Competition/Coexisitence of Magnetism and Superconductivity in Iron Pnictides Probed by Muon Spin Rotation

TL;DR

This study investigates the coexistence and competition of magnetism and superconductivity in iron pnictides using muon spin rotation, revealing phase separation, magnetic behavior, and differences in pairing mechanisms across compounds.

Contribution

It provides detailed experimental insights into phase separation, magnetic and superconducting phase relationships, and anisotropy of the order parameter in iron pnictides.

Findings

Simultaneous development of magnetism and superconductivity in LFAOF at 18 K.

Distinct phase segregation in CFCAF with magnetic transition near 80-120 K.

Hole-doped pnictides show larger gap parameters than electron-doped ones.

Abstract

The presence of macroscopic phase separation into superconducting and magnetic phases in LaFeAsO$_{1-x}$F$_x$ (LFAOF) and CaFe$_{1-x}$Co$_x$AsF (CFCAF) is demonstrated by muon spin rotation (muSR) measurement across their phase boundaries ($x=0.06$ for LFAOF and $x=0.075$--0.15 for CFCAF). In LFAOF, both magnetism and superconductivity develop simultaneously below a common critical temperature, $T_m =~ T_c =~ 18$ K, where the magnetism is characterized by strong randomness. A similar, but more distinct segregation of these two phases is observed in CFCAF, where the magnetic phase retains $T_m$ as close to that of the parent compound ($T_c << T_m =~ 80$--120 K) and the superconducting volume fraction is proportional to the Co content $x$. The close relationship between magnetism and superconductivity is discussed based on these experimental observations. Concerning superconducting phase, an assessment is made on the anisotropy of order parameter in the superconducting state of LFAOF, CFCAF, and Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (BKFA, $x=0.4$) based on the temperature dependence of superfluid density [$n_s(T)$] measured by muSR. The gap parameter, $2\Delta/k_BT_c$, determined from $n_s(T)$ exhibits a tendency that hole-doped pnictides (BKFA) is much greater than those in electron-doped ones (LFAOF, CFCAF), suggesting difference in the coupling to bosons mediating the Cooper pairs between relevant $d$ electron bands.