Superconductivity at Tc = 36.5 K in Na-Substituted SrFe2As2 Single Crystals

Abstract

We report superconductivity, which is achieved in Sr1-xNaxFe2As2 at Tc = 36.5 K for x = 0.47. Single crystals of Na-substituted SrFe2As2 up to 2 × 2 mm2 were grown from NaF flux. The superconducting transition temperature with a narrow width less than 2 K, confirmed by both magnetic and resistivity measurements, indicates the high quality of our samples. The upper critical fields (0) = 87 T and (0) = 217.5 T are extremely high. The anisotropy ratio γ ~ 2.5 is close to that of other hole-or electron-doped 122 phases but lower than γ ~ 4.3 – 5 of phases from the 1111 series and much lower than γ ~ 7 – 10 of superconducting cuprates. Magnetic relaxation measurements, which are consistent with the collective pinning model, reveal that the flux-creep rate in Na substituted 122 SrFe2As2 is smaller than in 1111 type iron pnictides.