NMR characterization of sulphur substitution effects in the KxFe2−ySe2−zSz high-Tc superconductor

We present a 77Se NMR study of the effect of S substitution in the high-Tc superconductor KxFe2−ySe2−zSz in a temperature range up to 250 K. We examine two S concentrations, with z=0.8 (Tc∼ 26 K) and z=1.6 (nonsuperconducting). The samples containing sulphur exhibit broader NMR line shapes than the KxFe2Se2 sample due to local disorder in the Se environment. Our Knight shift 77K data indicate that in all samples, uniform spin susceptibility decreases with temperature, and that the magnitude of the Knight shift itself decreases with increased S concentration. In addition, S substitution progressively suppresses low-frequency spin fluctuations. None of the samples exhibit an enhancement of low-frequency antiferromagnetic spin fluctuations near Tc in 1/T1T, as seen in FeSe.