Correlated Trends of Coexisting Magnetism and Superconductivity in Optimally Electron-Doped Oxypnictides

We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped ${\mathrm{SmFe}}_{1-x}{\mathrm{Ru}}_x{\mathrm{AsO}}_{0.85}{\mathrm{F}}_{0.15}$ for $0.1\le x\lesssim 0.5$. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a common critical threshold $x_c\sim 0.6$. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold $x_c$.

Figure 1

Muon diamagnetic shift (TF, ${\mu }_{0}H=20\mathrm{mT}$, ○) and zero field cooled (ZFC) SQUID susceptibility (${\mu }_{0}H=0.5\mathrm{mT}$, solid curve) for $x=0.05$. Inset: ZFC SQUID susceptibility $\chi (T)$ of representative ${\mathrm{SmFe}}_{1-x}{\mathrm{Ru}}_x{\mathrm{AsO}}_{0.85}{\mathrm{F}}_{0.15}$ samples (${\mu }_{0}H=0.5\mathrm{mT}$). No relevant differences are found from FC measurements (not shown).

Figure 2

Fraction of sample volume $V_M$ (▪) where muons detect an internal magnetic field and its width $\Delta {\mathrm{B}}_{\mu }$ (▴) vs $T$, for ${\mathrm{SmFe}}_{1-x}{\mathrm{Ru}}_x{\mathrm{AsO}}_{0.85}{\mathrm{F}}_{0.15}$ with $x=0.2$. Inset: time dependence of the normalized ZF muon asymmetry, with best fits, at $T>T_M$, $T\lesssim T_M$ and $T\ll T_M$.

Figure 3

(a) SQUID superconducting $T_c$ (▴) and $\mathrm{ZF}\mathrm{\text{−}}\mu \mathrm{SR}$ magnetic $T_M$ (●) of ${\mathrm{SmFe}}_{1-x}{\mathrm{Ru}}_x{\mathrm{AsO}}_{0.85}{\mathrm{F}}_{0.15}$ vs Ru content. The lines are a guide for the eye.

Figure 4

(a) NQR spectra, for $T=290\mathrm{K}$, with best fits and their components for selected ${\mathrm{SmFe}}_{1-x}{\mathrm{Ru}}_x{\mathrm{AsO}}_{0.85}{\mathrm{F}}_{0.15}$ samples; $x=0$, $T_c=52\mathrm{K}$, nominal F content 0.10, from Ref. 5, NQR measured at $T=160\mathrm{K}$ (the differences in nominal F and $T$ are irrelevant here). (b) NQR peak frequencies vs Ru content. Lines guide the eye to identify NN Ru configurations. (c) Normalized NQR weights, $a_j$, $j=0\cdots 4$ ($j=0$ is the sum of $0A$ and $0B$), vs Ru content, with fits (solid lines, see text). (d) Weights of $0A$, $0B$ ($j=0$ components).

Figure 5

$T_c$ and $T_M$ versus the NQR weights $a_{0A}$ and $a_{0B}$ respectively with their linear regressions.