Observation of the coherence peak in MgB2 thin film

The coherence peak is observed in the temperature dependence of the real part of the microwave conductivity (sigma(1)) of c-axis oriented MgB2 thin film, which is extracted from microwave surface impedance (Z(s)) measurements using a sapphire resonator at 17.9 GHz. The peak appears at a temperature around 0.6 times the critical temperature (T-c), which is greatly shifted downwards in temperature compared with conventional superconductors for which the peak occurs around 0.9 T-c. Calculations based on the two-band model suggest that this peak corresponds to the smaller of the two energy gaps. (C) 2004 Published by Elsevier B.V