Coupling of fully symmetric As phonon to magnetism in iron based superconductors

Raman coupling to the fully symmetric As phonon $A_g$(As) in iron based superconductors is forbidden for the XY scattering geometry with cross-polarized light along the Fe-As directions in the tetragonal phase, whereas it becomes allowed in the orthorhombic phase: The emerging mode’s intensity indicates the lattice orthorhombicity, which is expected to be small. However, in the orthorhombic phase of several families of parent compounds of Fe-based superconductors (BaFe$_2$As$_2$, NaFeAs, FeSe, and LaFeAsO) [1], as well as in the gold doped compounds Ba(Fe$_{1−x}$Au$_x$)$_2$As$_2$ [2], we find that the $A_g$(As) phonon intensity is significantly enhanced when the magnetic order sets in below the Neel temperature $T_N$. The $A_g$(As) phonon also shows an asymmetric line shape below $T_N$ and an anomalous linewidth broadening upon Au doping. By the Fano model analysis, we conclude the temperature dependence of light coupling amplitude to the $A_g$(As) phonon follows the evolution of the magnetic order parameter. We propose that the intensity enhancement of the $A_g$(As) phonon is due to electronic anisotropy induced by the collinear spin density wave order. [1]Phys.Rev.Research 2, 033140 (2020) [2]Phys.Rev B 102, 014501 (2020)