Effects of the Two- Gap Nature on the Microwave Conductivity of 39 K Polycrystalline MgB2 Films

The surface resistance ( R S ) and the real part ( σ 1 ) of the microwave complex conductivity of a ~380 nm-thick polycrystalline MgB 2 film with the critical temperature ( T C ) of 39.3 K were investigated at ~8.5 GHz as a function of temperature. Two coherence peaks were observed in the σ 1 versus temperature curve at temperatures of ~0.5 T C and ~0.9 T C , respectively, providing a direct evidence for the two-gap nature of MgB 2 . The film appeared to have a π band gap energy of 1.8 meV. For the MgB 2 film ion-milled down to the thickness of ~320 nm, two coherence peaks were still observed with the first conductivity peak at ~0.6 T C . Reduction of T C by 3 K and reduced normal-state conductivity at T C were observed along with an enhanced π -band gap energy of 2.1 meV and a reduced R S at temperatures below 15 K for the ion-milled film. Calculations based on the gap energies from the weak coupling Bardeen-Cooper-Schrieffer theory and the strong coupling theory suggest that both the σ – band and the π -band contribute to σ 1 of the polycrystalline MgB 2 films significantly. Our results are in contrast with the observation of single coherence peak at ~0.6 T C and dominant role of the π -band in the microwave conductivity of c -axis oriented MgB 2 films as reported by Jin et al . [Phys. Rev. Lett. 91, 127006 (2003)]. Variations in the inter-band coupling constants with the level of disorder can account for the different T C and σ 1 behavior for the as-grown and ion-milled films. Our results suggest that enhanced inter-band scattering can improve microwave properties of MgB 2 films at low temperatures due to the larger π -band gap despite the reduction of T C .