respect to microwave magnetic field in microwave frequency devices. The superconductor’s electrodynamics depends on the carrier density and varies with oxygen doping level. By annealing samples in nitrogen at temperatures between 250 and 400 degrees Celsius, the doping level is lowered. The altered hole doping changes the critical temperature, T_C, and increases the grain boundary losses, which lowers the quality factor and the frequency of a sapphire resonator constructed with the superconductor as an end-wall sample. When similar reduced temperatures (T/T_C) are matched together, the slope parameter, ΔX_S/ΔR_S, shows a linear variation with the hole density with values centered around unity at higher temperatures, which allows us to attribute the nonlinearity of underdoped samples to nucleation/annihilation hysteresis of Tinkham fluxons. Nonlinear microbridges have been associated with oxygen depletion, and larger slope parameters at lower temperatures as well as higher doping levels implicates resistive losses in grain boundaries in the microwave nonlinearity.

Publications and Presentations:
S.K. Remillard, J. Thomas, M.M. Bischak, C.J. Goodson; "Effect of Carrier Doping on Nonlinear Distortion of Microwave Signals by Superconducting Thin Films," Applied Superconductivity Conference, Portland, OR, October 8-12, 2012.

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