Dynamic conductivity and coherence peak in YBa₂Cu₃O₇ superconductors

We measure directly both the real and imaginary parts of the sub-band-gap conductivity σ in YBa₂Cu₃O₇ in the 15–80 cm^-1 frequency range using coherent time-domain terahertz spectroscopy. We observe a peak in the real part σ₁ 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.