Effect of pressure on the superconducting and spin-density-wave states of SmFeAsO1-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 [J. Am. Chem. Soc. 130, 3296 (2008)] discovered a T(c) up to 26 K in the layered La(O(1-x)F(x))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 RE(O(1-x)F(x))FeAs by compression. This later prediction appears to be supported by the pressure-induced T(c) increase in La(O(0.89)F(0.11))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 RE(O(1-x)F(x))FeAs and that the T(c) of La(O(1-x)F(x))FeAs and Sm(O(1-x)F(x))FeAs may be further raised to about 50 K.