Anomalous coherence peak in the microwave conductivity of MgB2 thin film at 7.2 GHz

We present here the temperature dependence of the real part of the microwave conductivity of MgB2 thin film measured at 7.2 GHz. A pronounced peak occurs at a temperature around 0.6 times the critical temperature, same position as it did at 17.9 GHz [Jin et. al., Phys. Rev. Lett. 91 (2003) 127006]. This frequency independence of the peak position confirms our previous conclusion, i.e., the peak comes from the coherence peak from the pi band contribution. Some discussions are given to the measurement and extraction procedures on the microwave conductivity. We conclude that it is crucial to measure the real and imaginary parts of the surface impedance simultaneously at the same frequency to get an accurate temperature dependence of the microwave conductivity through the superconducting transition.