Temperature dependence of phonon spectrum in Ba2Ti2Fe2As4O

The spin driven high-Tc supeconductivity picture was recently challenged by a stoichiometric compound Ba2Ti2Fe2As4O, which usually has a non-magnetic ground state, but shows an possible charge-density-wave (CDW) transition around T*≈125 K. Although the two dimensional structure makes the Ba2Ti2Fe2As4O could be regared as the close kin to the bilayer cuprates, the CDW order and relatively low Tc make it is similar to those conventional titanium oxide superconductors. The absence of spin fluctuations in the polycrystalline sample of Ba2Ti2Fe2As4O measured by inelastic neutron scattering experiments doubts the magnetic origin of the superconductivity. Signature of polarons and weak temperature dependence of phonon modes were also observed in spectroscopic measurements. Although we have observed weak spin fluctuations up to 15 meV in the single crystals of Ba2Ti2Fe2As4O, it is hard to distinguish the magnetic and phonon excitations by previous TAS measurement mode due to the strong background from incoherent scattering, and the nearly Q-indendent and broad phonon-like excitations seem not like the optical branches but closely resemble with the localized polaronic state as observed in cuprates.To figure out the temperature dependence of phonon spectrum across the CDW transition in Ba2Ti2Fe2As4O, we thus propose to do further TOF experiments on MERLIN spectrometer. The coming results will give important clues for the understanding of superconducting mechnanism in Ba2Ti2Fe2As4O.