Extreme sensitivity of superconductivity to stoichiometry inFe1+δSe

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The recently discovered iron arsenide superconductors appear to display a universal set of characteristic features, including proximity to a magnetically ordered state and robustness of the superconductivity in the presence of disorder. Here we show that superconductivity in ${\text{Fe}}_{1+\ensuremath{\delta}}\text{Se}$, which can be considered the parent compound of the superconducting arsenide family, is destroyed by very small changes in stoichiometry. Further, we show that nonsuperconducting ${\text{Fe}}_{1+\ensuremath{\delta}}\text{Se}$ is not magnetically ordered down to 5 K. 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.