Raman phonon anomalies in Sr(Fe_1-xCo_x)₂As₂

Phonon anomalies have been reported in iron-pnictide superconductors indicating a diverse interplay between different orders in the materials. Here, we report Raman scattering measurements on Sr(Fe_1-xCo_x)₂As₂ (x=0 and x=0.04) single crystals in the B_2g symmetry with respect to a 1 Fe unit cell. Upon cooling, we observe a larger split (13 cm^-1) of E_g Raman phonon modes pertaining to in-plane Fe and As displacements as the crystals undergo the tetragonal-to-orthorhombic structural phase transition, although a considerable split (9 cm^-1) has been reported in Ba(Fe_1-xCo_x)₂As₂. Furthermore, the splitting of E_g phonon modes is strongly reduced upon doping. We perform an order-parameter analysis revealing a similar doping dependence of E_g phonon splitting as reported in other compounds of the 122 family, indicating these phonon anomalies widely exist in 122 iron-based superconductors and might share the same mechanisms.