Kiyotaka Miyoshi, Eiko Kojima, Saki Ogawa, Yuta Shimojo, Jun Takeuchi
Superconducting transition under pressure ($P$) has been investigated for optimum-doped $R$FeAsF$_{\rm 1-x}$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$$\sim$3.0 GPa and then shows a monotonic decrease with increasing pressure. The plateau width decreases for the system with smaller lattice constants, and shrinks to almost zero for $R$=Sm. From the $T_{\rm c}$($P$) data, we construct the $T_{\rm c}$ evolution map on the height of the As atom from the Fe-plane $h_{\rm As}$ versus lattice constant $a$ or $c$ plane. It is shown that the characteristic $T_{\rm c}$ variations including the $T_{\rm c}$$-$$P$ plateau followed by a monotonic decrease and the drastic $T_{\rm c}$ decrease by changing $R$-element 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 the lattice constant gradient of $T_{\rm c}$ changes the sign from positive to negative, as a ridge on the $T_{\rm c}$($h_{\rm As}$, $a$ or $c$) surface.
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http://arxiv.org/abs/1303.0917
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