  It has been considered that FeAs-based high transition temperature (high-Tc)
superconductors need electron or hole doping by aliovalent ion substitution or
large off-stoichiometry in order to induce superconductivity. We report that
exposure of undoped SrFe2As2 epitaxial thin films to water vapor induces a
superconducting transition. These films exhibit a higher onset-Tc (25 K) and
larger magnetic field anisotropy than those of cobalt-doped SrFe2As2 epitaxial
films, suggesting that the mechanism for the observed superconducting
transition differs from that of the aliovalent-ion doped SrFe2As2. The present
finding provides a new approach to induce superconductivity with a higher Tc in
FeAs-based superconductors.
