Superconductivity under pressure in $R$FeAsO${}_{1-\mathrm{x}}$F${}_{\mathrm{x}}$ ($R$=La, Ce$-$Sm) by dc magnetization measurements

Superconducting transition under pressure ($P$) has been investigated for optimum-doped $R$FeAsO${}_{1-\mathrm{x}}$F${}_{\mathrm{x}}$ ($R$=La, Ce$-$Sm) by dc magnetization measurements. For $R$=La, $T_{\mathrm{c}}$ 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,weconstructthe$T_{\rm c}$ evolution map on the height of the As atom from the Fe-plane $h_{\mathrm{A}\mathrm{s}}$ versus lattice constant $a$ or $c$ plane. It is shown that the characteristic $T_{\mathrm{c}}$ variations under pressure for all compounds and the rapid decrease in $T_{\mathrm{c}}$ 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,suggestingthat$T_{\rm c}$ is determined by $h_{\mathrm{A}\mathrm{s}}$ and lattice constant. It is also suggested that there exists an upper limit of the lattice constant $a_{\mathrm{u}\mathrm{l}\mathrm{m}}$ (or $c_{\rm ulm}$),abovewhich$dT_{\rm c}da$ (or $dT_{\rm c}dc$) changes the sign from positive to negative.