Roles of Spin Fluctuations and Nematic Fluctuations for Superconductivity in FeSe(S,Te) Superconductors

While superconductivity mediated by spin or nematic fluctuations is theoretically well-established in iron-based superconductors (IBSCs), experimental identification has remained ambiguous. A significant challenge is the frequent intertwining of magnetism and nematicity in most IBSCs. However, sulfur or tellurium-doped FeSe systems exhibit isolated nematic phases without magnetic ordering, offering an excellent platform to investigate this issue. We conducted ⁷⁷Se nuclear magnetic resonance studies of FeSe(S,Te) to elucidate the roles of spin and nematic fluctuations in the superconductivity of this system. By systematic comparing spin fluctuations and the superconducting transition temperature, our findings reveal a change in the pairing mechanism across the phase diagram of FeSe(S,Te).