Isoelectronic Ru substitution at the iron site in ${\text{SmFe}}_{1-x}{\text{Ru}}_x{\text{AsO}}_{0.85}{\text{F}}_{0.15}$ and its effects on structural, superconducting, and normal-state properties

In this work we present a systematic experimental and theoretical study of the structural, transport, and superconducting properties of $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{As}\left({\text{O}}_{0.85}{\text{F}}_{0.15}\right)$ polycrystalline samples as a function of Ru content $\left(x\right)$ ranging from 0 to 1. The choice of Ru as isoelectronic substitution at Fe site of F-doped compounds allows to better clarify the role of structural disorder in modifying the normal and superconducting properties of these recently discovered multiband superconductors. Two different regions are identified: the Fe-rich phase $\left(x<0.5\right)$ where superconducting and normal-state properties are strongly affected by disorder induced by Ru substitution; the Ru-rich phase $\left(x>0.5\right)$ where the system is metallic and strongly compensated and the presence of Ru frustrates the magnetic moment on Fe ions. Here the lack of magnetic features and related spin fluctuations may be the cause for the suppression of superconductivity.

Figure 1

(Color online) Dependence of the cell parameters of $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{As}\left({\text{O}}_{0.85}{\text{F}}_{0.15}\right)$ on Ru content $x$ (▲). The calculated lattice constants for $\text{La}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$ (●), and $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$ (◼) are plotted as comparison.

Figure 2

(Color online) Dependence of selected bond angles as a function of Ru content in $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{As}\left({\text{O}}_{0.85}{\text{F}}_{0.15}\right)$ (exp) and $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$ (DFT), respectively.

Figure 3

(Color online) Calculated distances between the $TM$ and As planes as well as the $RE$ and O plane $\left(RE=\text{La},\text{Sm}\right)$; the corresponding experimental values for $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{As}\left({\text{O}}_{0.85}{\text{F}}_{0.15}\right)$ are plotted for comparison (▲). (●) refers to $\text{La}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$ and (◼) to $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$, respectively.

Figure 4

(Color online) Structure and magnetic moment of $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$ and $\text{La}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$ compounds shown along the basal $ab$ plane. Moment values in parenthesis refer to $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$.

Figure 5

(Color online) Total DOS for the $\text{La}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$ at different Ru concentrations.

Figure 6

(Color online) $\text{Fe}d$- and $\text{Ru}d$-states contribution to the PDOS inside an atomic sphere of $1.1\text{Å}$ for $\text{La}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$ system. The upper panel shows the PDOS on Fe and the lower one on Ru.

Figure 7

(Color online) Energy bands (in the nonmagnetic configuration) for $\text{La}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{AsO}$ at different Ru concentration. Most bands not crossing the Fermi energy are not shown for sake of clarity.

Figure 8

(Color online) Hall coefficients for $\text{LaFeAs}\left({\text{O}}_{0.85}{\text{F}}_{0.15}\right)$ (full lines) and $\text{LaRuAs}\left({\text{O}}_{0.85}{\text{F}}_{0.15}\right)$ (broken lines) as divided into hole and electron contributions (see text for further details).

Figure 9

(Color online) Resistivity vs $T$ for $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{As}\left({\text{O}}_{0.85}{\text{F}}_{0.15}\right)$ samples.

Figure 10

(Color online) $R_H$ as a function of $T$ for selected $x$ values. Inset: $R_H$ as a function of $x$ at 150 K; the dashed line is a guide for the eyes.

Figure 11

(Color online) Magnetoresistivity vs $B^2$ measured at 57 K for $x=0$, 0.05, 0.20, 0.36, 0.5, 0.56, and 0.75.

Figure 12

(Color online) ${\mu }_H$ evaluated by Hall effect (diamonds) evaluated and ${\mu }_{\text{MR}}$ evaluated by magnetoresistivity (squares) versus $x$. The mean-free path is reported in the right $y$ axis (see text for details).

Figure 13

(Color online) ${\mu }_0{H}_{\text{c}2}$ evaluated at the 90% of the resistive transition for $x=0$, 0.05, 0.10, 0.2, 0.3, and 0.36.

Figure 14

(Color online) The slopes ${\mu }_0\left|d{H}_{\text{c}2}/dT\right|$ are shown versus $x$. In the inset ${\mu }_0\left|d{H}_{\text{c}2}/d{T}_c\right|/T_c$ is plotted as a function of $T_c$. See text for details.

Figure 15

(a) $T_c$ versus $x$ and (b) $\rho \left(0\right)$ vs $x$ for $\text{Sm}\left({\text{Fe}}_{1-x}{\text{Ru}}_x\right)\text{As}\left({\text{O}}_{0.85}{\text{F}}_{0.15}\right)$ compounds, where $\rho \left(0\right)$ is the linear extrapolation to zero temperature of the curves in the range 250–300 K. The lines are guide for the eyes.