Effects of disorder on the microwave properties ofMgB2polycrystalline films

Abstract

The role of disorder in superconducting magnesium diboride $({\mathrm{MgB}}_{2})$ policrystalline films is investigated in the high frequency range by a coplanar microwave resonator technique. Two sources of disorder are considered, heavy-ion irradiation damage and sample ageing. Microwave measurements are analyzed in the framework of the two-gap model with strong interband scattering contribution. It turns out that disorder enhancement increases the interband scattering rate, resulting in a reduction of the surface resistance at low temperatures, due to a slight increase of the $\ensuremath{\pi}$ gap. Moreover, increasing disorder at grain boundaries induces a nonmonotonic residual surface resistance, showing the features of a resistive behavior for the highest disorder level. Finally, the effects of the different kinds of disorder on the intrinsic and on the grain-boundary properties of the ${\mathrm{MgB}}_{2}$ films are compared and discussed.