Doping Dependent Microwave Nonlinearity of Tl2Ba2CaCu2O8-x Superconductor
Dr. Stephen Remillard, Hope College
The carrier doping of the Tl2Ba2CaCu2O8-x superconductor is a potential tool for modifying the nonlinearity of the superconductor’s microwave response. The properties that respond to carrier doping level include the surface resistance (Rs) and the critical temperature (Tc) of the superconductor. Nitrogen annealing was used to modify carrier doping, and its effect on the samples was studied by using a sapphire/superconducting dielectric resonator to measure Rs and Tc. The influence of the induced surface current on surface resistance, Rs, called nonlinearity, was found to depend on the doping level, as was the critical temperature, Tc. This, in turn, provides a useful correlation of doping level to the temperature of the nitrogen anneal. Nitrogen annealing has thus been shown to provide a calibrated method to select a nonlinearity regime of the superconductor. A series of nitrogen anneals were also performed with 15 samples to show the effects of annealing on the Tc. Samples that had been annealed in nitrogen at 250°C, 300°C, 350°C, 375°C, and 400°C, as well as three unannealed samples were tested. Those that were unannealed were then annealed and re-tested at different temperatures resulting in 18 unique measurements. The average Tc was taken from the three samples at each temperature. N2 annealing was found to decrease the Tc, however, 250°C resulted in almost no change in Tc. 300°C resulted in a variety of changes ranging from a small decrease to no change. Significant changes became evident when the annealing temperature exceeded 300°C. Because nitrogen annealing has been found to change the doping level as shown by the Tc shift, it can be concluded then that nonlinearity depends on the doping level and can also be controlled through doping.
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