NMR investigation of iron-selenide and iron-arsenide high $T_{c}$ superconductors

We have investigated the electronic, magnetic, and superconducting properties of the iron-selenide high $T_{c}$ superconductor K$_{x}$Fe$_{2-y}$Se$_{z}$ ($T_{c}=33$\ K) with $^{77}$Se NMR [1]. We will compare the results with those observed for FeSe in ambient and applied pressures ($T_{c}>9$\ K) [2], and with iron-arsenides [3]. Similarities and dissimilarities will be pointed out, with primary focus on the anomalous normal state properties. Our latest work on K$_{x}$Fe$_{2-y}$Se$_{z}$ was carried out in collaboration with D. Torchetti, M. Fu, D. Christensen, K. Nelson (McMaster), H. Lei, and C. Petrovic (Brookhaven National Lab).\\[4pt] [1] D. Torchetti et al., PR {\bf B83}, 104508 (2011).\\[0pt] [2] T. Imai et al. PRL {\bf 102}, 177005 (2009).\\[0pt] [3] F.L. Ning et al., PRL {\bf 104}, 037001 (2010); JPSJ {\bf 78}, 103711 (2009).