A balancing act: Evidence for a strong subdominant d-wave pairing channel in ${\rm Ba_{0.6}K_{0.4}Fe_2As_2}$

TL;DR

This study provides evidence for a strong subdominant d-wave pairing channel in Ba0.6K0.4Fe2As2, revealing a Bardasis-Schrieffer exciton in Raman spectra and suggesting d-wave may be the dominant pairing symmetry in certain Fe-based superconductors.

Contribution

The paper demonstrates the existence of a significant subdominant d-wave pairing channel in Ba0.6K0.4Fe2As2 through Raman spectroscopy and theoretical analysis, indicating potential d-wave dominance.

Findings

Emergent sharp mode in Raman spectra below Tc

Binding energy suggests strong d-wave coupling

D-wave channel as strong as 60% of s+- channel

Abstract

We present an analysis of the Raman spectra of optimally doped ${\rm Ba_{0.6}K_{0.4}Fe_2As_2}$ based on LDA band structure calculations and the subsequent estimation of effective Raman vertices. Experimentally a narrow, emergent mode appears in the $B_{1g}$ ($d_{x^2-y^2}$) Raman spectra only below $T_c$, well into the superconducting state and at an energy below twice the energy gap on the electron Fermi surface sheets. The Raman spectra can be reproduced quantitatively with estimates for the magnitude and momentum space structure of the s$_{+-}$ pairing gap on different Fermi surface sheets, as well as the identification of the emergent sharp feature as a Bardasis-Schrieffer exciton, formed as a Cooper pair bound state in a subdominant $d_{x^2-y^2}$ channel. The binding energy of the exciton relative to the gap edge shows that the coupling strength in this subdominant $d_{x^2-y^2}$ channel is as strong as 60% of that in the dominant $s_{+-}$ channel. This result suggests that $d_{x^2-y^2}$ may be the dominant pairing symmetry in Fe-based sperconductors which lack central hole bands.