Microstructure and transport properties of Y-rich YBa ₂Cu ₃O _7-δ thin films

Epitaxial Y _xBa ₂Cu ₃O _7-δ (Y _xBCO) thin films were deposited by pulsed laser deposition on both single crystal SrTiO ₃ (100) and buffered polycrystalline metal substrates, where X=0.9, 1.0, 1.1, 1.2, and 1.3. The self-field critical current density (J _c) of Y _xBCO exhibits the best performance at X=1.1 for the samples on both single crystal substrates and metal substrates. Epitaxial Y ₂O ₃ nanoparticles were observed in Y-rich samples (X >1.0) by X-ray diffraction and cross-sectional transmission electron microscopy. The field-dependence and angular-dependence measurements show that Y ₂O ₃ nanoparticles improve the in-field J _c performance, especially in the low field regime, without reducing self-field J _c of YBCO films.