Effect of K-doping on the superconducting properties of FeSe0.5Te0.5 single crystals

Abstract

Fe1−xKxSe0.5Te0.5 ( x = 0, 0.025, 0.05, 0.075 and 0.1) single crystals were grown by self-flux method. The effects of K-doping on the length of c-axis, critical temperature, anisotropy and critical current density were investigated. With the increase of K-doping content x, the length of c-axis increases, the critical temperature rises at first and then falls, and the anisotropy decreases. The critical current density displays a significant increase at x = 0.075. Subsequently, the superconducting properties of Fe0.925K0.075Se0.5Te0.5 were thoroughly studied. The field-dependent pinning energy U(H) and the temperature-dependent upper critical field Hc2(T) were given. Additionally, the Dew-Hughes model was used to scale the flux pinning force, and the result suggests that the superconductor had a temperature-induced pinning mechanism from normal point pinning to normal surface pinning, in which twins play an important role. We also discussed the temperature-dependent critical current density Jc(T)=Jc(0)(1-T/T1)n and found that the n-value depends on different flux pinning regimes, including single flux pinning, strong collective pinning associated with twins and weak collective pinning associated with point defects.