Multiband Superconductivity in KCa 2 Fe 4 As 4 F 2

The paper reports complementary studies of the order parameter of the newly discovered iron-based superconductor, KCa 2 Fe 4 As 4 F 2 ( T c = 34 K), that consists of alternating blocks of KFe 2 As 2 (122) phase and CaFeAsF (1111) phase. We performed high-resolution intrinsic multiple Andreev reflection spectroscopy supplemented with self-field critical current measurements. Andreev spectra obtained by means of a break-junction technique reveal two distinct superconducting order parameters Δ L and Δ S . Their temperature dependencies were measured from 1.5 K to T c . The shapes of Andreev reflections dips, corresponding to the two gaps are symmetrical, which is a signature of s -wave order parameter. Temperature dependence of the self-field critical current can also be described by two s -wave gaps model.