Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ films: Growth and applications in dc SQUIDs and microwave devices
Tl-2212 thin films were grown ex situ in a two step process on a variety of substrates, including ones with artificially introduced high angle steps. Grain boundary weak links were formed on LaAlO/sub 3/ substrates with steps and on bicrystal substrates and were employed in dc SQUIDs. The white noise level of the SQUIDs was 50/spl mu//spl Phi//sub 0///spl radic/(Hz) at 77 K. A 12.5 GHz HEMT oscillator stabilized with a Tl-2212 resonator gave 8-10 dB lower phase noise than using a gold resonator.
Thallium-based high-Tc films with very low surface impedance
Abstract Epitaxial Tl 2 Ba 2 Ca 2 Cu 3 O x films with very low microwave losses have been prepared on LaAlO 3 substrates by the two-step method: sputtering of Tl free precusor films and post-annealing in a Tl-oxide atmosphere in the temperature range from 1161 K to 1168 K. We found that the annealing temperature has a large influence on the film quality. The highest-quality Tl 2 Ba 2 Ca 2 Cu 3 O x film, prepared at 1168 K, exhibits a surface resistance of R s = 6.5 m Ω at 77 K and 1 mΩ at 4.2 K and 87 GHz, and a penetration depth λ (0 K) = 170 nm. Furthermore we have observed for the first time a decrease of R s ( T ) below about 30 K for several TBCCO films. The surface resistance measured at different frequencies by means of two types of cavity measurement techniques exhibits a quadratic dependence on frequency at 77 K over the frequency range from 17.8 GHz to 87 GHz.
An analytical approach to the design of electrodes in high-power, fast-transverse-flow CO2 lasers
Abstract Usually, the design of the electrodes of fast-transverse-flow CO 2 lasers relies more on experimental data than on theoretical analysis. Traditional systems sustain a stable, high-power discharge but the current distribution generates a peaked, non-uniform small-signal-gain pattern. We present a theoretical model suitable for the design of electrodes that achieves a quasi uniform gain distribution. The analysis, based on a two-temperature model and the gas-transport equations, computes the electron density that supports a flat gain pattern. Combining the desired electron density with the electron-balance equations, the model determines the required electric field. The results were used to design a new set of electrodes for a home-made fast transverse flow CO 2 laser. A stable, large volume discharge ( 4 × 8 × 90 cm 3 ) with very good uniformity was obtained. The resulting gain distribution was registered in a bi-dimensional map. The peak gain rose from 1.2 m - 1 , attained with the old electrode set, to 1.6 m - 1 with the new one and the gain showed a homogeneous profile.