Growth and characterization of A1−xKxFe2As2(A = Ba, Sr) single crystals withx= 0–0.4

Single crystals of A 1− x K x Fe 2 As 2 (A = Ba, Sr) with high quality have been grown successfully by an FeAs self-flux method. The samples have sizes up to 4 mm with flat and shiny surfaces. The x-ray diffraction patterns suggest that they have high crystalline quality and c -axis orientation. The non-superconducting crystals show a spin-density-wave (SDW) instability at about 173 and 135 K for the Sr-based and Ba-based compound, respectively. After doping K as the hole dopant into the BaFe 2 As 2 system, the SDW transition is smeared, and superconducting samples of the compound Ba 1− x K x Fe 2 As 2 (0< x ≤0.4) are obtained. The superconductors, characterized by AC susceptibility and resistivity measurements, exhibit very sharp superconducting transitions at about 36, 32, 27 and 23 K for x = 0.40,0.28,0.25 and 0.23, respectively.