Josephson Transport through Spin–Orbit-Coupled SrIrO₃ and Sr₂IrO₄ Interlayers

The strontium iridate family includes both conducting and insulating phases with strong spin–orbit coupling. These materials can serve as interlayers in superconducting heterostructures. An epitaxial SrIrO₃ film grown on an NdGaO₃ substrate exhibited weakly nonmetallic resistivity and an approximately linear Hall response. A single-band analysis at 5 K yielded a carrier density of 2 × 1021 cm⁻³, a resistivity of 6 × 10⁻³ Ω cm, and a mobility of 0.6 cm² V⁻¹ s⁻¹. We fabricated Nb/iridate/YBa₂Cu₃O_7−δ superconducting mesa heterostructures. For heterostructures containing 15-nm SrIrO₃ interlayers, IcRN was 0.1–0.3 μV at 4.2 K. The corresponding magnetic diffraction pattern had minima close to zero and a Fraunhofer-like central lobe. For 5-nm Sr₂IrO₄ interlayers, IcRN was 41–79 μV, and the area-normalized normal resistance was about two orders of magnitude larger than that of the SrIrO₃ devices. The RNA ranges for heterostructures with SrIrO₃ and Sr₂IrO₄ interlayers were (3–8) × 10⁻⁸ and (0.8–1) × 10⁻⁵ Ω cm², respectively. Resonant Fiske steps in the current–voltage characteristics corroborated ac Josephson dynamics.