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

TL;DR

This study investigates how pressure affects superconductivity in $R$FeAsO$_{ m 1-x}$F$_{ m x}$ compounds with different rare-earth elements, revealing that $T_c$ depends on structural parameters like $h_{As}$ and lattice constants, with a universal behavior across compounds.

Contribution

It introduces a unified $T_c$($h_{As}$, lattice constants) surface model explaining pressure effects and rare-earth element variations on superconductivity in these materials.

Findings

$T_c$ remains constant up to 3.0 GPa for La-based compounds.

$T_c$ decreases monotonically with pressure for La, Ce, Sm compounds.

A universal $T_c$ surface describes the pressure and composition dependence.

Abstract

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.