Superconductivity under pressure in RFeAsO1-xFx (R = La, Ce-Sm) by dc magnetization measurements

Superconducting transition under pressure (P) has been investigated for optimum-doped RFeAsO1-xFx (R = La, Ce-Sm) by dc magnetization measurements. For R = La, T-c is found to be pressure independent up to P similar to 3.0 GPa and then shows a monotonic decrease with increasing pressure. The plateau width decreases for the system with smaller lattice constants and shrinks to almost zero for R = Sm. From the T-c(P) data, we construct the T-c evolution map on the height of the As atom from the Fe-plane h(As) versus lattice-constant a or c plane. It is shown that the characteristic T-c variations under pressure for all compounds and the rapid decrease in T-c induced by changing the R element from Sm to La can be described by a common T-c (h(As), a or c) surface, suggesting that T-c is determined by h(As) and lattice constant. It is also suggested that there exists an upper limit of the lattice constant a(ulm) (or c(ulm)), above which dT(c)/da (or dT(c)/dc) changes the sign from positive to negative.