Microscopic pairing fingerprint of the iron-based superconductor ${\rm Ba_{1-x}K_xFe_2As_2}$

TL;DR

This study uses Raman spectroscopy combined with theoretical modeling to identify the dominant and sub-dominant pairing interactions in the iron-based superconductor Ba_{1-x}K_xFe_2As_2, revealing a spin-fluctuation mediated pairing mechanism.

Contribution

It demonstrates how Raman spectroscopy can distinguish multiple pairing tendencies and confirms their nature through fRG and RPA calculations in an unconventional superconductor.

Findings

Identification of the leading s-wave pairing state.

Detection of two collective modes indicating sub-dominant d-wave tendencies.

Theoretical confirmation of multiple pairing channels matching experimental doping dependence.

Abstract

Resolving the microscopic pairing mechanism and its experimental identification in unconventional superconductors is among the most vexing problems of contemporary condensed matter physics. We show that Raman spectroscopy provides an avenue for this quest by probing the structure of the pairing interaction at play in an unconventional superconductor. As we study the spectra of the prototypical Fe-based superconductor ${\rm Ba_{1-x}K_xFe_2As_2}$ for $0.22\le x \le 0.70$ in all symmetry channels, Raman spectroscopy allows us to distill the leading $s$-wave state. In addition, the spectra collected in the $B_{1g}$ symmetry channel reveal the existence of two collective modes which are indicative of the presence of two competing, yet sub-dominant, pairing tendencies of $d_{x^2-y^2}$ symmetry type. A comprehensive functional Renormalization Group (fRG) and random-phase approximation (RPA) study on this compound confirms the presence of the two sub-leading channels, and consistently matches the experimental doping dependence of the related modes. The synopsis of experimental evidence and theoretical modelling supports a spin-fluctuation mediated superconducting pairing mechanism.