Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR

We report bulk superconductivity (SC) in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 single crystals by means of electrical resistivity, magnetic susceptibility and specific heat measurements with Tc ≃ 20 K and an antiferromagnetic (AFM) ordering of Eu2+ moments at TN ≃ 2.0 K in zero field. 75As NMR experiments have been performed in the two external field directions (H ∥ ab) and (H ∥ c). 75As-NMR spectra are analysed in terms of first-order quadrupolar interaction. Spin-lattice relaxation rates (1/T1) follow a T3 law in the temperature range 4.2-15 K. There is no signature of a Hebel-Slichter coherence peak just below the SC transition, indicating a non-s-wave or s± type of superconductivity. In the temperature range 160-18 K 1/T1T follows the C/(T+θ) law reflecting 2D AFM spin fluctuations. (paper)