Observation of selective surface element substitution in FeTe_(0.5)Se_(0.5) superconductor thin film exposed to ambient air by synchrotron radiation spectroscopy

A systematic investigation of oxidation on a superconductive FeTe_0.5Se_0.5 thin film, which was grown on Nb-doped SrTiO₃ (001) by pulsed laser deposition, has been carried out. The sample was exposed to ambient air for one month for oxidation. Macroscopically, the exposed specimen lost its superconductivity due to oxidation. The specimen was subjected to in situ synchrotron radiation photoelectron spectroscopy (PES) and x-ray absorption spectroscopy (XAS) measurements following cycles of annealing and argon ion etching treatments to unravel what happened in the electronic structure and composition after exposure to air. By the spectroscopic measurements, we found that the as-grown FeTe_0.5Se_0.5 superconductive thin film experienced an element selective substitution reaction. The oxidation preferentially proceeds through pumping out the Te and forming Fe-O bonds by O substitution of Te. In addition, our results certify that in situ vacuum annealing and low-energy argon ion etching methods combined with spectroscopy are suitable for depth element and valence analysis of layered structure superconductor materials.