Raman spectroscopy as a probe of correlations in iron based superconductors

I review Raman data on iron pnictide and chalcogenides high-Tc superconductors.Below Tc one or two sharp well defined in-gap modes are observed for cross polarized Raman geometry. Next, I constrain the possible pairing symmetry relying on this data as well as ARPES, and neutron scattering.The in-gap modes are interpreted as Bardasis-Schrieffer and particle-hole excitons respectively consistent with the generalized s-wave pairing. Next I'll talk about the normal state data on parent compounds, AFe2As2, with A=Sr,Eu. The theory that yields a quasi-elastic scattering in terms of spin/orbital nematic correlations is presented.