With single crystal X-ray diffraction studies, we compare the structures of three sample showing optimal superconductivity, K0.774(4)Fe1.613(2)Se2, K0.738(6)Fe1.631(3)Se2 and Cs0.748(2)Fe1.626(1)Se2. All have an almost identical ordered vacancy structure with a (√5 x √5 x 1) super cell. The tetragonal unit cell, space group I4/m, possesses lattice parameters at 250K of a = b = 8.729(2) Å and c = 14.120(3) Å, a = b = 8.7186(12) Å and c = 14.0853(19) Å and at 295 K, a = b = 8.8617(16) Å and c = 15.304(3) Å for the three crystals, respectively. The structure contains two iron sites; one is almost completely empty, whilst the other is fully occupied. There are similarly two alkali metal sites that are occupied in the range of 72.2(2) % to 85.3(3) %. The inclusion of alkali metals and the presence of vacancies within the structure allows for considerable relaxation of the FeSe4 tetrahedron, compared with members of the Fe(Te, Se, S) series, and the resulting shift of the Se -F -Se bond angles to less distorted geometry could be important in understanding the associated increase in the superconducting transition temperature. The structure of these superconductors distinguishes themselves from the structure of the non-superconducting phases by an almost complete absence of Fe on the (0 0.5 0.25) site as well as lower alkali metal occupancy that ensures an exact Fe 2+ oxidation state, which are clearly critical parameters in the promotion of superconductivity.

We report the physical properties of a layered transition metal pnictide, CaMnBi2, which has a crystal structure similar to that of the superconducting iron pnictides. This compound is a bad metal with a long-range antiferromagnetic order at TN = 270 K. The linear temperature dependence of magnetic susceptibility above TN suggests that strong antiferromagnetic correlations exist in the paramagnetic state. A linear magnetic field dependence of the magnetoresistance implies the existence of the linear energy dispersion, which may result in the giant in-plane magnetoresistance (about 105% in 10 T at 2.5 K for H∥c). The results of de Haas-van Alphen effect are consistent with the presence of Dirac fermions.

We report the successful growth of high-quality single crystals of potassium intercalated iron selenide K x Fe 2−y Se 2 by Bridgeman method. The effect of iron vacancies on transport properties was investigated by electrical resistivity and magnetic susceptibility measurements. With varying iron content, the system passes from semiconducting/insulating to superconducting state. Comparing with superconductivity, the anomalous "hump" effect in the normal state resistivity is much more sensitive to the iron deficiency. The electrical resistivity exhibits a perfect metallic behavior (R 300K /R 35K ≈42) for the sample with little iron vacancies. Our results suggest that the anomalous "hump" effect in the normal state resistivity may be due to the ordering process of the cation vacancies in this non-stoichiometric compound rather than magnetic/structure transition. A trace of superconductivity extending up to near 44 K was also detected in some crystals of K x Fe 2−y Se 2 , which has the highest T c of the reported iron selenides.

We carried out high resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of K0.68Fe1.79Se2 (Tc=32 K) and (Tl0.45K0.34)Fe1.84Se2 (Tc=28 K) superconductors. In addition to the electron-like Fermi surface near M(π,π), two electronlike Fermi pockets are revealed around the zone center Γ(0,0) in K0.68Fe1.79Se2. This observation makes the Fermi surface topology of K0.68Fe1.79Se2 consistent with that of (Tl,Rb)xFe2−ySe2 and (Tl,K)xFe2−ySe2 compounds. A nearly isotropic superconducting gap (∆) is observed along the electron-like Fermi pocket near the M point in K0.68Fe1.79Se2 (∆∼ 9 meV) and (Tl0.45K0.34)Fe1.84Se2 (∆∼ 8 meV). The establishment of a universal picture on the Fermi surface topology and superconducting gap in the AxFe2−ySe2 (A=K, Tl, Cs, Rb and etc.) superconductors will provide important information in understanding the superconductivity mechanism of the iron-based superconductors.

Magnetic field penetration and magnetization hysteresis loops (MHLs) have been measured in KxFe2−ySe2 single crystals. The magnetic field penetration shows a two-step feature with a very small full-magnetic-penetration field (≈ 300 Oe at 2 K), and accordingly the MHL exhibits an abnormal vanishing of the central peak near zero field below 13 K. The width of the MHL in KxFe2−ySe2 at the same temperature is in general much smaller than that measured in Ba0.6K0.4Fe2As2 and Ba(Fe0.92Co0.08)2As2, and the MHLs in the latter two samples show a normal central peak near zero field. All these anomalies found in KxFe2−ySe2 can be understood in the sense that the sample is percolative with weakly coupled superconducting islands.