We have fabricated and characterized electrically tunable high temperature superconductor coplanar microstrip resonators incorporating tunable SrTiO3 ferroelectric thin films. The low frequency capacitance of the SrTiO3 capacitor is measured directly. High frequency capacitance and loss information are extracted from the observed resonances and compared with the low frequency data. Hysteresis loops display an onset of ferroelectricity at 160 K. The spontaneous charge and coercive voltage (at 10 kHz) as a function of temperature are extracted from these loops.

We investigate the nature of low-power nonlinear effects in high-temperature superconducting microwave devices by measuring third harmonic generation at 76 K in coplanar waveguide transmission lines of different geometries fabricated from YBa2Cu3O7−δ thin films. The measured power in the third-harmonic signal changes systematically with film thickness, center conductor linewidth, and line length. We analyze these results using a simple model for a transmission line with a nonlinear inductance arising from a current-dependent superconducting penetration depth. This analysis describes quantitatively the observed differences in harmonic generation for transmission lines of different dimensions, and yields a single geometry-independent parameter (the nonlinear scaling current density J0) to quantify the observed nonlinear behavior. For the thin film samples studied here J0=3.0×107 A/cm2 at 76 K for all geometries investigated. These results provide the means to establish a lower limit for the expected nonlinear response of superconducting components of arbitrary geometry at microwave frequencies.

Measurements and modeling of the microwave-frequency ͑rf͒ power dependence of the impedance in Y-Ba-Cu-O thin-film grain-boundary Josephson junctions ͑JJ's͒ are presented. Microwave impedance measurements were performed using a stripline resonator with an engineered grain-boundary JJ as a function of rf current ͑10 Ϫ4 -1 A͒ and temperature ͑5-70 K͒. To understand the observed power dependence, we have developed a long-junction model which allows for Josephson-vortex creation, annihilation, and motion. The impedance calculated using the long-junction model fits the measured data qualitatively. We show that Josephson vortices generated by the rf fields cause nonlinearities in the impedance, resulting in increases in both resistance and reactance with steps in the resistance due to flux quantization. These observations and analysis of Josephson-vortex effects at microwave frequencies have important implications for understanding rf power handling in high-T c films.

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