Hope College Physics Department
Research Experiences for Undergraduates
Summer 2013
Project Summary


Project Title: Effect of Carrier Density on Nonlinear Emissions from Superconducting Resonators
Student Name: Michael Bischak
Student's Home Institution: Hope College
Research Advisor: Dr. Stephen Remillard
Source of Support:

Funding for this project was provided by the National Science Foundation under NSF Grants DMR 1206149 and PHY/DMR 1004811, a Cottrell College Science Award from the Research Corporation for Science Advancement, and the Hope College Dean for the Natural and Applied Sciences Office.

Superconductors made from Tl2Ba2CaCu2O8-x exhibit nonlinear behavior of the complex surface impedance, ZS=RS+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 of the already overdoped film is increased further from the optical phase diagram location for superconductivity. The altered hole doping changes the critical temperature, TC, 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/TC) are matched together, the slope parameter, ΔXS/ΔRS, 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: