Since the discovery of superconductivity in doped LaOFeAs with T_c=26 K [J. Am. Chem. Soc. 130, 3296 (2008)], a series of superconducting iron-based compounds were found, such as AeFe₂As₂ (Ae= K, Sr, Ba) [Phys. Rev. Lett. 101, 107007 (2008), Phys. Rev. B 80, 024506 (2009), Phys. Rev. Lett. 101, 107006 (2008)], LiFeAs [Phys. Rev. B 78, 060505(R) (2008)], FeSe [Proc. Natl. Acad. Sci. U.S.A. 105, 14262 (2008)], etc.
　  Among them, FeSe has a substantially simplified structure stacked only by FeSe layers and no intercalating cations. Superconductivity at 8 K was firstly reported under ambient pressure. Subsequently, by tellurium doping or the exertion of high pressure, the T_c can increase up to 15.2 K and 37 K [EPL 84, 37002 (2008), Phys. Rev. B 80, 064506 (2009)], respectively. These results imply that the superconductivity of FeSe-based compounds is sensitive to the structure and carriers modulation.
　  Recently, Prof. Xiaolong Chen and his colleagues from the Institute of Physics, Chinese Academy of Sciences reported the superconductivity at about 30 K in a new FeSe-layer compound KFe₂Se₂ (Figure 1), which is isostructural to BaFe₂As₂. It is the highest value of T_c reported for FeSe-layer materials so far under ambient pressure. More interestingly, KFe₂Se₂ has T_c comparable to those of FeAs-based materials, while without arsenic and less toxic.
　  Jiangang Guo, Prof. Xiaolong Chen et al. have carefully investigated the phase forming behavior of K_xFe₂Se₂ (0≤x≤1). As shown in Figure 2, with increasing K contents, the FeSe (101) diffraction peak gradually weakens and the KFe₂Se₂ (103) diffraction peak gradually enhances. When x is above 0.8, the FeSe (101) diffraction peak finally disappears, while below this value, the samples contain both KFe₂Se₂ and FeSe phases. The temperature-dependent magnetization (Figure 3) for K_0.4Fe₂Se₂ polycrystalline sample clearly exhibits two magnetic singularities at ~8 K and ~31 K, respectively.
　  The sharp drop occurring at ~8 K is attributed to the known FeSe, while the magnetic anomaly at ~31 K might be correlated with KFe₂Se₂. The magnetization of K_xFe₂Se₂ single phase (Figure 4) shows a clear diamagnetic response (Meissner effect) at 31 K. The superconducting volume fraction estimated from the ZFC magnetization at 10 K is ~60%. The temperature dependence of in-plane electrical resistance of K_xFe₂Se₂ crystal shows the onset transition temperature at 30.1 K and zero resistance at 27.2 K, which clearly indicates that the superconductive phase with T_c at ~31 K is K_xFe₂Se₂. The right inset of Figure 4 shows the M-H curve at 5 K. From the M-H curve, the lower critical magnetic field (H_c1) is around 0.2 T and the estimated upper critical magnetic field (H_c2) is higher than 9 T.
　  The results demonstrate the existence of new iron-based superconductors with high T_c apart from the known FeAs-layer materials and provide a new opportunity to better understanding the underlying mechanism of iron-based superconductors. The results were published in [Physical Review B 82, 182520 (R) (2010)] and then highlighted with a Synopsis on the APS Website.

Figure 1. Powder x-ray diffraction and Ritveld refinement profile of KFe2Se2at 297 K. The inset shows the schematic crystal structure of KFe2Se2(ThCr2Si2-type).

Figure 2. The intercalated concentration dependence of PXRD peaks of the FeSe (101) and the KxFe2Se2(103) diffraction peaks collected at room temperature.

Figure 3. Temperature-dependent magnetization for representative member K0.4Fe2Se2polycrystalline sample.

Figure 4. The magnetization of K0.8Fe2Se2crystal as a function of temperature with theHparallel 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 versusHat 5 K.