Effect of pressure on the superconducting and spin-density-wave states ofSmFeAsO1−xFx

High-temperature superconductors with a $T_c$ above 40 K have been found to be strongly correlated electron systems and to have a layered structure. Guided by these rules, Kamihara et al. [J. Am. Chem. Soc. 130, 3296 (2008)] discovered a $T_c$ up to 26 K in the layered $\text{La}\left({\text{O}}_{1-x}{\text{F}}_x\right)\text{FeAs}$. By replacing La with trivalence rare-earth (RE) elements of smaller ionic radii, $T_c$ has subsequently been raised to 41–52 K. Many theoretical models have been proposed that emphasize the important magnetic origin of superconductivity in this compound system and a possible further $T_c$ enhancement in $\text{RE}\left({\text{O}}_{1-x}{\text{F}}_x\right)\text{FeAs}$ by compression. This later prediction appears to be supported by the pressure-induced $T_c$ increase in $\text{La}\left({\text{O}}_{0.89}{\text{F}}_{0.11}\right)\text{FeAs}$ observed. Here we show that, in contrast to previous expectations, pressure can either suppress or enhance $T_c$, depending on the doping level, suggesting that a $T_c$ exceeding 50 K may be found only in the yet-to-be discovered compound systems related to but different from $\text{RE}\left({\text{O}}_{1-x}{\text{F}}_x\right)\text{FeAs}$ and that the $T_c$ of $\text{La}\left({\text{O}}_{1-x}{\text{F}}_x\right)\text{FeAs}$ and $\text{Sm}\left({\text{O}}_{1-x}{\text{F}}_x\right)\text{FeAs}$ may be further raised to about 50 K.