Hydrothermally Obtaining Superconductor Single Crystal of FeSe0.2Te0.8 without Interstitial Fe

We report a hydrothermal route to remove interstitial excess Fe in non-superconducting iron chalcogenide Fe1+δSe1-xTex single crystals.The extra-Fe-free(δ～0)FeSe0.2Te0.8 single crystal thus obtained shows bulk superconductivity at Te～13.8 K,which is about 2 K higher than the FeSe0.2Te0.8 sample obtained by usual post-annealing process.The upper critical field μ0Hc2 is estimated to be～42.5T,similar to the annealed FeSe0.2Te0.8.It is surprising to find that the hydrothermal FeSe0.2Te0.8 exhibits a remarkably small isothermal magnetization hysteresis loop at T=3K.This yields an extremely low critical current density Jc～1.1×102 A·cm-2(over 100 times smaller than the annealed FeSe0.2Te0.8)and indicates more free vortices in the hydrothermal FeSe0.2Te0.8.