Argument on superconductivity pairing mechanism from cobalt impurity doping in FeSe: spin ($s_{\pm}$) or orbital ($s_{++}$) fluctuation

TL;DR

This study investigates how cobalt impurity doping affects superconductivity in FeSe to determine whether spin ($s_{\u2212\u2212}$) or orbital ($s_{++}$) fluctuations are responsible for pairing, providing insights into the pairing mechanism.

Contribution

It provides experimental evidence that challenges the $s_{\u2212\u2212}$ pairing model by showing weaker suppression of $T_c$ than predicted, suggesting a different pairing mechanism in FeSe.

Findings

Superconductivity and structural/orbital order are suppressed by Co doping and vanish above x=0.036.

The suppression of $T_c$ is weaker than expected for a sign-reversal $s_{\u2212\u2212}$ model.

Results favor an orbital fluctuation-mediated pairing mechanism in FeSe.

Abstract

In high-superconducting transition temperature ($T_{\rm c}$) iron-based superconductors, interband sign reversal ($s_{\rm \pm}$) and sign preserving ($s_{\rm ++}$) $s$-wave superconducting states have been primarily discussed as the plausible superconducting mechanism. We study Co impurity scattering effects on the superconductivity in order to achieve an important clue on the pairing mechanism using single crystal Fe$_{1-x}$Co$_x$Se and depict a phase diagram of a FeSe system. Both superconductivity and structural transition / orbital order are suppressed by the Co replacement on the Fe sites and disappear above $x$ = 0.036. These correlated suppressions represent a common background physics behind these physical phenomena in the multiband Fermi surfaces of FeSe. By comparing experimental data and theories so far proposed, the suppression of $T_{\rm c}$ against the residual resistivity is shown to be much weaker than that predicted in the case of a general sign reversal and a full gap $s_{\pm}$ models. The origin of the superconducting paring in FeSe is discussed in terms of its multiband electronic structure.