Raman scattering study of Spin-Density-Wave order and electron-phonon coupling in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

TL;DR

This study uses Raman scattering to investigate the electronic and phononic properties of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ across different doping levels, revealing insights into SDW gap formation, electronic transitions, and electron-phonon coupling.

Contribution

It provides a detailed analysis of SDW-induced electronic transitions and phonon coupling, challenging orbital ordering scenarios and supporting a band-folding model with Dirac cones.

Findings

Spectral weight redistribution in the SDW phase.

Identification of two SDW-induced electronic transitions.

Evidence of strong electron-phonon coupling and in-plane anisotropy.

Abstract

We report Raman scattering measurements on iron-pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with varying cobalt $x$ content. The electronic Raman continuum shows a strong spectral weight redistribution upon entering the magnetic phase induced by the opening of the Spin Density Wave (SDW) gap. It displays two spectral features that weaken with doping, which are assigned to two SDW induced electronic transitions. Raman symmetry arguments are discussed to identify the origin of these electronic transitions in terms of orbital ordering in the magnetic phase. Our data do not seem consistent with an orbital ordering scenario and advocate for a more conventional band-folding picture with two types of electronic transitions in the SDW state, a high energy transition between two anti-crossed SDW bands and a lower energy transition involving a folded band that do not anti-cross in the SDW state. The latter transition could be linked to the presence of Dirac cones in the electronic dispersion of the magnetic state. The spectra in the SDW state also show significant coupling between the arsenide optical phonon and the electronic continuum. The symmetry dependence of the arsenide phonon intensity indicates a strong in-plane anisotropy of the dielectric susceptibility in the magnetic state.