Study of nonlinear microwave properties of high-temperature superconducting film

Abstract

In this thesis, we have explored some of the possible origins of nonlinear properties of high-temperature superconducting films at microwave frequencies. The effect of patterning on the nonlinear microwave surface resistance of HTS films was studied by comparing the results of a dielectric-resonator technique and a stripline-resonator technique. We have concluded that patterning process contributes to neither the low-power surface resistance nor the power dependence of the surface resistance. To understand the grain-boundary contributions to the nonlinear microwave properties, we introduced a coupled-grain RSJ series-array model which simulates an HTS film as a network of ideal superconducting grains, coupled by resistively shunted Josephson junctoins with a distribution of I and Rn parameters. The agreement between the measured and modeled microwave surface resistance suggests the validity of this model. The discrepancy in the microwave surface reactance implies possible long Josephson junction effects. We have investigated the nonlinear microwave properties of engineered HTS grain boundaries systematically, in order to further understand the contributions of grain boundaries. The microwave impedance, second- and third-harmonic generation, and the third-order intermodulation distortion of engineeried bicrystal grain boundaries with misorientation angles of [theta] = 2⁰, 5⁰, 7.5⁰, 10⁰, and 24⁰ were measured using a suspended microstrip resonator with temperature ranging from 5 to 75 K.

(cont.) From the measured rf and dc magnetic field dependence of the microwave impedance and harmonic generation of the 24⁰ grain boundaries, we observed signs of Josephson-vortex dynamics. To explore the effects of Josephson vortices quantitatively, we developed a long-Josephson-junction model based on the nonlinear sine-Gordon equation to describe the large-angle grain boundaries. Combining the modeled and measured results, we were able to identify the penetrating and exiting events of individual mesoscopic Josephson vortices in large-angle grain boundaries. We have concluded that the microwave losses associated with Josephson vortices induced by either rf or dc magnetic fields in long-junction-like large-angle grain boundaries contribute dramatically to the nonlinear impedance and harmonic generation in HTS films. We found that grain boundaries with misorientation angles.