  Magnesium diboride (MgB2) is attractive for RF cavity application for
particle accelerators because it might not show an increase of RF surface
losses at high magnetic surface fields, a phenomenon that has prevented high-Tc
superconducting materials such as YBCO from being used for this application. We
have measured the RF surface resistance (Rs) at 21 GHz of a MgB2 sample
fabricated using Hot Isostatic Press (HIP) at 200 MPa and 1000 C. The results
show that polishing with 0.1-micron diamond lapping film followed by a 1500-psi
DI water rinse in a clean room reduced the Rs by a factor of 6.2 at 15 K and it
is the lowest compared to other published data. The Rs data near the lowest
temperature (~13 K) scatter between 0.6 and 1.3 mOhm. The penetration depth and
energy gap were estimated to be 263 nm and 1.9-2.7, respectively, for the
polished surface.
