Evolution of pairing symmetry in FeSe1−xSx as probed by in-plane strain tuning of Tc

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, in-plane strain (ε) tuning of the superconducting transition temperature Tc [∆Tc(ε) = αε + βε2] offers a unique approach to investigating the evolution of pairing symmetry if both s and d wave pairing instabilities are relevant. Here, we employ in-plane strain to tune the Tc of FeSe1−xSx, in which both nematicity and superconductivity un?dergo significant changes with doping. While Tc is usually suppressed quadratically with ε in optimally doped BaFe2As2, ∆Tc(ε) in FeSe1−xSx dominated by ∆Tc(ε) = βε2 changes its sign from β < 0 in FeSe to β > 0 in FeSe1−xSx (x ≳ 0.10), indicating an evolution of the pairing symmetry from an s± state towards an s + d wave state. These findings highlight the ∆Tc(ε) 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.