Effects of the two-gap nature on the microwave conductivity of polycrystalline MgB2 films with a critical temperature of 39 K -: art. no. 104514

The surface resistance (R-S) and the real part (sigma(1)) of the microwave complex conductivity of a similar to 380-nm-thick polycrystalline MgB2 film with the critical temperature (T-C) of 39.3 K were investigated at frequency similar to 8.5 GHz as a function of temperature. Two distinct coherence peaks were observed in the sigma(1) vs temperature curve at T similar to 0.5T(C) and similar to 0.9T(C), respectively, providing a direct evidence for the two-gap nature of MgB2. The film is shown to have a pi-band gap energy of similar to 1.7 meV. For the MgB2 film ion milled down to the thickness of similar to 320 nm, two coherence peaks were still observable with the first conductivity peak at similar to 0.6 T-C. The inferred pi-band gap energy of similar to 2.0 meV is higher than before ion milling. Reduced normal-state conductivity at T-C and R-S at temperatures below 15 K were found for the ion-milled film. Calculations based on the weak-coupling BCS theory and the strong coupling theory suggest that both sigma and pi bands contribute to sigma(1) of the polycrystalline MgB2 films significantly. Our results are in contrast with the observation of single coherence peak, ascribed to the dominant role of the pi band, in the microwave conductivity of c-axis-oriented MgB2 films reported by Jin [Phys. Rev. Lett. 91, 127006 (2003)]. Variations in the interband coupling constants with the level of disorder can account for the different T-C and sigma(1) behavior of the as-grown and ion-milled films.