Reduction of the field-dependent microwave surface resistance in YBa_2Cu_3O_7 with sub-micrometric BaZrO_3 inclusions as a function of BaZrO_3 concentration
arXiv:0811.2351 · doi:10.1063/1.3056179
Abstract
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 YBaCuO thin films, where sub-micrometric BaZrO particles have been incorporated. As a function of the BaZrO content, we observe that the absolute losses slightly decrease up to a BaZrO 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 concentration: at small concentration (2.5%) the field-induced losses increase, but large reduction of the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance we estimate vortex pinning-related parameters. We find that BaZrO inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, thus leaving room for further improvements.
7 pages, 7 figures