RESISTIVE LOSS AT 10 GHZ IN C-AXIS-ALIGNED INSITU-GROWN YBA2CU3O7 FILMS

The resistive losses near 10 GHz are measured at 4.2 K for ten c-axis-oriented YBa2Cu3O7 thin films deposited in situ on MgO substrates. The losses range from 16 +/- 3-mu-OMEGA, the lowest value reported to date for a thin film of YBa2Cu3O7 to 740 +/- 150-mu-OMEGA, a difference of more than a factor of 40. Among the same films, the dc resistivity at 100 K ranges from 50 to 155-mu-OMEGA cm, about a factor of 3. We show that the losses in the higher-loss samples can be accounted for by the presence of small fractions of c-axis-aligned grains highly misaligned in the plane of those films. Volume fractions of highly misaligned c-axis-oriented grains as low as 4% lead to losses above 500-mu-OMEGA, more than 25 times higher than the loss in the lowest-loss film reported here. Among the lower-loss films having less than 0.15 vol % highly misaligned grains, a substantial portion of the losses are due to additional mechanisms. Among these films, the losses are found to be lower in the films with lower normal-state resistivities and higher transition temperatures (T(c)). For all samples, the losses increase with increasing deviations from structural or electrical behavior expected for ideal single crystals. The loss in even the lowest-loss film is orders of magnitude above that expected for an ideal isotropic BCS superconductor having the same T(c). The observed loss level is consistent with the presence of a large density of uncondensed carriers.