DYNAMIC CONDUCTIVITY AND COHERENCE PEAK IN YBA2CU3O7 SUPERCONDUCTORS

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