Quasi-degenerate superconductivity in the `122' iron-pnictide superconductors

In iron-based superconductors, especially so-called `122' systems, the superconducting gap symmetry is still a debated issue both theoretically and experimentally. Most early experimental studies in BaK-122 and BaCo-122 families including ARPES, thermal conductivity, and penetration depth measurements indicate a fully-gapped superconducting state, although the consensus for the gap structure is still lacking. Here we report the magnetic penetration depth measured down to 0.01$T$/$T_{c}$ clearly shows the presence of line nodes on the superconducting gap structure in some `122' iron-pnictide superconductors. This result indicates that non-universal gap symmetry is realized in the `122' systems, depending on the substituting materials and/or its doping levels.