Superconductivity in Ru-substituted polycrystalline ${\text{BaFe}}_{2-x}{\text{Ru}}_x{\text{As}}_2$

The occurrence of bulk superconductivity at $\sim 22\text{K}$ is reported in polycrystalline samples of ${\text{BaFe}}_{2-x}{\text{Ru}}_x{\text{As}}_2$ for nominal Ru content in the range of $x=0.75–1.125$. A systematic suppression of the spin-density-wave transition temperature $\left(T_{\text{SDW}}\right)$ precedes the appearance of superconductivity in the system. A phase diagram is proposed based on the measured $T_{\text{SDW}}$ and superconducting transition temperature $\left(T_{\text{C}}\right)$ variations as a function of Ru composition. Band-structure calculations indicate introduction of electron carriers in the system upon Ru substitution. The calculated magnetic moment on Fe shows a minimum at $x=1.0$, suggesting that the suppression of the magnetic moment is associated with the emergence of superconductivity. Results of low-temperature and high-field Mössbauer measurements are presented. These indicate weakening of magnetic interaction with Ru substitution.

Figure 1

(Color online) Variation in the $a$- and $c$-lattice parameters as a function of Ru fraction substituted.

Figure 2

(Color online) Variation in normalized resistivity with temperature in ${\text{BaFe}}_{2-x}{\text{Ru}}_x{\text{As}}_2$ for various nominal Ru fractions, $x$, as indicated. The origin is shifted for each composition for clarity. The SDW transitions are indicated by stars.

Figure 3

(Color online) dc magnetization as a function of temperature for nominal $x=0.75$, 0.875, 1.0, and 1.125.

Figure 4

(Color online) The variation in the SDW transition temperatures and superconducting onsets as a function nominal Ru fraction $x$. SDW and superconductivity states coexist for the $x=0.75$ sample.

Figure 5

(Color online) Variation in resistance versus temperature for various external magnetic fields indicated, for Ru fraction, $x=0.875$. Inset shows the field dependence of superconducting transition temperatures, normalized to their zero-field counterparts, for $x=0.75$, 0.875, and 1.125.

Figure 6

(Color online) A comparison of the spin-polarized density of states obtained from first-principles calculations in (a) ${\text{BaFe}}_2{\text{As}}_2$ and (b) ${\text{BaFe}}_{1.0}{\text{Ru}}_{1.0}{\text{As}}_2$. (c) A schematic of the structure of the superstructure used in the calculations.

Figure 7

(Color online) Plots of the Hall coefficient in (a) ${\text{BaFe}}_2{\text{As}}_2$ single crystals and (b) in polycrystalline ${\text{BaFe}}_{2-x}{\text{Ru}}_x{\text{As}}_2$ for $x=0.75$; the measuring field and current used in the experiments are indicated.

Figure 8

(Color online) The variations in the maximum magnetic moment and the averaged magnetic moment with increase in Ru concentration as obtained from spin-polarized calculations. The arrows indicate the directions and magnitude of the magnetic moment at the four distinct sites in a Fe/Ru plane of the supercell obtained from the calculations.

Figure 9

(Color online) Evolution of the Fermi surface obtained from spin-polarized DFT calculations for the spin-up bands in ${\text{BaFe}}_{2-x}{\text{Ru}}_x{\text{As}}_2$ for (a) $x=0.0$, (b) $x=0.5$, (c) $x=1.0$, and (d) $x=2.0$.

Figure 10

(Color online) Variation in the transmitted intensity versus the velocity of the Mössbauer spectra measured at 5 K in ${\text{BaFe}}_{2-x}{\text{Ru}}_x{\text{As}}_2$ samples with (a) $x=0.0$, (b) $x=0.5$, (c) $x=1.0$, and (d) $x=1.0$ with an external magnetic field of 5 T. Inset in (a) shows the probability distribution of the magnetic field for $x=0.0$ and 0.5 samples, obtained from fits of the corresponding data shown in (a) and (b).