Doping-dependent optical properties in YBCO superconducting films via BaHfO₃ nanocrystal addition

This study investigates the effect of BaHfO₃ (BHO) addition on the optical properties of YBa₂Cu₃O_7–δ (YBCO) superconducting thin films using spectroscopic ellipsometry. Through Raman spectroscopy and SEM analysis, optimal 10-min Ar ion etching effectively removes surface a-axis-oriented grains and Ba–Cu–O impurities, enhancing surface quality. Optical conductivity analysis reveals a doping-dependent evolution: 10% BHO doping maximizes free carrier density and interband transition efficiency, attributed to optimized Cu–O bond contraction and reduced lattice distortions. Higher doping induces defect clustering, carrier scattering, and redshifted transitions due to lattice expansion. Dielectric function and loss function analyses confirm enhanced plasmonic behavior and flux pinning at 10% doping, while excessive doping degrades electronic transitions. These results highlight the critical role of controlled BHO addition and surface treatment in tailoring the optical and superconducting properties of YBCO, offering insights into the interplay among doping, carrier dynamics, and electronic structure in high-temperature superconductors (HTS).