Observation of a peak below T_c in the RF-conductivity Re{ σ(T)} of YBa₂Cu₃O₇

We report the observation of a peak in the real part of the in-plane rf-conductivity, Re{σ(T)} of YBaCuO thin films. The measurements were made on high-quality films in the frequency range from 100 kHz to 500 MHz. The peak was detected at frequencies up to 500 MHz but was substantially sharper and larger at lower frequencies. All peaks were narrower (∼0.5 K) than expected from the BCS quasiparticle density of states (∼30 K), i.e. the Hebel-Slichter peak. However, they could be interpreted as a measurement artifact resulting from a broadening of the resistive tramsition. © 1991.