Enhanced flux pinning isotropy by tuned nanosized defect network in superconducting YBa2Cu3O6+x films

Striving to improve the critical current density Jc of superconducting YBa 2 Cu 3 o 6+x (YBCO) thin films via enhanced vortex pinning, the interplay between film growth mechanisms and the formation of nanosized defects, both natural and artificial, is systematically studied in undoped and BaZrO 3 (BZO)-doped YBCO thin films. The films were grown via pulsed laser deposition (PLD), varying the crystal grain size of the targets in addition to the dopant content. The microstructure of the PLD target has been observed to have a great impact on that of the deposited thin films, including the formation of vortex pinning centers, which has direct implications on the superconducting performance, especially on the isotropy of flux pinning properties. Based on experimentally measured angular dependencies of Jc, coupled with a molecular dynamics (MD) simulation of flux pinning in the YBCO films, we present a quantitative model of how the splay and fragmentation of BZO nanorods artifically introduced into the YBCO film matrix explain the majority of the observed critical current anisotropy. To obtain the freedom to engineer future high-temperature superconductor (HTS) applications for desired operating magnetic field and temperature ranges, it is necessary to optimize the vortex pinning landscape for an enhanced, isotropic flux pinning performance 1-6. In addition to naturally formed crystalline defects, which typically have spatial dimensions distinctly below the superconducting coherence length, defect-engineering with artificially produced pinning centers (APCs) with dimensionalities of 1D-3D have been observed to be extremely effective 7-10. However, the complex nucleation process of YBCO during PLD process, that leads to growth island size variation, and the manner in which this could affect the size and distribution of the nanoscale structural defects is chiefly neglected. Especially, a clear gap exists in the current literature regarding how ordered arrays of nanoscale defects can also influence and regulate the distribution and growth of more effective APCs and thus decrease the anisotropy by allowing vortices to be trapped in a wider angular range 11. Partly, the clear lack of information on the subject is arguably be due to the rather general assumption that during PLD process, the film growth method of our choice, the target material is largely decomposed on the atomic level, and thus its properties should not have an effect on the formation and nucleation of particles on the substrate surface. This assumption, which our studies have led us to challenge, would precariously force one to downplay the potential importance of target microstructure on the functional properties of derived films. The angular dependence of the J c has an excellent physical importance providing an approach to the problem of flux pinning and vortex dynamics anisotropy in HTSs, both from the experimental and theoretical point of view. For instance, in the angular dependent critical current plots, one can easily observe how the various types of pinning centers such as correlated linear, columnar or planar defects and, on the other hand, defects based on growth mechanisms together with YBCO's intrinsic pinning can dramatically alter the angular dependence of J c (B) 4. For understanding the origin of angular dependent flux pinning J c (θ), experimental tools like transmission electron microscopy (TEM) are often exploited to probe the structural properties and features, such as the defects naturally formed during the film growth, as well as the size, shape, orientation and distribution of the artificially produced and self-assembled pinning centers 5,12,13. However, methods like positron annihilation spectroscopy,