Superconducting transition under pressure ($P$) has been investigated for optimum-doped $R$FeAsO$_{ \rm 1-x}$F$_{ \rm x}$ ($R$=La, Ce$-$Sm) by dc magnetization measurements. For $R$=La, $T_{ \rm c}$ is found to be pressure independent up to $P$$\sim$3.0 GPa and then shows a monotonic decrease with increasing pressure. The plateau width decreases for the system with smaller lattice constants, and shrinks to almost zero for $R$=Sm. From the $T_{ \rm c}$($P$) data, we construct the $T_{ \rm c}$ evolution map on the height of the As atom from the Fe-plane $h_{ \rm As}$ versus lattice constant $a$ or $c$ plane. It is shown that the characteristic $T_{ \rm c}$ variations under pressure for all compounds and the rapid decrease in $T_{ \rm c}$ induced by changing $R$-element from Sm to La can be described by a common $T_{ \rm c}$($h_{ \rm As}$, $a$ or $c$) surface, suggesting that $T_{ \rm c}$ is determined by $h_{ \rm As}$ and lattice constant. It is also suggested that there exists an upper limit of the lattice constant $a_{ \rm ulm}$ (or $c_{ \rm ulm}$), above which $dT_{ \rm c}$$da$ (or $dT_{ \rm c}$$dc$) changes the sign from positive to negative.
Categories: cond-mat.supr-con