We have measured the surface impedance of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single crystals and have evaluated the conductivity ${\mathrm{\sigma }}_1$ at millimeter-wave frequencies. We observe a peak below ${\mathit{T}}_{\mathit{c}}$ which we suggest is due to case-II coherence factors, like the Hebel-Slichter peak observed in the NMR relaxation rate of classical superconductors. A comparison with theory leads to a large gap, suggesting strong coupling.

• Received 17 January 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.152