Supersaturation-driven defect engineering for enhanced flux pinning in FF-MOD YBCO films via yttrium stoichiometry control

Supersaturation serves as the primary thermodynamic driving force for nucleation, governing the crystallographic orientation and microstructural evolution of superconducting films. In this study, we investigated the impact of the yttrium stoichiometry ( x = 0.8, 1.0, 1.2) of Y x Ba 2 Cu 3 O 7- δ films grown via fluorine-free metal-organic deposition. By tuning the yttrium content, we effectively modulated the transient liquid-phase supersaturation, driving a fundamental transition in the defect architecture and vortex pinning dynamics. The Y-deficient film (Y 0.8 BCO), grown under lower supersaturation, lacks a competing dense 3D defect network. Consequently, its microstructural landscape is dominated by localized planar stacking faults, which provide robust 2D correlated pinning and sustain a broader vortex trapping angle ( θ T ). Conversely, high supersaturation in the Y-rich film (Y 1.2 BCO) generates a dense 3D network of isotropic defects. These fine-scale disorders dimensionally match a reduced in-plane coherence length ( ξ ab ), acting as efficient core pinning centers that enhance the self-field critical current density ( J c ) to 2.0 MA cm −2 at 77 K. However, a crucial high-field trade-off emerges: under magnetic fields (>2.7 T), excessive lattice disorder depresses local superconducting properties, rendering the pristine stoichiometric YBCO film more robust. Ultimately, balancing the transition between 2D planar and 3D isotropic pinning regimes via stoichiometry-driven supersaturation engineering provides an effective strategy for optimizing the in-field performance of REBCO coated conductors.