Synopsis: Success without pressure

Iron selenide is transformed into an ambient high-temperature superconductor by a purely chemical method, substituting extremely high pressure.

In the last two years, many distinct families of pnictide and chalcogenide superconductors have been discovered as a result of concerted efforts in this field. In almost all cases, an “ordinary” parent compound is turned into a superconductor either by chemical doping, or by application of pressure. One of the structurally simpler materials available is iron selenide, which has as basic building blocks layers of ${\text{FeSe}}_{4}$ tetrahedra. Application of extremely high pressure has yielded a superconducting critical temperature ${T}_{c}$ above $30\phantom{\rule{0.333em}{0ex}}\text{K}$, approximately half of the highest reported ${T}_{c}$ of $55\phantom{\rule{0.333em}{0ex}}\text{K}$ in oxygen-deficient $\text{SmFeAsO}$.

Now, in a Rapid Communication appearing in Physical Review B, Jiangang Guo and collaborators from the Chinese Academy of Sciences in Beijing, report the synthesis of ${\text{K}}_{x}{\text{Fe}}_{2}{\text{Se}}_{2}$, a superconducting compound with a critical temperature ${T}_{c}$ slightly above $30\phantom{\rule{0.333em}{0ex}}\text{K}$ at ambient pressure. That value sets a new record for the highest critical temperature at ambient pressure for this family of compounds. Structurally, these materials are related to the pnictide $122$ compounds $A{\text{Fe}}_{2}{\text{As}}_{2}$ (). In this case, the potassium is intercalated between the $\text{FeSe}$ layers, and the authors claim that the enhancement of ${T}_{c}$ is due to the optimization of the carrier density in the $\text{FeSe}$ layers and also due to the $\text{Fe-Se}$ tetrahedra achieving a shape close to the ideal. – Alexios Klironomos

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