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+ δ Se 1– x Te x single crystals. The extra-Fe-free ( δ ∼ 0) FeSe 0.2 Te 0.8 single crystal thus obtained shows bulk superconductivity at T c ∼ 13.8 K, which is about 2 K higher than the FeSe 0.2 Te 0.8 sample obtained by usual post-annealing process. The upper critical field μ 0 H c2 is estimated to be ∼ 42.5 T, similar to the annealed FeSe 0.2 Te 0.8 . It is surprising to find that the hydrothermal FeSe 0.2 Te 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 × 10 2 A⋅cm −2 (over 100 times smaller than the annealed FeSe 0.2 Te 0.8 ) and indicates more free vortices in the hydrothermal FeSe 0.2 Te 0.8 .