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-delta films with and without sub-micrometer BaZrO3 inclusions. We explore the temperature and field ranges 60 K, < T < T-c and 0 <mu H-0 < 0.8 T, with the field applied along the c-axis. The magnetic field induces a much smaller increase of the microwave resistivity, Delta rho(1)(H)+i Delta rho(2)(H), in YBa2Cu3O7-delta/BaZrO3 with respect to pure YBa2Cu3O7-delta.Delta rho(1)(H) is slightly superlinear in pure YBa2Cu3O7-delta (suggesting a possible contribution of thermal activation), but linear or sublinear in YBa2Cu3O7-delta/BaZrO3 (suggesting a possible suppression of thermal activation as a consequence of BaZrO3 inclusions). These features persist up to close to T-c. We discuss our data in terms of the ratio r = Delta rho(2)(H)/Delta rho(1)(H) in the framework of the models for the microwave surface impedance in the mixed state. Large r are found in YBa2Cu3O7-delta/BaZrO3, with little field dependence. By contrast, smaller values and stronger field dependences are found in pure YBa2Cu3O7-delta. We discuss the different field dependence of the pinning constant. (C) 2008 Elsevier B.V. All rights reserved.