Hydrothermally Obtaining Superconductor Single Crystal of FeSe_0.2Te_0.8 without Interstitial Fe

We report a hydrothermal route to remove interstitial excess Fe in non-superconducting iron chalcogenide Fe_1+δSe_1–xTe_x single crystals. The extra-Fe-free (δ ∼ 0) FeSe_0.2Te_0.8 single crystal thus obtained shows bulk superconductivity at T_c ∼ 13.8 K, which is about 2 K higher than the FeSe_0.2Te_0.8 sample obtained by usual post-annealing process. The upper critical field μ₀H_c2 is estimated to be ∼ 42.5 T, similar to the annealed FeSe_0.2Te_0.8. It is surprising to find that the hydrothermal FeSe_0.2Te_0.8 exhibits a remarkably small isothermal magnetization hysteresis loop at T = 3 K. This yields an extremely low critical current density J_c ∼ 1.1 × 10² A⋅cm⁻² (over 100 times smaller than the annealed FeSe_0.2Te_0.8) and indicates more free vortices in the hydrothermal FeSe_0.2Te_0.8.