High Tc superconductivity in CaKFe$_4$As$_4$ in absence of nematic fluctuations

TL;DR

This study uses polarization-resolved Raman spectroscopy to investigate CaKFe$_4$As$_4$, revealing the absence of nematic fluctuations and identifying multiple superconducting gap features, suggesting a sub-dominant d-wave interaction.

Contribution

It provides the first Raman spectroscopic evidence of the absence of nematic fluctuations in CaKFe$_4$As$_4$ and characterizes its superconducting gap structure and electron-phonon coupling.

Findings

No signatures of nematic fluctuations in Raman response.

Identification of three superconducting pair-breaking peaks.

Weak electron-phonon coupling estimated at λ^Γ=0.0015.

Abstract

We employ polarization-resolved Raman spectroscopy to study multi-band stoichiometric superconductor CaKFe$_4$As$_4$. The B$_{2g}$ symmetry Raman response shows no signatures of Pomeranchuk-like electronic nematic fluctuations which is observed for many other Fe-based superconductors. In the superconducting state, we identify three pair-breaking peaks at 13.8, 16.9 and 21 meV and full spectral weight suppression at low energies. The pair-breaking peak energies in Raman response are about 20% lower than twice the gap energies as measured by single-particle spectroscopy, implying a sub-dominant $d$-wave symmetry interaction. We analyze the superconductivity induced phonon self-energy effects and give an estimation of weak electron-phonon coupling constant $\lambda^\Gamma$=0.0015.