Doctor of Philosophy (Ph.D.)
Harlan E Schone
The microwave surface impedance of granular high temperature superconductors is an important figure of merit for technological applications. Because the behavior of the granular materials deviates significantly from that of the ideal defect free superconductors, the loss mechanisms are not fully understood. This dissertation seeks to quantify the contribution of granularity to centimeter wave and millimeter wave losses. By understanding these losses, the superconductive coupling between neighboring grains can also be understood.;The weakly coupled grain model is used as a phenomenological description of the microwave surface impedance. The granular superconducting surface is modelled as an effective resistively shunted Josephson junction. The measured surface impedance is compared to the model by plotting the normalized surface resistance versus the normalized surface reactance.;The model offers a quantitative explanation of many features observed in the surface impedance data including a local maximum in the surface reactance versus static magnetic field. The model also predicts the weaker than quadratic BCS frequency dependence of the surface resistance. The surface impedance of granular superconductors is always observed to saturate in high static magnetic fields. From analysis with the weakly coupled grain model it is concluded that the saturation is due to superconducting microshorts with properties which are independent of magnetic field.;Finally, measurement of surface resistance with an open Fabry-Perot resonator is treated within as a mini-dissertation. The loss mechanisms in the open resonator geometry are considered. The ohmic losses are computed numerically from a vector theory, and Bethe diffraction theory is used to compute a lower limit for losses arising from mode mixing.
© The Author
Remillard, Stephen Keith, "The effects of granularity on the microwave surface impedance of high kappa superconductors" (1993). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539623844.