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

Abstract

We report bulk superconductivity (SC) in Eu${}_{0.2}$Sr${}_{0.8}$(Fe${}_{0.86}$Co${}_{0.14}$)${}_2$As${}_2$ single crystals by means of electrical resistivity, magnetic susceptibility, and specific heat measurements with $T$${}_{\mathrm{c}}$ $\simeq$ 20 K with an antiferromagnetic (AFM) ordering of Eu${}^{2+}$ moments at $T$${}_{\mathrm{N}}$ $\simeq$ 2.0 K in zero field. ${}^{75}$As NMR experiments have been performed in the two external field directions (H$\Vert ab$) and (H$\Vert c$). ${}^{75}$As-NMR spectra are analyzed in terms of first order quadrupolar interaction. Spin-lattice relaxation rates (1/$T_1$) follow a $T^3$ law in the temperature range 4.2-15 K. There is no signature of Hebel-Slichter coherence peak just below the SC transition indicating a non s-wave or s${}_{\pm }$ type of superconductivity. The increase of 1/$T_{1}T$ with lowering the temperature in the range 160-18 K following $\frac{C}{T+\theta }$ law reflecting 2D AFM spin fluctuations.

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