Single-crystal neutron diffraction studies on superconductors $A_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$, where $A=\mathrm{Rb}$, Cs, (Tl, Rb), and (Tl, K) ($T_c\sim 30\mathrm{K}$), uncover the same Fe vacancy ordered crystal structure and the same block antiferromagnetic order as in ${\mathrm{K}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$. The Fe order-disorder transition occurs at $T_S=500–578\mathrm{K}$, and the antiferromagnetic transition at $T_N=471–559\mathrm{K}$ with an ordered magnetic moment $\sim 3.3{\mu }_B/\mathrm{Fe}$ at 10 K. Thus, all recently discovered $A$ intercalated iron selenide superconductors share the common crystalline and magnetic structure, which are very different from previous families of Fe-based superconductors, and constitute a distinct new 245 family.

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the (Tl0.58Rb0.42)Fe1.72Se2 superconductor with a Tc=32\,\,K. The Fermi surface topology consists of two electronlike Fermi surface sheets around the $\Gamma${} point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of $\sim${}12\,\,meV. The large Fermi surface near the $\Gamma${} point also shows a nearly isotropic superconducting gap of $\sim${}15\,\,meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.

122 type pnictide superconductors are of particular interest for high-field applications because of their large upper critical fields H_c2 (> 100 T) and low anisotropy γ (<2). Successful magnet applications require fabrication of polycrystalline superconducting wires that exhibit large critical current density J_c, which is limited by poor grain coupling and weak-link behavior at grain boundaries. Here, we report our recent achievement in the developing Sr_0.6K_0.4Fe₂As₂ tapes with transport J_c up to 0.1 MA/cm² at 10 T and 4.2 K. This value is by far the highest ever recorded for iron based superconducting wires and has surpassed the threshold for practical application. The synergy effects of enhanced grain connectivity, alleviation of the weak-link behavior at grain boundaries, and the strong intrinsic pinning characteristics led to the superior J_c performance exhibited in our samples. This advanced J_c result opens up the possibility for iron-pnictide superconducting wires to win the race in high-field magnet applications.

We observed bulk superconductivity in the FeTe${}_{1-x}$Se${}_x$ crystals with lower Se concentrations after annealed in air. A revised magnetism and superconductivity phase diagram is obtained via resistance and magnetic-susceptibility measurements for the FeTe${}_{1-x}$Se${}_x$ system. It is found that bulk superconductivity coexists with antiferromagnetic order in the crystals with 0.05 $\le$ x $\le$ 0.18. The phase diagram is very similar to the case of the K-doping or Co-doping BaFe${}_2$As${}_2$ as well as that of the SmFeAsO${}_{1-x}$F${}_x$ system, perhaps indicating that all the iron-based systems have a generic phase diagram, although the antiferromagnetic wave vectors in the parent compounds of both iron pnictides and FeTe${}_{1-x}$Se${}_x$ systems are remarkably different.