Intrinsic Anisotropy and Pinning Anisotropy in Nanostructured YBa$_2$Cu$_3$O$_{7-\delta}$ from Microwave Measurements

Abstract

Anisotropy is an intrinsic factor that dictates the magnetic properties of YBCO, thus with great impact for many applications. Artificial pinning centres are often introduced in an attempt to mitigate its effect, resulting in less anisotropic electrical and magnetic properties. However, the nanoengineering of the superconductor makes the quantification of the anisotropy itself uncertain: the intrinsic anisotropy due to the layered structure, quantified by the anisotropy factor $\gamma$, mixes up with the additional anisotropy due to pinning. As a consequence, there is no consensus on the experimental anisotropy factor $\gamma$ that can result in YBCO when directional (twin planes, nanorods) or isotropic defects are present. We present here measurements of the magnetic field and angular dependent surface impedance in very different nanostructured YBCO films, grown by chemical route and by pulsed laser deposition, with different kind of defects (nanorods, twin planes, nanoparticles). We show that the surface impedance measurements are able to disentangle the intrinsic anisotropy from the directional pinning anisotropy, thanks to the possibility to extract the true anisotropic flux--flow resistivity and by correctly exploiting the angular scaling. We find in all films that the intrinsic anisotropy $\gamma = 5.3\pm0.7$. By contrast, the pinning anisotropy determines a much complex, feature--rich and nonuniversal, sample--dependent angular landscape.