Direct observation of nanoscale interface phase in the superconducting chalcogenide ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ with intrinsic phase separation

We have used scanning micro x-ray diffraction to characterize different phases in superconducting ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature ($T_c\sim 32$ K), phase separation temperature ($T_{ps}\sim 520$ K), and iron-vacancy order temperature ($T_{vo}\sim 580$ K). In addition to the iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic minority filamentary phase, we have found clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below $\sim 300$ K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides clear evidence of the formation of protected metallic percolative paths in the majority texture with large magnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the $T_{ps}$ and $T_c$ is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$.

Figure 1

(a) Temperature evolution of the (004) Bragg peak of superconducting ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ single crystal showing intrinsic phase separation at 520 K and a tetragonal-to-orthorhombic structural transition of the minority metallic phase below 500 K. (b) Temperature dependence of the lattice parameters for different phases. Color shades are used to show superconducting and tetragonal nonmagnetic regions in (b).

Figure 2

Normalized intensity of (004) peak of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ for different phases as a function of temperature, representing the evolution of their relative weights. The majority phase decreases sharply (red) due to the appearance of the new phase (blue) below the phase separation temperature ($T_{ps}\sim 520$ K). Further cooling results in the appearance of an interface phase (green) at $\sim 300$ K.

Figure 3

Intensity of the $\sqrt{5}\times \sqrt{5}$ superstructure peak normalized with respect to the corresponding (004) diffraction peak is shown as a function of temperature. A sharp jump at $\sim 580$ K due to iron-vacancy order is apparent. A small deviation at $\sim 520$ K indicates changing microstructure due to phase separation. A zoom over the low temperature is shown as an inset revealing anomalous change in the microstructure properties below 300 K, and at $T_c\sim 32$ K.

Figure 4

(a) The line profile of (004) diffraction peak (inset) along $a^{*}$ direction at 15 K for ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$. The line profile contains three peaks (3 Lorentzians and a model fit is also shown), with the central peak being due to the interface phase while the other two are due to the minority orthorhombic phase. (b) Spatial distribution of different phases at 15 K for a selected area of $80\times 80 \mu \mathrm{m}$. The interface phase (green) is clearly visible between the majority (red) and the minority (blue) phases. The lower panels show the spatial distribution of different phases at several temperatures, revealing evolution of percolative paths (blue) below the phase separation temperature, getting protection of the interface phase (green) at low temperature.