Persistent Josephson tunneling between Bi2Sr2CaCu2O8+x flakes twisted by 45° across the superconducting dome

There is a heated debate on the Josephson effect in twisted Bi2Sr2CaCu2O8+x flakes. Latest results suggest the presence of either anomalously isotropic pairing or exotic d + id-wave pairing, in addition to the commonly believed d-wave one. Here, we address this controversy by fabricating ultraclean junctions with uncompromised crystalline quality at the junction interface. In the optimally doped regime, we obtain prominent Josephson coupling (2-4 mV) in multiple junctions with the twist angle of 45 degrees , in sharp contrast to a recent report that shows two orders of magnitude suppression around 45 degrees from the value at 0 degrees. We further extend this study to the previously unexplored overdoped regime and observe pronounced Josephson tunneling at 45 degrees together with Josephson diode effect up to 50 K. Our study helps establish the persistent presence of an isotropic pairing component across the entire superconducting phase diagram.