Grain boundary junctions of FeSe_0.5Te_0.5 thin films on SrTiO₃ bi-crystal substrates

Grain boundary junctions were fabricated in the epitaxial FeSe_0.5Te_0.5 thin films on [001] tilt SrTiO₃ bi-crystal substrates with a CeO₂ buffer layer. Critical current densities across the junctions with different mis-orientation angles of 4°, 7°, 15°, and 24° were measured at magnetic fields up to 30T. It was found that the 4° and 7° junctions carry critical current densities comparable to that of the intra-grain film while those of the 15° and 24° junctions were suppressed drastically. A critical mis-orientation angle of around 9° was identified that separates the strong coupling region from the weak link region. We found that the critical current densities across the grain boundary with a 24° mis-orientation angle are modulated by the magnetic field, indicating a Josephson Effect. This junction is estimated to be in the intermediate-size regime with an effective transverse junction width L∼2.6-2.8μm and a Josephson penetration depth λ_J∼1.2μm.