Competing orders in underdoped (Ba_1–xK_x)Fe₂As₂

We report ⁷⁵As Nuclear Magnetic Resonance (NMR) measurements in the high-T_c superconductor (Ba_1−xK_x)Fe₂As₂ in the underdoped regime. A structural transition at T_s ≊ 110 K is followed by an antiferromagnetic (AFM) order at T_N ≊ 102 K for our x = 0.16 single crystal [1]. Superconductivity (SC) also appears at T_c ≊ 20 K. We find that the ordered Fe moment (S) is reduced upon hole-doping. Both spectrum analysis and relaxation measurements indicate that pinned vortices are present below T_c and SC is coexisting with AFM fluctuations.