  The new superconductor MgB2 may prove useful for microwave applications at
intermediate temperatures. MgB2 films with the thickness of 300 - 400 nm have
surface resistance Rs less than 0.01 mOhms at a frequency (f) of 8.5 GHz and 7
K,and ~ 1.5 mOhms at 87 GHz and 4.2 K. The critical temperature (Tc) of these
films is ~ 39 K when they are prepared under optimum conditions. The Rs appears
to scale as f^2 up to 30 K. After surface ion-milling, a reduction of the Tc
and an enhanced resistivity rho(Tc) are observed consistently at 8.5 GHz and 87
GHz along with a reduced Rs at low temperatures. The observed rho(Tc) - Tc
relation and the uncorrelated behavior between rho(Tc) and Rs values measured
at low temperatures are well explained in terms of the two-gap model, with the
observed rho(Tc) - Tc relation attributed to the properties of the large gap,
and the Rs at lower temperatures reflecting the properties of the small gap,
with an enhanced small gap energy due to increased interband scattering. This
study suggests that the interband scattering should be enhanced to improve the
low temperature microwave properties of MgB2 films and that the ion-milling
process must be performed with great care to preserve the high quality of MgB2
films.
