Microwave field dependence of surface resistance for high-tc superconducting yba2cu3o7-x films

We have studied the dependence of the microwave field at 13 GHz on the surface resistance of highTc superconducting YBa2Cu3O7− x films. The microwave field varies from the weak field (Meissner effect region) to the field about 400 A/m. The sample YBa2Cu3O7− x film with c-axis normal to the surface was prepared by laser ablation method on to a copper substrate 36 mm in diameter. From the H dependence of the surface resistance the rf cr itical current density Jcrf (T ) was determined by grain boundary weaklinks model. Jcrf about 2.8×105 A / cm at 20 K and about 4×104 A / cm at 77 K were obtained. Jcrf (T ) was expressed as Jc (T ) ∝ (1 − T / Tc )m with m 0.6, 1.2 and 1.4 for our three high-Tc samples.