Extreme Sensitivity of Superconductivity to Stoichiometry in FeSe   (Fe1+dSe)
  The recently discovered iron arsenide superconductors, which display superconducting transition temperatures as high as 55 K, appear to share a number of general features with high-Tc cuprates, including proximity to a magnetically ordered state and robustness of the superconductivity in the presence of disorder. Here we show that superconductivity in Fe1+dSe, the parent compound of the superconducting arsenide family, is destroyed by very small changes in stoichiometry. Further, we show that non-superconducting Fe1+dSe is not magnetically ordered down to low temperatures. These results suggest that robust superconductivity and immediate instability against an ordered magnetic state should not be considered as intrinsic characteristics of iron-based superconducting systems, and that Fe1+dSe may present a unique opportunity for determining which materials characteristics are critical to the existence of superconductivity in high Tc iron arsenide superconductors and which are not. 
T. M. McQueen, Q. Huang, V. Ksenofontov, C. Felser, Q. Xu, H. W.   Zandbergen, Y. S. Hor, J. Allred, A. J. Williams, D. Qu, ...