Transport and anisotropy in single-crystallineSrFe2As2andA0.6K0.4Fe2As2(A=Sr, Ba) superconductors

Abstract

We have successfully grown high-quality single crystals of ${\text{SrFe}}_{2}{\text{As}}_{2}$ and ${A}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$($A=\text{Sr}$, Ba) using flux method. The resistivity, specific heat, and Hall coefficient have been measured. For parent compound ${\text{SrFe}}_{2}{\text{As}}_{2}$, an anisotropic resistivity with ${\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{ab}$ as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivities due to the spin-density-wave (SDW) instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows twofold 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 ${A}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$($A=\text{Sr}$, Ba), the SDW instability is suppressed and the superconductivity appears with ${T}_{c}$ above 35 K. The rather low anisotropy in upper critical field between $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$ indicates that interplane coupling plays an important role in hole-doped Fe-based superconductors.