Reduction in the field-dependent microwave surface resistance in YBa_2Cu_3O_(7-δ) with submicrometric BaZrO_3 inclusions as a function of BaZrO_3 concentration

摘要

In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa_2Cu_3O_(7-δ) thin films, where submicrometric BaZrO_3 particles have been incorporated. As a function of the BaZrO_3 content, we observed that the absolute losses slightly decrease up to a BaZrO_3 content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently in that they are not monotonic with increasing BaZrO_3 concentration. At small concentration (2.5%) the field-induced losses increase, but large reduction in the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO_3 concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance, we estimate vortex pinning-related parameters. We found that BaZrO_3 inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO_3 content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, leaving room for further improvements.