<i>Anomalous Critical Current Temperature Evolution of Bi₂Sr₂CaCu₂O_8+xTwist Josephson Junctions</i>

The high temperature superconductor Bi₂Sr₂CaCu₂O_8+x (Bi-2212) represents a prototypical d-wave cuprate superconductor, with weakly bonded van der Waals layers. Using a novel cryogenic van der Waals pickup technique, we have fabricated Josephson junctions between two exfoliated Bi-2212 crystals with controlled relative twist angles. To preserve the integrity of the air- and heat-sensitive Bi-2212 surface, we handled the devices entirely within an argon or high-vacuum environment and kept the devices cold throughout the fabrication process. The resulting junctions support a Josephson critical current density of similar magnitude as the bulk c-axis intrinsic junctions, and with Tc within a few Kelvin of the bulk value. With no need of a post-stacking anneal step, the interface shows minimal signs of surface degradation or reconstruction in cross-sectional TEM images. The junctions’ IcRn(θ) evolves as expected for a d-wave superconductor. For devices with θ close to 45°, the critical current magnitude anomalously decreases as temperature cools. Our new fabrication methods opens the possibility of creating arbitrarily complex, monolayer Bi-2212 heterostructures.