Overdoping of Superconducting TLAG - YBa₂Cu₃O_7-δ Films

Since the discovery of superconductivity in the early 20th century, high-temperature superconductors have been a breakthrough, expanding applicability in various fields. REBCO, a cuprate family superconductor, boasts exceptional properties like high critical current density and irreversibility field, enabling its use in high-field NMR, fault current limiters, compact fusion, electric aviation, and more. The successful integration of REBCO into potential applications were possible due to the development of highly textured coated conductors (CC). However, the high cost-performance ratio on the production of CCs, limits their widespread use. A key approach to reduce this is increasing the growth rate [1]. With this aim, the novel Transient Liquid Assisted Growth (TLAG) method combined with the traditional chemical solution deposition approach is a success in achieving high growth rates above 1000nm/s and critical current density in the range of 3MA/cm² at 77K [2-5].

In addition to improving the cost-performance ratio, researchers are actively working to enhance the performance of coated conductors (CCs) under magnetic field by increasing the pinning strength of vortices, as vortex motion is the primary cause of dissipation. Currently, two main strategies are being pursued: the development of superconducting nanocomposites and overdoping. The former involves introducing nanoscale defects to act as artificial pinning centres (APCs), thereby pinning vortices more effectively. The latter, a more recent approach, aims to increase charge carrier density by achieving the overdoped state in REBCO coated conductors, where maximum critical current density (Jc) can be reached due to enhanced condensation energy. Recent studies are hinting a combined effect of these two methods - overdoped nanocomposites - can push the current carrying capabilities closer to the depairing current density (J_d) [6]. Since the oxygen doping state significantly impacts REBCO's superconducting properties, incorporating oxygen into the lattice is a critical step during fabrication. Although low-temperature oxygenation supports overdoping, it suffers from slow diffusion. We are addressing this challenge by investigating the overdoped state in TLAG YBCO films using different oxygenation methods, such as oxygen-ozone mixtures and Ag surface decoration layers, to improve oxygen incorporation. This study will provide a comprehensive examination of the key oxygenation parameters that influence the properties of superconducting films, including temperature, dwell time, and ozone concentration (in the case of oxygen-ozone mixture), as each plays a crucial role in determining the final oxygen content in the material. The impact of these parameters on achieving the desired overdoped state will be thoroughly analyzed. Additionally, the research will delve into the characteristics of the overdoped state specifically in TLAG YBCO films, highlighting how they differ from the overdoped TFA and PLD films [7] in terms of critical current density and other physical properties. This comparison aims to identify potential advantages of the TLAG method in producing high-performance superconducting films.

Furthermore, the study will include preliminary results on the extension of overdoping into TLAG nanocomposites, providing insights into their pinning capabilities and potential to enhance critical current density. The inclusion of these early-stage findings will help lay the groundwork for future development of overdoped TLAG nanocomposite films.

[1] T. Puig et al, Nat Rev Phys (2024), [2] L. Soler et al, Nature Communications (2020), [3] S. Rasi et al, Advance Science (2022), [4] L. Saltarelli et al, ACS Appl Materials and Interfaces (2022), [5] L. Saltarelli et al, ACS Appl Materials and Interfaces (2024), [6] X. Obradors et al, Nature Materials (2024), [7] A. Stangl et al, Sci Report 11, 8176 (2021)