Microwave Frequency Power Dependence in High - Tc Thin Films , Grain Boundaries

A comprehensive study of the dc and rf electrodynamic properties of YBa 2 Cu 307-, (YBCO) thin films, grain boundaries and Josephson junctions (JJ) is presented. The surface impedance Z, = R, + iX, of YBCO thin films is shown to have a dependence on rf current, which leads to nonlinear power losses at levels of rf power that are relevant for device applications. To understand the loss mechanisms in high-Tc thin films, measurements and modeling of the microwave-frequency (rf) power dependence of the impedance in YBCO thin-film grain-boundary Josephson junctions (JJ) are presented. Microwave impedance measurements were performed using a stripline resonator technique on engineered grain-boundary JJs as a function of rf current (10-4 to 1 A) and temperature (5 to 70 K). To understand the observed power dependence, a long-junction model which allows for Josephson-vortex creation, annihilation, and motion is introduced. The impedance calculated using the long-junction model fits the measured data qualitatively. It is shown 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. The modeling results also show second harmonic generation and this is explored experimentally. In addition, a comparative study of the rf power-dependence of thin-film YBCO grain boundaries engineered on sapphire bicrystal substrates with misorientation angles of 0 = 20, 50, 10 , and 240 and films grown on single-crystal substrates is presented. The rf results are compared to dc measurements performed on a four-point test structure on the same substrate as the resonator. The measurements demonstrate that lowangle grain boundaries (0 < 100) have little effect on the rf power handling, while the high-angle grain boundaries (0 = 24') cause large nonlinear losses due to Josephson vortices created by rf currents. Thesis Supervisor: Mildred S. Dresselhaus Title: Institute Professor