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

TL;DR

This study demonstrates bulk superconductivity and antiferromagnetic order in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 single crystals, revealing unconventional pairing and 2D AFM spin fluctuations through NMR and other measurements.

Contribution

First detailed NMR analysis of Eu0.2Sr0.8(Fe0.86Co0.14)2As2 revealing superconducting pairing symmetry and magnetic fluctuations.

Findings

Superconductivity at Tc ≈ 20 K confirmed by resistivity, susceptibility, and heat capacity.

Absence of Hebel-Slichter peak suggests non-s-wave or s± pairing symmetry.

Observation of 2D AFM spin fluctuations above Tc.

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$\|ab$) and (H$\|c$). $^{75}$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.