Reduction in the field-dependent microwave surface resistance inYBa2Cu3O7-delta with submicrometric BaZrO3 inclusions as a function of BaZrO3 concentration

Abstract

In order to study the vortex pinning determined by artificiallyintroduced pinning centers in the small-vortex displacement regime, wemeasured the microwave surface impedance at 47.7 GHz in the mixed stateof YBa2Cu3O7-delta thin films, where submicrometric BaZrO3 particleshave been incorporated. As a function of the BaZrO3 content, weobserved that the absolute losses slightly decrease up to a BaZrO3content of 5%, and then increase. We found that themagnetic-field-induced losses behave differently in that they are notmonotonic with increasing BaZrO3 concentration. At small concentration(2.5%) the field-induced losses increase, but large reduction in thelosses themselves, by factors up to 3, is observed upon furtherincreasing the BaZrO3 concentration in the target up to 7%. Usingmeasurements of both surface resistance and surface reactance, weestimate vortex pinning-related parameters. We found that BaZrO3inclusions introduce deep and steep pinning wells. In particular, theminimum height of the energy barrier for single vortices is raised. Atlarger BaZrO3 content (5% and 7%) the phenomenon is at its maximum, butit is unclear whether it shows a saturation or not, leaving room forfurther improvements. (C) 2009 American Institute of Physics. [DOI:10.1063/1.3056179]