Effect of pressure on the superconducting and spin-density-wave states of SmFeAsO1-x Fx

High-temperature superconductors with a Tc above 40 K have been found to be strongly correlated electron systems and to have a layered structure. Guided by these rules, Kamihara [J. Am. Chem. Soc. 130, 3296 (2008)] discovered a Tc up to 26 K in the layered La (O1-x Fx) FeAs. By replacing La with trivalence rare-earth (RE) elements of smaller ionic radii, Tc 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 Tc enhancement in RE (O1-x Fx) FeAs by compression. This later prediction appears to be supported by the pressure-induced Tc increase in La (O0.89 F0.11) FeAs observed. Here we show that, in contrast to previous expectations, pressure can either suppress or enhance Tc, depending on the doping level, suggesting that a Tc exceeding 50 K may be found only in the yet-to-be discovered compound systems related to but different from RE (O1-x Fx) FeAs and that the Tc of La (O1-x Fx) FeAs and Sm (O1-x Fx) FeAs may be further raised to about 50 K.