Manipulating fractional Shapiro steps in twisted cuprate Josephson junctions

Abstract

High$-$quality Josephson junctions made of twisted cuprate superconductors offer unprecedented opportunities in addressing fundamental problems and realizing next$-$generation superconducting devices at relatively high temperatures. Whether or not the twisted cuprates possess high$-$temperature topological superconductivity remains an outstanding issue. Here, we tackle this problem via an in$-$depth study of the key predicted feature $--$ half$-$integer Shapiro steps. We show that half$-$integer Shapiro steps do occur in samples at a twist angle of 45$^\circ$ but are unstable with thermal cycling. Interestingly, fractional steps can be introduced by training the sample with a small magnetic field or annealing with a large electrical current, attesting to a tunable current$-$phase relation (CPR) in twisted cuprates. We extend the current annealing to realize fractional steps with odd denominators too. Furthermore, half$-$integer steps can be induced in the regime that is well beyond the expectation of topological superconductivity, favoring an alternative mechanism involving trapped vortices. Our results not only caution the direct association of half$-$integer Shapiro steps with the exotic mechanism but also open a distinct pathway toward a Josephson junction with electrically tunable CPR at high temperatures.