Growth optimization of YBa₂Cu₃O_7-x thin films using MBE flux gradients and closed-loop electronic transport

Efforts to probe superconductor-based devices for qubits and advanced computing has renewed the interest to optimize high-temperature cuprate superconductors. We present a systematic approach to optimize superconductivity in YBa₂Cu₃O_7-x thin films using ozone assisted molecular beam epitaxy. Our experiments take a combinatorial approach to optimize stoichiometry and minimize defects with direct closed-loop feedback on superconductivity using ex-situ electronic transport measurements. We identify T_C0 as a single parameter that is susceptible to all types of defects and inhomogeneities. By arranging the effusion cells in a geometry that provides two orthogonal spatial directions across the sample surface where the relative atomic ratios Y/Cu and Ba/Cu can be maintained, we carry out experimental growths with multiple STO(001) substrates (cleaved from the same crystal) strategically mounted on a puck. The positional dependance of T_C0 is used to provide iterative feedback to adjust the center stoichiometry of the next sample. With this process our optimized c-axis YBa₂Cu₃O_7-x films (of thickness 12 to 36 nm) show T_C0 values as high as 89 K.