We have patterned thin film YBa2Cu3O7-delta (YBCO) coplanar resonators
to characterise the surface impedance of our films[l]. Our best film,
with growth conditions optimised for good dc transport properties, yi
elds a resonator with an unloaded quality factor in excess of 40000 at
15K and 8GHz, with a corresponding surface resistance around 25muOMEG
A. The impedance is constant for microwave fields H(rf) up to 10(4)Am-
1, above which it increases rapidly. This contrasts with the case of l
ess well optimised films; although the surface resistances of these fi
lms may be less than a factor of 2 larger than those for optimised fil
ms, the non-linear effects are much more pronounced and occur at much
lower microwave fields, and they are therefore less suitable for high
field applications.