Charge Nematicity and Electronic Raman Scattering in Iron-based Superconductors

TL;DR

This paper reviews electronic Raman scattering measurements of charge nematic fluctuations in iron-based superconductors, proposing a theoretical framework and analyzing data that support an electronically driven structural phase transition.

Contribution

It introduces a simple $d$-wave Pomeranchuk transition model to interpret Raman spectra and demonstrates the effectiveness of Raman spectroscopy in probing electronic nematicity.

Findings

Low energy quasi-elastic peak indicates nematic fluctuations.

Raman data are consistent with an electronic-driven structural transition.

Quasi-elastic peak becomes a finite frequency resonance in the superconducting state.

Abstract

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 BaFe$_2$As$_2$, 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.