  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 et al. discovered a Tc up to 26 K in the layered
La(O1-xFx)FeAs. By replacing La with tri-valence rare-earth elements RE of
smaller ionic radii, Tc has subsequently been raised to 41-52 K. Many
theoretical models have been proposed emphasizing the important magnetic origin
of superconductivity in this compound system and a possible further
Tc-enhancement in RE(O1-xFx)FeAs by compression. This later prediction appears
to be supported by the pressure-induced Tc-increase in La(O0.89F0.11)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's K may be found only in the yet-to-be discovered compound
systems related to but different from R(O1-xFx)FeAs and that the Tc of
La(O1-xFx)FeAs and Sm(O1-xFx)FeAs may be further raised to 50's K.
