Phonon and magnetic excitations in the novel BaFe1.8Co0.2As2 superconductor

Phonon and magnetic excitations in the BaFe1.8Co0.2As2 superconductor single crystal were studied by inelastic neutron scattering using the ARCS time-of-flight spectrometer at the Spallation Neutron Source. Most of the phonon branches show a good agreement with the density functional theory calculations. However, the As-Raman vibrations along the c-axis demonstrate strong softening contrary to the flat behavior expected from the LDA calculations. The softening is strongest along the (0.5, 0.5, L) direction, by up to 4 meV. At the same time a sharp magnetic response was found along the same (0.5, 0.5, L) direction over a wide range of L-values at energy transfer of 10-25 meV. This dynamic magnetic responds indicates on a 2-D character of antiferromagnetic spin fluctuations in the superconducting phase, in strong contrast to the 3-D static antiferromagnetism in the undoped non-superconducting BaFe2As2. Thus, in Fe-As based superconductors magnetism shows strong sensitivity to the lattice, suggesting a possibility of spin-phonon coupling playing a role in superconductivity.