We perform theoretical analysis of polarization-sensitive Raman spectroscopy on ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}, \mathrm{EuFe}2{\mathrm{As}}_2, {\mathrm{SrFe}}_2{\mathrm{As}}_2$, and $\mathrm{Ba}\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right){}_2{\mathrm{As}}_2$, focusing on two features seen in the $B_{1g}$ symmetry channel (in one Fe unit cell notation): the strong temperature dependence of the static, uniform Raman response in the normal state and the existence of a collective mode in the superconducting state. We show that both features can be explained by the coupling of fermions to pairs of magnetic fluctuations via the Aslamazov-Larkin process. We first analyze magnetically mediated Raman intensity at the leading two-loop order and then include interactions between pairs of magnetic fluctuations. We show that the full Raman intensity in the $B_{1g}$ channel can be viewed as the result of the coupling of light to Ising-nematic susceptibility via Aslamazov-Larkin process. We argue that the singular temperature dependence in the normal state is the combination of the temperature dependencies of the Aslamazov-Larkin vertex and of Ising-nematic susceptibility. We discuss two scenario for the resonance below $T_c$. One is the resonance due to development of a pole in the fully renormalized Ising-nematic susceptibility. Another is orbital excitonic scenario, in which spin fluctuations generate attractive interaction between low-energy fermions.

• Received 11 November 2015
• Revised 5 January 2016

DOI:https://doi.org/10.1103/PhysRevB.93.075106