Change of strength of vortex pinning in YBCO due to BaZrO3 inclusions

We probe the short-range pinning properties with the application of microwave currents at very high driving frequencies (47.7 GHz) on YBa2Cu3O7−δ films with and without sub-micrometer BaZrO3 inclusions. We explore the temperature and field ranges 60  K < T < Tc and 0 <μ0H < 0.8 T, with the field applied along the c-axis. The magnetic field induces a much smaller increase of the microwave resistivity, Δρ1(H)+iΔρ2(H), in YBa2Cu3O7−δ/BaZrO3 with respect to pure YBa2Cu3O7−δ. Δρ1(H) is slightly superlinear in pure YBa2Cu3O7−δ (suggesting a possible contribution of thermal activation), but linear or sublinear in YBa2Cu3O7−δ/BaZrO3 (suggesting a possible suppression of thermal activation as a consequence of BaZrO3 inclusions). These features persist up to close to Tc. We discuss our data in terms of the ratio r=Δρ2(H)/Δρ1(H) in the framework of the models for the microwave surface impedance in the mixed state. Large r are found in YBa2Cu3O7−δ/BaZrO3, with little field dependence. By contrast, smaller values and stronger field dependences are found in pure YBa2Cu3O7−δ. We discuss the different field dependence of the pinning constant.