Impact of valence states on the superconductivity of iron telluride and iron selenide films with incorporated oxygen

Abstract

We report on the local electronic structure of oxygen incorporated FeTe and FeSe films and how this relates to superconductivity observed in these films. In the case of FeTe, initially grown films are measured to be nonsuperconducting, but become superconducting following oxygen incorporation. In FeSe the opposite happens; initially grown films are measured to be superconducting, but experience a quenching of superconductivity following oxygen incorporation. X-ray photoemission and total fluorescence yield (TFY) x-ray absorption experiments show that oxygen incorporation changes the initial Fe valence state in both the as grown FeTe and FeSe films to mainly Fe${}^{3+}$ in the oxygen incorporated films. In contrast we observe that while Te moves to a mixed Te${}^{0}$/Te${}^{4+}$ valence state, the Se always remains Se${}^{0}$. This work highlights how different responses of the electronic structure by the respective chalcogenides to oxidation could be related to the mechanisms which are inducing superconductivity in FeTe and quenching superconductivity in FeSe.