Abstract
We present measurements of the magnetic field dependent microwave surface resistance in laser-ablated YBa2Cu3O7-δ films on SrTiO3 substrates. BaZrO3 crystallites were included in the films using composite targets containing BaZrO3 inclusions with mean grain size smaller than 1 μm. X-ray diffraction showed single epitaxial relationship between BaZrO3 and YBa2Cu3O7-δ. The effective surface resistance was measured at 47.7 GHz for 60<T<90 K and 0<μ0H<0.8 T. The magnetic field had a very different effect on pristine YBa2Cu3O7-δ and YBa2Cu3O7-δ/BaZrO3, while for μ0H=0 only a reduction of Tc in the YBa2Cu3O7-δ/BaZrO3 film was observed, consistent with dc measurements. At low enough T, in moderate fields YBa2Cu3O7-δ/BaZrO3 exhibited an intrinsic thin film resistance lower than the pure film. The results clearly indicate that BaZrO3 inclusions determine a strong reduction of the field-dependent surface resistance. From the analysis of the data in the framework of simple models for the microwave surface impedance in the mixed state we argue that BaZrO3 inclusions determine very steep pinning potentials.
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