Doping dependence of the lattice dynamics in Ba(Fe1-xCox)2As2 studied by Raman spectroscopy

We report Raman scattering measurements on iron-pnictide superconductor Ba(Fe1-xCox)(2)As-2 single crystals with varying cobalt x content. Upon cooling through the tetragonal-to-orthorhombic transition, we observe a large splitting of the E-g in-plane phonon modes involving Fe and As displacements. The splitting of the in-plane phonons at the transition is strongly reduced upon doping and disappears for x=0.06 qualitatively following the trend displayed by the Fe magnetic moment. The origin of the splitting is discussed in terms of magnetic frustration inherent to iron-pnictide systems and we argue that such enhanced splitting may be linked to strong spin-phonon coupling.