Nematic resonance in the Raman response of iron-based superconductors

TL;DR

This paper investigates how nematic correlations near a quantum critical point modify the Raman response in iron-based superconductors, revealing a nematic resonance below twice the superconducting gap that aligns with experimental observations.

Contribution

It introduces the concept of a nematic resonance in the Raman response, highlighting how nematic fluctuations alter the pair-breaking peak in superconductors.

Findings

Nematic correlations transform the pair-breaking peak into a nematic resonance.

The nematic resonance energy remains below 2Δ and is enhanced at the quantum critical point.

Experimental data on iron-based superconductors support the presence of nematic correlations.

Abstract

In a fully-gapped superconductor the electronic Raman response has a pair-breaking peak at twice the superconducting gap $\Delta$, if the Bogoliubov excitations are uncorrelated. Motivated by the iron based superconductors, we study how this peak is modified if the superconducting phase hosts a nematic-structural quantum critical point. We show that, upon approaching this point by tuning, e.g., doping, the growth of nematic correlations between the quasiparticles transforms the pair-breaking peak into a nematic resonance. The mode energy is below 2$\Delta$, and stays finite at the quantum critical point, where its spectral weight is sharply enhanced. The latter is consistent with recent experiments on electron-doped iron based superconductors and provides direct evidence of nematic correlations in their superconducting phases.