Systematic modulation of superconducting gap dynamics in YBCO|BNT|YBCO Josephson Junctions through THz field interaction and BNT ferroelectric barrier

The integration of ferroelectric barriers into high temperature Josephson junctions offers a pathway to tunable superconducting quantum devices. Here, we demonstrate robust Josephson coupling in YBa₂Cu₃O₇₋ₓ (YBCO) | Bi₀.₅Na₀.₅TiO₃ (BNT) | YBCO trilayer junctions incorporating a relatively thick (40 nm) ferroelectric BNT barrier. Epitaxial trilayers were fabricated using pulsed laser deposition on SrTiO₃ substrates and systematically investigated under broadband terahertz (THz) irradiation in the range of 0.5 to 2.5 THz. At 1.5 THz, which is close to the Josephson plasma resonance, the BNT polarization increased significantly, from 33.4 to 46.4 µC/cm² at 30 K, leading to enhanced superconducting transport. The junctions exhibited a well-defined zero voltage supercurrent branch, with critical current I _c ​(T) remaining nearly constant up to 60 K and reaching a peak value of 468.2 µA under 1.5 THz excitation, evidencing strong phase coherence. Scanning tunneling spectroscopy revealed enhanced superconducting gaps in YBCO|BNT|YBCO (Δ = 1.3 meV) compared to pure YBCO (Δ = 1.05 meV), while optical conductivity measurements showed a systematic reduction in conductivity and gap with increasing temperature, consistent with BCS theory under THz excitation. To exclude defect mediated transport, atomic force microscopy and X ray reflectivity confirmed uniform morphology and sharp interfaces. Magnetic field modulation of I _c (B) exhibited a canonical Fraunhofer interference pattern, and resistance mapping revealed alternating lobes of high and low dissipation, both of which are strong indicators of coherent Josephson tunneling. These results establish that Josephson coupling is intrinsic to the YBCO|BNT|YBCO junctions, enabled by the dipolar character and dynamic THz response of the BNT barrier. This study positions BNT as a viable and tunable barrier material for next generation superconducting quantum devices based on high-T _c materials.