Strong reduction of field-dependent microwave surface resistance in YBa₂Cu₃O₇₋δ with sub-micrometric BaZrO₃ inclusions

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We observe a strong reduction of the field induced thin film surface resistance measured at high microwave frequency (ν=47.7 GHz) in YBa₂Cu₃O₇₋δ thin films grown on SrTiO₃ substrates, as a consequence of the introduction of sub-micrometric BaZrO₃ particles. The field increase of the surface resistance is smaller by a factor of 3 in the film with BaZrO₃ inclusions, while the zero-field properties are not much affected. Combining surface resistance and surface reactance data we conclude (a) that BaZrO₃ inclusions determine very deep and steep pinning wells and (b) that the pinning changes nature with respect to the pure film.