Raman investigation of the magneto-structural transition in electron doped Ba(FeAs)$_2$*

We report a doping dependent Raman scattering study of the magneto-structural transition in Co doped Ba(FeAs)2. Several zone centered phonons display significant anomalies at the tetragonal to orthorhombic transition. In particular, the doubly degenerate in-plane Eg phonon shows an enhanced splitting in the ortho phase. The splitting weakens considerably with doping and gives evidence for strong spin-phonon coupling in iron-pnictides. The electronic Raman continuum displays a systematic upturn at low energy around the magneto-structural transition. This quasi-elastic scattering is similar to magnetic energy fluctuations usually observed in magnetic insulators. Interestingly significant fluctuations are observed at low temperature even for x=0.065 doping, where the Neel temperature goes to zero and optimal Tc is reached. At high energy and low doping, the electronic Raman continuum displays clear signatures Fermi surface reconstruction due to the opening of the spin density wave gap at the magnetic transition.