Transport and anisotropy in single-crystalline SrFe₂As₂ and A₀.₆K₀.₄Fe₂As₂ (A = Sr, Ba) superconductors

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We have successfully grown high quality single crystals of SrFe₂As₂ and A₀.₆K₀.₄Fe₂As₂(A=Sr, Ba) using flux method. The resistivity, specific heat and Hall coefficient have been measured. For parent compound SrFe₂As₂, an anisotropic resistivity with ρc / ρab as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivity due to the SDW instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows 2-fold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K doped A₀.₆K₀.₄Fe₂As₂(A=Sr. Ba), the SDW instability is suppressed and the superconductivity appears with Tc above 35 K. The rather low anisotropy in upper critical field between H and H indicates inter-plane coupling play an important role in hole doped Fe-based superconductors.