Role of magnetism in superconductivity of $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$: Study of $5d$ Au-doped crystals

We investigate properties of $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ (122) single crystals upon gold doping, which is the transition metal with the highest atomic weight. The Au substitution into the FeAs-planes of 122 crystal structure (Au-122) is only possible up to a small amount of $\sim 3\%$. We find that $5d$ is more effective in reducing magnetism in 122 than its counter $3d$ Cu, and this relates to superconductivity. We provide evidence of short-range magnetic fluctuations and local lattice inhomogeneities that may prevent strong percolative superconductivity in $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{A}{\mathrm{u}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$.

Figure 1

For Au-122, (a) room-temperature powder x-ray diffraction pattern for $x=0.031$. Red circles represent observed data; black and green solid lines represent the calculated intensity and difference between the observed and calculated intensity; blue vertical bars indicate the Bragg reflection positions. (b) Refined lattice parameters for $0\le x\le 0.031$, inset is cell volume $V$ versus x.

Figure 2

For Au-122, $0\le x\le 0.031$, (a) temperature dependence of magnetic susceptibility (a) along $ab$- and (b) $c$-lattice directions. (c) $\chi \left(\mathrm{T}\right)$ behavior enlarged for $x=0.031$ below 100 K at 1 T; the inset is data below 4 K taken at 10 Oe.

Figure 3

For Au-122, temperature dependent resistivity for (a) $0\le x\le 0.031$ normalized to 380 K (inset has arbitrary $\rho$). (b) $\rho \left(\mathrm{T}\right)$ for $x=0.031$ below 100 K, with field dependence in inset. (c) Hall coefficient for $x=0$, 0.012, and 0.031.

Figure 4

For Au-122, heat capacity for $0\le x\le 0.031$ (a) below 150 K, with inset showing the enlarged data around transitions; (b) in form of $C/T$ versus ${\mathrm{T}}^2$ below 10 K; (c) below 15 K at applied fields for $x=0.031$, with inset measuring below 3 K.

Figure 5

For Au-122 with $x=0.005$, (a), (b) the neutron and (c) the x-ray diffraction data. The temperature dependence of magnetic $(\mathrm{½}\mathrm{½}5{)}_{\mathrm{T}}$ and nuclear ${\left(220\right)}_{\mathrm{T}}$ reflections gives the onset of the AF transition below $T_{\mathrm{N}}=122\mathrm{K}$ and orthorhombic transition below $T_{\mathrm{O}}=128\mathrm{K}$. (c) Tracking the (1 1 2) reflection with temperature clearly broadens with weak splitting to the orthorhombic (2 0 2) and (0 2 2) evident at 20 K.

Figure 6

$d\rho /dT$ of the $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{A}{\mathrm{u}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$ system.

Figure 7

$T-x$ phase diagram for Au-122. Inset shows $T_N$ vs $x$ for Au-122 and Cu-122 (Cu-122 data is retrieved from Ref. [29]).