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

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 ($\epsilon$) tuning of the superconducting transition temperature $T_c$ [$\Delta T_c(\epsilon )=\alpha \epsilon +\beta {\epsilon }^2$] 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 $\epsilon$ in optimally doped BaFe${}_2$As${}_2$, $\Delta T_c(\epsilon )$ in FeSe${}_{1-x}$S${}_x$ dominated by $\Delta T_c(\epsilon )=\beta {\epsilon }^2$ changes its sign from $\beta$ < $0$ in FeSe to $\beta$ > $0$ in FeSe${}_{1-x}$S${}_x$ ($x\gtrsim 0.10$), indicating an evolution of the pairing symmetry from an $s_{\pm }$ state towards an $s+d$ wave state. These findings highlight the $\Delta T_c(\epsilon )$ 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.

6 pages, 4 figures. Supplementary is available upon reasonable request