Superconductivity under pressure inRFeAsO1−xFx(R=La, Ce−Sm) by dc magnetization measurements

Abstract

Superconducting transition under pressure ($P$) has been investigated for optimum-doped $R$FeAsO${}_{1\ensuremath{-}x}$F${}_{x}$ ($R=\text{La}$, Ce--Sm) by dc magnetization measurements. For $R=\text{La}$, ${T}_{c}$ is found to be pressure independent up to $P\ensuremath{\sim}3.0$ 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 $R=\text{Sm}$. From the ${T}_{c}$($P$) data, we construct the ${T}_{c}$ evolution map on the height of the As atom from the Fe-plane ${h}_{\mathrm{As}}$ 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 ${T}_{c}$ (${h}_{\mathrm{As}}$, $a$ or $c$) surface, suggesting that ${T}_{c}$ is determined by ${h}_{\mathrm{As}}$ and lattice constant. It is also suggested that there exists an upper limit of the lattice constant ${a}_{\mathrm{ulm}}$ (or ${c}_{\mathrm{ulm}}$), above which $d{T}_{c}$$/$$da$ (or $d{T}_{c}$$/$$dc$) changes the sign from positive to negative.