Superconductivity in the iron selenide ${\text{K}}_x{\text{Fe}}_2{\text{Se}}_2$ $\left(0\le x\le 1.0\right)$

We report the superconductivity at above 30 K in a FeSe-layer compound ${\text{K}}_{0.8}{\text{Fe}}_2{\text{Se}}_2$ (nominal composition) achieved by metal K intercalating in between FeSe layers. It is isostructural to ${\text{BaFe}}_2{\text{As}}_2$ and possesses the highest $T_c$ for FeSe-layer materials so far under ambient pressure. Hall effect indicates the carriers are dominated by electron in this superconductor. We confirm that the observed superconductivity at above 30 K is due to this FeSe-based 122 phase. Our results demonstrate that FeSe-layer materials are really remarkable superconductors via structure and carrier modulation.

This article appears in the following collection:

Figure 1

(Color online) Powder x-ray diffraction and Ritveld refinement profile of ${\text{KFe}}_2{\text{Se}}_2$ at 297 K. The inset shows the schematic crystal structure of ${\text{KFe}}_2{\text{Se}}_2$ (${\text{ThCr}}_2{\text{Si}}_2$ type).

Figure 2

(Color online) The x-ray diffraction pattern of ${\text{K}}_{0.8}{\text{Fe}}_2{\text{Se}}_2$ crystal indicates that the $\left(00l\right)$ $\left(l=2n\right)$ reflections dominate the pattern. The asterisk shows an unknown reflection. Inset shows the photography of the ${\text{K}}_{0.8}{\text{Fe}}_2{\text{Se}}_2$ crystal (length scale 1 mm).

Figure 3

The temperature dependence of electrical resistance for the ${\text{K}}_{0.8}{\text{Fe}}_2{\text{Se}}_2$ crystal sample. The lower inset shows the details of superconducting transition from 10 to 40 K. The upper inset shows temperature dependence of normal-state Hall coefficient for crystal sample.

Figure 4

(Color online) The magnetization of ${\text{K}}_{0.8}{\text{Fe}}_2{\text{Se}}_2$ crystal as a function of temperature with the $H$ parallel to $c$ axis. The left inset shows the expanded view of the temperature dependence of the magnetization near the onset of superconducting transition. The right inset shows the magnetization versus $H$ at 5 K.

Figure 5

(Color online) (a) The intercalated concentration dependence of PXRD peaks of the FeSe (101) and the ${\text{K}}_x{\text{Fe}}_2{\text{Se}}_2$ (103) reflections collected at room temperature. The inset shows the powder x-ray diffraction patterns for ${\text{K}}_x{\text{Fe}}_2{\text{Se}}_2$ $\left(0\le x\le 1.0\right)$ polycrystalline samples at room temperature. Dots mark the reflections of FeSe. (b) Temperature-dependent magnetization for representative member ${\text{K}}_{0.4}{\text{Fe}}_2{\text{Se}}_2$ polycrystalline sample (arrow shows the transition point).