Non-linear microwave surface impedance of patterned YBa 2Cu 3O 7 thin films

Abstract

We have patterned thin film YBa 2Cu 3O 7−δ (YBCO) coplanar resonators to characterise the surface impedance of our films[1]. Our best film, with growth conditions optimised for good dc transport properties, yields a resonator with an unloaded quality factor in excess of 40000 at 15K and 8GHz, with a corresponding surface resistance around 25μΩ. The impedance is constant for microwave fields H rf up to 10 4Am −1, above which it increases rapidly. This contrasts with the case of less well optimised films; although the surface resistances of these films may be less than a factor of 2 larger than those for optimised films, 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.