Electronic Phase Separation in the Slightly Underdoped Iron Pnictide Superconductor ${\mathrm{Ba}}_{1-x}{\mathbf{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$

Here we present a combined study of the slightly underdoped novel pnictide superconductor ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ by means of x-ray powder diffraction, neutron scattering, muon-spin rotation ($\mu \mathrm{SR}$), and magnetic force microscopy (MFM). Static antiferromagnetic order sets in below $T_m\approx 70\mathrm{K}$ as inferred from the neutron scattering and zero-field-$\mu \mathrm{SR}$ data. Transverse-field $\mu \mathrm{SR}$ below $T_c$ shows a coexistence of magnetically ordered and nonmagnetic states, which is also confirmed by MFM imaging. We explain such coexistence by electronic phase separation into antiferromagnetic and superconducting- or normal-state regions on a lateral scale of several tens of nanometers. Our findings indicate that such mesoscopic phase separation can be considered an intrinsic property of some iron pnictide superconductors.

Figure 1

Room-temperature XRPD data. (a) Scattered x-ray intensity at $T=300\mathrm{K}$ as a function of diffraction angle $2\Theta$ ($\lambda =0.7\AA$) fitted to the tetragonal $I4/mmm$ space group. For $2\Theta >17°$, the plot is enlarged by a factor of 3. The fit includes a few weight% of tetragonal $\beta$ tin from the flux as an impurity phase and some reflections of the brass sample holder as indicated by the reflection markers in (b). (c) The difference $\Delta$ between the experimental points and the fitting curve. The inset shows the sharp and reproducible SC transition measured by dc susceptibility on four randomly selected samples.

Figure 2

Elastic neutron-scattering data measured in the vicinity of the $(10\overline{3}{)}_{\mathrm{O}}$ magnetic Bragg peak (O stands for orthorhombic notation). (a) Scans along $(h0\overline{3}{)}_{\mathrm{O}}$ at three different temperatures. (b) Temperature evolution of the magnetic intensity. The solid symbols are measured in a magnetic field of 13.5 T applied parallel to the FeAs layers.

Figure 3

$\mu \mathrm{SR}$ data. (a) Time dependence of the muon-spin asymmetry in zero field. (b) Temperature dependence of the asymmetry in a weak transverse field, showing coexistence of magnetic and nonmagnetic phases. (c) Temperature dependence of the relaxation rate in a transverse field.

Figure 4

(a) Cartoonish representation of the phase-separated coexistence of AFM and SC/normal states. (b) MFM image measured at 10 K in the absence of external field, revealing weak magnetic contrast on the lateral scale of $\sim 65\mathrm{nm}$, as can be estimated from the Fourier-transformed image in panel (c). Panel (d) shows the corresponding spatial frequency profile integrated within the dotted rectangle. The arrow marks the highest-frequency peak in the spectrum, responsible for the 65 nm modulations.