Evolution of pairing symmetry in FeSe$_{1-x}$S$_x$ as probed by uniaxial-strain tuning of $T_c$

TL;DR

This study uses uniaxial strain to investigate how the pairing symmetry in FeSe$_{1-x}$S$_x$ superconductors evolves from an $s_{ ext{±}}$ to an $s+d$ wave state, revealing the interplay between nematicity and superconductivity.

Contribution

It demonstrates that the strain-induced change in $T_c$ can serve as a probe for pairing symmetry evolution in nematic FeSCs, showing a transition from $s_{ ext{±}}$ to $s+d$ wave pairing.

Findings

$eta$ changes sign from negative to positive with doping.

$T_c$ suppression is quadratic in strain in optimally doped samples.

Evidence of pairing symmetry evolution from $s_{ ext{±}}$ to $s+d$ wave.

Abstract

In iron-based superconductors (FeSCs), the interplay between electronic nematicity and superconductivity is essential for understanding the exotic superconducting ground state. In the nematic regime, uniaxial-strain ($\varepsilon$) tuning of the superconducting transition temperature $T_c$ [$\Delta T_c(\varepsilon)=\alpha\varepsilon+\beta\varepsilon^2$] offers a unique approach to investigating the evolution of pairing symmetry if both $s$ and $d$ wave pairing instabilities are relevant. Here, we employ uniaxial strain to tune the $T_c$ of FeSe$_{1-x}$S$_x$, in which both nematicity and superconductivity undergo significant changes with doping. While $T_c$ is usually suppressed quadratically with $\varepsilon$ in optimally doped BaFe$_2$As$_2$, $\Delta T_c(\varepsilon)$ in FeSe$_{1-x}$S$_x$ dominated by $\Delta T_c(\varepsilon)=\beta\varepsilon^2$ changes its sign from $\beta$ < $0$ in FeSe to $\beta$ > $0$ in FeSe$_{1-x}$S$_x$ ($x\gtrsim0.10$), indicating an evolution of the pairing symmetry from an $s_{\pm}$ state towards an $s+d$ wave state. These findings highlight the $\Delta T_c(\varepsilon)$ as a powerful probe for elucidating the superconducting pairing symmetry in the nematic regime of FeSCs and provide new insights into the evolution of pairing symmetry in FeSCs.