Fluorine doped Fe(Se, Te) has been successfully synthesized using the melting method. A dual oscillation effect was found in F-doping sample, which combined both the microstructural oscillation and the chemical compositional oscillation. The microstructural oscillation could be attributed to alternate growth of tetragonal b-Fe(Se, Te) and hexagonal d-Fe(Se, Te) which formed a pearlite-like structure, and led to the enhancement of dl flux pinning due to the alternating distributed non-superconducting d-Fe(Se, Te) phase. The chemical compositional oscillations in b-Fe(Se, Te) phase was owing to the inhomogeneously distributed Se and Te, which changes the pinning mechanism from surface pinning in undoped sample to Dk pinning in 5% F-doped one. As a result, the critical current, upper critical field and thermally-activated-flux-flow activation energy of FeSe_0.45Te_0.5F_0.05 were enhanced by 7, 2 and 3 times, respectively. Our work revealed the physical insights of F-doping rendering high performance Fe(Se, Te) superconductors, and inspired a new approach to optimize superconductivities in iron-based superconductors through phase and element manipulations.

Keywords: iron-based superconductors, chemical doping, interface effect, flux pinning