Microwave stripline resonator measurements are presented of Rs on large-area YBCO (Y1Ba2Cu3O7-x) films sputter deposited on cleaved MgO substrates. Dependence of Rs on temperature, frequency, and applied input power was determined. The films exhibited excellent dc properties with typical Tcs equals approx. 85 K, and Jcs equals 1 MA/cm2 at 77 K. At 1.2 GHz the inferred microwave Rs equals approx. 100 (mu) (Omega) at 4.2 K and Rs equals approx. 2 m(Omega) at 78 K for these same films. Losses in the patterned YBCO center strips showed large changes over an 80 dBm range for input power. These patterned center strips exhibited a much stronger dependence on input power than did unpatterned ground planes. This may be related to peak currents and high Hrf fields inside the stripline near the edges of the patterned center strip. Film degradation due to environmental effects has also been observed, with films stored for over 6 months degrading by a factor of two in Rs when retested. With respect to frequency, Rs approximates an f2 dependence when higher harmonics of the resonator are measured on these YBCO films.
Thin films of YBCO were used as the active element in an optically triggered fast-opening
switch. These epitaxial films, up to 1 .tm thick, were deposited either by coevaporation or by if
magnetron sputtering. A patterned switch structure was current-biased and subjected to 150 ps
pulses from a Nd:YAG laser. Switching from the superconducting to the normal state was
observed to occur as fast as 1 ns. A model is presented that can account for the observed
switching characteristics in terms of the thermal and electrical times of the films; the results clearly
indicate substantial film heating above the ambient temperature. Results are encouraging for
potential application to a high-current opening switch.
The response to short infrared pulses of some epitaxial YBCO films prepared by sputter deposition and by electron-beam evaporation is reported. The response is found to be essentially bolometric on the ns timescale, with some indirect hints of nonequilibrium electron transport on the ps scale. Fast switching could be obtained either by biasing the switch close to the critical current or by cooling the film below about 20 K. These results are encouraging for potential application to a high-current optically-triggered opening switch.
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