Superconductivity and fluctuation in Ba$_{1-p}$K$_p$Fe$_2$As$_2$ and Ba(Fe$_{1-n}$Co$_n$)$_2$As$_2$

TL;DR

This study investigates the role of spin fluctuations and their relation to superconductivity in BaFe$_2$As$_2$ compounds doped with potassium and cobalt, revealing that fluctuations are present but may not significantly influence pairing.

Contribution

It provides detailed Raman spectroscopic analysis of fluctuations and gap excitations across different doping levels, highlighting the limited impact of fluctuations on Cooper pairing.

Findings

Fluctuations are stronger in Co-doped samples and originate from specific spin exchanges.

Superconducting gaps are strongly anisotropic with near-nodal features in Co-doped samples.

The presence of Bardasis-Schrieffer modes suggests fluctuations influence pairing only in certain doping regimes.

Abstract

We study the interplay of fluctuations and superconductivity in BaFe$_2$As$_2$ (Ba-122) compounds with Ba and Fe substituted by K ($p$ doping) and Co ($n$ doping), respectively. To this end we measured electronic Raman spectra as a function of polarisation and temperature. We observe gap excitations and fluctuations for all doping levels studied. The response from fluctuations is much stronger for Co substitution and, according to the selection rules and the temperature dependence, originates from the exchange of two critical spin fluctuations with characteristic wave vectors $(\pm\pi, 0)$ and $(0,\pm\pi)$. At 22\% K doping ($p=0.22$), we find the same selection rules and spectral shape for the fluctuations but the intensity is smaller by a factor of 5. Since there exists no nematic region above the orthorhombic spin-density-wave (SDW) phase the identification of the fluctuations via the temperature dependence is not possible. The gap excitations in the superconducting state indicate strongly anisotropic near-nodal gaps for Co substitution which make the observation of collective modes difficult. The variation with doping of the spectral weights of the $A_{1g}$ and $B_{1g}$ gap features does not support the influence of fluctuations on Cooper pairing. Therefore, the observation of Bardasis-Schrieffer modes inside the nearly clean gaps on the K-doped side remains the only experimental evidence for the relevance of fluctuations for pairing.