Charge nematicity and electronic Raman scattering in iron-based superconductors

We review the recent developments in electronic Raman scattering measurements of charge nematic fluctuations in iron-based superconductors. A simple theoretical framework of a d-wave Pomeranchuk transition is proposed in order to capture the salient features of the spectra. We discuss the available Raman data in the normal state of 122 iron-based systems, particularly Co-doped BaFe2As2, and we show that the low-energy quasi-elastic peak, the extracted nematic susceptibility and the scattering rates are consistent with an electronic-driven structural phase transition. In the superconducting state with a full gap, the quasi-elastic peak transforms into a finite-frequency nematic resonance, evidences for which are particularly strong in the electron-doped systems. A crucial feature of the analysis is the fact that the electronic Raman signal is unaffected by the acoustic phonons. This makes Raman spectroscopy a unique probe of electronic nematicity. (C) 2015 Academie des sciences. Published by Elsevier Masson SAS.