Resistive loss at 10 GHz in c-axis-aligned in-situ-grown ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ films

The resistive losses near 10 GHz are measured at 4.2 K for ten c-axis-oriented ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ thin films deposited in situ on MgO substrates. The losses range from 16±3 μΩ, the lowest value reported to date for a thin film of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ to 740±150 μΩ, a difference of more than a factor of 40. Among the same films, the dc resistivity at 100 K ranges from 50 to 155 μΩ 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 μΩ, 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 (${\mathit{T}}_{\mathit{c}}$). 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 ${\mathit{T}}_{\mathit{c}}$. The observed loss level is consistent with the presence of a large density of uncondensed carriers.