Controllable Self-Assembly of Nanostructured Artificial Pinning Centers (APCs) in High Temperature Superconductor Epitaxial Thin Films

Abstract

A Superconductor exhibits dc zero-resistance below a critical temperature (Tc). The possible uses of superconductors in a high temperature range of 50-77 K are greatly expanded by the discovery of high temperature superconductors (HTS) in 1986. One of the most important parameters for the industrial applications of high temperature superconductors (HTS) is a high value of critical current density Jc, in applied magnetic fields (H) up to tens of Teslas. In HTS, magnetic flux would be expected to penetrate the superconductor in the form of filaments containing one flux quantum (Φo) in each filament. These flux lines are surrounded by circulating current that acts as screening current and give rise to the mixed state known as vortex state. Application challenges involve preventing vortex motion in HTS and determining the high value of Jc at the high magnetic field (H). The vortex motion increases with increasing applied field and hence decreases the Jc due to dissipation induced by the vortex motion. Obtaining a high Jc(H) requires stoppage of vortex motion in HTS. This can be done using pinning centers which capture the vortices and prevent their motion. Therefore, reaching high Jc(H) in HTS requires the insertion of strong pinning centers of dimension comparable to the superconducting coherence length on the order of few nanometers. Such pinning centers improve the critical current density and strengthen the pinning force density. Various innovative approaches have been developed in the last decade to generate optimally efficient artificial pinning centers (APCs) in YBa2Cu3O7-x (YBCO) nanocomposite films. However, controllable generation of self-assembled nanostructures during sample growth stage remains a challenge. Therefore, in this study, we generate a landscape of one-dimensional (1D) plus three-dimensional (3D) APCs of flexible elastic materials to improve strong and isotropic pinning which is beneficial for many industrial applications such as motors and generators. Specifically, a study of 3 vol.% Y2O3+2-6 vol.% BaHfO3 (BHO) double doped YBa2Cu3O7-x epitaxial thin films is carried out and compared to the same concentration of BaZrO3 (BZO) doping materials to explore the morphologic adaptation of the c-axis aligned 1D APCs to the 3D APCs. A significant reduction of Jc anisotropy is found for low doping BaHfO3 and 3 vol.% Y2O3 doped YBCO nanocomposite films (BHO double doped films). The self-assembly of 1D APCs in YBCO film matrix driven by the strain field is influenced by the lattice mismatch at the APC/YBCO interface. To answer the fundamental question on how the pinning efficiency of 1D APCs is affected by the APC/YBCO interface, electrical transport properties Jc (H, T) of the comparable diameter of BaZrO3 and BaHfO3 1D APCs on single doped YBCO nanocomposite films have been studied. The pinning force density is found to be significantly larger for a coherent, a less defective, BHO 1D APC/YBCO interface compared to a semicoherent, defective and oxygen deficient, BZO 1D APC/YBCO interface of epitaxial YBCO nanocomposite thin films. Transmission Electron Microscopy (TEM) images are utilized to study the difference of the nanostructures’ morphology, and 1D APC/YBCO interface of single and double doped nanocomposite thin films. It is found that less rigid BHO material forms a mixed APCs morphology reducing Jc anisotropy to about 20 % for 2 vol.% BHO double doped YBCO thin film at temperature of 65 K and at magnetic field of 9.0 T. A coherent APC/YBCO interface enhances the pinning efficiency of 1D APCs in BHO doped YBCO thin films. Significantly reduced pinning efficiency of BZO 1D APCs is observed for a defective BZO/YBCO interface. A method of repairing defective APC/YBCO interface through calcium doping is explored and recommended to enhance the pinning efficiency of one-dimensional APCs.