Research progress on YBa₂Cu₃O_7–<italic>_δ</italic> thin films and Josephson junctions

High-temperature superconducting (HTS) YBa2Cu3O7–δ (YBCO) thin films and Josephson junctions are pivotal for both fundamental research into HTS mechanisms and practical electronic devices operating at liquid nitrogen temperatures. This article provides a comprehensive review of recent advances in the fabrication techniques, physical properties, and applications status of YBCO thin films and Josephson junctions, describing the path from fundamental research toward technological implementation.Substantial progress has been made in the growth of YBCO thin films. Advanced deposition methods, including pulsed laser deposition (PLD), magnetron sputtering, and metal-organic chemical vapor deposition (MOCVD), have enabled the production of films with enhanced superconducting properties over large areas. Critical parameters such as the superconducting transition temperature (Tc), critical current density (Jc), and surface smoothness have seen continuous improvement. Through strategies such as substrate strain, the use of buffer layers (e.g., CeO2), the construction of superlattices (e.g., PrBa2Cu3O7/YBCO), and annealing with active oxygen sources, researchers can effectively suppress twinning rate, control oxygen doping homogeneity, and modulate electronic structures. These capabilities are indispensable not only for probing fundamental phenomena like the pseudogap phase and charge density waves but also for meeting the exacting material specifications required for high-performance devices. Nevertheless, challenges persist. The microwave surface resistance (Rs) of even the best films, while far superior to conventional metals, has not yet reached its theoretical minimum. Great achievements have been realized in the fabrication of YBCO Josephson junctions, the fundamental building blocks of HTS electronics. Superconducting quantum interference filters (SQIFs), comprising arrays of junctions, exhibiting an improved range of single-valued and linear range of the magnetic field-voltage response, enable highly sensitive absolute magnetometry. The high-temperature superconducting terahertz mixer utilizes the nonlinear current-voltage characteristics of the Josephson junction to achieve frequency conversion of microwave/terahertz signals. Compared to other types, it has the potential for lower noise temperature and a lighter cooling system. Series arrays of thousands of junctions are also being developed for a voltage standard through alternating current Josephson effect. Furthermore, the phase-sensitive technique of YBCO untwinned thin films and Josephson junctions provides crucial evidence for revealing the pairing symmetry of HTS materials. The community has developed a variety of methods for preparing Josephson junctions, and each technique has its own unique characteristics. Bi-crystal junctions, defined by a grain boundary on a specially prepared substrate, offer a relatively simple and reproducible structure. Step-edge junctions, created by depositing YBCO over a lithographically defined substrate step, provide good design flexibility for individual devices. A step-edge SNS junction is formed by depositing metal on the steeper steps, as the junction region contracts with grain boundaries. Inclined-edge junctions have excellent stability and anti-interference capabilities with top electrodes. In particular, the emergence of focused helium ion beam (He-FIB) technology represents a breakthrough. This direct-write technique with a high degree of freedom utilizes a sub-nanometer helium ion probe to locally modify the oxygen content or crystal structure, creating a tunable barrier within the range limited by the superconducting coherence length. Its greatest strength lies in enabling the fabrication of large-scale, dense arrays of junctions with complex geometries, a capability important for integrated devices.Despite these promising developments, critical challenges should be addressed to achieve industrialization and commercialization. For junction arrays, the statistical scatters in key parameters like critical current (Ic) and normal resistance (Rn) need further optimization for device stability and reproducibility. Future progress depends on a co-optimization strategy: advancing film growth to achieve outstanding and homogeneous superconducting performance and perfect surface flatness; minimizing damage and improving uniformity in nanofabrication processes of junctions; establishing microstructure-function relationships to guide empirical optimization with physical insight.