Microwave surface resistance peak at T-c in Bi2Sr2CaCu2O8+x film

We observe an anomalous peak in the microwave surface resistance of a Bi2Sr2CaCu2O8+x (Bi:2212) thin film, at a temperature close, but different, to the transition temperature. The peak is frequency-dependent. Measurements of the resistive transitions between 19.5 and 26.5 GHz show that the peak temperature decreases with increasing frequency. The peak is most clearly visible at high frequency. We show that the peak is not due to physical phenomena in the superconductor, but instead it is due to substrate effects. A recently developed simplified model for the microwave surface resistance of a thin superconducting film, together with a two-fluid expression for the conductivity, can fit both the resistive transition, including the peak, and the frequency dependence of the peak. So, we do not find it necessary to invoke a distribution of T-c values, or inhomogeneities of the sample.