Coexistence of superconductivity and density wave in Ba2Ti2Fe2As4O: An optical spectroscopy study

We performed an optical spectroscopy measurement on single crystals of Ba2Ti2Fe2As4O, which is a newly discovered superconductor showing a coexistence of superconductivity and density wave orders. The study reveals spectral changes associated with both density wave and superconductivity phase transitions. The density wave phase transition at T-DW approximate to 125 K leads to the reconstruction of Fermi surfaces which removes about half of Drude spectral weight. The ratio of 2 Delta(DW)/k(B)T(DW) approximate to 11.9 is considerably larger than the mean-field value based on the weak-coupling BCS theory. At the lowest temperature in the superconducting state, further spectral change associated with the superconducting condensate is observed. The low frequency optical conductivity could be well modeled within the Mattis-Bardeen approach with two isotropic gaps of Delta(1)(0) = 3.4 meV and Delta(2)(0) = 7.9 meV. The superconducting properties of the Ba2Ti2Fe2As4O compound are similar to those of BaFe1.85Co0.15As2.