NMR study of hole-doped iron-pnictide superconductor Ba_1−xK_xFe₂As₂ (x = 0.27−1)

We performed the ⁷⁵As nuclear magnetic resonance measurements of Ba_1−xK_xFe₂As₂ (BKFA) for x = 0.27, 0.39, 0.58, 0.64, 0.69 and 1 to understand gap structure in this system. Temperature dependence of spin lattice relaxation rate (1/T₁) below superconducting (SC) transition temperature T_c changes gradually from x = 0.39 to 1.0. This result indicates that the SC gap structure changes smoothly from full gap state with maximum T_c into nodal-line structure state for x = 1 (KFe₂As₂). BKFA does not have distinct SC symmetry change. The nodal gap structure appears continuously and develops gradually with increasing x.