Significant reduction of the microwave surface resistance of MgB2 films bysurface ion milling

The microwave surface resistance R-S of MgB2 films with the zero-resistance temperature of similar to 39 K was measured at 8.0-8.5 GHz. The MgB2 films were prepared by deposition of boron films on c-cut sapphire, followed by annealing in a magnesium vapor environment. The R-S appeared significantly reduced by ion milling of the as-grown MgB2 film surface, with the observed R-S of similar to0.8 m Omega at 24 K for an ion-milled MgB2 film as small as 1/15 of the value for the corresponding as-grown MgB2 film. The reduced R-S of the ion-milled MgB2 films is attributed to the effects of the Mg-ric h metallic layer existing at the surfaces of the as-grown MgB2 films. (C) 2 001 American Institute of Physics.