Coupling Of Fully Symmetric As Phonon To Magnetism In Ba(Fe1-Xaux)(2)As-2

We study the coupling of the fully symmetric vibration mode of arsenic atoms to magnetism in a Ba(Fe1-xAux)(2)As-2 system by polarization-resolved Raman spectroscopy and neutron diffraction. In this system, there are two phase transitions: a tetragonal-to-orthorhombic structural phase transition at temperature T-S and a magnetic phase transition into collinear spin-density wave (SDW) state at temperature T-N (<= T-S). T-S and T-N almost coincide in the pristine compound, whereas they differ by as much as 8 K for compounds with dilute gold substitution for iron. Raman coupling to the A(g)(As) phonon is forbidden for the XY scattering geometry in the tetragonal phase above T-S, whereas it becomes allowed in the orthorhombic phase below T-S: The emerging mode's intensity indicates the lattice orthorhombicity. We find that upon cooling below T-S, first, weak A(g)(As) phonon mode intensity appears in the XY scattering geometry spectra; however, the mode's intensity is significantly enhanced in the magnetic phase below T-N. The A(g)(As) phonon also shows an asymmetric line shape below T-N and an anomalous linewidth broadening upon Au doping. We describe the anomalous behavior of the A(g)(As) mode in the XY scattering geometry using a Fano model involving the A(g)(As) phonon interacting with the B-2g(D-4h)-symmetry-like electron-hole continuum. We conclude that the temperature dependence of light coupling amplitude to the A(g)(As) phonon follows the evolution of the magnetic order parameter M( T). We propose that the intensity enhancement of the A(g)(As) phonon in the XY scattering geometry below T-N is due to electronic anisotropy induced by the collinear SDW order parameter.