Evidence for multi-gap symmetry from grain boundary effects in polycrystalline microbridge

We studied the grain boundaries within mechanically formed polycrystalline BaKFe2As2 micro-bridges. By tunneling current across grain boundaries, current–voltage characteristics (IVCs) demonstrated the typical Josephson weak links behavior in micro-bridges. Shapiro steps were observed for the junctions under the microwave radiation at 10 GHz. The temperature dependence of the critical current I c was observed as a shoulder, corresponding to a multi-gap symmetry but not a single-gap s-wave or d-wave.