We measure directly both the real and imaginary parts of the sub-band-gap conductivity σ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ in the 15–80 ${\mathrm{cm}}^{\mathrm{-}1}$ frequency range using coherent time-domain terahertz spectroscopy. We observe a peak in the real part ${\mathrm{\sigma }}_1$ similar to the coherence peak expected in an s-wave BCS superconductor. The absence of such a peak in NMR relaxation-rate experiments suggests that its origin lies in a strongly temperature-dependent inelastic-scattering rate rather in coherence factors.

• Received 9 April 1991

DOI:https://doi.org/10.1103/PhysRevLett.66.3305