77Se NMR Study of K{x}Fe{2-y}Se{2-z}S{z}

We will present a $^{77}$Se NMR study of the superconducting properties of the recently discovered K$_{x}$Fe$_{2-y}$Se$_{2}$ (T$_{c} \sim$ 33 K), in a temperature range of 4 to 250 K [1]. Our Knight shift data reflect the progressive decrease in uniform spin susceptibility with temperature, in analogy with FeSe and iron-arsenide systems. Nuclear spin-lattice relaxation rate data shows no Hebel-Slichter coherence peak, nor any enhancement of antiferromagnetic spin fluctuations (AFSF) toward T$_{c}$. We have also conducted $^{77}$Se NMR measurements on K$_{x}$Fe$_{2-y}$Se$_{0.4}$S$_{1.6}$ (non-superconducting) and K$_{x}$Fe$_{2-y}$Se$_{1.2}$S$_{0.8}$ (T$_{c} \sim$ 21 K) to study the effect of sulphur substitution in this superconductor [2]. Sulphur applies a chemical pressure on the lattice, because it has the same valence as Selenium but less than half the ionic radius. We again measure NMR Knight shift and nuclear spin-lattice relaxation rate 1/T$_{1}$, and find that both are suppressed with S substitution. We will discuss these results in comparison with K$_{x}$Fe$_{2-y}$Se$_{2}$. \\[4pt] [1] D. Torchetti et. al., PRB 83, 104508 (2011)\\[0pt] [2] D. Torchetti et. al., arXiv:1111.2552 (2011)