Raman resonance in iron-based superconductors: The magnetic scenario

TL;DR

This paper provides a theoretical explanation for Raman spectroscopy features in iron-based superconductors, linking them to magnetic fluctuations and nematic susceptibility, and discusses possible origins of the superconducting resonance.

Contribution

It introduces a comprehensive theoretical framework connecting Raman features to magnetic fluctuations and nematic susceptibility, including two scenarios for the superconducting resonance.

Findings

Raman response features explained by fermion coupling to magnetic fluctuations

Temperature dependence linked to Aslamazov-Larkin vertex and nematic susceptibility

Two scenarios proposed for the superconducting resonance mechanism

Abstract

We perform theoretical analysis of polarization-sensitive Raman spectroscopy on NaFe$_{1-x}$Co$_x$As, EuFe$2$As$_2$, SrFe$_2$As$_2$, and Ba(Fe$_{1-x}$Co$_x$)$_2$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 scenarios for the resonance below $T_c$. In one, the resonance is due to the development of a pole in the fully renormalized Ising-nematic susceptibility. The other is the orbital excitonic scenario, in which spin fluctuations generate an attractive interaction between low-energy fermions.