Synopsis: Superconductivity can be sensitive
Following the explosion of interest in the iron-based (pnictide) superconductors, many compounds belonging to the same family of materials have been discovered. FeSe is a recent new addition, composed of layers analogous to the layers of the prototypical pnictide , but with an overall simpler structure . However, the robustness of this material’s physical properties with respect to changes in stoichiometry is not established.
In Physical Review B, Robert Cava and Nai Phuan Ong of Princeton University, and collaborators in the USA and Europe, report a variety of techniques to grow and characterize that contains an excess of iron. They find that compared to , destroys seemingly robust superconductivity, which they believe most likely results from disorder in the crystal structure caused by the additional iron. Moreover, while stoichiometric FeSe exhibits magnetic order at temperatures above the superconductivity transition, the magnetism apparently disappears in .
These results point to the role of deviations from stoichiometry and pose intriguing theoretical questions regarding the interplay of magnetism and superconductivity, with implications for the entire family of iron-based superconductors. – Alexios Klironomos
 Michelle Johannes, Physics 1, 28 (2008).