Microwave surface resistance close to 10 GHz has been measured as a function of temperature for epitaxial thin films of YBa2Cu3O7 (YBCO) on LaAlO3 in the film thickness range of 0.2–0.8 μm. The films were made by a reduced-temperature post-anneal technique. The surface resistance (Rs) scaled to 10 GHz decreases with increasing film thickness as is expected due to the finite film thickness with respect to the magnetic penetration depth. Below about 70 K there is an increase in Rs for the thickest films, attributed to a change in microstructure from c axis normal to the substrate plane, to c axis in the plane of the substrate; the resulting minimum in Rs occurs at a film thickness of 0.6 μm. The critical current density (Jc) at 77 K is highest for the thinner films, so that films with the highest Jc do not have the lowest measured Rs. These results suggest that the optimum YBCO film thickness for microwave devices patterned from these films may be about 0.6 μm, depending on operating temperature.
A prototype superconducting microwave bandpass filter whose center frequency can be tuned electrically is designed, modeled, fabricated, ~ and tested. The tuning is accomplished via self-heating of a superconducting control line located next to a superconducting resonating element.Both the single-pole microstrip filter and the control line are patterned from a single thin-film layer of Y B a 2 C U 3 0 7. When the critical current of the control line is exceeded, at an operating temperature of 77 K and below, the center frequency of 13 GHz shifts down by as much as 50 MHz, with only a modest increase of insertion loss. This performance is successfully modeled in terms of the change in the kinetic inductance of the filter element caused by heating the control line. Switching speeds in excess of 1 kHz are demonstrated. Implications of these results for the design of an improved tunable hight e m p e r a t u r e are discussed. sup er c o n d u c t iiig f i 1 t e r
YBa2Cu3O7 (YBCO) thin films with improved properties have been recently reported by post-annealing in a low partial pressure of oxygen, similar to that used by in situ methods, compared to the usual post-annealing in 1 atm of oxygen. Here it is shown that the improvements extend to the microwave surface resistance. The surface resistance was measured at close to 10 GHz; the scaled value at 10 GHz and at 77 K is 240 μΩ. This value is as low as has been reported for YBCO thin films measured around 10 GHz made by any method, and is two orders of magnitude lower than the surface resistance of copper at the same temperature and frequency.
Room-temperature thermal conductivity values were determined for sputtered c-axis oriented YBazCuJ07-a thin films. These values were measured through the plane of the films (KL! using a thermal comparator technique. and calculated in the plane of the films (7) using the Wiedemann-Franz-Lorenz conversion of electrical sheet con ucttvity values. The films show a thermal conductivity anisotropy, with values of KI IT 0.26 W m-'K-' and Kll E 2.44.5 W m-'K-'. The thermal conductivities are depressed in both directions from expected values based on room-temperature measurements of single-crystal thermal conductivities. While some anisotropy is expected due to the crystallographic orientation of the films, the observed anisotropy is greater than that typically seen in single crystals of YBazCu301..6. Possible explanations are presented based on thin film microstructure and phonon and charge carrier scattering mechanisms, but due to the sholt room-temperature mean free path of this material, phonon scattering mechanisms such as the size effect or scaner from Sacking faults may be discounted as significant sources of thermal conductivity reduction. Instead, the formation of a second phase during film deposition is the most likely source of thermal resistance in the films.
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