~22 K*

Hiramatsu et al.¹ first found non-bulk superconductivity at ~22 K in SrFe₂As₂ exposed to water vapor. Our own work with exposure to water vapor gave varying results, as well as up-to-severe degradation of the crystal surface after exposure. The present work explores exposure of single crystal BaFe₂As₂ to other reactive gases, with consistent results from exposure to fluorine (5% in Argon) gas for 20 minutes at room temperature. The depth of the fluorine is ≤ 10 µ, and XPS data indicates that fluorine replaces As in the lattice. The superconductivity is observed in the resistivity but not in the magnetic susceptibility. A tentative model to explain what is happening microscopically to cause the superconductivity is proposed.
1. H. Hiramatsu, T. Takase, T. Kamiya, M. Hirano, and H. Hosono, Phys. Rev. B 80, 052501 (2009).