Fe-based superconductivity with Tc=31 K bordering an antiferromagnetic insulator in (Tl,K) FexSe2

(Tl,K)FexSe2 single crystals were first successfully synthesized with the Bridgeman method. The physical properties are characterized by electrical resistivity, magnetic susceptibility and Hall coefficient. We found that the (Tl,K)FexSe2 (1.30 <= x <= 1.65) compounds show an antiferromagetic (AFM) insulator behavior, which may be associated with the Fe-vacancy ordering in the crystals. While in the 1.70 <= x < 1.78 crystals, superconductivity (SC) coexists with an insulating phase. As Fe content further increases, the bulk SC with T-c = 31K (and a T-c(onset) as high as 40 K) appears in the 1.78 <= x <= 1.88 crystals. Our discovery represents the first Fe-based high-temperature superconductivity (HTSC) at the verge of an AFM insulator. Copyright (C) EPLA, 2011