Abstract
We study the coupling of the fully symmetric vibration mode of arsenic atoms to magnetism in a Ba(Fe1−xAux)2As2 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 TS and a magnetic phase transition into collinear spin-density wave (SDW) state at temperature TN (≤TS). TS and TN 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 Ag(As) phonon is forbidden for the XY scattering geometry in the tetragonal phase above TS, whereas it becomes allowed in the orthorhombic phase below TS: The emerging mode's intensity indicates the lattice orthorhombicity. We find that upon cooling below TS, first, weak Ag(As) phonon mode intensity appears in the XY scattering geometry spectra; however, the mode's intensity is significantly enhanced in the magnetic phase below TN. The Ag(As) phonon also shows an asymmetric line shape below TN and an anomalous linewidth broadening upon Au doping. We describe the anomalous behavior of the Ag(As) mode in the XY scattering geometry using a Fano model involving the Ag(As) phonon interacting with the B2g(D4h)-symmetry-like electron-hole continuum. We conclude that the temperature dependence of light coupling amplitude to the Ag(As) phonon follows the evolution of the magnetic order parameter M(T). We propose that the intensity enhancement of the Ag(As) phonon in the XY scattering geometry below TN is due to electronic anisotropy induced by the collinear SDW order parameter.
