The microwave surface resistance at 2.95 GHz of a very-high-quality crystal of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ exhibits unusual nonmonotonic temperature dependence. After a sharp drop below ${\mathit{T}}_{\mathit{c}}$ by a factor of 5000 the loss rises to a peak at 35 K and then falls at lower temperature. The peak is due to a rapid decrease in the scattering of thermally activated quasiparticles below ${\mathit{T}}_{\mathit{c}}$ and this suppression of scattering suggests that the excitations responsible for the large resistivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ are gapped below ${\mathit{T}}_{\mathit{c}}$. A technological implication is that disorder may lower the microwave loss.

• Received 15 July 1991

DOI:https://doi.org/10.1103/PhysRevLett.68.2390